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Volume 33 Issue 5

May  2016

Article Contents
Article >  ADVANCES IN ATMOSPHERIC SCIENCES  > 2016 >  33(5): 599-613

Impact of Atmospheric and Oceanic Conditions on the Frequency and Genesis Location of Tropical Cyclones over the Western North Pacific in 2004 and 2010

  • 1.

    College of Meteorology and Oceanography, PLA University of Science and Technology, Nanjing 211101

  • 2.

    International Center for Climate and Environment Science, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029

  • 3.

    Beijing Institute of Aeronautical Meteorology, Beijing 100085


doi: 10.1007/s00376-015-5046-2

  • This study examines the impact of atmospheric and oceanic conditions during May-August of 2004 and 2010 on the frequency and genesis location of tropical cyclones over the western North Pacific. Using the WRF model, four numerical experiments were carried out based on different atmospheric conditions and SST forcing. The numerical experiments indicated that changes in atmospheric and oceanic conditions greatly affect tropical cyclone activity, and the roles of atmospheric conditions are slightly greater than oceanic conditions. Specifically, the total number of tropical cyclones was found to be mostly affected by atmospheric conditions, while the distribution of tropical cyclone genesis locations was mainly related to oceanic conditions, especially the distribution of SST. In 2010, a warmer SST occurred west of 140°E, with a colder SST east of 140°E. On the one hand, the easterly flow was enhanced through the effect of the increase in the zonal SST gradient. The strengthened easterly flow led to an anomalous boundary layer divergence over the region to the east of 140°E, which suppressed the formation of tropical cyclones over this region. On the other hand, the colder SST over the region to the east of 140°E led to a colder low-level air temperature, which resulted in decreased CAPE and static instability energy. The decrease in thermodynamic energy restricted the generation of tropical cyclones over the same region.
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  • Bengtsson L., K. I. Hodges, M. Esch, N. Keenlyside, L. Kornblueh, J. J. Luo, and T. Yamagata, 2007: How may tropical cyclones change in a warmer climate? Tellus, 59( 4), 539- 561.10.1111/j.1600-0870.2007.00251.x9e4f45e1-f3b7-4f75-a93d-92c0574142d5decde5c4f07bcf285183fc4de08a9b70http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1111%2Fj.1600-0870.2007.00251.x%2Ffullrefpaperuri:(201e45b204c1bc48ad003876e626aaaa)http://onlinelibrary.wiley.com/doi/10.1111/j.1600-0870.2007.00251.x/fullTropical cyclones (TC) under different climate conditions in the Northern Hemisphere have been investigated with the Max Planck Institute (MPI) coupled (ECHAM5/MPI-OM) and atmosphere (ECHAM5) climate models. The intensity and size of the TC depend crucially on resolution with higher wind speed and smaller scales at the higher resolutions. The typical size of the TC is reduced by a factor of 2.3 from T63 to T319 using the distance of the maximum wind speed from the centre of the storm as a measure. The full three-dimensional structure of the storms becomes increasingly more realistic as the resolution is increased. For the T63 resolution, three ensemble runs are explored for the period 1860 until 2100 using the IPCC SRES scenario A1B and evaluated for three 30 yr periods at the end of the 19th, 20th and 21st century, respectively. While there is no significant change between the 19th and the 20th century, there is a considerable reduction in the number of the TC by some 20% in the 21st century, but no change in the number of the more intense storms. Reduction in the number of storms occurs in all regions. A single additional experiment at T213 resolution was run for the two latter 30-yr periods. The T213 is an atmospheric only experiment using the transient sea surface temperatures (SST) of the T63 resolution experiment. Also in this case, there is a reduction by some 10% in the number of simulated TC in the 21st century compared to the 20th century but a marked increase in the number of intense storms. The number of storms with maximum wind speeds greater than 50 m s 1 increases by a third. Most of the intensification takes place in the Eastern Pacific and in the Atlantic where also the number of storms more or less stays the same. We identify two competing processes effecting TC in a warmer climate. First, the increase in the static stability and the reduced vertical circulation is suggested to contribute to the reduction in the number of storms. Second, the increase in temperature and water vapour provide more energy for the storms so that when favourable conditions occur, the higher SST and higher specific humidity will contribute to more intense storms. As the maximum intensity depends crucially on resolution, this will require higher resolution to have its full effect. The distribution of storms between different regions does not, at first approximation, depend on the temperature itself but on the distribution of the SST anomalies and their influence on the atmospheric circulation. Two additional transient experiments at T319 resolution where run for 20 yr at the end of the 20th and 21st century, respectively, using the same conditions as in the T213 experiments. The results are consistent with the T213 study. The total number of TC were similar to the T213 experiment but were generally more intense. The change from the 20th to the 21st century was also similar with fewer TC in total but with more intense cyclones.
    Broccoli A. J., S. Manabe, 1990: Can existing climate models be used to study anthropogenic changes in tropical cyclone climate? Geophys. Res. Lett., 17( 11), 1917- 1920.
    Camargo S. J., S. E. Zebiak, 2002: Improving the detection and tracking of tropical cyclones in atmospheric general circulation models. Wea. Forecasting, 17( 6), 1152- 1162.10.1049/pi-a.1962.01507f8273ae-e96d-47ff-91f9-283abafd80396a4023f482d0c1119e79a46cc9800142http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D5242661refpaperuri:(e986ddea60f1c86322cdd54ba1a72ffd)http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5242661Dynamical seasonal forecasts of tropical storm frequency require robust and efficient algorithms for detection and tracking of tropical storms in atmospheric general circulation models (AGCMs). Tropical storms are generally detected when dynamic and thermodynamic variables meet specified criteria. Here, it is shown that objectively defined model- and basin-dependent detection criteria improve simulations of tropical storm climatology and interannual variability in low-resolution AGCMs. An improved tracking method provides more realistic tracking and accurate counting of storms.
    Camargo S. J., A. H. Sobel, 2005: Western North Pacific tropical cyclone intensity and ENSO. J. Climate, 18( 15), 2996- 3006.10.1175/JCLI3457.1f4a2f6bf47d61f1b984d7765878a520dhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2005JCli...18.2996Chttp://adsabs.harvard.edu/abs/2005JCli...18.2996CAbstract The influence of the El Ni09o–Southern Oscillation (ENSO) on tropical cyclone intensity in the western North Pacific basin is examined. Accumulated cyclone energy (ACE), constructed from the best-track dataset for the region for the period 1950–2002, and other related variables are analyzed. ACE is positively correlated with ENSO indices. This and other statistics of the interannually varying tropical cyclone distribution are used to show that there is a tendency in El Ni09o years toward tropical cyclones that are both more intense and longer-lived than in La Ni09a years. ACE leads ENSO indices: during the peak season (northern summer and fall), ACE is correlated approximately as strongly with ENSO indices up to six months later (northern winter), as well as simultaneously. It appears that not all of this lead–lag relationship is easily explained by the autocorrelation of the ENSO indices, though much of it is. Interannual variations in the annual mean lifetime, intensity, and number of tropical cyclones all contribute to the ENSO signal in ACE, though the lifetime effect appears to be the most important of the three.
    Cha D. H., Y. Q. Wang, 2013: A dynamical initialization scheme for real-time forecasts of tropical cyclones using the WRF model. Mon. Wea. Rev., 141( 3), 964- 986.10.1175/MWR-D-12-00077.1ce73077f74b59d867f728be643481c87http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2013MWRv..141..964Chttp://adsabs.harvard.edu/abs/2013MWRv..141..964CAbstract To improve the initial conditions of tropical cyclone (TC) forecast models, a dynamical initialization (DI) scheme using cycle runs is developed and implemented into a real-time forecast system for northwest Pacific TCs based on the Weather Research and Forecasting (WRF) Model. In this scheme, cycle runs with a 6-h window before the initial forecast time are repeatedly conducted to spin up the axisymmetric component of the TC vortex until the model TC intensity is comparable to the observed. This is followed by a 72-h forecast using the Global Forecast System (GFS) prediction as lateral boundary conditions. In the DI scheme, the spectral nudging technique is employed during each cycle run to reduce bias in the large-scale environmental field, and the relocation method is applied after the last cycle run to reduce the initial position error. To demonstrate the effectiveness of the proposed DI scheme, 69 forecast experiments with and without the DI are conducted for 13 TCs over the northwest Pacific in 2010 and 2011. The DI shows positive effects on both track and intensity forecasts of TCs, although its overall skill depends strongly on the performance of the GFS forecasts. Compared to the forecasts without the DI, on average, forecasts with the DI reduce the position and intensity errors by 10% and 30%, respectively. The results demonstrate that the proposed DI scheme improves the initial TC vortex structure and intensity and provides warm physics spinup, producing initial states consistent with the forecast model, thus achieving improved track and intensity forecasts.
    Chan J. C. L., 1985: Tropical cyclone activity in the northwest Pacific in relation to the El Niñ0/Southern Oscillation phenomenon. Mon. Wea. Rev., 113( 4), 599- 606.10.1175/1520-0493(1985)113<0599:TCAITN>2.0.CO;2513ba66e-b46c-4169-9d76-5d59b488a1bb232a4ba68862471024cdd0b0d4640d52http%3A%2F%2Fonlinelibrary.wiley.com%2Fresolve%2Freference%2FADS%3Fid%3D1985MWRv..113..599Crefpaperuri:(f49c986da99e8f70e6a47025cf636b4c)http://onlinelibrary.wiley.com/resolve/reference/ADS?id=1985MWRv..113..599CAbstract The interannual variations in tropical cyclone activity in the northwest Pacific (NWPAC) and their relationships with the El Ni09o/Southern Oscillation (ENSO) phenomenon were studied using the method of spectral analyses. Time series of a Southern Oscillation Index (SOI, defined as the sea-level pressure difference between Easter Island and Darwin) and tropical cyclone activity in the entire (NWPAC) ocean basin as well as in different regions of the NWPAC were analyzed. Two spectral peaks are apparent in all these time series. One corresponds to the generally accepted Southern Oscillation with a period of 653 to 3.5 years and another at the quasi-biennial oscillation (QBO) frequency. Cross-spectral analyses between the SOI and tropical cyclone activity show significant coherence in these two spectral peaks. The dominant peak is at the Southern Oscillation frequency with the SOI leading typhoon activity by almost a year. At the QBO frequency, the two series are almost in phase. Cyclone activity in the eastern part of NWPAC, however, is 65180° out of phase with the SOI series at the Southern Oscillation frequency. It appears that fluctuations of cyclone activity at the dominant Southern Oscillation frequency may be explained in terms of the change in the horizontal and vertical circulations in the atmosphere during periods of low SOI. The establishment of an anomalous Walker Circulation shifts areas of enhanced or suppressed convection, leading to the observed variations in cyclone activity.
    Chan J. C. L., 2000: Tropical cyclone activity over the western North Pacific associated with El Niñ0 and La Niña events. J. Climate, 13( 16), 2960- 2972.
    Chen G. H., R. H. Huang, 2006: The effect of warm pool thermal states on tropical cyclone in West Northwest Pacific. J. Trop. Meteor., 22( 6), 527- 532. (in Chinese)10.1016/S1872-2032(06)60022-Xec1ee05b0dc2a384a7b4a4205908ec12http%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTOTAL-RDQX200606002.htmhttp://en.cnki.com.cn/Article_en/CJFDTOTAL-RDQX200606002.htmThe influence of thermal states in the warm pool on tropical cyclones (TCs) in west North Pacific is investigated. There are fewer typhoons during warm years of the warm pool in which tropical storms tend to form in the northwest quadrant and move westward. Inversely, the number of typhoons, which tend to recurve northeastward to the southeast of Japan, increases in the southeast quadrant during cold years. Based on a composite analysis, the circulation-induced dynamic factors rather than thermal factors are identified as responsible for TCs activities. During the warm state, the monsoon trough retreats westwards, which leads to anomalous vorticity in low-level and divergence in high-level in the western part of west Pacific. Above-normal TCs activity is found in this area. Furthermore, wind anomalies in 500hPa determine the main track types. On the contrary, when the warm pool is in cold state, the atmospheric circulation is related to the formation of more TCs in the southeast quadrant. There are easterly shear in the south and westerly shear in the north of northwest Pacific, which is favorable for tropical cyclone development.
    Chou M. D., M. J. Suarez, 1994: An efficient thermal infrared radiation parameterization for use in general circulation models. NASA Tech. Memo,84 pp.b0bc9a47c9147ed3d9ee73e7355bd6c8http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F2329334_An_Efficient_Thermal_Infrared_Radiation_Parameterization_For_Use_In_General_Circulation_Modelshttp://www.researchgate.net/publication/2329334_An_Efficient_Thermal_Infrared_Radiation_Parameterization_For_Use_In_General_Circulation_Models
    Dare R. A., J. L. McBride, 2011: Sea surface temperature response to tropical cyclones. Mon. Wea. Rev., 139( 12), 3798- 3808.10.1175/MWR-D-10-05019.17c3a40f56715c0da15d1396a28ec70edhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2011MWRv..139.3798Dhttp://adsabs.harvard.edu/abs/2011MWRv..139.3798DThe response of sea surface temperature (SST) to tropical cyclones is studied using gridded SST data and global cyclone tracks from the period 1981-2008. A compositing approach is used whereby temperature time series before and after cyclone occurrence at individual cyclone track positions are averaged together. Results reveal a variability of several days in the time of maximum cooling with respect to cyclone passage, with the most common occurrence 1 day after cyclone passage. When compositing is carried out relative to the day of maximum cooling, the global average response to cyclone passage is a local minimum SST anomaly of 0903’0.900°C. The recovery of the ocean to cyclone passage is generally quite rapid with 44%% of the data points recovering to climatological SST within 5 days, and 88%% of the data points recovering within 30 days. Although differences exist between the mean results from the separate tropical cyclone basins, they are in broad agreement with the global mean results. Storm intensity and translation speed affect both the size of the SST response and the recovery time. Cyclones occurring in the first half of the cyclone season disrupt the seasonal warming trend, which is not resumed until 20-30 days after cyclone passage. Conversely, cyclone occurrences in the later half of the season bring about a 0.500°C temperature drop from which the ocean does not recover due to the seasonal cooling cycle.
    Emanuel K. A., 2005: Divine Wind,the History and Science of Hurricanes. Oxford University Press, New York, 296 pp.001f0fdcbaf8ad6d3fe9df767ce57439http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2005diwi.book.....Ehttp://adsabs.harvard.edu/abs/2005diwi.book.....EImagine standing at the center of a Roman coliseum that is 20 miles across, with walls that soar 10 miles into the sky, towering walls with cascades of ice crystals falling along its brilliantly white surface. That's what it's like to stand in the eye of a hurricane. In Divine Wind , Kerry Emanuel, one of the world's leading authorities on hurricanes, gives us an engaging account of these awe-inspiring meteorological events, revealing how hurricanes and typhoons have literally altered human history, thwarting military incursions and changing the course of explorations. Offering an account of the physics of the tropical atmosphere, the author explains how such benign climates give rise to the most powerful storms in the world and tells what modern science has learned about them. Interwoven with this scientific account are descriptions of some of the most important hurricanes in history and relevant works of art and literature. For instance, he describes the 17th-century hurricane that likely inspired Shakespeare's The Tempest and that led to the British colonization of Bermuda. We also read about the Galveston Hurricane of 1900, by far the worst natural calamity in U.S. history, with a death toll between 8,000 and 12,000 that exceeded the San Francisco earthquake, the Johnstown Flood, and the Okeechobee Hurricane combined. Boasting more than one hundred color illustrations, from ultra-modern Doppler imagery to classic paintings by Winslow Homer, Divine Wind captures the profound effects that hurricanes have had on humanity. Its fascinating blend of history, science, and art will appeal to weather junkies, science buffs, and everyone who read Isaac's Storm .
    Emanuel K. A., 2013: Downscaling CMIP5 climate models shows increased tropical cyclone activity over the 21st century. Proceedings of the National Academy of Sciences of United States of America, 110( 30), 12219- 12224.10.1073/pnas.1301293110238366464023907795b7da4b30c99b7676a03e0bhttp%3A%2F%2Fmed.wanfangdata.com.cn%2FPaper%2FDetail%2FPeriodicalPaper_PM23836646http://med.wanfangdata.com.cn/Paper/Detail/PeriodicalPaper_PM23836646A recently developed technique for simulating large [O(10(4))] numbers of tropical cyclones in climate states described by global gridded data is applied to simulations of historical and future climate states simulated by six Coupled Model Intercomparison Project 5 (CMIP5) global climate models. Tropical cyclones downscaled from the climate of the period 1950-2005 are compared with those of the 21st century in simulations that stipulate that the radiative forcing from greenhouse gases increases by over preindustrial values. In contrast to storms that appear explicitly in most global models, the frequency of downscaled tropical cyclones increases during the 21st century in most locations. The intensity of such storms, as measured by their maximum wind speeds, also increases, in agreement with previous results. Increases in tropical cyclone activity are most prominent in the western North Pacific, but are evident in other regions except for the southwestern Pacific. The increased frequency of events is consistent with increases in a genesis potential index based on monthly mean global model output. These results are compared and contrasted with other inferences concerning the effect of global warming on tropical cyclones.
    Emanuel K. A., R. Sundararajan, and J. Williams, 2008: Hurricanes and global warming: Results from downscaling IPCC AR4 simulations. Bull. Amer. Meteor. Soc., 89( 3), 347- 367.10.1175/BAMS-89-3-347e02dc3798ddd814c0179eda78e9d149ahttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2008BAMS...89..347Ehttp://adsabs.harvard.edu/abs/2008BAMS...89..347EAbstract Changes in tropical cyclone activity are among the more potentially consequential results of global climate change, and it is therefore of considerable interest to understand how anthropogenic climate change may affect such storms. Global climate models are currently used to estimate future climate change, but the current generation of models lacks the horizontal resolution necessary to resolve the intense inner core of tropical cyclones. Here we review a new technique for inferring tropical cyclone climatology from the output of global models, extend it to predict genesis climatologies (rather than relying on historical climatology), and apply it to current and future climate states simulated by a suite of global models developed in support of the most recent Intergovernmental Panel on Climate Change report. This new technique attacks the horizontal resolution problem by using a specialized, coupled ocean-揳tmosphere hurricane model phrased in angular momentum coordinates, which provide a high resolution of the core at low cost. This model is run along each of 2,000 storm tracks generated using an advection-and-beta model, which is, in turn, driven by large-scale winds derived from the global models. In an extension to this method, tracks are initiated by randomly seeding large areas of the tropics with weak vortices and then allowing the intensity model to determine their survival, based on large-scale environmental conditions. We show that this method is largely successful in reproducing the observed seasonal cycle and interannual variability of tropical cyclones in the present climate, and that it is more modestly successful in simulating their spatial distribution. When applied to simulations of global climate with double the present concentration of carbon dioxide, this method predicts substantial changes and geographic shifts in tropical cyclone activity, but with much variation among the global climate models used. Basinwide power dissipation and storm intensity generally increase with global warming, but the results vary from model to model and from basin to basin. Storm frequency decreases in the Southern Hemisphere and north Indian Ocean, increases in the western North Pacific, and is indeterminate elsewhere. We demonstrate that in these simulations, the change in tropical cyclone activity is greatly influenced by the increasing difference between the moist entropy of the boundary layer and that of the middle troposphere as the climate warms.
    Fierro A. O., R. F. Rogers, F. D. Marks, and D. S. Nolan, 2009: The impact of horizontal grid spacing on the microphysical and kinematic structures of strong tropical cyclones simulated with the WRF-ARW model. Mon. Wea. Rev., 137( 11), 3717- 3743.10.1175/2009MWR2946.187e188134ff77f8718352e52e53341f6http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2009MWRv..137.3717Fhttp://adsabs.harvard.edu/abs/2009MWRv..137.3717FUsing the Advanced Weather Research and Forecasting numerical model, the impact of horizontal grid spacing on the microphysical and kinematic structure of a numerically simulated tropical cyclone (TC), and their relationship to storm intensity was investigated with a set of five numerical simulations using input data for the case of Hurricane Rita (2005). The horizontal grid spacing of the parent domain was systematically changed such that the horizontal grid spacing of the inner nest varied from 1 to 5 km by an increment of 1 km, this while keeping geographical dimensions of the domains identical. Within this small range of horizontal grid spacing, the morphology of the simulated storms and the evolution of the kinematic and microphysics field showed noteworthy differences. As grid spacing increased, the model produced a wider, more tilted eyewall, a larger radius of maximum winds, and higher-amplitude, low wavenumber eyewall asymmetries. The coarser-resolution simulations also produced larger volume, areal coverage, and mass flux of updraft speeds =5 m s-1; larger volumes of condensate and ice-phase particles aloft; larger boundary layer kinetic energy; and a stronger secondary circulation. While the contribution of updrafts =5 m s-1 to the total updraft mass flux varied little between the five cases, the contribution of downdrafts =-2 m s-1 to the total downdraft mass flux was by far the largest in the finest-resolution simulation. Despite these structural differences, all of the simulations produced storms of similar intensity, as measured by peak 10-m wind speed and minimum surface pressure, suggesting that features in the higher-resolution simulations that tend to weaken TCs (i.e., smaller area of high surface fluxes and weaker total updraft mass flux) compensate for features that favor TC intensity (i.e., smaller-amplitude eyewall asymmetries and larger radial gradients). This raises the possibility that resolution increases in this range may not be as important as other model features (e.g., physical parameterization and initial condition improvements) for improving TC intensity forecasts.
    Fu B., T. Li, M. Peng, and F. Weng, 2007: Analysis of tropical cyclone genesis in the western North Pacific for 2000 and 2001. Wea. Forecasting, 22( 4), 763- 780.
    Gao J. Y., X. Z. Zhang, Z. H. Jiang, and L. J. You, 2008: Anomalous western North Pacific monsoon trough and tropical cyclone activities. Acta Oceanologica Sinica, 30( 3), 35- 47. (in Chinese)268da7da-cd40-4d5e-bfee-95859749f473124262008303885f7d094409ae594045b6f7badd6e6c0http%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTOTAL-SEAC200803005.htmhttp://en.cnki.com.cn/Article_en/CJFDTOTAL-SEAC200803005.htmA general survey was made of 1979-2005 tropical cyclones(TC) before a sequence is established of the western North Pacific monsoon trough(MT) tropical cyclones(MTTC).Statistics show that in May October,1979-2005,TCs originating from the South China Sea(SCS) and western North Pacific(WNP) totaled 672,of which 491 had the origin in the monsoon trough(i.e.,MTTCs),making up 73.1% of the total and 79.2% of the TCs landing on China in this period.It follows that grasp of their activity features means the grasp of the main activity laws of WNP and China-landing TCs.Based on the analysis of day-to-day circulation fields the MTs are categorized as(1) SCS type,(2) SCS-WNP type,(3) reversal type,(4) triple-flow type and(5) WNP type.In terms of the strength and position of May to October monsoon troughs,subtropical highs and cross-equatorial flows on an annual basis,the 1979-2005 monsoon troughs are separated into such annual types as the southwest-northwest-,and eastward of mean as well as normal.For the first three MT anomalous annual types diagnostic study is conducted of TC formation favorable SST and large-scale circulation fields,vapor transport,atmospheric apparent heat sources,apparent vapor sinks,and zonal wind vertical shear,indicating that for the different MT annual types differences in Pacific SST fields cause the disparity in the intensity of the Hadley and Walker circulations,the positions of western Pacific subtropical high and South Asian high as well as the intensity of cross-equatorial flows,thus producing TC genesis favorable thermal,dynamic and environmental conditions,leading to remarkable differences in MTTC genesis area,frequency,track and TC China-landing location.
    Gleixner S., N. Keenlyside, K. I. Hodges, W. L. Tseng, and L. Bengtsson, 2014: An inter-hemispheric comparison of the tropical storm response to global warming. Climate Dyn., 42( 7-8), 2147- 2157.10.1007/s00382-013-1914-66bae95a54009bb58603f018c77ac47a2http%3A%2F%2Flink.springer.com%2F10.1007%2Fs00382-013-1914-6http://onlinelibrary.wiley.com/doi/10.1111/j.1743-7563.2008.00158.x/pdfModel studies do not agree on future changes in tropical cyclone (TC) activity on regional scales. We aim to shed further light on the distribution, frequency, intensity, and seasonality of TCs that society can expect at the end of the twenty-first century in the Southern hemisphere (SH). Therefore, we investigate TC changes simulated by the atmospheric model ECHAM5 with T213 (~6002km) horizontal resolution. We identify TCs in present-day (20C; 1969–1990) and future (21C; 2069–2100) time slice simulations, using a tracking algorithm based on vorticity at 85002hPa. In contrast to the Northern hemisphere (NH), where tropical storm numbers reduce by 602%, there is a more dramatic 2202% reduction in the SH, mainly in the South Indian Ocean. While an increase of static stability in 21C may partly explain the reduction in tropical storm numbers, stabilization cannot alone explain the larger SH drop. Large-scale circulation changes associated with a weakening of the Tropical Walker Circulation are hypothesized to cause the strong decrease of cyclones in the South Indian Ocean. In contrast the decrease found over the South Pacific appears to be partly related to increased vertical wind shear, which is possibly associated with an enhanced meridional sea surface temperature gradient. We find the main difference between the hemispheres in changes of the tropical cyclones of intermediate strength with an increase in the NH and a decrease in the SH. In both hemispheres the frequency of the strongest storms increases and the frequency of the weakest storms decreases, although the increase in SH intense storms is marginal.
    Gualdi S., E. Scoccimarro, and A. Navarra, 2008: Changes in tropical cyclone activity due to global warming: Results from a high-resolution coupled general circulation model. J. Climate, 21( 20), 5204- 5228.10.1175/2008JCLI1921.1c2fc5982657f73dc49a1fd7c83af4a62http%3A%2F%2Fpapers.ssrn.com%2Fsol3%2Fpapers.cfm%3Fabstract_id%3D1366806http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1366806Abstract This study investigates the possible changes that greenhouse global warming might generate in the characteristics of tropical cyclones (TCs). The analysis has been performed using scenario climate simulations carried out with a fully coupled high-resolution global general circulation model. The capability of the model to reproduce a reasonably realistic TC climatology has been assessed by comparing the model results from a simulation of the twentieth century with observations. The model appears to be able to simulate tropical cyclone-搇ike vortices with many features similar to the observed TCs. The simulated TC activity exhibits realistic geographical distribution, seasonal modulation, and interannual variability, suggesting that the model is able to reproduce the major basic mechanisms that link TC occurrence with large-scale circulation. The results from the climate scenarios reveal a substantial general reduction of TC frequency when the atmospheric CO 2 concentration is doubled and quadrupled. The reduction appears particularly evident for the tropical western North Pacific (WNP) and North Atlantic (ATL). In the NWP the weaker TC activity seems to be associated with reduced convective instabilities. In the ATL region the weaker TC activity seems to be due to both the increased stability of the atmosphere and a stronger vertical wind shear. Despite the generally reduced TC activity, there is evidence of increased rainfall associated with the simulated cyclones. Finally, the action of the TCs remains well confined to the tropical region and the peak of TC number remains equatorward of 20 latitude in both hemispheres, notwithstanding the overall warming of the tropical upper ocean and the expansion poleward of warm SSTs.
    Haarsma R. J., J. F. B. Mitchell, and C. A. Senior, 1993: Tropical disturbances in a GCM. Climate Dyn., 8( 5), 247- 257.10.1007/BF001986193308dd11-d46d-4c4e-b624-0dfdae0f44c6eb212795a6e5bc20babea7ebd848a50ehttp%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2FBF00198619refpaperuri:(effa53e290305bf4a1ed3825460b27bc)http://link.springer.com/article/10.1007/BF00198619ABSTRACT We have analyzed the tropical disturbances in a 11-layer atmospheric general circulation model (GCM) on a 2.5 3.75 horizontal grid coupled to a 50 m-mixed layer ocean. Due to the coarse resolution, the GCM is unable to resolve adequately tropical cyclones. The tropical disturbances simulated by the GCM are much weaker and have a much larger horizontal extent. However, they still display much of the essential physics of tropical cyclones, including low-level convergence of mass and moisture, upper tropospheric outflow and a warm core. For most ocean basins the spatial and temporal distribution of the simulated tropical disturbances compares well with the observed tropical cyclones. On doubling the CO2 concentration, the number of simulated tropical disturbances increases by about 50%. There is a relative increase in the number of more intense tropical disturbances, whose maximum windspeed increases by about 20%. This agrees with the theoretical estimate of Emanuel. However, because the low-resolution of the GCM severely restricts their maximum possible intensity, simulated changes in tropical disturbance intensity should be interpreted cautiously.
    Held I. M., M. Zhao, 2011: The response of tropical cyclone statistics to an increase in CO2 with fixed sea surface temperatures. J. Climate, 24( 20), 5353- 5364.9ddf2e90-173a-4abf-86b1-315227dd0c87f4b5c77727752a25303aecd57e2735dfhttp%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2011JCli...24.5353H%26db_key%3DPHY%26link_type%3DABSTRACTrefpaperuri:(533c4b23f3bffbf2d9e0aa157d58f396)/s?wd=paperuri%3A%28533c4b23f3bffbf2d9e0aa157d58f396%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2011JCli...24.5353H%26db_key%3DPHY%26link_type%3DABSTRACT&ie=utf-8
    Ho C. H., J. H. Kim, H. S. Kim, C. H. Sui, and D. Y. Gong, 2005: Possible influence of the Antarctic Oscillation on tropical cyclone activity in the western North Pacific. J. Geophys. Res., 110( D19), D19104.10.1029/2005JD005766d75d8c224862c8e8673b0d51b529659chttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2005JD005766%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/2005JD005766/fullAbstract Top of page Abstract 1.Introduction 2.Data 3.AAO Index and Large-Scale Teleconnection Patterns 4.TC Activity Associated With AAO Variations 5.Concluding Remarks Acknowledgments References Supporting Information [1] The present study investigates how large-scale atmospheric circulation in the Southern Hemisphere (SH) modulates tropical cyclone (TC) activity in the western North Pacific (WNP) during a typhoon season (July, August, and September; boreal summer). The variation of the SH circulation of interest is the Antarctic Oscillation (AAO). In the positive phase of AAO relative to its negative phase, two anomalous highs develop over the western Pacific in both hemispheres: a huge anticyclone in southeastern Australia and a relatively weak anticyclone in the East China Sea. These teleconnection patterns are examined and compared with previous analyses. Related to the AAO variations, a statistically significant alteration of TC activities is found over the WNP. The difference in the mean TC passage numbers over the East China Sea (120°–140°E, 20°–40°N) between the eight highest-AAO years and the eight lowest-AAO years is as large as 2, equivalent to a 50–100% increase from the climatology. This change is primarily a result of more TCs forming over the eastern Philippine Sea. On the other hand, TC passage numbers slightly decrease over the South China Sea. These changes in TC activity are predominant in August and are consistent with changes in low-level vorticity over the subtropical WNP. The influence of SH circulation variability on large-scale environments and tropical convection in the subtropical NH suggest a possible usage of AAO variation for long-range forecasting of TC activity over the WNP.
    Holland, G. J., 1997: The maximum potential intensity of tropical cyclones. J. Atmos. Sci., 54( 21), 2519- 2541.10.1175/1520-0469(1997)054<2519:TMPIOT>2.0.CO;29135551c263fcc7d34469151c7211ffehttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1997JAtS...54.2519Hhttp://adsabs.harvard.edu/abs/1997JAtS...54.2519HFocuses on a study which looked at a thermodynamic approach to estimating the maximum potential intensity (MPI) of tropical cyclones. Methodology used in the study; What are cyclones; How cyclone intensity is measured; Findings of the study.
    Holland, G., C. L. Bruy\`ere, 2014: Recent intense hurricane response to global climate change. Climate Dyn., 42( 3-4), 617- 627.10.1007/s00382-013-1713-0de498f38f1ffc45189030362265c996fhttp%3A%2F%2Flink.springer.com%2F10.1007%2Fs00382-013-1713-0http://link.springer.com/10.1007/s00382-013-1713-0An Anthropogenic Climate Change Index (ACCI) is developed and used to investigate the potential global warming contribution to current tropical cyclone activity. The ACCI is defined as the difference between the means of ensembles of climate simulations with and without anthropogenic gases and aerosols. This index indicates that the bulk of the current anthropogenic warming has occurred in the past four decades, which enables improved confidence in assessing hurricane changes as it removes many of the data issues from previous eras. We find no anthropogenic signal in annual global tropical cyclone or hurricane frequencies. But a strong signal is found in proportions of both weaker and stronger hurricanes: the proportion of Category 4 and 5 hurricanes has increased at a rate of ~25–3002% per °C of global warming after accounting for analysis and observing system changes. This has been balanced by a similar decrease in Category 1 and 2 hurricane proportions, leading to development of a distinctly bimodal intensity distribution, with the secondary maximum at Category 4 hurricanes. This global signal is reproduced in all ocean basins. The observed increase in Category 4–5 hurricanes may not continue at the same rate with future global warming. The analysis suggests that following an initial climate increase in intense hurricane proportions a saturation level will be reached beyond which any further global warming will have little effect.
    Hong S. Y., Y. Noh, and J. Dudhia, 2006: A new vertical diffusion package with an explicit treatment of entrainment processes. Mon. Wea. Rev., 134( 9), 2318- 2341.10.1175/MWR3199.1afb57a27-5c9f-45c9-b677-05ded3e0e41c79f98ee85a3853a6bfee0ec84e90c901http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F252170327_A_New_Vertical_Diffusion_Package_with_an_Explicit_Treatment_of_Entrainment_Processesrefpaperuri:(fd0cba578920821c17be8d8e91464f15)http://www.researchgate.net/publication/252170327_A_New_Vertical_Diffusion_Package_with_an_Explicit_Treatment_of_Entrainment_ProcessesAbstract This paper proposes a revised vertical diffusion package with a nonlocal turbulent mixing coefficient in the planetary boundary layer (PBL). Based on the study of Noh et al. and accumulated results of the behavior of the Hong and Pan algorithm, a revised vertical diffusion algorithm that is suitable for weather forecasting and climate prediction models is developed. The major ingredient of the revision is the inclusion of an explicit treatment of entrainment processes at the top of the PBL. The new diffusion package is called the Yonsei University PBL (YSU PBL). In a one-dimensional offline test framework, the revised scheme is found to improve several features compared with the Hong and Pan implementation. The YSU PBL increases boundary layer mixing in the thermally induced free convection regime and decreases it in the mechanically induced forced convection regime, which alleviates the well-known problems in the Medium-Range Forecast (MRF) PBL. Excessive mixing in the mixed layer in the presence of strong winds is resolved. Overly rapid growth of the PBL in the case of the Hong and Pan is also rectified. The scheme has been successfully implemented in the Weather Research and Forecast model producing a more realistic structure of the PBL and its development. In a case study of a frontal tornado outbreak, it is found that some systematic biases of the large-scale features such as an afternoon cold bias at 850 hPa in the MRF PBL are resolved. Consequently, the new scheme does a better job in reproducing the convective inhibition. Because the convective inhibition is accurately predicted, widespread light precipitation ahead of a front, in the case of the MRF PBL, is reduced. In the frontal region, the YSU PBL scheme improves some characteristics, such as a double line of intense convection. This is because the boundary layer from the YSU PBL scheme remains less diluted by entrainment leaving more fuel for severe convection when the front triggers it.
    Horn M., Coauthors, 2014: Tracking scheme dependence of simulated tropical cyclone response to idealized climate simulations. J. Climate, 27( 24), 9197- 9213.5c55e726688b84335650ee9a6188646ahttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2014JCli...27.9197H/s?wd=paperuri%3A%28ed245fcc5c2a9978872a02fe49eba24f%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2014JCli...27.9197H&ie=utf-8
    Hoyos C. D., P. A. Agudelo, P. J. Webster, and J. A. Curry, 2006: Deconvolution of the factors contributing to the increase in global hurricane intensity. Science, 312( 5770), 94- 97.3287476319388207751876292223222216543416338153712c09c87fcf5cf0454ef126eb6f6http%3A%2F%2Fwww.bioone.org%2Fservlet%2Flinkout%3Fsuffix%3Di0026-6493-93-2-335-Hoyos1%26dbid%3D8%26doi%3D10.3417%252F0026-6493%282006%2993%5B335%253AMPAHFI%5D2.0.CO%253B2%26key%3D16543416/s?wd=paperuri%3A%28ade7b4d76aaf319388a20de77518f7c6%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fwww.bioone.org%2Fservlet%2Flinkout%3Fsuffix%3Di0026-6493-93-2-335-Hoyos1%26dbid%3D8%26doi%3D10.3417%252F0026-6493%282006%2993%5B335%253AMPAHFI%5D2.0.CO%253B2%26key%3D16543416&ie=utf-8
    Huang L. N., J. Y. Gao, J. Sun, and J. L. Wu, 2013: Abnormal climatic features of accumulated cyclone energy over the Northwest Pacific. Meteorological Monthly, 39( 8), 995- 1003. (in Chinese)41c9c735099a054e50b87f355b902a57http%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTOTAL-QXXX201308006.htmhttp://en.cnki.com.cn/Article_en/CJFDTOTAL-QXXX201308006.htmBased on JTWC tropical cyclone data,NCEP reanalysis data and Hadly Center SST data,the abnormal year's climatic features and background for accumulated cyclone energy(ACE) over the Northwest Pacific are analyzed.The results show that ACE has significant interannual and interdecadal variations.In the abnormal years of ACE,due to the notable difference of the longitude that the eastern flow from monsoon trough reaches,the passageway and intensity difference of cross-equatorical flows,and the location difference of subtropical high,the frequency,intensity and the persistence of typhoons are different.The abnormality of ACE is closely connected with tropical Pacific SSTA in May-ugust,When SST anomaly at equatorial central and eastern Pacific is positive and that of Southwest Pacific is negative,as a result of favorable cross-equatorical southern flow provided by downflow branches of abnormal Walker and Hadley Circulations,the year's ACE tends to be positively abnormal.Otherwise,ACE tends to be negatively abnormal.
    Janjić, Z. I., 1994: The step-mountain eta coordinate model: Further developments of the convection, viscous sublayer, and turbulence closure schemes. Mon. Wea. Rev., 122( 5), 927- 945.21adf0bb81ffbc01b483730fef95b787http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1994mwrv..122..927j/s?wd=paperuri%3A%28aac9b35271007f30b34d46f5bcb35c64%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1994mwrv..122..927j&ie=utf-8
    Kim H. S., G. A. Vecchi, T. R. Knutson, W. G. Anderson, T. L. Delworth, A. Rosati, F. R. Zeng, and M. Zhao, 2014: Tropical cyclone simulation and response to CO2 doubling in the GFDL CM2.5 high-resolution coupled climate model. J. Climate, 27( 21), 8034- 8054.10.1175/JCLI-D-13-00475.11c582291-105d-49d2-a513-a58105439605ffdeaf46a758ea8ff85d7d74bd583086http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F267567912_Tropical_Cyclone_Simulation_and_Response_to_CO2_Doubling_in_the_GFDL_CM2.5_High-Resolution_Coupled_Climate_Modelrefpaperuri:(cc64fd2160b268e6cf0ed6f39fb0615f)http://www.researchgate.net/publication/267567912_Tropical_Cyclone_Simulation_and_Response_to_CO2_Doubling_in_the_GFDL_CM2.5_High-Resolution_Coupled_Climate_ModelGlobal tropical cyclone (TC) activity is simulated by the Geophysical Fluid Dynamics Laboratory (GFDL) Climate Model, version 2.5 (CM2.5), which is a fully coupled global climate model with a horizontal resolution of about 50 km for the atmosphere and 25 km for the ocean. The present climate simulation shows a fairly realistic global TC frequency, seasonal cycle, and geographical distribution. The model has some notable biases in regional TC activity, including simulating too few TCs in the North Atlantic. The regional biases in TC activity are associated with simulation biases in the large-scale environment such as sea surface temperature, vertical wind shear, and vertical velocity. Despite these biases, the model simulates the large-scale variations of TC activity induced by El Ni01±o-Southern Oscillation fairly realistically. The response of TC activity in the model to global warming is investigated by comparing the present climate with a CO2 doubling experiment. Globally, TC frequency decreases (0903’19%) while the intensity increases (+2.7%) in response to CO2 doubling, consistent with previous studies. The average TC lifetime decreases by 0903’4.6%, while the TC size and rainfall increase by about 3% and 12%, respectively. These changes are generally reproduced across the different basins in terms of the sign of the change, although the percent changes vary from basin to basin and within individual basins. For the Atlantic basin, although there is an overall reduction in frequency from CO2 doubling, the warmed climate exhibits increased interannual hurricane frequency variability so that the simulated Atlantic TC activity is enhanced more during unusually warm years in the CO2-warmed climate relative to that in unusually warm years in the control climate.
    Knutson, T. R., Coauthors, 2010: Tropical cyclones and climate change. Nature Geoscience, 3( 3), 157- 163.10.1038/ngeo779bea02010-bf0f-435b-8a8a-ebfd472df0c9728fc728fd4bc9c06ee812b7720a2c0ehttp%3A%2F%2Fwww.nature.com%2Fngeo%2Fjournal%2Fv3%2Fn3%2Fabs%2Fngeo779.htmlrefpaperuri:(f55bfcc5db6cb0130b2314c07dd1fb05)http://www.nature.com/ngeo/journal/v3/n3/abs/ngeo779.htmlWhether the characteristics of tropical cyclones have changed or will change in a warming climate — and if so, how — has been the subject of considerable investigation, often with conflicting results. Large amplitude fluctuations in the frequency and intensity of tropical cyclones greatly complicate both the detection of long-term trends and their attribution to rising levels of atmospheric greenhouse gases. Trend detection is further impeded by substantial limitations in the availability and quality of global historical records of tropical cyclones. Therefore, it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes. However, future projections based on theory and high-resolution dynamical models consistently indicate that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2–11% by 2100. Existing modelling studies also consistently project decreases in the globally averaged frequency of tropical cyclones, by 6–34%. Balanced against this, higher resolution modelling studies typically project substantial increases in the frequency of the most intense cyclones, and increases of the order of 20% in the precipitation rate within 100 km of the storm centre. For all cyclone parameters, projected changes for individual basins show large variations between different modelling studies.
    Land er, M. A., 1994: An exploratory analysis of the relationship between tropical storm formation in the western North Pacific and ENSO. Mon. Wea. Rev., 122( 4), 636- 651.
    Lau K. H., N. C. Lau, 1990: Observed structure and propagation characteristics of tropical summertime synoptic scale disturbances. Mon. Wea. Rev., 118( 9), 1888- 1913.10.1175/1520-0493(1990)118<1888:OSAPCO>2.0.CO;27e1bac0a83d85475ef35af3d09978797http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1990MWRv..118.1888Lhttp://adsabs.harvard.edu/abs/1990MWRv..118.1888LAbstract The three-dimensional structure and propagation characteristics of tropical synoptic scale transients during the northern summer we studied with twice daily ECMWF global gridded analyses for the 1980-1987 period. Regions of enhanced variability in relative vorticity at 850 mb are identified in the western Pacific, eastern Pacific, Bay of Bengal/northern India and eastern Atlantic/western Africa sectors. Dominant spectral peaks with time scales ranging from 3 to 8 days are noted in the power spectra for these locations. The lag-correlation and regression statistics of tropical fluctuations with synoptic time scales are examined. Strong teleconnectivity and temporal coherence are found over all of the active sites with enhanced vorticity variance, as well as over the western Atlantic/Caribbean and the Indochinese Peninsula. These results indicate that a substantial amount of synoptic scale variability in the tropics is associated with propagating wavelike disturbances that remain coherent over several days. The disturbances in all active regions tend to travel west/northwestward. The eastern portion of each active site is characterized by rapid growth of the disturbances, whereas decay typically occurs in the western portion. The transient behavior throughout the tropics is also investigated using Extended Empirical Orthogonal Function (EEOF) techniques. The sites of activity thus identified coincide with the locations inferred from the lag-correlation analyses. Using time series of the EEOF coefficients as a reference, the temporal evolution as well as the horizontal and vertical structure of the disturbances occurring in each active region are delineated by composites of selected meteorological variables. Well-defined in vorticity, vertical velocity, temperature and humility at various tropospheric levels, as well as convective activity (deduced from the outgoing longwave radiation field), are discernible in the disturbances at various sites. Phase relationships among different variables are interpreted in terms of dynamical and physical processes operating within the disturbances. The horizontal phase tilt of the fluctuations and their positions relative to the ambient mean circulation suggest a tendency for kinetic energy transfer from the quasi-stationary flow to the transient eddies. Most of the findings reported here are in accord with previous investigations based on different analysis tools and more limited datasets. Whereas considerable similarities are noted among disturbances occurring over various active maritime sites, the perturbations over central and western Africa exhibit structural characteristics that are unique to that region. Specifically, two propagation tracks are identified in the African sector. The northern track along southern Sahara consists mostly of eddies commonly found over arid zones, with ascent of warm and dry air over surface troughs. The southern track is collocated with the climatological rainfall maximum at about 10N, and is associated with moist convective systems.
    Lau R., M. Y. Chan, 1993: Equatorial stratospheric flow patterns and quasi-biennial/pentaennial oscillations. East Asia and Western Pacific Meteorology and Climate, 2, 31- 38.68ab755f-5d39-42bf-8654-bf9f5f36a78f6ce38a41bd1d1c45e4365029663a32a8http%3A%2F%2F203.129.68.43%2Fpublica%2Freprint%2Fr246.pdfhttp://203.129.68.43/publica/reprint/r246.pdfGenerally speaking, stratospheric flow patterns are less complex and more stable than tropospheric flows. In terms of forecasting general pattern changes therefore, one may be excused to suggest that it would be comparatively simpler at stratospheric levels than at
    Lei X. T., L. S. Chen, 2001: An overview on the interaction between tropical cyclone and mid-latitude weather systems. J. Trop. Meteor., 17( 4), 452- 461. (in Chinese)
    Liebmann B., H. H. Hendon, and J. D. Glick, 1994: The relationship between tropical cyclones of the western Pacific and Indian Oceans and the Madden-Julian oscillation. J. Meteor. Soc.Japan, 72( 3), 401- 412.10.1175/1520-0469(1994)051<1559:RODIAO>2.0.CO;23f29ac5bbfc30a31a8eca575e0629660http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F110001807345http://ci.nii.ac.jp/naid/110001807345We document a relationship between tropical cyclones of the Indian and western Pacific oceans and the Madden-Julian oscillation (MJO). Cyclones preferentially occur during the convective phase of the oscillation, and cluster around the low-level cyclonic vorticity and divergence anomalies that appear poleward and westward of the large-scale convective anomaly along the equator. Although the absolute numbers of storms and typhoons are enhanced during the convective phase of the oscillation, the ratios of storms and typhoons formed per depression are the same in the convective phase as the dry phase of the oscillation. There exist more storms and typhoons simply by there being more depressions in the convective phase. The third result of this study is that the increase in cyclone activity during active periods of convection is not restricted to MJO activity. In fact, we find that an equal increase occurs during the convective phase of an arbitrarily chosen, completely independent band from the MJO. We conclude that the MJO does not influence tropical cyclones in a unique fashion, but it is important because of the relatively large proportion of tropical variance that is explained by it.
    Lin, I. I., Coauthors, 2013: An ocean coupling potential intensity index for tropical cyclones. Geophys. Res. Lett., 40( 9), 1878- 1882.10.1002/grl.50091d4c4c3287e052aa65e4bc68ba8743096http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fgrl.50091%2Fabstracthttp://onlinelibrary.wiley.com/doi/10.1002/grl.50091/abstractAbstract [1] Timely and accurate forecasts of tropical cyclones (TCs, i.e., hurricanes and typhoons) are of great importance for risk mitigation. Although in the past two decades there has been steady improvement in track prediction, improvement on intensity prediction is still highly challenging. Cooling of the upper ocean by TC-induced mixing is an important process that impacts TC intensity. Based on detail in situ air-deployed ocean and atmospheric measurement pairs collected during the Impact of Typhoons on the Ocean in the Pacific (ITOP) field campaign, we modify the widely used Sea Surface Temperature Potential Intensity (SST_PI) index by including information from the subsurface ocean temperature profile to form a new Ocean coupling Potential Intensity (OC_PI) index. Using OC_PI as a TC maximum intensity predictor and applied to a 1465year (1998–2011) western North Pacific TC archive, OC_PI reduces SST_PI-based overestimation of archived maximum intensity by more than 50% and increases the correlation of maximum intensity estimation from r 2 65=650.08 to 0.31. For slow-moving TCs that cause the greatest cooling, r 2 increases to 0.56 and the root-mean square error in maximum intensity is 1165m65s 611 . As OC_PI can more realistically characterize the ocean contribution to TC intensity, it thus serves as an effective new index to improve estimation and prediction of TC maximum intensity.
    Mann H. B., D. R. Whitney, 1947: On a test of whether one of two random variables is stochastically larger than the other. Ann. Math. Statis., 18( 1), 50- 60.8a2fa28c6c7d31e16d527e83e6630a55http%3A%2F%2Fjnnp.bmj.com%2Fexternal-ref%3Faccess_num%3D10.1214%2Faoms%2F1177730491%26link_type%3DDOIhttp://jnnp.bmj.com/external-ref?access_num=10.1214/aoms/1177730491&amp;link_type=DOI
    Mlawer E. J., S. J. Taubman, P. D. Brown, M. J. Iacono, and S. A. Clough, 1997: Radiative transfer for inhomogeneous atmospheres: RRTM, a validated correlated-k model for the longwave. J.Geophys. Res., 102, 16 663- 16 682.10.1029/97JD00237cd97feda-8613-4507-b607-8e01cbc0152abf5f762e845a497b1ec8058223fb6df8http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F97JD00237%2Fpdfrefpaperuri:(98daaed043b544401196cd274fa354f5)http://onlinelibrary.wiley.com/doi/10.1029/97JD00237/pdfABSTRACT A rapid and accurate radiative transfer model (RRTM) for climate applications been developed and the results extensively evaluated. The current version of RRTM calculates fluxes and cooling rates for the longwave spectral region (10-3000 cm-1) for an arbitrary clear atmosphere. The molecular species treated in the model are water vapor, carbon dioxide, ozone, methane, nitrous oxide, and the common halocarbons. The radiative transfer in RRTM is performed using the correlated-k method: the k distributions are attained directly from the LBLRTM line-by-line model, which connects the absorption coefficients used by RRTM to high-resolution radiance validations done with observations. Refined methods have been developed for treating bands containing gases with overlapping absorption, for the determination of values of the Planck function appropriate for use in the correlated-k approach, and for the inclusion of minor absorbing species in a band. The flux and cooling rate results of RRTM are linked to measurement through the use of LBLRTM, which has been substantially validated with observations. Validations of RRTM using LBLRTM have been performed for the midlatitude summer, tropical, midlatitude winter, subarctic winter, and four atmospheres from the Spectral Radiance Experiment campaign. On the basis of these validations the longwave accuracy of RRTM for any atmosphere is as follows: 0.6 W m-2 (relative to LBLRTM) for net flux in each band at all altitudes, with a total (10-3000 cm-1) error of less than 1.0 W m-2 at any altitudes; 0.07 K d-1 for total cooling rate error in the troposphere and lower stratosphere, and 0.75 K d-1 in the upper stratosphere and above. Other comparisons have been performed on RRTM using LBLRTM to gauge its sensitivity to changes in the abundance of specific species, including the halocarbons and carbon dioxide. The radiative forcing due to doubling the concentration of carbon dioxide is attained with an accuracy of 0.24 W m-2, an error of less than 5%. The speed of execution of RRTM compares favorably with that of other rapid radiation models, indicating that the model is suitable for use in general circulation models.
    Nakano M., Sawada M., Nasuno T., andSatoh M., 2015: Intraseasonal variability and tropical cyclogenesis in the western North Pacific simulated by a global nonhydrostatic atmospheric model. Geophys. Res. Lett., 42, 565- 571.10.1002/2014GL06247962a9302798cef70dccbf5844a6fc4a26http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F269726970_Intraseasonal_variability_and_tropical_cyclogenesis_in_the_western_North_Pacific_simulated_by_a_global_nonhydrostatic_atmospheric_model_Tropical_cyclogenesis_in_WNP_by_NICAMhttp://www.researchgate.net/publication/269726970_Intraseasonal_variability_and_tropical_cyclogenesis_in_the_western_North_Pacific_simulated_by_a_global_nonhydrostatic_atmospheric_model_Tropical_cyclogenesis_in_WNP_by_NICAMABSTRACT
    Nakazawa T., 2006: Madden-Julian oscillation activity and typhoon landfall on Japan in 2004. Sola, 2, 136- 139.10.2151/sola.2006-035c5e6680106840aac207f14174ba39638http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F30014990399%2Fhttp://ci.nii.ac.jp/naid/30014990399/ABSTRACT The reason for the record-breaking typhoon landfall on Japan in 2004 was examined by analyzing the behavior of tropical intraseasonal variation associated with the Madden-Julian Oscillation (MJO, Madden and Julian 1972). There were three active phases of MJO in June, August and early October over the western North Pacific during the typhoon season in 2004. The monsoon trough was enhanced during the active phase of the MJO over the western North Pacific and generated most of the typhoons over the region. The steering flow anomaly persisted, due to the long duration of the active phase of the MJO over the western Pacific, and directed westward-northwestward from the TC genesis region with both anti-cyclonic (cyclonic) centers over east of Japan (east of the Philippines), and we suggest that the persisted steering flow anomaly provided a favorable condition for typhoon landfall on Japan in 2004.
    Peduzzi P., B. Chatenoux, H. Dao, A. D. Bono, C. Herold, J. Kossin, F. Mouton, and O. Nordbeck, 2012: Global trends in tropical cyclone risk. Nature Climate Change, 2( 4), 289- 294.10.1038/nclimate1410b4c0d13c-a523-4084-a95a-044313618b29eacdb096782d5a2eade419ec2825eb40http%3A%2F%2Fwww.nature.com%2Fnclimate%2Fjournal%2Fv2%2Fn4%2Ffull%2Fnclimate1410.htmlrefpaperuri:(eb41d5619d310bc21e5079922521603e)http://www.nature.com/nclimate/journal/v2/n4/full/nclimate1410.htmlThe impact of tropical cyclones on humans depends on the number of people exposed and their vulnerability, as well as the frequency and intensity of storms. How will the cumulative effects of climate change, demography and vulnerability affect risk? Conventionally, reports assessing tropical cyclone risk trends are based on reported losses, but these figures are biased by improvements to information access. Here we present a new methodology based on thousands of physically observed events and related contextual parameters. We show that mortality risk depends on tropical cyclone intensity, exposure, levels of poverty and governance. Despite the projected reduction in the frequency of tropical cyclones, projected increases in both demographic pressure and tropical cyclone intensity over the next 20 years can be expected to greatly increase the number of people exposed per year and exacerbate disaster risk, despite potential progression in development and governance.
    Ren S. L., Y. M. Liu, and G. X. Wu, 2007: Interactions between typhoon and subtropical anticyclone over western pacific revealed by numerical experiments. Acta Meteorologica Sinica, 65( 3), 329- 340. (in Chinese)10.1002/jrs.1570d40818c3-df4f-4d2b-8886-abb8e73adf5a5584200734702be07aea9216bc4a5e20311307e0e7http%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTotal-QXXB200703002.htmhttp://en.cnki.com.cn/Article_en/CJFDTotal-QXXB200703002.htmThree familiar kinds of typhoon tracks are sorted based on a set of typhoon data from 1958 to 1998.The result of composite analysis confirms that different typhoon tracks correspond to different patterns of the subtropical anticyclone over Western Pacific(SAWP).When the tracks are westward the subtropical high is strong with a zonal form,and stretches westward;when tracks are recurving ones,the main body of the subtropical high moves eastward and breaks near longitude 160E;and when tracks are northward the subtropical high locates much more east of normal position.Based on this result and using the GOALS R42L9 climate model,a temperature disturbance is added into two different initial fields to force the formation of typhoon.Westward tracking and northward tracking typhoons are successfully simulated,thus verifying the finding that different patterns of SAWP have different effects on typhoon tracks.Results also show that typhoons can induce barotropic Rossby waves propagating towards the mid and high latitudes.Under different background zonal flows,wave trains triggered by typhoons of westward and northward tracks are also different,and their effects on the mid and high latitude circulation and the SAWP are different: compared to the northward tracking typhoon,the westward tracking typhoon is able to induce positive geopotential height anomaly to its north and northwest,resulting in the strengthening and westward stretching of the SAWP.
    Rogers E., T. Black, B. Ferrier, Y. Lin, D. Parrish, and J. DiMego, 2001: Changes to the NCEP Meso Eta Analysis and Forecast System: Increase in resolution, new cloud microphysics, modified precipitation assimilation, modified 3DVAR analysis. NWS Technical Procedures Bulletin, No. 488.
    [ Available online at http://www.emc.ncep.noaa.gov/mmb/mmbpll/eta22tpb/]7766ada3e21641908ed59f135f4ec641http%3A%2F%2Fwww.emc.ncep.noaa.gov%2Fmmb%2Fmmbpll%2Feta12tpb%2Fhttp://www.emc.ncep.noaa.gov/mmb/mmbpll/eta12tpb/higher resolution runs because intense precipitation features are more explicitly resolved by Although both forecasts predicted a weak pressure gradient in the Baltimore/Washington Reisner,J., RM Rasmussen, and RT Bruintjes, 1998: Explicit forecasting of supercooled liquid
    Santer, B. D., Coauthors, 2006: Forced and unforced ocean temperature changes in Atlantic and Pacific tropical cyclogenesis regions. Proceedings of the National Academy of Sciences of United States of American, 103( 38), 13 905- 13 910.10.1073/pnas.0602861103169687811df217cfaeca851e62320b533d9f614dhttp%3A%2F%2Fmed.wanfangdata.com.cn%2FPaper%2FDetail%2FPeriodicalPaper_PM16968781http://med.wanfangdata.com.cn/Paper/Detail/PeriodicalPaper_PM16968781Previous research has identified links between changes in sea surface temperature (SST) and hurricane intensity. We use climate models to study the possible causes of SST changes in Atlantic and Pacific tropical cyclogenesis regions. The observed SST increases in these regions range from 0.32 degrees C to 0.67 degrees C over the 20th century. The 22 climate models examined here suggest that century-timescale SST changes of this magnitude cannot be explained solely by unforced variability of the climate system. We employ model simulations of natural internal variability to make probabilistic estimates of the contribution of external forcing to observed SST changes. For the period 1906-2005, we find an 84% chance that external forcing explains at least 67% of observed SST increases in the two tropical cyclogenesis regions. Model "20th-century" simulations, with external forcing by combined anthropogenic and natural factors, are generally capable of replicating observed SST increases. In experiments in which forcing factors are varied individually rather than jointly, human-caused changes in greenhouse gases are the main driver of the 20th-century SST increases in both tropical cyclogenesis regions.
    Scoccimarro E., S. Gualdi, G. Villarini, M. Zhao, K. Walsh, and A. Navarra, 2014: Intense precipitation events associated with landfalling tropical cyclones in response to a warmer climate and increased CO2. J. Climate, 27( 12), 4642- 4654.10.1175/JCLI-D-14-00065.1113711cf24a1b73b66d77abeb0fec570http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2014JCli...27.5119Bhttp://adsabs.harvard.edu/abs/2014JCli...27.5119BABSTRACT
    Shen X. Y., W. D. Zhu, J. Du, and W. Y. Pan, 2010: The seasonal forecasting experiment of typhoon from July to September of 2006. Scientia Meteorologica Sinica, 30( 5), 676- 683. (in Chinese)10.3788/HPLPB20102207.1462f3232b5875c7128359cc3418f7dbb20bhttp%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTOTAL-QXKX201005014.htmhttp://en.cnki.com.cn/Article_en/CJFDTOTAL-QXKX201005014.htmBased on the NCEP/NCAR data with 6 h time resolution and 2.5°×2.5° horizontal resolution,WRF model of 27×27 km horizontal resolution,which has 38 levels in vertical direction and data output per 6 h,is employed to give a seasonal forecasting experiment for TCs in the Pacific Northwest(NWP) basin.According to low-level vorticity,wind speed at 10 meters,sea level pressure,warm core structure and duration,the generation and tracks of the TCs are determined.From July 1 to September 30,the proportions of well simulated typhoons,STSs,TSs and TDs are 4/9,1/3,0/1,and 0/1,bad simulations account for 3/9,1/3,1/1,1/1,respectively,while missing report rates are 2/9,1/3,0/1,0/1.And 2 typhoons,9 STSs and 3TSs are false predicted.The possible reasons for simulated typhoons' weaker intensity and declinational tracks are related to the model's resolution,parameter of microphysical processes and integral time.The causes of the false simulated typhoons,STSs and TSs may be explained by the feature of model,integral time,characteristic of the relational sea area and the tropical waves.
    Skamarock, W. C., Coauthors, 2008: A description of the Advanced Research WRF version 3. NCAR Tech. Note NCAR/ TN-475+STR,113 pp.133fdf5edd3fc85654e5fe959ecf2a0ahttp%3A%2F%2Fntis.library.gatech.edu%2Fhandle%2F123456789%2F2086http://ntis.library.gatech.edu/handle/123456789/2086The development of the Weather Research and Forecasting (WRF) modeling system is a multi-agency effort intended to provide a next-generation mesoscale forecast model and data assimilation system that will advance both the understanding and prediction of mesoscale weather and accelerate the transfer of research advances into operations. The model is being developed as a collaborative effort among the NCAR Mesoscale and Microscale Meteorology (MMM) Division, the National Oceanic and Atmospheric Administration's (NOAA) National Centers for Environmental Prediction (NCEP) and Forecast System Laboratory (FSL), the Department of Defense's Air Force Weather Agency (AFWA) and Naval Research Laboratory (NRL), the Center for Analysis and Prediction of Storms (CAPS) at the University of Oklahoma, and the Federal Aviation Administration (FAA), along with the participation of a number of university scientists. The WRF model is designed to be a flexible, state-of-the-art, portable code that is efficient in a massively parallel computing environment. A modular single-source code is maintained that can be configured for both research and operations. It offers numerous physics options, thus tapping into the experience of the broad modeling community. Advanced data assimilation systems are being developed and tested in tandem with the model. WRF is maintained and supported as a community model to facilitate wide use, particularly for research and teaching, in the university community. It is suitable for use in a broad spectrum of applications across scales ranging from meters to thousands of kilometers. Such applications include research and operational numerical weather prediction (NWP), data assimilation and parameterized-physics research, downscaling climate simulations, driving air quality models, atmosphere-ocean coupling, and idealized simulations (e.g boundary-layer eddies, convection, baroclinic waves). With WRF as a common tool in the university and operational centers, closer ties will be promoted between these communities, and research advances will have a direct path to operations. These hallmarks make the WRF modeling system unique in the history of NWP in the United States.
    Sugi M., A. Noda, and N. Sato, 2002: Influence of the global warming on tropical cyclone climatology: An experiment with the JMA global model. J. Meteor. Soc.Japan, 80( 2), 249- 272.10.2151/jmsj.80.249ba778edbdddd6e52ba3ec7994f676991http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F110001807733http://ci.nii.ac.jp/naid/110001807733The influence of the global warming on tropical cyclones has been examined using a high resolution AGCM. Two ten-year integrations were performed with the JMA global model at T106 horizontal resolution. For the control experiment, the observed SST for the period 1979-1988 is prescribed, while for the doubling CO2 (2 CO2) experiment, SST anomaly due to the global warming estimated from a coupled model transient CO2 experiment (Tokioka et al. 1995) is added to the SST used in the control experiment. The results of experiments show that a significant reduction in the frequency of tropical cyclones is possible in response to the greenhouse gas-induced global warming. The most significant decrease is indicated over the North Pacific. On the other hand, a considerable increase in tropical cyclone frequency is indicated for the North Atlantic. As for the maximum intensity of tropical cyclones, no significant change has been noted. It has been found that the regional change in tropical cyclone frequency is closely related to the distribution of the SST anomaly, and the change in convective activity associated with it. The results of the experiment indicate that the change in tropical cyclogenesis is strongly controlled by dynamical factors associated with the change in SST distribution, rather than the thermodynamical factors associated with the change in absolute value of local SST. On the other hand, for the decrease in the global total number of tropical cyclones on doubling CO2, a weakening of tropical circulation associated with the stabilization of the atmosphere (the increase in dry static stability), seems to be responsible. It is found that the rate of increase in the tropical precipitation due to the global warming is much less than the rate of increase in the atmospheric moisture. With this little increase in precipitation (convective heating), a considerable increase in the dry static stability of the atmosphere leads to a weakening of the tropical circulation.
    Sugi M., H. Murakami, and J. Yoshimura, 2012: On the mechanism of tropical cyclone frequency changes due to global warming. J. Meteor. Soc.Japan, 90, 397- 408.10.2151/jmsj.2012-A248230d676f07d3e649843b2f38bb6a2cchttp%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F130004435202http://ci.nii.ac.jp/naid/130004435202In order to explore the hypothesized mechanisms for the reduction of global tropical cyclone (TC) frequency due to greenhouse warming, an experiment has been conducted using a most recent version of MRI-AGCM with a new convection scheme. In addition to a present climate run (HPA run) and a future climate run (HFA run), two more runs are conducted. In CO2F run, future values of CO2 and other greenhouse gas (GHG) concentrations are used with present value of sea surface temperature (SST), while in the SSTF run, future value of SST is used with present values of CO2 and other GHG concentrations. The reductions of global TC frequency in HFA run, CO2F run and SSTF run from HPA run are 25%, 9% and 18%, respectively. These results are basically consistent with previous studies.Based on the results of the experiment, we examined three key relations in our hypothesized mechanism for the reduction of TC frequency. First, the relation between changes in atmospheric radiative cooling and precipitation is confirmed to be valid in the experiment, in which not only CO2 but other GHG is increased. It is also confirmed that the effect of increasing CO2 is decreasing precipitation, while the effect of increasing other GHG is increasing precipitation. Second, the relation between changes in precipitation and upward mass flux is clarified by using a simple approximate thermodynamic equation. Third, regarding the relation between changes in upward mass flux and TC genesis frequency, we examined the changes in four parameters (precipitation, upward mass flux, vertical wind shear and mid-troposphere saturation deficit) which are closely related to deep convective activities in the tropics and may affect TC genesis frequency. The results of our experiment support the idea suggested by the previous studies that the reduction of TC frequency is closely related to a reduction of upward mass flux, although the chain of causality linking the two remains unclear. In addition, our experiment suggests a possibility that the changes in mid-troposphere saturation deficit may also contribute to the changes in TC genesis frequency.
    Sun L., 2011: Analysis of features and causation for tropical cyclone activities over the Western North Pacific in 2010. Meteorological Monthly, 37( 8), 929- 935. (in Chinese)10.3724/SP.J.1146.2006.01085bd1507268fe1cd4f6b4e10a1df8c4fa8http%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTOTAL-QXXX201108005.htmhttp://en.cnki.com.cn/Article_en/CJFDTOTAL-QXXX201108005.htmCharacteristics and causation for tropical cyclone(TO activities over the Western North Pacific (WNP) and the South China Sea(SCS) in 2010 were analyzed by using the data of TCs,reanalysis data of NCEP/NCAR in this paper.TC activities are characterized by(1) forming the least,and the highest percentages of landing,(2) concentrated and westward genesis,(3) higher proportion of strong TCs and short life cycle TCs,(4) the later for the first TC and last TC,and(5) TC landing fewer in July-ugust and more than normal in September,5 of 7 TCs landing on Fujian.The causes were studied and the results were listed as follows.Firstly,the warm mode in the East Indian Ocean in 2010 was one of the key extra forcing factors resulting in the least TC genesis in WNP and SCS in 2010,especially the lag response of extratropical atmosphere to El Nino.Secondly,the atmospheric circulation abnormity was the key reasons. It showed the combined impacts of multi-factors including the larger and stronger Northwest Pacific subtropical high extending abnormally west,the abnormally westward monsoon trough and large vertical shear anomalies.
    Sun Y., Z. Zhong, L. Wei, and Y. J. Hu, 2014a: Why are tropical cyclone tracks over the western North Pacific sensitive to the cumulus parameterization scheme in regional climate modeling? A case study for Megi (2010). Mon. Wea. Rev., 142( 3), 1240- 1249.7a407a98-c17f-4870-ac7a-76df79e60b67491af2707abe20e0f50ce9b544555167http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2014MWRv..142.1240S%26db_key%3DPHY%26link_type%3DABSTRACT%26high%3D545944fc4722343refpaperuri:(8bdc23195050615b769e3426b53f4f9e)/s?wd=paperuri%3A%288bdc23195050615b769e3426b53f4f9e%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2014MWRv..142.1240S%26db_key%3DPHY%26link_type%3DABSTRACT%26high%3D545944fc4722343&ie=utf-8
    Sun Y., Z. Zhong, L. Yi, Y. Ha, and Y. M. Sun, 2014b: The opposite effects of inner and outer sea surface temperature on tropical cyclone intensity. J. Geophys. Res.: Atmos., 119( 5), 2193- 2208.10.1002/2013JD02135405320a274771c2247fef69d5c300bb5fhttp%3A%2F%2Fcpfd.cnki.com.cn%2FArticle%2FCPFDTOTAL-ZGQX201510023150.htmhttp://cpfd.cnki.com.cn/Article/CPFDTOTAL-ZGQX201510023150.htmA suite of semi-idealized numerical experiments are conducted to investigate the sensitivity of tropical cyclone(TC) intensity to changes of sea surface temperature(SST) over different radial extents. It is found that the increase of inner SST within the range 1.5-2.0 times the radius of maximum wind(RMW), defined as the effective radius(ER), contributes greatly to the increase of TC intensity and the reduction of TC inner-core size, whereas the increase of outer SST(defined as SST outside the ER)reduces TC intensity and increases TC inner-core size. Further analysis suggests that,the effects of SST inside and outside the ER on TC intensity rely on the factors that influence the TC development.As the SST increases inside the ER, more surface enthalpy flux enters TC eyewall and less enters the outer spiral rainbands. This will decrease the RMW, leading to a smaller eyewall radius where strong latent heating is released. As a result, the central pressure of the TC deepens with stronger radial pressure gradient. Meanwhile,difference between SST and upper-tropospheric temperature increases. All above factors contribute to TC intensification as the inner SST increases. The opposite happen as the SST increases outside the ER. How TC intensity responds to the change of entire SST depends on the competitive and opposite effects of inner and outer SST.Moreover, understanding the mechanisms is vital to the forecast of variations in TC intensity and inner-core size when a TC comes across an ocean cold or warm pool.
    Tory K. J., S. S. Chand , R. A. Dare, and J. L. McBride, 2013: The development and assessment of a model-, grid-, and basin-independent tropical cyclone detection scheme. J. Climate, 26( 15), 5493- 5507.
    Wada A., J. C. L. Chan, 2008: Relationship between typhoon activity and upper ocean heat content. Geophys. Res. Lett., 35( 17), L17603.10.1029/2008GL035129564438f01c0e73c63ec4e7b7886c3091http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2008GL035129%2Fcitedbyhttp://onlinelibrary.wiley.com/doi/10.1029/2008GL035129/citedby[1] A 44-year mean distribution of tropical cyclone heat potential (TCHP), a measure of the oceanic heat content from the surface to the 26C-isotherm depth, shows that TCHP is locally high in the western North Pacific (WNP). TCHP varies on interannual time scales and has a relationship with tropical cyclone (TC) activity. The third mode of an empirical orthogonal function analysis of TCHP shows that an increase in the total number of TCs is accompanied with a warm central Pacific and cool WNP. Negative TCHP anomalies in the WNP suggest that an increase in total number of TCs results in cooling due to their passages. On the other hand, the first mode shows that the number of super typhoons increases in mature El Ni&ntilde;o years. An increase in accumulated TCHP is related to the increase in the number of super typhoons due to long duration.
    Walsh K. J. E., M. Fiorino, C. W. Land sea, and K. L. Mcinnes, 2007: Objectively determined resolution-dependent threshold criteria for the detection of tropical cyclones in climate models and reanalyses. J. Climate, 20( 10), 2307- 2314.10.1175/JCLI4074.1e8baa2c3739440c804e3eb9d2252d935http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2007JCli...20.2307Whttp://adsabs.harvard.edu/abs/2007JCli...20.2307WAbstract Objectively derived resolution-dependent criteria are defined for the detection of tropical cyclones in model simulations and observationally based analyses. These criteria are derived from the wind profiles of observed tropical cyclones, averaged at various resolutions. Both an analytical wind profile model and two-dimensional observed wind analyses are used. The results show that the threshold wind speed of an observed tropical cyclone varies roughly linearly with resolution. The criteria derived here are compared to the numerous different criteria previously employed in climate model simulations. The resulting method provides a simple means of comparing climate model simulations and reanalyses.
    Wang B., J. C. L. Chan, 2002: How strong ENSO events affect tropical storm activity over the western North Pacific. J. Climate, 15( 13), 1643- 1658.10.1175/1520-0442(2002)015<1643:HSEEAT>2.0.CO;2f54b877e-3649-4945-a9e9-654243b303a2248bb77da7a55c482abfa57c1313880ehttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2002JCli...15.1643Wrefpaperuri:(1b6bf47549cbe306b97493eb81c0fa23)http://adsabs.harvard.edu/abs/2002JCli...15.1643WAbstract An analysis of 35-yr (1965–99) data reveals vital impacts of strong (but not moderate) El Ni09o and La Ni09a events on tropical storm (TS) activity over the western North Pacific (WNP). Although the total number of TSs formed in the entire WNP does not vary significantly from year to year, during El Ni09o summer and fall, the frequency of TS formation increases remarkably in the southeast quadrant (0°–17°N, 140°E–180°) and decreases in the northwest quadrant (17°–30°N, 120°–140°E). The July–September mean location of TS formation is 6° latitude lower, while that in October–December is 18° longitude eastward in the strong warm versus strong cold years. After the El Ni09o (La Ni09a), the early season (January–July) TS formation in the entire WNP is suppressed (enhanced). In strong warm (cold) years, the mean TS life span is about 7 (4) days, and the mean number of days of TS occurrence is 159 (84) days. During the fall of strong warm years, the number of TSs, which recurve northward across 35°N, is 2.5 times more than during strong cold years. This implies that El Ni09o substantially enhances poleward transport of heat–moisture and impacts high latitudes through changing TS formation and tracks. The enhanced TS formation in the SE quadrant is attributed to the increase of the low-level shear vorticity generated by El Ni09o–induced equatorial westerlies, while the suppressed TS generation over the NW quadrant is ascribed to upper-level convergence induced by the deepening of the east Asian trough and strengthening of the WNP subtropical high, both resulting from El Ni09o forcing. The WNP TS activities in July–December are noticeably predictable using preceding winter–spring Ni09o-3.4 SST anomalies, while the TS formation in March–July is exceedingly predictable using preceding October–December Ni09o-3.4 SST anomalies. The physical basis for the former is the phase lock of ENSO evolution to the annual cycle, while for the latter it is the persistence of Philippine Sea wind anomalies that are excited by ENSO forcing but maintained by local atmosphere–ocean interaction.
    Wang H. J., K. Fan, 2007: Relationship between the Antarctic oscillation in the western North Pacific typhoon frequency. Chinese Science Bulletin, 52( 4), 561- 565.10.1007/s11434-007-0040-460cedd0986c81c9cf5caa8fc8229972chttp%3A%2F%2Fd.wanfangdata.com.cn%2FPeriodical_kxtb-e200704020.aspxhttp://d.wanfangdata.com.cn/Periodical_kxtb-e200704020.aspx
    Wang H. J., J. Q. Sun, and K. Fan, 2007: Relationships between the North Pacific Oscillation and the typhoon/hurricane frequencies. Science in China Series D: Earth Sciences, 50( 9), 1409- 1416.10.1007/s11430-007-0097-65d8b6147c183607f3af260fb8ae99689http%3A%2F%2Flink.springer.com%2F10.1007%2Fs11430-007-0097-6http://d.wanfangdata.com.cn/Periodical_zgkx-ed200709013.aspxRelationships between the North Pacific Oscillation (NPO) and the typhoon as well as hurricane frequencies are documented. The correlation between NPO index in June-July-August-September and the annual typhoon number in the western North Pacific is 0.37 for the period of 1949-1998. The NPO is correlated with the annual hurricane number in the tropical Atlantic at - 0.28 for the same period. The variability of NPO is found to be concurrent with the changes of the magnitude of vertical zonal wind shear, sea-level pressure patterns, as well as the sea surface temperature, which are physically associated with the typhoons and hurricanes genesis. The NPO associated atmospheric circulation variability is analyzed to explain how NPO is linked with variability of the tropical atmospheric circulation in the western Pacific and the tropical Atlantic, via the atmospheric teleconnection.
    Wang Y. Q., T. Y. Song, J. Liang, and W. Y. Pan, 2012: Simulation of seasonal tropical cyclone activity over the western North Pacific by using the WRF model. Transactions of Atmospheric Sciences, 35( 1), 24- 31. (in Chinese)10.1007/s11783-011-0280-z0a0ddecfdbd7ebab50240dfb18883d3ahttp%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTotal-NJQX201201002.htmhttp://en.cnki.com.cn/Article_en/CJFDTotal-NJQX201201002.htmBy using the WRF(weather research and forecasting) model,the paper simulated the seasonal tropical cyclone activity over western North Pacific from 1 July to 30 September 2006.Results show that:1) the simulated total number of Typhoons is close to that from Best-track data set.The simulated landfall TC number is less than that of real Typhoon number and it is the same for separate monthly landfall number.The simulated intensity of Typhoons is weaker than that from Best-track data set;2) the model shows better ability in reproducing Typhoon and the height and wind vertical shear fields in July,but the ability tends to be weaker as the model time increases.This suggests that the model can be used to simulate the feature of seasonal tropical cyclone activity but the time limit needs further exploration.
    Webster P. J., G. J. Holland , J. A. Curry, and H. R. Chang, 2005: Changes in tropical cyclone number, duration, and intensity in a warming environment. Science, 309( 5742), 1844- 1846.10.1126/science.1116448161665144b3da70b2014d037261218dece85528dhttp%3A%2F%2Fmed.wanfangdata.com.cn%2FPaper%2FDetail%2FPeriodicalPaper_PM16166514http://med.wanfangdata.com.cn/Paper/Detail/PeriodicalPaper_PM16166514We examined the number of tropical cyclones and cyclone days as well as tropical cyclone intensity over the past 35 years, in an environment of increasing sea surface temperature. A large increase was seen in the number and proportion of hurricanes reaching categories 4 and 5. The largest increase occurred in the North Pacific, Indian, and Southwest Pacific Oceans, and the smallest percentage increase occurred in the North Atlantic Ocean. These increases have taken place while the number of cyclones and cyclone days has decreased in all basins except the North Atlantic during the past decade.
    Yoshimura J., M. Sugi, 2005: Tropical cyclone climatology in a high-resolution AGCM-Impacts of SST warming and CO2 increase. SOLA, 1, 133- 136.10.2151/sola.2005-035abf41708-6d4b-46e9-8302-621740961a24f1654ff791faeb817a101ceefa61eeechttp%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F130004448365refpaperuri:(027a29ff5d8fe8ca00f59738e54e27f5)http://ci.nii.ac.jp/naid/130004448365Using a high-horizontal-resolution atmospheric general circulation model (AGCM), impacts of SST warming and CO2 increase on the tropical cyclone (TC) climatology are investigated. The SST effect is examined from numerical experiments in which SST is uniformly higher/lower by 2 K, without changing the atmospheric CO2 concentration. The CO2 effect is shown from doubled and quadrupled CO2 experiments with a fixed SST condition. The results demonstrate that the increases in CO2 have large impacts to reduce TC frequency globally, while the SST changes have relatively small influences on the TC frequency. The SST warming causes significant increase in climatological precipitation, and this indicates intensification of convective heating and should have some influences to activate the atmospheric circulation in terms of vertical mass flux in the tropics. In the high-SST experiment, however, larger warming in the upper troposphere causes higher dry static stability, which should have some impacts to weaken the atmospheric circulation. It seems that these two conflicting factors, in terms of TC frequency, may cancel out to a large extent. As the effect of CO2 enhancement, precipitation decreases significantly in the tropics, which may lead to the reduction in TC frequency.
    Zhao, M., Coauthors, 2013: Response of global tropical cyclone frequency to a doubling of CO2 and a uniform SST warming multi-model intercomparison. U. S. CLIVAR Hurricane Workshop, Geophysical Fluid Dynamics Laboratory, Princeton, US, 5-7June 2013.
    Zhou B. T., X. Cui, 2008: Hadley circulation signal in the tropical cyclone frequency over the western North Pacific. J. Geophys. Res.: Atmos., 113( D16), D16107.10.1029/2007JD0091569dd1da81699a514cc970efe51d13a1e7http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2007JD009156%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/2007JD009156/fullAbstract Top of page Abstract 1.Introduction 2.Data and Definition of the HC Index 3.Spring HC and Summer TC Frequency Over the Western North Pacific 4.Possible Reason for the Spring HC Impact on the Summer Atmospheric Circulations 5.Conclusion and Discussion Acknowledgments References [1] The relationship between the boreal spring (March-ay) Hadley circulation (HC) and the following summer (June-揝eptember) tropical cyclone (TC) frequency expressed as the total number of TCs in the western North Pacific is investigated through using the observed data. Results show that the spring HC is negatively correlated to the summer TC frequency. Such a relationship can be explained by the changes of the atmospheric circulations related to anomalous spring HC. A strong spring HC is followed by the weaker East Asian monsoon, stronger vertical zonal wind shear, and reduced convection over the western North Pacific in summer, which are unfavorable for TC genesis, and vice versa. The potential mechanism of how the spring HC affects the summer atmospheric circulations is also preliminarily identified. It is found that sea surface temperature (SST) anomalies in the Indian Ocean and the South China Sea may play important roles, since an anomalous spring HC can excite simultaneous SST anomalies, which can persist to the following summer and in turn give rise to the atmospheric anomalies associated with TC activities. Thus the variation of the spring HC can be a potential indicator in predicting summer TC activities over the western North Pacific.
    Zhou B. T., X. Cui, 2014: Interdecadal change of the linkage between the North Atlantic Oscillation and the tropical cyclone frequency over the western North Pacific. Science China Earth Sciences, 57( 9), 2148- 2155.10.1007/s11430-014-4862-z177139014d992bdfe166a00afb244a4a48ec4307http%3A%2F%2Fwww.cnki.com.cn%2FArticle%2FCJFDTotal-JDXG201409015.htmhttp://www.cnki.com.cn/Article/CJFDTotal-JDXG201409015.htmThe relationship between the North Atlantic Oscillation (NAO) and the tropical cyclone frequency over the western North Pacific (WNPTCF) in summer is investigated by use of observation data. It is found that their linkage appears to have an interdecadal change from weak connection to strong connection. During the period of 1948-1977, the NAO was insignificantly correlated to the WNPTCF. However, during the period of 1980-2009, they were significantly correlated with stronger (weaker) NAO corresponding to more (fewer) tropical cyclones in the western North Pacific. The possible reason for such a different relationship between the NAO and the WNPTCF during the former and latter periods is further analyzed from the perspective of large-scale atmospheric circulations. When the NAO was stronger than normal in the latter period, an anomalous cyclonic circulation prevailed in the lower troposphere of the western North Pacific and the monsoon trough was intensified, concurrent with the eastward-shifting western Pacific subtropical high as well as anomalous low-level convergence and high-level divergence over the western North Pacific. These conditions favor the genesis and development of tropical cyclones, and thus more tropical cyclones appeared over the western North Pacific. In contrast, in the former period, the impact of the NAO on the aforementioned atmospheric circulations became insignificant, thereby weakening its linkage to the WNPTCF. Further study shows that the change of the wave activity flux associated with the NAO during the former and latter periods may account for such an interdecadal shift of the NAO-WNPTCF relationship.
    Zhou B. T., X. Cui, and P. Zhao, 2008: Relationship between the Asian-Pacific oscillation and the tropical cyclone frequency in the western North Pacific. Science in China Series D: Earth Sciences, 51( 3), 380- 385.10.1007/s11430-008-0014-70c9834403f1d27efb471b557da2e88e4http%3A%2F%2Flink.springer.com%2F10.1007%2Fs11430-008-0014-7http://www.cnki.com.cn/Article/CJFDTotal-JDXG200803007.htmThe relationship between the Asian-Pacific oscillation (APO) and the tropical cyclone frequency over the western North Pacific (WNP) in summer is preliminarily investigated through an analysis of ob- served data. The result has shown clearly that APO is significantly and positively correlated to the tropical cyclone frequency in the WNP. If APO is above (below) the normal in summer, more (less) tropical cyclones will tend to appear in the WNP. The present study also addresses the large-scale at- mospheric general circulation changes underlying the linkage between APO and the WNP tropical cy- clone frequency. It follows that a positive phase of summer APO is concurrent with weakened as well as northward and eastward located western Pacific subtropical high (WPSH), low-level convergence and high-level divergence, and reduced vertical zonal wind shear in the WNP, providing favorable envi- ronment for the tropical cyclone genesis, and thus more tropical cyclones will come into being, and vice versa.
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    • Manuscript History

    Manuscript received: 07 February 2015
    Manuscript revised: 20 September 2015
    Manuscript accepted: 12 October 2015
    通讯作者: 陈斌, bchen63@163.com
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      沈阳化工大学材料科学与工程学院 沈阳 110142

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    Impact of Atmospheric and Oceanic Conditions on the Frequency and Genesis Location of Tropical Cyclones over the Western North Pacific in 2004 and 2010

    • 1. College of Meteorology and Oceanography, PLA University of Science and Technology, Nanjing 211101
    • 2. International Center for Climate and Environment Science, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029
    • 3. Beijing Institute of Aeronautical Meteorology, Beijing 100085
    • Manuscript received: 2015-02-07
    • Manuscript revised: 2015-09-20
    • Manuscript accepted: 2015-10-12

    Abstract: This study examines the impact of atmospheric and oceanic conditions during May-August of 2004 and 2010 on the frequency and genesis location of tropical cyclones over the western North Pacific. Using the WRF model, four numerical experiments were carried out based on different atmospheric conditions and SST forcing. The numerical experiments indicated that changes in atmospheric and oceanic conditions greatly affect tropical cyclone activity, and the roles of atmospheric conditions are slightly greater than oceanic conditions. Specifically, the total number of tropical cyclones was found to be mostly affected by atmospheric conditions, while the distribution of tropical cyclone genesis locations was mainly related to oceanic conditions, especially the distribution of SST. In 2010, a warmer SST occurred west of 140°E, with a colder SST east of 140°E. On the one hand, the easterly flow was enhanced through the effect of the increase in the zonal SST gradient. The strengthened easterly flow led to an anomalous boundary layer divergence over the region to the east of 140°E, which suppressed the formation of tropical cyclones over this region. On the other hand, the colder SST over the region to the east of 140°E led to a colder low-level air temperature, which resulted in decreased CAPE and static instability energy. The decrease in thermodynamic energy restricted the generation of tropical cyclones over the same region.

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    1. Introduction

    • The western North Pacific (WNP) is the world's most active tropical cyclone basin (Emanuel, 2005; Peduzzi et al., 2012; Lin et al., 2013). The interannual variability of tropical cyclone activities, including the frequency, intensity, location and landfalling, is complicated and has been found to be closely related to large-scale circulation, including ENSO (Chan, 1985, 2000; Lander, 1994; Wang and Chan, 2002; Camargo and Sobel, 2005; Wada and Chan, 2008), MJO (Liebmann et al., 1994; Nakazawa, 2006; Nakano et al., 2015), quasi-biennial oscillation (Chan, 1985; Lau and Chan, 1993), Asian-Pacific Oscillation (Zhou et al., 2008), North Pacific oscillation (Wang et al., 2007), Antarctic oscillation (Ho et al., 2005; Wang and Fan, 2007), North Atlantic oscillation (Zhou and Cui, 2014), and Hadley circulation (Zhou and Cui,2008). In addition, many researchers consider global warming, and related SST change, to be a key driver of changes in tropical cyclone activity (Webster et al., 2005; Emanuel et al., 2008; Sugi et al., 2002, 2012; Gleixner et al., 2014; Scoccimarro et al., 2014; Holland and Bruy\`ere, 2014). However, the exact impacts that changes in SST may have on tropical cyclone activity remain ambiguous, especially when changes are not uniform.

      The effects of climate change on tropical cyclones have been a prominent issue for a number of years. In recent decades, SST in major tropical cyclone generation regions has increased several tenths of a degree Celsius (Santer et al., 2006). As a result of the recent increase in the capabilities of climate models, such models have captured some of the essential physical relationships that govern the links between the climate and tropical cyclones. Early climate model simulations, however, suggested some ambiguity in changes of tropical cyclone characteristics induced by warmer SST. While many models projected fewer tropical cyclones globally (Sugi et al., 2002; Bengtsson et al., 2007; Gualdi et al., 2008; Knutson et al., 2010), other climate models suggested some increase in future numbers (Broccoli and Manabe, 1990; Haarsma et al., 1993; Emanuel, 2013). (Yoshimura and Sugi, 2005) investigated impacts of SST warming on the tropical cyclone climatology using a high-horizontal-resolution AGCM. The results of numerical experiments in which SST was uniformly higher by 2 K demonstrated that the changes in SST had a relatively small influence on the tropical cyclone frequency. Similar to (Yoshimura and Sugi, 2005), (Held and Zhao, 2011) carried out an experiment in which SST was uniformly higher by 2 K, and found that the tropical cyclone frequency decreased by 10%. (Zhao et al., 2013) compared Hurricane Working Group model responses for various simulations. They found that most of the models showed decreases in global tropical cyclone frequency when the model was run with 2 K higher SST. In addition to the experiments with a uniformly higher SST, (Sugi et al., 2002) found that the regional change in tropical cyclone frequency was closely related to the distribution of the SST anomaly and the change in convective activity associated with it. (Chen and Huang, 2006) pointed out that when the West Pacific warm pool was warm, the tropical cyclone numbers over the WNP were lower than those when it was cold. Additionally, when the West Pacific warm pool was warm, the locations of tropical cyclones were mostly in the northwest, whereas the tropical cyclones occurred in the southeast more frequently when the warm pool was relatively cold. These studies show that the exact influence of SST changes on tropical cyclone activity is still a matter for debate. Moreover, the uniform increase or decrease in SST and non-uniform SST changes may affect the tropical cyclone activity differently.

      The horizontal scale of tropical cyclone ranges from hundreds of kilometers to thousands of kilometers, both of which are small relative to the scale of global climate models. However, tropical cyclones always involve substantial energy exchange and complex thermodynamic mechanisms, which cannot be depicted in detail even by the highest resolution global climate models. The WRF model, as one of the most popular numerical models in studying mesoscale weather systems, has been widely used in tropical cyclone case studies (Fierro et al., 2009; Cha and Wang, 2013; Sun et al., 2014a). (Shen et al., 2010) and (Wang et al., 2012) attempted the seasonal prediction of tropical cyclones over the WNP in 2006 and indicated that WRF was capable of tropical cyclone seasonal forecasting. However, they did not discuss the impacts of atmospheric environment change and SST change on tropical cyclone activity.

      In the present study, the impacts of the different atmospheric environments and SST between 2004 and 2010 on tropical cyclone activity were examined using the WRF model. The rationale for the choice of 2004 and 2010 as abnormal years for tropical cyclone activity is introduced in section 2. After a brief description of the model and experiments in section 3, the results of the experiments and the possible reasons for the changes in tropical cyclone activities due to changes in atmospheric environments and SST are presented and discussed in section 4, followed by a summary in section 5.

    2. Choice of abnormal years for tropical cyclone activity, and the climate background

      2.1. Analysis of tropical cyclone frequency

    • Tropical cyclone numbers were analyzed using the best track dataset of the China Meteorological Administration (CMA) (http://tcdata.typhoon.gov.cn/en/index.html) for the period 2000-2012 to identify abnormal years for tropical cyclone activity. Figure 1 shows that the annual total number of tropical cyclones was highest in 2004 and lowest in 2010. In addition, the average number of tropical cyclones during May-August was also highest in 2004 and lowest in 2010. Therefore, 2004 and 2010 were chosen as abnormal years for this study. The red and blue lines in Fig. 1 show the same pattern, meaning that the variation in the total number of tropical cyclones during May-August (MJJA) can also indicate the variation in the total number of tropical cyclones in a year.

      The numbers of tropical cyclones for each month between May and August of 2000 to 2012 are given in Table 1. The total number of tropical cyclones in 2004 and 2010 was 34 and 18, respectively. The number of tropical cyclones during MJJA in 2004 and 2010 was 20 and 8, respectively. For this reason, we chose the months of MJJA in 2004 and 2010 as the case study period.

      In addition to the difference in total tropical cyclone numbers, we considered whether there was any difference in the activity region or distribution of the genesis location of tropical cyclones between the two years. Figure 2 shows the distribution of tropical cyclone genesis location over the WNP from the best track dataset. There are clear differences in tropical cyclone numbers and locations between 2004 and 2010. Tropical cyclone numbers in 2004 are considerably greater than those in 2010, and the locations are scattered across a range from 100°E to 180°E in 2004, whereas in 2010 they are mainly located to the west of 140°E. In this study, we sought to determine why such great differences in total numbers and locations existed between the two years. We also sought to determine what the differences were in the atmospheric and oceanic situation that influenced the tropical cyclone activity between these two abnormal years.

      Figure 1.  Tropical cyclone numbers recorded in the CMA best track dataset over the WNP during the period 2000-2012: annual total number (red line); total during MJJA (blue line).

      Figure 2.  The tracks and locations from the best track dataset during MJJA (a) 2004 and (b) 2010 over the WNP. Different colors represent different intensities: weaker than tropical depression ($<$TD); tropical depression (TD); tropical storm (TS); severe tropical storm (STS); typhoon (TY); severe typhoon (STY); and super typhoon (SuperTY).

      Figure 3.  The average MJJA (a, b) geopotential height (contours; interval of solid contours: 4 dagpm; units: dagpm) at 500 hPa and (c, d) stream fields at 850 hPa with tropical cyclone genesis locations (red dots) in (a, c) 2004 and (b, d) 2010. Fields were derived from NCEP FNL; tropical cyclone genesis locations were from best track dataset of CMA.

      • 2.2. Atmospheric and oceanic background of 2004 and 2010

    • We examined the differences in the subtropical high, WNP monsoon trough and SST during MJJA between 2004 and 2010. The datasets used were all from the NCEP FNL (final) Analysis (http://rda.ucar.edu/datasets/ds083.2/) at 1° resolution and 6-h intervals.

      The forecasting of circulation is a very important part in the process of tropical cyclone seasonal forecasting (Wang et al., 2012). In the typhoon season, the southeast trade winds in the south of the western Pacific subtropical high directly influence the main genesis locations of tropical cyclones, as well as their generation, development and track (Lei and Chen, 2001; Huang et al., 2013). Figures 3a and b demonstrate the circulation at 500 hPa in 2004 and 2010. In 2004, the subtropical high (area of 588 dagpm line) was weaker than that in 2010. The western ridge was near 140°E in 2004, while it expanded to 122°E in 2010. Downward air-flows in regions controlled by a subtropical high are unfavorable for the formation of tropical cyclones. Therefore, tropical cyclones always activate in the south and west of the western Pacific subtropical high (Ren et al., 2007; Sun, 2011). This explains why the locations of tropical cyclones in 2004 were scattered over the broad region in the south and west of the subtropical high. In 2010, the subtropical high extended southward to 10°N. In general, tropical cyclones do not originate in such low latitude regions, so the locations of tropical cyclones in 2010 were located mostly in the west of the subtropical high, and fewer tropical cyclones were activated in total.

      The western North Pacific monsoon trough is a low pressure belt formed by convergence of the southwest monsoon or cross-equatorial flow and southeast trade wind in the south of the subtropical high. It is a part of the ITCZ. The WNP monsoon trough is a region of considerable convective activity, and such activities are conducive to the generation of low pressure perturbations and therefore the generation and development of tropical cyclones (Gao et al., 2008). Figures 3c and d show the monsoon trough in 2004 and 2010. The monsoon trough in 2004 was clearly stronger than it was in 2010. The monsoon trough extended eastward to 150°E with more tropical cyclones in 2004; while in 2010, it did not reach 130°E and therefore the tropical cyclone locations were further west, and fewer tropical cyclones were activated in total.

      From the analysis of atmospheric circulations, we can see that the atmospheric conditions were clearly distinct between the two years in question. The location and strength of the subtropical high and monsoon troughs strongly influenced the tropical cyclone activity. Following the description of these different atmospheric situations, we next examined whether the oceanic situation in 2004 and 2010 was also noticeably different. Previous research has indicated that tropical cyclone activity is closely related to SST (Holland, 1997; Hoyos et al., 2006; Dare and McBride, 2011; Sun et al., 2014b). Therefore, this paper also analyzes the variation in the SST in 2004 and 2010.

      Figure 4 shows the distribution of oceanic heat (SST) difference between 2004 and 2010 (expressed as 2010 minus 2004). Figure 4 shows that there was a significant difference in SST distribution between 2004 and 2010. The SST difference of the warm pool [located at approximately (0°-20°N, 110°-150°E)] and the ocean to the east of the warm pool were totally opposite. The warm pool in 2010 was warmer than that in 2004, while the ocean to the east of the warm pool was colder than that in 2004.

      The SST in 2004 and 2010 was also compared with the climatology (not shown). The results were consistent with those in Fig. 4, e.g., the warm pool in 2004 had a negative anomaly, but there was a positive anomaly in 2010. The observations of tropical cyclone genesis locations in 2004 and 2010 show that the tropical cyclone numbers over the east of the warm pool in 2010 were lower than those of 2004. This indicates that the number of tropical cyclones formed in the east of the warm pool decreases when the warm pool has a warm anomaly. This relationship between different thermal conditions of the warm pool and tropical cyclone activity is consistent with the results of (Chen and Huang, 2006).

      Figure 4.  Difference in average SST (units: $^\circ$C) between 2004 and 2010.

    3. Model and experiments

      3.1. Model configuration

    • The WRF model (version 3.3.1) (Skamarock et al., 2008) was employed in this study. A 10-km resolution domain with 36 vertical levels was set up for the simulation. The model domain was centered at (30°N, 140°E), with 380 (east-west) × 340 (north-south) grid points, including complex topography and land-sea contrast. The initial and lateral boundary conditions, as well as the SST forcing data, were derived from the datasets used in section 2.2. The simulation started at 0000 UTC 1 May and ended at 1800 UTC 31 August, with a total of 123 days' integration. The physical parameterizations used included the Eta microphysics scheme (Rogers et al., 2001), and the Betts-Miller-Jajic cumulus parameterization scheme (Janjić, 1994). The other schemes were the Yonsei University planetary boundary layer scheme (Hong et al., 2006), the Rapid Radiative Transfer Model (Mlawer et al., 1997) for longwave radiation, and the Goddard scheme (Chou and Suarez, 1994) for shortwave radiation. The sst_update option in the WRF model was opened to update the SST every six hours.

      • 3.2. Experiments

    • From the analyses above, we can see that both oceanic and atmospheric conditions were significantly different between 2004 and 2010. There seems to be a close link between the difference in SST and tropical cyclone locations in 2004 and 2010. Based on these observations, we next considered what influence the SST changes may have on tropical cyclone activity when the atmospheric conditions remain the same. To answer this question, four experiments were implemented.

      The atmospheric initial and lateral boundary conditions and oceanic SST forcing of EXP1 were from the 2004 FNL dataset. EXP2 was the same as EXP1 except that the SST forcing was taken from 2010. The atmospheric initial and lateral boundary conditions and oceanic SST forcing of EXP3 were from the 2010 FNL dataset. EXP4 was the same as EXP3 except that the SST forcing was taken from 2004.

      • 3.3. Criteria for selecting tropical cyclones

    • An essential first step in the analysis was to select a method for detecting and tracking the cyclones in the model outputs. A number of such schemes have been developed over the years. For example, the model and basin-threshold dependent scheme (Camargo and Zebiak, 2002), the structure-based threshold scheme (Walsh et al., 2007), and the circulation-based scheme (Tory et al., 2013). These schemes have some key differences but also share many common characteristics: (1) near-surface wind speed; (2) low-level relative vorticity; (3) a warm core; (4) a difference in wind speed between the upper and low levels; and (5) duration. Based on Hurricane Working Group simulations, (Horn et al., 2014) examined the sensitivity of the variation in tropical cyclone numbers to different schemes. In general, after corrections were made, there was considerably greater agreement than disagreement on the sign of the model response between different tracking schemes. Nevertheless, it was possible to obtain a different sign of the response for the same experiment by using different schemes.

      This was demonstrated by (Camargo and Zebiak, 2002), who used basin-and model-dependent threshold criteria to improve the climatology and interannual statistics of model tropical cyclones. The methods used in the present study were basically the same as those used by (Camargo and Zebiak, 2002). Values of 850-hPa relative vorticity, sea level pressure, and 10 m wind speed were chosen to locate the tropical cyclone. The structure was described from three different aspects: 700, 500 and 300 hPa temperature anomalies, to depict the warm core structure; the difference between the 300 and 850 hPa temperature anomalies, to depict the vertical structure of the warm core; and the difference in wind speed between 300 and 850 hPa, to depict the vertical structure of the tropical cyclone. These thresholds were confirmed by (Camargo and Zebiak, 2002). Seven sets of criteria were used as follows:

      (1) An 850-hPa relative vorticity of ≥ 3.6 × 10-5 s-1 near the cyclone center;

      (2) A maximum surface wind speed in a centered 7°× 7° grid box of >11.8 m s-1;

      (3) A sea level pressure that is the local minimum and carries a value of <1020 hPa;

      (4) An average temperature difference from the area mean of the surrounding region at 300, 500 and 700 hPa of >1.9 °C;

      (5) A local temperature anomaly at 300 hPa that is greater than that at 850 hPa;

      (6) An average speed that is larger at 850 hPa than at 300 hPa;

      (7) A duration of at least 1.5 days (for six-hourly output).

      (Camargo and Zebiak, 2002) pointed out that the tracks obtained by these criteria are usually very short. Visual examination of the corresponding relative vorticity fields showed that the cyclone structure is visible well before and after the detection criteria are met. This suggests that relaxing the detection criteria would produce longer tracks. To verify the given method, it was used to detect tropical cyclones from the FNL dataset ranging from 0000 UTC 1 May 2004 to 1800 UTC 31 August 2004. Figure 5 shows the result from the model and the best track. The results show that the method is able to successfully detect the tropical cyclone locations and tracks.

    4. Results and discussion

      4.1. Results

    • The numbers of tropical cyclones and tropical cyclone "dots" simulated by the numerical experiments are shown in Table 2. A tropical cyclone "dot" is a grid location that meets all seven of the criteria mentioned above. A longer track was obtained by tracking backward and forward based on the tropical cyclone dot. Both the number of tropical cyclones and number of tropical cyclone dots were the most in EXP1, while in EXP3 they were both the least. These results agreed well with the CMA best track dataset. EXP1 simulated three fewer tropical cyclones than the best track of 2004 and EXP3 simulated two fewer tropical cyclones than the best track of 2010, i.e., the simulated tropical cyclone numbers were lower than observed, which is in agreement with the result of (Wang et al., 2012). Comparison of EXP1 and EXP2 indicates that the SST of 2010 (EXP2) reduced the number of tropical cyclone dots by nearly 15% and tropical cyclone numbers by nearly 29%. Meanwhile, comparison of EXP3 and EXP4 indicates that the SST of 2010 (EXP3) decreased the number of tropical cyclone dots by nearly 35% and tropical cyclone numbers by nearly 25%. Comparison of EXP1 and EXP4 indicates that the atmospheric environment of 2010 (EXP4) reduced the number of tropical cyclone dots by nearly 44% and tropical cyclone numbers by nearly 53%. Meanwhile, comparison of EXP2 and EXP3 indicates that the atmospheric environment of 2010 (EXP3) decreased the number of tropical cyclone dots by nearly 57% and tropical cyclone numbers by nearly 50%. Thus, we can conclude that such changes in atmospheric conditions and oceanic conditions (SST) greatly affect tropical cyclone activity. Moreover, the roles of changes in atmospheric conditions were slightly greater than those of oceanic conditions.

      Figure 5.  Tracks detected from the FNL dataset using the Camargo (2002) method (blue) and the tracks of the best track dataset from the CMA (red).

      Figure 6.  The (a, b) tracks and (c, d) locations of model outputs (blue) and best track dataset (red) in (a, c) 2004 (EXP1) and (b, d) 2010 (EXP3).

      The tracks and locations of tropical cyclones simulated in EXP1 and EXP3 are shown alongside the best track results in Fig. 6. EXP1 was a simulation of 2004, while EXP3 was a simulation of 2010. Although the simulated tracks were not able to remain consistent with the best track one by one, the results nevertheless captured general features of tracks and the distribution of locations. The tropical cyclone genesis locations simulated in EXP1 were essentially in accordance with the best track results for 2004, with a slight westward deviation. The results of EXP3 show that the tropical cyclone genesis locations in 2010 were mostly located to the west of 140°E.

      Figure 7.  The simulated (a-d) tracks and (e-h) locations in the four experiments.

      Figure 8.  Difference in average SST (a, b) (contours; above zero: solid line; below zero: dashed line; interval: 0.4$^\circ$C), geopotential height at 500 hPa (c, d) (contours; above zero: solid line; below zero: dashed line; interval: 0.5 dagpm) and precipitation rate (shaded; units: mm d$^{-1}$). Pattern correlations between SST, geopotential height at 500 hPa and precipitation are also shown in parentheses.

      Figure 9.  Difference in the average stream field (vectors; units: m s$^{-1}$) at 850 hPa and vertical velocity (colored shading; units: m s$^{-1}$) at 500 hPa.

      Figure 10.  Difference in (a, b) CAPE (units: J kg$^{-1}$), (c, d) static stability (units: $^\circ$), (e, f) relative humidity (units: percent) at 700 hPa, and (g, h) OLR (units: W m$^-2$) and SST (contours; above zero: solid line; below zero: dashed line; interval: 0.4$^\circ$C). Pattern correlations between CAPE, static stability, relative humidity at 700 hPa, OLR and SST are also shown in parentheses.

      Figure 11.  Difference in (a, b) CAPE (units: J kg$^{-1}$), (c, d) static stability (units: $^\circ$), (e, f) relative humidity (units: percent) at 700 hPa, and (g, h) OLR (units: W m$^-2$) and geopotential height at 500 hPa (contours; above zero: solid line; below zero: dashed line; interval: 0.5 dagpm). Pattern correlations between CAPE, static stability, relative humidity at 700 hPa, OLR and geopotential height at 500 hPa are also shown in parentheses.

      Figure 7 shows the tracks and locations of the four experiments. The number of tropical cyclones that originated from east of 140°E in EXP2 was lower than in EXP1 (Fig. 7e). In other words, SST in 2010 reduced the number of tropical cyclones originating from east of 140°E. This is in agreement with the best track results, which show that the locations in 2010 were in the west. The number of tropical cyclones originating from the region east of 140°E was higher in EXP4 than in EXP3 (Fig. 7f). In other words, SST in 2004 increased the number of tropical cyclones originating from east of 140°E. Again, this is in agreement with the best track results, which show that the tropical cyclone genesis locations in 2004 were scattered widely both east and west of 140°E. Figure 7g shows many tropical cyclones originating from east of 140°E in both EXP1 and EXP4 with SST in 2004. The atmospheric conditions in 2010 reduced the total number of tropical cyclones. Figure 7h shows that tropical cyclones in EXP2 and EXP3 with SST in 2010 were mainly scattered west of 140°E. The atmospheric conditions in 2004 increased the total number of tropical cyclones. These model results were all in accordance with the observations.

      • 4.2. Discussion

    • The comparisons between the experiments reported above indicate that any changes of atmospheric conditions and oceanic conditions can greatly affect tropical cyclone activity. We now attempt to explain this process by analyzing the impacts of their changes on the convective activity and its associated quantities.

      Figure 8 shows the differences in average atmospheric conditions (geopotential height at 500 hPa: H500), oceanic conditions (SST) and precipitation rate. We can see good agreement between the pattern of the SST anomaly and the pattern of the precipitation difference in both Figs. 8a and b. The pattern correlation (8°-30°N) reaches 0.44 and 0.42, respectively. All the pattern correlation coefficients in this paper passed the u-test (Mann and Whitney, 1947), with a significance level of 0.05. The regions with a positive SST anomaly have a positive precipitation difference, and vice versa. Thus, over the region east of 140°E, the patterns of changes in tropical cyclone numbers, precipitation and the SST anomaly resemble each other. There is also good agreement between the H500 and the pattern of the precipitation difference in both Figs. 8c and d. The pattern correlation (8°-30°N) reaches -0.38 and -0.43, respectively. The regions with a positive H500 anomaly have a negative precipitation difference. This suggests a chain of links from the geopotential height anomaly at 500 hPa and the SST anomaly to convective activity (or precipitation), from convective activity to circulation, and from circulation to cyclogenesis. Besides, we can see that the spatial patterns of SST and precipitation rate in Figs. 8a and b possess apparent differences in zonal direction, while the spatial patterns of H500 and precipitation rate in Figs. 8c and d do not show any obvious zonal difference.

      To understand the changes in convective activity, we examined the changes in the distribution of the mean fields of quantities associated with convection. Figure 9 shows the difference in the wind field at 850 hPa and the vertical velocity at 500 hPa (W500). In Fig. 9a, there is weak anticyclonic circulation in the east of the warm pool between 10°N and 20°N, and weak anticyclonic circulation in the same regions shown in Fig. 9b. (Zhao et al., 2013) showed that decreases in global tropical cyclone frequency for the 2 K higher SST run were most closely related to 500 hPa vertical velocity. (Kim et al., 2014), using GFDL CM2.5, obtained the same result; that the reduction of tropical cyclone frequency was strongly related to weakening of vertical velocity in the mid-troposphere. In the 500 hPa vertical velocity field in Fig. 9a, there is a downward W500 in the east of the warm pool where an anticyclone is located in the lower level, which is not suitable for tropical cyclone generation. In Fig. 9b, there is also downward W500 in the east of the warm pool with anticyclonic circulation in the lower level, which is not suitable for tropical cyclone formation. The anticyclone and downward W500 can be seen in Figs. 9c and d over the region to the west of the warm pool, and also to the east of the warm pool. We can conclude that the atmospheric circulation triggered by the atmospheric condition and SST in 2010 was not conductive to cyclogenesis.

      In addition to the dynamic variables, Figs. 10 and 11 show the patterns of differences in average CAPE, static stability, relative humidity at 700 hPa, and OLR. In Fig. 10a, negative anomalous CAPE with a negative SST difference is apparent in the east of the warm pool. Over these regions, anomalous convection is negative (Fig. 8a). On the other hand, the same relationship can be seen in Figs. 10b and 8b. The pattern correlation in Figs. 10a and b reach 0.54 and 0.66, respectively. Here, the static stability is defined as the difference in potential temperature at 500 hPa and 1000 hPa (denoted as THETA for convenience). Figure 10c shows negative anomalous static stability east of the warm pool, and Fig. 10d also shows negative anomalous static stability in the same region. The pattern correlation in Figs. 10c and d reaches 0.82 and 0.81, respectively. In Fig. 10e, there are positive and negative relative humidity centers in the east of the warm pool with a negative anomalous SST. In Fig. 10f, there are also positive and negative relative humidity centers in the region where the anomalous SST is negative. Therefore, the pattern correlation in Figs. 10e and f only reaches -0.04 and 0.12, respectively. In Fig. 10g, there is positive anomalous OLR in the warm pool and east of the warm pool, whereas Fig. 10h shows positive anomalous OLR in the east of the warm pool with a negative anomalous SST and negative anomalous OLR in the warm pool with a positive anomalous SST. The pattern correlation in Figs. 10g and h reaches -0.20 and -0.16, respectively. Moreover, there are zonal differences in all of these quantities' distributions. The consistency between the distribution of SST and these quantities associated with convection means that the distribution of SST influenced the distribution of tropical cyclone genesis locations.

      Figure 11 shows the difference in CAPE, static stability, relative humidity at 700 hPa, OLR, and H500, in EXP4 versus EXP1, and EXP3 versus EXP2. Compared with Fig. 10, we can see that the distributions of these quantities in Fig. 11 possess no obvious zonal differences. The differences in CAPE, static stability, and relative humidity at 700 hPa are negatively correlated with the differences in H500, while the differences in OLR are positively correlated with the differences in H500. The pattern correlations in Figs. 11a-h reach -0.22, -0.25, -0.17, -0.24, -0.39, -0.53, 0.34 and 0.36, respectively. This means that the increase in H500 in 2010 restrained the convective activity.

      Besides, we also calculated the pattern correlation with SST and H500 of the relative vorticity at 850 hPa and divergence at 200 hPa, separately. Figure 10 shows the roles of SST fields (EXP1 versus EXP2, EXP3 versus EXP4) in tropical cyclone activity in the WNP, and Fig. 11 shows the roles of the atmospheric environment (EXP1 versus EXP4, EXP2 versus EXP3) in tropical cyclone activity in the WNP. We averaged the pattern correlation in Figs. 10a and b to represent the impacts of SST on CAPE. Similarly, we averaged the pattern correlation in Figs. 11a and b to represent the impacts of the atmospheric environment (H500) on CAPE. The results are shown in Table 3. It is clear that changes in atmospheric environment have greater impacts on changes in RH, OLR, relative vorticity and divergence, compared with changes in ocean conditions. However, changes in ocean conditions have greater impacts on changes in CAPE and THETA, compared with changes in atmospheric environment.

      The above analysis may help us to better understand why colder SST over the region to the east of 140°E in 2010 decreased tropical cyclone numbers over the same region. In 2010, a greater SST warming occurred west of 140°E; meanwhile, there was colder SST east of 140°E. As a result, the zonal SST gradient increased across the whole region (90°-180°E). The increased zonal SST gradient strengthened the easterly flow, which led to an increase in boundary layer divergence over the region to the east of 140°E (wind fields in Figs. 9a and b). As we know, tropical cyclones originate from tropical disturbances (Lau and Lau, 1990; Fu et al., 2007); therefore, the anomalous boundary layer divergence over that region suppressed the formation of tropical cyclones. In addition to the unfavorable dynamic conditions, the colder SST east of 140°E led to a colder low-level air temperature, which resulted in a decrease in CAPE and static instability energy (Figs. 10a-d). The decrease in thermodynamic energy restricted the generation of tropical cyclones.

    5. Summary

    • In this study, the influence of different atmospheric and oceanic conditions on tropical cyclone frequency and location in the WNP was examined using the WRF model. Four experiments that employed different atmospheric initial and lateral boundary conditions, as well as different SST forcing, were performed for the MJJA period of 2004 and 2010.

      The results of the experiments showed that changes in atmospheric environment have greater impacts on changes in RH at 700 hPa, OLR, relative vorticity at 850 hPa, and divergence at 200 hPa, compared with changes in ocean conditions. However, changes in ocean conditions have greater impacts on changes in CAPE and static stability. The differences in the locations of tropical cyclones in 2004 and 2010 were closely related to the distribution of the SST anomaly, and the change in its associated convective activity. In the warmer (colder) region, low-level static instability enhanced (weakened), mid-low-level moisture increased (decreased), and CAPE increased (decreased). The increase (decrease) of low-level instability energy and moisture directly affected the convective activity. Accompanied by low-level cyclonic (anticyclonic) circulation and upward (downward) vertical velocity in the mid-troposphere, tropical cyclone activity over this region was promoted (restrained).

      It has been found that the total number of tropical cyclones was mostly affected by atmospheric conditions, while the distribution of tropical cyclone genesis locations was mainly influenced by oceanic conditions, especially the distribution of SST. The physical processes responsible for the decreased numbers of tropical cyclones over the region to the east of 140°E in 2010, caused by colder SST over the same region, are discussed below. In 2010, on the one hand, the easterly flow was enhanced through the effect of the increase of the zonal SST gradient. The strengthened easterly led to anomalous boundary layer divergence over the region to the east of 140°E. The anomalous boundary layer divergence suppressed the formation of tropical cyclones. On the other hand, the colder SST over the region to the east of 140°E led to colder low-level air temperature, which resulted in decreased CAPE and static instability energy. The decrease in thermodynamic energy restricted the generation of tropical cyclones.

      Although the simulated numbers and locations of tropical cyclones in 2004 and 2010 were close to the best track on the seasonal timescale, there were still clear differences on the monthly timescale. Model errors accumulated in long-term simulations and the choice of parameterization schemes may contribute to this issue. Improving our understanding of the mechanisms through which changes of atmospheric conditions and oceanic conditions influence tropical cyclone activity requires more detailed analyses and experiments in the future.

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