## 2012 Vol. 29, No. 4

Display Method:
2012, 29(4): 647-660. doi: 10.1007/s00376-012-1235-4
Abstract:
The El Nino-Southern Oscillation (ENSO) is modulated by many factors; most previous studies have emphasized the roles of wind stress and heat flux in the tropical Pacific. Freshwater flux (FWF) is another environmental forcing to the ocean; its effect and the related ocean salinity variability in the ENSO region have been of increased interest recently. Currently, accurate quantifications of the FWF roles in the climate remain challenging; the related observations and coupled ocean-atmosphere modeling involve large elements of uncertainty. In this study, we utilized satellite-based data to represent FWF-induced feedback in the tropical Pacific climate system; we then incorporated these data into a hybrid coupled ocean-atmosphere model (HCM) to quantify its effects on ENSO. A new mechanism was revealed by which interannual FWF forcing modulates ENSO in a significant way. As a direct forcing, FWF exerts a significant influence on the ocean through sea surface salinity (SSS) and buoyancy flux (QB) in the western-central tropical Pacific. The SSS perturbations directly induced by ENSO-related interannual FWF variability affect the stability and mixing in the upper ocean. At the same time, the ENSO-induced FWF has a compensating effect on heat flux, acting to reduce interannual QB variability during ENSO cycles. These FWF-induced processes in the ocean tend to modulate the vertical mixing and entrainment in the upper ocean, enhancing cooling during La Nina and enhancing warming during El Nino, respectively. The interannual FWF forcing-induced positive feedback acts to enhance ENSO amplitude and lengthen its time scales in the tropical Pacific coupled climate system.
2012, 29(4): 661-674. doi: 10.1007/s00376-012-1028-9
Abstract:
In this study, tropical cyclone data from China Meteorological Administration (CMA) and the ECMWF reanalysis data for the period 1958--2001 was used to propose a possible mechanism for the impacts of air--sea interaction on the activity of tropical cyclones (TCs) affecting China. The frequency of TCs affecting China over past 40 years has trended downward, while during the same period, the air--sea interaction in the two key areas of the Pacific region has significantly weakened. Our diagnoses and simulations suggest that air--sea interactions in the central North Pacific tropics and subtropics (Area 1) have an important role in adjusting typhoon activities in the Northwest Pacific in general, and especially in TC activity affecting China. On the contrary, impacts of the air--sea interaction in the eastern part of the South Pacific tropics (Area 2) were found to be rather limited. As both observational analysis and modeling studies show that, in the past four decades and beyond, the weakening trend of the latent heat released from Area 1 matched well with the decreasing Northwest Pacific TC frequency derived from CMA datasets. Results also showed that the weakening trend of latent heat flux in the area was most likely due to the decreasing TC frequency over the Northwest Pacific, including those affecting China. Although our preliminary analysis revealed a possible mechanism through which the air--sea interaction may adjust the genesis conditions for TCs, which eventually affect China, other relevant questions, such as how TC tracks and impacts are affected by these trends, remain unanswered. Further in-depth investigations are required.
2012, 29(4): 675-689. doi: 10.1007/s00376-012-1157-1
Abstract:
2012, 29(4): 690-694. doi: 10.1007/s00376-012-1145-5
Abstract:
A new invariant called the generalized Ertel--Rossby invariant (GER) was developed in this study. The new invariant is given by the dot product of the generalized vorticity and the generalized velocity. The generalized vorticity is the absolute vorticity minus the cross--product of the gradient of Lagrangian--time integrated temperature and the gradient of entropy. The generalized velocity is the absolute velocity minus the sum of the gradient of Lagrangian--time integrated kinetic potential and the Lagrangian--time integrated temperature multiplied by the gradient of entropy. In addition to the traditional potential vorticity, the GER invariant may provide another useful tool to study the atmospheric dynamic processes for weather phenomena ranging from large scales to small scales.
2012, 29(4): 695-704. doi: 10.1007/s00376-012-1153-5
Abstract:
The dominant patterns of summer rainfall anomalies in East China were studied using Empirical Orthogonal Function (EOF) analysis. The results indicate that after the late 1970s, the first and second dominant patterns switched. During the period before the late 1970s, the spatial pattern of the first (second) dominant mode was the Yangtze River pattern'' (the South China pattern''), but this changed to the South China pattern'' (the Yangtze River pattern'') after the late 1970s. This decadal change in the dominant patterns resulted from a significant decadal change in summer rainfall over South China after the late 1970s, i.e., a negative phase during 1978--1992 and a positive phase during 1993--2006. When the decadal variation of rainfall in East China is omitted from the analysis, the first and second dominant patterns represent the Yangtze River pattern'' and the South China pattern'', respectively. These results suggest that when decadal variation is included, the rainfall in China may be dominated by one mode during certain periods and by another in other periods. For the interannual variability when decadal variation is excluded, however, the first and second modes can be easily distinguished, and their order has been stable since at least 1951.
2012, 29(4): 705-716. doi: 10.1007/s00376-012-1174-0
Abstract:
This study examines the time and regime dependencies of sensitive areas identified by the conditional nonlinear optimal perturbation (CNOP) method for forecasts of two typhoons. Typhoon Meari (2004) was weakly nonlinear and is herein referred to as the linear case, while Typhoon Matsa (2005) was strongly nonlinear and is herein referred to as the nonlinear case. In the linear case, the sensitive areas identified for special forecast times when the initial time was fixed resembled those identified for other forecast times. Targeted observations deployed to improve a special time forecast would thus also benefit forecasts at other times. In the nonlinear case, the similarities among the sensitive areas identified for different forecast times were more limited. The deployment of targeted observations in the nonlinear case would therefore need to be adapted to achieve large improvements for different targeted forecasts. For both cases, the closer the forecast time, the higher the similarities of the sensitive areas. When the forecast time was fixed, the sensitive areas in the linear case diverged continuously from the verification area as the forecast period lengthened, while those in the nonlinear case were always located around the initial cyclones. The deployment of targeted observations to improve a special forecast depends strongly on the time of deployment. An examination of the efficiency gained by reducing initial errors within the identified sensitive areas confirmed these results. In general, the greatest improvement in a special time forecast was obtained by identifying the sensitive areas for the corresponding forecast time period.
2012, 29(4): 717-730. doi: 10.1007/s00376-012-1259-9
[Abstract](422) [PDF 552KB](2105)
Abstract:
In this study, the application of artificial intelligence to monthly and seasonal rainfall forecasting in Queensland, Australia, was assessed by inputting recognized climate indices, monthly historical rainfall data, and atmospheric temperatures into a prototype stand-alone, dynamic, recurrent, time-delay, artificial neural network. Outputs, as monthly rainfall forecasts 3 months in advance for the period 1993 to 2009, were compared with observed rainfall data using time-series plots, root mean squared error (RMSE), and Pearson correlation coefficients. A comparison of RMSE values with forecasts generated by the Australian Bureau of Meteorology's Predictive Ocean Atmosphere Model for Australia (POAMA)-1.5 general circulation model (GCM) indicated that the prototype achieved a lower RMSE for 16 of the 17 sites compared. The application of artificial neural networks to rainfall forecasting was reviewed. The prototype design is considered preliminary, with potential for significant improvement such as inclusion of output from GCMs and experimentation with other input attributes.
2012, 29(4): 731-743. doi: 10.1007/s00376-012-1176-y
Abstract:
In this study, the major features of a heavy rainfall event in the Yangtze River region on 3--7 June 2011 and its event-related large-scale circulation and predictability were studied. Both observational analysis and model simulation were used, the latter being based on the Weather Research and Forecasting (WRF) model forced by NCEP Global Forecast System (GFS) datasets. It was found that, during 3--5 June, the western Pacific subtropical high apparently extended to the west and was much stronger, and the Indian summer monsoon trough was slightly weaker than in normal years. The east--west oriented shear line over the middle and lower reaches of the Yangtze River was favorable for the transportation and convergence of water vapor, and the precipitation band was located slightly to the south of the shear line. During 6--7 June, the western Pacific subtropical high retreated eastward, while the trough over the Okhotsk Sea deepened. The low vortex in Northeast China intensified, bringing much more cold air to the middle and lower reaches of the Yangtze River, and the shear line over this area moved slightly southward. The convection band moved southward and became weaker, so the rainfall during 6--7 June weakened and was located slightly to the south of the previous precipitation band. Many of the observed features, including background circulation and the distribution and amount of precipitation, were reproduced reasonably by the WRF, suggesting a feasibility of this model for forecasting extreme weather events in the Yangtze River region.
2012, 29(4): 744-754. doi: 10.1007/s00376-012-1121-0
Abstract:
The present study investigates modulation of western North Pacific (WNP) tropical cyclone (TC) genesis in relation to different phases of the intraseasonal oscillation (ISO) of ITCZ convection during May to October in the period 1979--2008. The phases of the ITCZ ISO were determined based on 30--80-day filtered OLR anomalies averaged over the region (5o--20oN, 120o--150oE). The number of TCs during the active phases was nearly three times more than during the inactive phases. The active (inactive) phases of ISO were characterized by low-level cyclonic (anticyclonic) circulation anomalies, higher (lower) midlevel relative humidity anomalies, and larger (smaller) vertical gradient anomalies of relative vorticity associated with enhanced (weakened) ITCZ convection anomalies. During the active phases, TCs tended to form in the center of the ITCZ region. Barotropic conversion from the low-level mean flow is suggested to be the major energy source for TC formation. The energy conversion mainly depended on the zonal and meridional gradients of the zonal flow during the active phases. However, barotropic conversion weakened greatly during the inactive phases. The relationship between the meridional gradient of absolute vorticity and low-level zonal flow indicates that the sign of the absolute vorticity gradient tends to be reversed during the two phases, whereas the same sign between zonal flow and the absolute vorticity gradient is more easily satisfied in the active phases. Thus, the barotropic instability of low-level zonal flow might be an important mechanism for TC formation over the WNP during the active phases of ISO.
2012, 29(4): 755-768. doi: 10.1007/s00376-012-1217-6
Abstract:
The significant differences of atmospheric circulation between flooding in the Huaihe and Yangtze River valleys during early mei-yu (i.e., the East Asian rainy season in June) and the related tropical convection were investigated. During the both flooding cases, although the geopotential height anomalies always exhibit equivalent barotropic structures in middle to high latitudes at middle and upper troposphere, the phase of the Rossby wave train is different over Eurasian continent. During flooding in the Huaihe River valley, only one single blocking anticyclone is located over Baikal Lake. In contrast, during flooding in the Yangtze River valley, there are two blocking anticyclones. One is over the Ural Mountains and the other is over Northeast Asia. In the lower troposphere a positive geopotential height anomaly is located at the western ridge of subtropical anticyclone over Western Pacific (SAWP) in both flooding cases, but the location of the height anomaly is much farther north and west during the Huaihe River mei-yu flooding. Furthermore, abnormal rainfall in the Huaihe River valley and the regions north of it in China is closely linked with the latent heating anomaly over the Arabian Sea and Indian peninsula. However, the rainfall in the Yangtze River valley and the regions to its south in China is strongly related to the convection over the western tropical Pacific. Numerical experiments demonstrated that the enhanced latent heating over the Arabian Sea and Indian peninsula causes water vapor convergence in the region south of Tibetan Plateau and in the Huaihe River valley extending to Japan Sea with enhanced precipitation; and vapor divergence over the Yangtze River valley and the regions to its south with deficient precipitation. While the weakened convection in the tropical West Pacific results in moisture converging over the Yangtze River and the region to its south, along with abundant rainfall.
2012, 29(4): 769-781. doi: 10.1007/s00376-012-2018-7
Abstract:
South China (SC) experienced persistent heavy rain in June 2010. The climatic anomalies and related mechanism are analyzed in this study. Results show that the large-scale circulation pattern favorable for precipitation was maintained. In the upper level, the South Asian High and westerly jet stream provided a divergent circulation over SC. In the middle and low levels, an anomalous strong subtropical high (STH) extended to the South China Sea. The southwesterly monsoon flow along the northwest flank of the STH transported abundant water vapor from the western North Pacific, the Bay of Bengal, and the South China Sea to SC. The precipitation can be classified into two types: the West Siberia low (WSL)-induced low-level cyclone mode, and the STH-induced low-level jet mode. STH and WSL indices are defined to estimate the influence of these two systems, respectively. Analysis shows that both are critical for precipitation, but their respective contributions differ from year to year. In 2010, both were important factors for the heavy rainfall in June.
2012, 29(4): 782-794. doi: 10.1007/s00376-012-1249-y
Abstract:
Two important questions are addressed in this paper using the Global Ensemble Forecast System (GEFS) from the National Centers for Environmental Prediction (NCEP): (1) How many ensemble members are needed to better represent forecast uncertainties with limited computational resources? (2) What is the relative impact on forecast skill of increasing model resolution and ensemble size? Two-month experiments at T126L28 resolution were used to test the impact of varying the ensemble size from 5 to 80 members at the 500-hPa geopotential height. Results indicate that increasing the ensemble size leads to significant improvements in the performance for all forecast ranges when measured by probabilistic metrics, but these improvements are not significant beyond 20 members for long forecast ranges when measured by deterministic metrics. An ensemble of 20 to 30 members is the most effective configuration of ensemble sizes by quantifying the tradeoff between ensemble performance and the cost of computational resources. Two representative configurations of the GEFS---the T126L28 model with 70 members and the T190L28 model with 20 members, which have equivalent computing costs---were compared. Results confirm that, for the NCEP GEFS, increasing the model resolution is more (less) beneficial than increasing the ensemble size for a short (long) forecast range.
2012, 29(4): 795-809. doi: 10.1007/s00376-012-1170-4
Abstract:
A basin-wide ocean general circulation model (OGCM) of the Pacific Ocean is employed to estimate the uptake and storage of anthropogenic CO2 using two different simulation approaches. The simulation (named BIO) makes use of a carbon model with biological processes and full thermodynamic equations to calculate surface water partial pressure of CO2, whereas the other simulation (named PTB) makes use of a perturbation approach to calculate surface water partial pressure of anthropogenic CO2. The results from the two simulations agree well with the estimates based on observation data in most important aspects of the vertical distribution as well as the total inventory of anthropogenic carbon. The storage of anthropogenic carbon from BIO is closer to the observation-based estimate than that from PTB. The Revelle factor in 1994 obtained in BIO is generally larger than that obtained in PTB in the whole Pacific, except for the subtropical South Pacific. This, to large extent, leads to the difference in the surface anthropogenic CO2 concentration between the two runs. The relative difference in the annual uptake between the two runs is almost constant during the integration processes after 1850. This is probably not caused by dissolved inorganic carbon (DIC), but rather by a factor independent of time. In both runs, the rate of change in anthropogenic CO2 fluxes with time is consistent with the rate of change in the growth rate of atmospheric partial pressure of CO2.
2012, 29(4): 810-822. doi: 10.1007/s00376-012-1082-3
Abstract:
To examine effects of sea spray evaporation and dissipative heating on structure and intensity of a real tropical cyclone, the sea spray flux parameterization scheme was incorporated into the fifth-generation Pennsylvania State University--National Center for Atmospheric Research Mesoscale Model (MM5). Sensitivity tests were performed with varying the spray source function intensities and with and without dissipation heating. The numerical results indicate that sea spray evaporation increases the interfacial sensible heat flux, which is increased by 16% for the moderate spray and 47% for the heavy spray, but has little effect on the interfacial latent heat flux. The net effect of sea spray evaporation is to decrease the total sensible heat flux and to increase the total latent heat flux. The total enthalpy flux is increased by 1% and 12% with moderate and strong spray amounts, respectively. Consistent with these results, the intensity of the tropical cyclone is increased by 5% and 16% in maximum 10-m wind speed, respectively, due to sea spray evaporation. Sea spray evaporation and dissipative heating modify the tropical cyclone structure in important but complex ways. The effect of sea spray on the near-surface temperature and moisture depends on the spray amounts and its location within the tropical cyclone. Within the high-wind region of a tropical cyclone, the lower atmosphere becomes cooler and moister due to the evaporation of sea spray. However, the dissipative heating offsets the cooling due to sea spray evaporation, which makes the lower atmosphere warmer.
2012, 29(4): 823-837. doi: 10.1007/s00376-011-1022-7
Abstract:
When altimetric data is assimilated, 3DVAR and Ensemble Optimal Interpolation (EnOI) have different ways of projecting the surface information downward. In 3DVAR, it is achieved by minimizing a cost function relating the temperature, salinity, and sea level. In EnOI, however, the surface information is propagated to other variables via a stationary ensemble. In this study, the differences between the two methods were compared and their impacts on the simulated variability were evaluated in a tropical Pacific model. Sea level anomalies (SLA) from the TOPEX/Poseidon were assimilated using both methods on data from 1997 to 2001 in a coarse resolution model. Results show that the standard deviation of sea level was improved by both methods, but the EnOI was more effective in the central/eastern Pacific. Meanwhile, the SLA evolution was better reproduced with EnOI than with 3DVAR. Correlations of temperature with the reanalysis data increased with EnOI by 0.1--0.2 above 200 m. In the eastern Pacific below 200 m, the correlations also increased by 0.2. However, the correlations decreased with 3DVAR in many areas. Correlations with the independent TAO profiles were also compared at two locations. While the correlations increased by up to 0.2 at some depths with EnOI, 3DVAR generally reduced the correlations by 0.1--0.3. Though both methods were able to reduce the model--data difference in climatological sense, 3DVAR appears to have degraded the simulated variability, especially during El Nino--Southern Oscillation events. For salinity, similar results were found from the correlations. This tendency should be considered in future SLA assimilations, though the comparisons may vary among different model implementations.
2012, 29(4): 838-854. doi: 10.1007/s00376-012-1229-2
Abstract:
Using the global chemistry and transport model MOZART, the simulated distributions of tropospheric hydroxyl free radicals (OH) over China and its sensitivities to global emissions of carbon monoxide (CO), nitrogen oxide (NOx), and methane (CH4) were investigated in this study. Due to various distributions of OH sources and sinks, the concentrations of tropospheric OH in east China are much greater than in west China. The contribution of NO + perhydroxyl radical (HO2) reaction to OH production in east China is more pronounced than that in west China, and because of the higher reaction activity of non-methane volatile organic compounds (NMVOCs), the contributions to OH loss by NMVOCs exceed those of CO and take the dominant position in summer. The results of the sensitivity runs show a significant increase of tropospheric OH in east China from 1990 to 2000, and the trend continues. The positive effect of double emissions of NOx on OH is partly offset by the contrary effect of increased CO and CH4 emissions: the double emissions of NOx will cause an increase of OH of 18.1%--30.1%, while the increases of CO and CH4 will cause a decrease of OH of 12.2%--20.8% and 0.3%--3.0%, respectively. In turn, the lifetimes of CH4, CO, and NOx will increase by 0.3%--3.1% with regard to double emissions of CH4, 13.9%--26.3% to double emissions of CO and decrease by 15.3%--23.2% to double emissions of NOx.
2012, 29(4): 855-866. doi: 10.1007/s00376-012-1177-x
Abstract:
In this paper the bromine family and radiative effects are considered in an updated box model under the framework of ozone--temperature feedback, in order to further analyze the possible behavior of atmospheric ozone in the lower mid-latitude stratosphere. Results show that this updated photochemical system can present several different solutions, within a certain domain of parameters, with fixed-point and periodic states appearing in turn. The temperature feedback effect introduced in this box model has not changed the topology of the ozone system. This result presents nonlinear characteristics of the ozone system, and possible trends in the stratospheric atmosphere between complex chemistry and radiation processes.
2012, 29(4): 867-886. doi: 10.1007/s00376-012-1238-1
Abstract:
The impacts of solar activity on climate are explored in this two-part study. Based on the principles of atmospheric dynamics, Part I propose an amplifying mechanism of solar impacts on winter climate extremes through changing the atmospheric circulation patterns. This mechanism is supported by data analysis of the sunspot number up to the predicted Solar Cycle 24, the historical surface temperature data, and atmospheric variables of NCEP/NCAR Reanalysis up to the February 2011 for the Northern Hemisphere winters. For low solar activity, the thermal contrast between the low- and high-latitudes is enhanced, so as the mid-latitude baroclinic ultra-long wave activity. The land-ocean thermal contrast is also enhanced, which amplifies the topographic waves. The enhanced mid-latitude waves in turn enhance the meridional heat transport from the low to high latitudes, making the atmospheric heat engine" more efficient than normal. The jets shift southward and the polar vortex is weakened. The Northern Annular Mode (NAM) index tends to be negative. The mid-latitude surface exhibits large-scale convergence and updrafts, which favor extreme weather/climate events to occur. The thermally driven Siberian high is enhanced, which enhances the East Asian winter monsoon (EAWM). For high solar activity, the mid-latitude circulation patterns are less wavy with less meridional transport. The NAM tends to be positive, and the Siberian high and the EAWM tend to be weaker than normal. Thus the extreme weather/climate events for high solar activity occur in different regions with different severity from those for low solar activity. The solar influence on the mid- to high-latitude surface temperature and circulations can stand out after removing the influence from the El Nino-Southern Oscillation. The atmospheric amplifying mechanism indicates that the solar impacts on climate should not be simply estimated by the magnitude of the change in the solar radiation over solar cycles when it is compared with other external radiative forcings that do not influence the climate in the same way as the sun does.
2012, 29(4): 887-908. doi: 10.1007/s00376-012-1239-0
Abstract:
Part II of this study detects the dominant decadal-centennial timescales in four SST indices up to the 2010/2011 winter and tries to relate them to the observed 11-yr and 88-yr solar activity with the sunspot number up to Solar Cycle 24. To explore plausible solar origins of the observed decadal-centennial timescales in the SSTs and climate variability in general, we design a simple one-dimensional dynamical system forced by an annual cycle modulated by a small-amplitude single- or multi-scale solar activity.'' Results suggest that nonlinear harmonic and subharmonic resonance of the system to the forcing and period-doubling bifurcations are responsible for the dominant timescales in the system, including the 60-yr timescale that dominates the Atlantic Multidecadal Oscillation. The dominant timescales in the forced system depend on the system's parameter setting. Scale enhancement among the dominant response timescales may result in dramatic amplifications over a few decades and extreme values of the time series on various timescales. Three possible energy sources for such amplifications and extremes are proposed. Dynamical model results suggest that solar activity may play an important yet not well recognized role in the observed decadal-centennial climate variability. The atmospheric dynamical amplifying mechanism shown in Part I and the nonlinear resonant and bifurcation mechanisms shown in Part II help us to understand the solar source of the multi-scale climate change in the 20th century and the fact that different solar influenced dominant timescales for recurrent climate extremes for a given region or a parameter setting. Part II also indicates that solar influences on climate cannot be linearly compared with non-cyclic or sporadic thermal forcings because they cannot exert their influences on climate in the same way as the sun does.