## 2012 Vol. 29, No. 6

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2012, 29(6): 1129-1141. doi: 10.1007/s00376-012-1248-z
Abstract:
In this study, the relationship between El Nino-Southern Oscillation (ENSO) and winter rainfall over Southeast China (SC) is demonstrated based on instrumental and reanalysis data. The results show that ENSO and SC winter rainfall (ENSO-SC rainfall) are highly correlated and intimately coupled through an anomalous high pressure over the northwestern Pacific. In mature phase, El Nino (La Nina) events can cause more (less) rainfall over SC in winter. Due to the persistence and spring barrier of ENSO, SC winter rainfall has potential predictability of about half a year ahead with ENSO as a predictor. Besides, the ENSO-SC rainfall relationship exhibits decadal variability, closer before the early 1970s (0.47) and after the early 1990s (0.76), but weaker (0.12) between these times. In different periods, atmospheric teleconnection patterns have large differences and the predictability of SC winter rainfall also changes dramatically. For the most recent 20 years, the ENSO-SC rainfall relationship is closest and the prediction of SC winter rainfall anomalies based on ENSO is most creditable. In addition, the causes and mechanisms of the decadal modulation of the relationship between ENSO and SC winter rainfall need to be further studied.
2012, 29(6): 1142-1158. doi: 10.1007/s00376-012-1183-z
Abstract:
We investigated the impact of tuning the length scale of the background error covariance in the Weather Research and Forecasting (WRF) three-dimensional variational assimilation (3DVAR) system. In particular, we studied the effect of this parameter on the assimilation of high-resolution surface data for heavy rainfall forecasts associated with mesoscale convective systems over the Korean Peninsula. In the assimilation of high-resolution surface data, the National Meteorological Center method tended to exaggerate the length scale that determined the shape and extent to which observed information spreads out. In this study, we used the difference between observation and background data to tune the length scale in the assimilation of high-resolution surface data. The resulting assimilation clearly showed that the analysis with the tuned length scale was able to reproduce the small-scale features of the ideal field effectively. We also investigated the effect of a double-iteration method with two different length scales, representing large and small-length scales in the WRF-3DVAR. This method reflected the large and small-scale features of observed information in the model fields. The quantitative accuracy of the precipitation forecast using this double iteration with two different length scales for heavy rainfall was high; results were in good agreement with observations in terms of the maximum rainfall amount and equitable threat scores. The improved forecast in the experiment resulted from the development of well-identified mesoscale convective systems by intensified low-level winds and their consequent convergence near the rainfall area.
2012, 29(6): 1159-1173. doi: 10.1007/s00376-012-1253-2
Abstract:
The persistence barrier of sea surface temperature anomalies (SSTAs) in the North Pacific was investigated and compared with the ENSO spring persistence barrier. The results show that SSTAs in the central western North Pacific (CWNP) have a persistence barrier in summer: the persistence of SSTAs in the CWNP shows a significant decline in summer regardless of the starting month. Mechanisms of the summer persistence barrier in the CWNP are different from those of the spring persistence barrier of SSTAs in the central and eastern equatorial Pacific. The phase locking of SSTAs to the annual cycle does not explain the CWNP summer persistence barrier. Remote ENSO forcing has little linear influence on the CWNP summer persistence barrier, compared with local upper-ocean process and atmospheric forcing in the North Pacific. Starting in wintertime, SSTAs extend down to the deep winter mixed layer then become sequestered beneath the shallow summer mixed layer, which is decoupled from the surface layer. Thus, wintertime SSTAs do not persist through the following summer. Starting in summertime, persistence of summer SSTAs until autumn can be explained by the atmospheric forcing through a positive SSTAs--cloud/radiation feedback mechanism because the shallow summertime mixed layer is decoupled from the temperature anomalies at depth, then the following autumn--winter--spring, SSTAs persist. Thus, summer SSTAs in the CWNP have a long persistence, showing a significant decline in the following summer. In this way, SSTAs in the CWNP show a persistence barrier in summer regardless of the starting month.
2012, 29(6): 1174-1184. doi: 10.1007/s00376-012-1102-3
Abstract:
In this paper the optimal precursors for wintertime Eurasian blocking onset are acquired by solving a nonlinear optimization problem whose objective function is constructed based on a blocking index with a triangular T21, three-level, quasi-geostrophic global spectral model. The winter climatological state is chosen as the reference basic state. Numerical results show that the optimal precursors are characterized by a baroclinic pattern with a westward tilt with height, which are mainly located upstream of the blocking region. For an optimization time of 5 days, these perturbations are mainly localized over the Northeast Atlantic Ocean and continental Europe. With the extension of the optimization time to 8 days, these perturbations are distributed more upstream and extensively in the zonal direction. Wave spectrum analysis reveals that the optimal precursors are composed of not only synoptic-scale (wave numbers 5--18) waves, but planetary-scale (wave numbers 0--4) waves as well. The synoptic-scale optimal precursors are mainly located in the mid-latitude area, while the planetary-scale optimal precursors focus primarily on the high-latitude region. The formation of a strong planetary-scale positive blocking anomaly is accompanied by the reinforcement of synoptic-scale perturbations and further fragmentation into two branches, in which the northern branch is generally stronger than the southern one. The eddy forcing arising from the self-interaction of synoptic-scale disturbances is shown to be crucial in triggering the dipole blocking anomaly, and the planetary-scale optimal precursor provides the initial favorable background conditions for blocking onset.
2012, 29(6): 1185-1199. doi: 10.1007/s00376-012-1257-y
Abstract:
The direct radiative forcing (DRF) of sulfate aerosols depends highly on the atmospheric sulfate loading and the meteorology, both of which undergo strong regional and seasonal variations. Because the optical properties of sulfate aerosols are also sensitive to atmospheric relative humidity, in this study we first examine the scheme for optical properties that considers hydroscopic growth. Next, we investigate the seasonal and regional distributions of sulfate DRF using the sulfate loading simulated from NCAR CAM-Chem together with the meteorology modeled from a spectral atmospheric general circulation model (AGCM) developed by LASG-IAP. The global annual-mean sulfate loading of 3.44 mg m-2 is calculated to yield the DRF of -1.03 and -0.57 W m-2 for clear-sky and all-sky conditions, respectively. However, much larger values occur on regional bases. For example, the maximum all-sky sulfate DRF over Europe, East Asia, and North America can be up to -4.0 W m-2. The strongest all-sky sulfate DRF occurs in the Northern Hemispheric July, with a hemispheric average of -1.26 W m-2. The study results also indicate that the regional DRF are strongly affected by cloud and relative humidity, which vary considerably among the regions during different seasons. This certainly raises the issue that the biases in model-simulated regional meteorology can introduce biases into the sulfate DRF. Hence, the model processes associated with atmospheric humidity and cloud physics should be modified in great depth to improve the simulations of the LASG-IAP AGCM and to reduce the uncertainty of sulfate direct effects on global and regional climate in these simulations.
2012, 29(6): 1200-1214. doi: 10.1007/s00376-012-1226-5
Abstract:
Responses of the East Asian winter monsoon (EAWM) in future projections were studied based on two core future projections of CMIP5 in coordinated experiments with the IAP-coupled model FGOALS2-s. The projected changes of EAWM in climatology, seasonality, and interannual variability are reported here; the projections indicated strong warming in winter season. Warming increased with latitude, ranging from 1oC to 3oC in the Representative Concentration Pathways simulation RCP4.5 projection (an experiment that results in additional radiative forcing of ～4.5 W m-2 in 2100) and from 4oC to 9oC in the RCP8.5 projection (an experiment that results in additional radiative forcing of ～8.5 W m-2 in 2100). The northerly wind along the East Asian coastal region became stronger in both scenarios, indicating a stronger EAWM. Accordingly, interannual variability (described by the standard deviation of temperature) increased around the South China Sea and lower latitudes and decreased over eastern China, especially in North China. The two EAWM basic modes, defined by the temperature EOF analysis over East Asia, were associated with the Arctic Oscillation (AO) and stratospheric polar vortex. The future projections revealed more total variance attributable to the secondary mode, suggesting additional influences from the stratosphere. The correlation between AO and the leading mode decreased, while the correlation between AO and the secondary mode increased, implying increased complexity regarding the predictability of EAWM interannual variations in future projections.
2012, 29(6): 1215-1226. doi: 10.1007/s00376-012-1186-9
Abstract:
The mesoscale vortex associated with a mesoscale low-level jet (mLLJ) usually causes heavy rainfall in the col field. The col field is defined as a region between two highs and two lows, with the isobaric surface similar to a col. Using a two-dimensional shallow water model, the meso- scale vortex couplets (MVCs) induced by eight types of mesoscale wind perturbations in an ideal col field were numerically simulated. With the sizes of ～100 km, the MVCs induced by northerly perturbation (NP) and southerly perturbation (SP) moved toward the col point. The sizes of MVCs induced by southwesterly perturbation (SWP), southeasterly perturbation (SEP), northwesterly perturbation (NWP), and northeasterly perturbation (NEP) were relatively small for the perturbations moving toward dilatation axis. The MVC induced by easterly perturbation (EP) and westerly perturbation (WP) could not develop because they quickly moved away from the col point, before the circulation could form. The size of the circulation was determined by the distance between the vortex and the col point. The closer to the col point the vortex was, the larger the size of vortex. The comparisons of maximum vorticity and vorticity root mean square error (RMSE) of the NP, the SWP, and the WP show that the maximum vorticity and the vorticity RMSE of the NP decreased slower than other perturbations. Therefore, the weak environment of the col field favors the maintenance of vorticity and the formation of vortex. When a mesoscale vortex forms near the col point or moves toward the col point, it may maintain a quasi-stationary state in the stable col field.
2012, 29(6): 1227-1237. doi: 10.1007/s00376-012-1066-3
Abstract:
The impact of ground heating on flow fields in street canyons under different ambient wind speed conditions was studied based on numerical methods. A series of numerical tests were performed, and three factors including height-to-width (H/W) ratio, ambient wind speed and ground heating intensity were taken into account. Three types of street canyon with H/W ratios of 0.5, 1.0 and 2.0, respectively, were used in the simulation and seven speed values ranging from 0.0 to 3.0 m s-1 were set for the ambient wind speed. The ground heating intensity, which was defined as the difference between the ground temperature and air temperature, ranged from 10 to 40 K with an increase of 10 K in the tests. The results showed that under calm conditions, ground heating could induce circulation with a wind speed of around 1.0 m s-1, which is enough to disperse pollutants in a street canyon. It was also found that an ambient wind speed threshold may exist for street canyons with a fixed H/W ratio. When ambient wind speed was lower than the threshold identified in this study, the impact of the thermal effect on the flow field was obvious, and there existed a multi-vortex flow pattern in the street canyon. When the ambient wind speed was higher than the threshold, the circulation pattern was basically determined by dynamic effects. The tests on the impact of heating intensity showed that a higher ground heating intensity could strengthen the vortical flow within the street canyon, which would help improve pollutant diffusion capability in street canyons.
2012, 29(6): 1238-1248. doi: 10.1007/s00376-012-1191-z
Abstract:
A season-reliant empirical orthogonal function (S-EOF) analysis was applied to the seasonal mean SST anomalies (SSTAs) based on the HadISST1 dataset with linear trend removed at every grid point in the South Pacific (60.5o--19.5oS, 139.5oE--60.5oW) during the period 1979--2009. The spatiotemporal characteristics of the dominant modes and their relationships with ENSO were analyzed. The results show that there are two seasonally evolving dominant modes of SSTAs in the South Pacific with interannual and interdecadal variations; they account for nearly 40% of the total variance. Although the seasonal evolution of spatial patterns of the first S-EOF mode (S-EOF1) did not show remarkable propagation, it decays with season remarkably. The second S-EOF mode (S-EOF2) showed significant seasonal evolution and intensified with season, with distinct characteristics of eastward propagation of the negative SSTAs in southern New Zealand and positive SSTAs southeast of Australia. Both of these two modes have significant relationships with ENSO. These two modes correspond to the post-ENSO and ENSO turnabout years, respectively. The S-EOF1 mode associated with the decay of the eastern Pacific (EP) and the central Pacific (CP) types of ENSO exhibited a more significant relationship with the EP/CP type of El Nino than that with the EP/CP type of La Nina. The S-EOF2 mode contacted with the EP type of El Nino changing into the EP/CP type of La Nina showed a more significant connection with the EP/CP type of La Nina.
2012, 29(6): 1249-1263. doi: 10.1007/s00376-012-1125-9
Abstract:
Based on monthly ECMWF reanalysis-Interim (ERA-Interim) reanalysis data, along with monthly precipitation and temperature data, the Dynamic Plateau Monsoon Index (DPMI) is defined. The results of a contrast analysis of the DPMI versus the Traditional Plateau Monsoon Index (TPMI) are described. The response of general circulation to northern Qinghai-Xizang Plateau summer monsoon anomalies and the correlation of the DPMI with general circulation anomalies are investigated. The results show that, the DPMI reflected meteorological elements better and depicted climate variation more accurately than the TPMI. In years when the plateau summer monsoon is strong, the low over the plateau and the trough near the eastern coast of Asia are deeper and higher than normal over South China. This correlation corresponds to two anomalous cyclones over the plateau and the eastern coast of Asia and an anomalous anticyclone in South China. The plateau and its adjacent regions are affected by anomalous southwesterly winds that transport more moisture to South China and cause more precipitation. The lower reaches of the Yangtze River appear to receive more precipitation by means of the strong westerly water vapor flow transported from the large triangle affecting the region". In years when the plateau summer monsoon is weak, these are opposite. The plateau monsoon is closely related to the intensity and position of the South Asian high, and the existence of a teleconnection pattern in the mid-upper levels suggests a possible linkage of the East Asian monsoon and the Indian monsoon to the plateau summer monsoon.
2012, 29(6): 1264-1278. doi: 10.1007/s00376-012-1052-9
Abstract:
Turbulence characteristics of an atmospheric surface layer over a coastal mountain area were investigated under different coordinate frames. Performances of three methods of coordinate rotation: double rotation (DR), triple rotation (TR), and classic planar-fit rotation (PF) were examined in terms of correction of eddy covariance flux. Using the commonly used DR and TR methods, unreasonable rotation angles are encountered at low wind speeds and cause significant run-to-run errors of some turbulence characteristics. The PF method rotates the coordinate system to an ensemble-averaged plane, and shows large tilt error due to an inaccurate fit plane over variable terrain slopes. In this paper, we propose another coordinate rotation scheme. The observational data were separated into two groups according to wind direction. The PF method was adapted to find an ensemble-averaged streamline plane for each group of hourly runs with wind speed exceeding 1.0 m s-1. Then, the coordinate systems were rotated to their respective best-fit planes for all available hourly observations. We call this the PF10 method. The implications of tilt corrections for the turbulence characteristics are discussed with a focus on integral turbulence characteristics, the spectra of wind-velocity components, and sensible heat and momentum fluxes under various atmospheric stabilities. Our results show that the adapted application of PF provides greatly improved estimates of integral turbulence characteristics in complex terrain and maintains data quality. The comparisons of the sensible heat fluxes for four coordinate rotation methods to fluxes before correction indicate that the PF10 scheme is the best to preserve consistency between fluxes.
2012, 29(6): 1279-1291. doi: 10.1007/s00376-012-1031-1
Abstract:
The interannual variability of occurrence of multiple tropical cyclone (MTC) events during June--October in the western North Pacific (WNP) was examined for the period 1979--2006. The number of the MTC events ranged from 2 to 9 per year, exhibiting a remarkable year-to-year variation. Seven active and seven inactive MTC years were identified. Compared to the inactive years, tropical cyclone genesis locations extended farther to the east and in the meridional direction during the active MTC years. A composite analysis shows that inactive MTC years were often associated with the El Nino decaying phase, as warm SST anomalies in the equatorial eastern-central Pacific in the preceding winter transitioned into cold sea surface temperature (SST) anomalies in the concurrent summer. Associated with the SST evolution were suppressed low-level cyclonic vorticity and weakened convection in the WNP monsoon region. In addition to the mean flow difference, significant differences between active and inactive MTC years were also found in the strength of the atmospheric intraseasonal oscillation (ISO). Compared with inactive MTC years, ISO activity was much stronger along the equator and in the WNP region during active MTC years. Both westward- and northward-propagating ISO spectrums strengthened during active MTC years compared to inactive years. The combined mean state and ISO activity changes may set up a favorable environment for the generation of MTC events.
2012, 29(6): 1292-1304. doi: 10.1007/s00376-012-1149-1
Abstract:
By analyzing the climatologically averaged wind stress during 2000--2007, it is found that the easterly wind stress in the northern tropical Pacific Ocean from Quick Scatterometer (QSCAT) data was stronger than those from Tropical Atmosphere Ocean (TAO) data and from National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis I. As a result, the Intertropical Convergence Zone (ITCZ) in the Pacific Ocean is more southward in the QSCAT data than in the NCEP/NCAR data. Relative to the NCEP wind, the southern shift of the ITCZ in the QSCAT data led to negative anomaly of wind stress curl north of a latitude of 6oN. The negative anomaly results in downward Ekman pumping in the central Pacific. The excessive local strong easterly wind also contributes to the downward Ekman pumping. This downward Ekman pumping suppresses the thermocline ridge, reduces the meridional thermocline slope and weakens the North Equatorial Countercurrent (NECC). These effects were confirmed by numerical experiments using two independent ocean general circulation models (OGCMs). Furthermore, the excessive equatorial easterly wind stress was also found to contribute to the weaker NECC in the OGCMs. A comparison between the simulations and observation data indicates that the stronger zonal wind stress and its southern shift of QSCAT data in the ITCZ region yield the maximum strength of the simulated NECC only 33% of the magnitude derived from observation data and even led to a missing" NECC in the western Pacific.
2012, 29(6): 1305-1317. doi: 10.1007/s00376-012-1171-3
Abstract:
This study produced a novel characterization of the troposphere-to-stratosphere transport (TST) over the Asian monsoon region during boreal summer, using a comprehensive analysis of 60-day backward trajectories initialized in the stratosphere. The trajectory datasets were derived from the high-resolution Lagrangian particle dispersion model (FLEXPART) simulation driven by the wind fields acquired from the National Center for Environmental Prediction (NCEP). The results indicate that the distribution of residence time (tTST) of tropopause-crossing trajectories in the lowermost stratosphere represents a horizontal signature of the Asian summer monsoon. Vertically, the distribution of tTST can be roughly separated into two layers: a consistent lower layer with tTST<5 days forming a narrow band, corresponding to a layer ～3 km thick following the location of the tropopause, and an upper layer at a larger distance from the local tropopause. The maximum residence time was ～20 days, especially within the Asian high anticyclone consistent with its confinement effects. In general, the overall geographical distribution of dehydration points was not coincident with the location of tropopause crossing. TST trajectories, which were initialized in the stratosphere, underwent their Lagrangian cold points mostly in the tropics and subtropics 1--4 days after the TST event; they were characterized by a wide range of temperature differences, with a mean value of 3--12 K. The vertical extent of the influence of tropospheric intrusion on the Asian monsoon region in the stratosphere exhibited a peak at ～16.5--18.5 km, and the uppermost height was ～21 km.
2012, 29(6): 1318-1329. doi: 10.1007/s00376-012-1233-6
Abstract:
2012, 29(6): 1330-1342. doi: 10.1007/s00376-012-1227-4
Abstract:
The Atmospheric Environmental Monitoring Network successfully undertook the task of monitoring the atmospheric quality of Beijing and its surrounding area during the 2008 Olympics. The results of this monitoring show that high concentrations of PM2.5 pollution exhibited a regional pattern during the monitoring period (1 June--30 October 2008). The PM2.5 mass concentrations were 53 μg m-3, 66 μg m-3, and 82 μg m-3 at the background site, in Beijing, and in the Beijing--Tianjin--Hebei urban agglomerations, respectively. The PM2.5 levels were lowest during the 2008 Olympic Games (8--24 August): 35 μg m-3 at the background site, 42 μg m-3 in Beijing and 57 μg m-3 in the region. These levels represent decreases of 49%, 48%, and 56%, respectively, compared to the prophase mean concentration before the Olympic Games. Emission control measures contributed 62%--82% of the declines observed in Beijing, and meteorological conditions represented 18%--38%. The concentration of fine particles met the goals set for a Green Olympics".
2012, 29(6): 1343-1359. doi: 10.1007/s00376-012-1198-5
Abstract:
The local features of transient kinetic energy and available potential energy were investigated using ECMWF (European Centre for Medium-Range Weather Forecasts) Interim Reanalysis data for the stratospheric sudden warming (SSW) event of January 2009. The Western Europe high plays important roles in the propagation of the energy from North America to Eurasian. When the Western Europe high appeared and shifted eastward, energy conversions increased and energy propagated from North America to Eurasian as a form of interaction energy flow. The baroclinic conversion between transient-eddy kinetic energy (Ke) and transient-eddy available potential energy (Ae) and the horizontal advection of geopotential height were approximately one order of magnitude less than Ke and Ae generation terms. So, these terms were less important to this SSW event.
2012, 29(6): 1360-1373. doi: 10.1007/s00376-012-1262-1
Abstract:
The aquatic eco-environment is significantly affected by temporal and spatial variation of the mixed layer depth (MLD) in large shallow lakes. In the present study, we simulated the three-dimensional water temperature of Taihu Lake with an unstructured grid with a finite-volume coastal ocean model (FVCOM) using wind speed, wind direction, short-wave radiation and other meteorological data measured during 13--18 August 2008. The simulated results were consistent with the measurements. The temporal and spatial distribution of the MLD and the possible relevant mechanisms were analyzed on the basis of the water temperature profile data of Taihu Lake. The results indicated that diurnal stratification might be established through the combined effect of the hydrodynamic conditions induced by wind and the heat exchange between air and water. Compared with the net heat flux, the changes of the MLD were delayed approximately two hours. Furthermore, there were significant spatial differences of the MLD in Taihu Lake due to the combined impact of thermal and hydrodynamic forces. Briefly, diurnal stratification formed relatively easily in Gonghu Bay, Zhushan Bay, Xukou Bay and East Taihu Bay, and the surface mixed layer was thin. The center of the lake region had the deepest surface mixed layer due to the strong mixing process. In addition, Meiliang Bay showed a medium depth of the surface mixed layer. Our analysis indicated that the spatial difference in the hydrodynamic action was probably the major cause for the spatial variation of the MLD in Taihu Lake.
2012, 29(6): 1374-1389. doi: 10.1007/s00376-012-1184-y
Abstract:
Diagnosis of changes in the winter stratospheric circulation in the Fifth Coupled Model Intercomparison Project (CMIP5) scenarios simulated by the Flexible Global Ocean-Atmosphere-Land System model, second version spectrum (FGOALS-s2), indicates that the model can generally reproduce the present climatology of the stratosphere and can capture the general features of its long-term changes during 1950--2000, including the global stratospheric cooling and the strengthening of the westerly polar jet, though the simulated polar vortex is much cooler, the jet is much stronger, and the projected changes are generally weaker than those revealed by observation data. With the increase in greenhouse gases (GHGs) effect in the historical simulation from 1850 to 2005 (called the HISTORICAL run) and the two future projections for Representative Concentration Pathways (called the RCP4.5 and RCP8.5 scenarios) from 2006 to 2100, the stratospheric response was generally steady, with an increasing stratospheric cooling and a strengthening polar jet extending equatorward. Correspondingly, the leading oscillation mode, defined as the Polar Vortex Oscillation (PVO), exhibited a clear positive trend in each scenario, confirming the steady strengthening of the polar vortex. However, the positive trend of the PVO and the strengthening of the polar jet were not accompanied by decreased planetary-wave dynamical heating, suggesting that the cause of the positive PVO trend and the polar stratospheric cooling trend is probably the radiation cooling effect due to increase in GHGs. Nevertheless, without the long-term linear trend, the temporal variations of the wave dynamic heating, the PVO, and the polar stratospheric temper
2012, 29(6): 1390-1394. doi: 10.1007/s00376-012-1269-7
Abstract:
Data from Goddard cumulus ensemble model experiment are used to study temporal and spatial scale dependence of tropical rainfall separation analysis based on cloud budget during Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE). The analysis shows that the calculations of model domain mean or time-mean grid-scale mean simulation data overestimate the rain rates of the two rainfall types associated with net condensation but they severely underestimate the rain rate of the rainfall type associated with net evaporation and hydrometeor convergence.
2012, 29(6): 1279-1291. doi: 10.1007/s00376-012-1031-1
Abstract:
The interannual variability of occurrence of multiple tropical cyclone (MTC) events during June--October in the western North Pacific (WNP) was examined for the period 1979--2006. The number of the MTC events ranged from 2 to 9 per year, exhibiting a remarkable year-to-year variation. Seven active and seven inactive MTC years were identified. Compared to the inactive years, tropical cyclone genesis locations extended farther to the east and in the meridional direction during the active MTC years. A composite analysis shows that inactive MTC years were often associated with the El Nino decaying phase, as warm SST anomalies in the equatorial eastern-central Pacific in the preceding winter transitioned into cold sea surface temperature (SST) anomalies in the concurrent summer. Associated with the SST evolution were suppressed low-level cyclonic vorticity and weakened convection in the WNP monsoon region. In addition to the mean flow difference, significant differences between active and inactive MTC years were also found in the strength of the atmospheric intraseasonal oscillation (ISO). Compared with inactive MTC years, ISO activity was much stronger along the equator and in the WNP region during active MTC years. Both westward- and northward-propagating ISO spectrums strengthened during active MTC years compared to inactive years. The combined mean state and ISO activity changes may set up a favorable environment for the generation of MTC events.