Atmospheric Circulation Cells Associated with Anomalous East Asian Winter Monsoon

Atmospheric circulation cells associated with anomalous East Asian Winter Monsoon (EAWM) were studied using the 1948/49 to 2002/03 NCEP/NCAR reanalysis and NCAR CAM3 AGCM simulations with monthly global sea surface temperatures from 1950 to 2000. Several atmospheric cells in the Pacific [i.e., the zonal Walker cell (ZWC) in the tropic, the Hadley cell in the western Pacific (WPHC), the midlatitude zonal cell (MZC) over the central North Pacific, and the Hadley cell in the eastern Pacific (EPHC)] are associated with anomalous EAWM. When the EAWM is strong, ZWC, WPHC, and MZC are enhanced, as opposed to EPHC. The anomalous enhanced ZWC is characterized by air parcels rising in the western tropical Pacific, flowing eastward in the upper troposphere, and descending in the tropical central Pacific before returning to the tropical western Pacific. The enhanced MZC has characteristics opposite those of the enhanced ZWC in the central North Pacific. The anomalous WPHC shows air parcels rising in the western Pacific, as in the case of ZWC, followed by flowing northward in the upper troposphere and descending in the west North Pacific, as in the case of the enhanced MZC before returning to the western tropical Pacific. The anomalous EPHC is opposite in properties to the anomalous WPHC. Opposite characteristics are found during the weak EAWM period. The model simulations and the observations show similar characteristics and indicate the important role of sea surface temperature. A possible mechanism is proposed to link interannual variation of EAWM with the central-eastern tropical Pacific sea surface temperature anomaly (SSTA).     

Assessment of ERA5 and ERA-Interim in Reproducing Mean and Extreme Climates over West Africa

In situ data in West Africa are scarce, and reanalysis datasets could be an alternative source to alleviate the problem of data availability. Nevertheless, because of uncertainties in numerical prediction models and assimilation methods, among other things, existing reanalysis datasets can perform with various degrees of quality and accuracy. Therefore, a proper assessment of their shortcomings and strengths should be performed prior to their usage. In this study, we examine the performance of E...     

Northern poleward edge of regional Hadley cell over western Pacific during boreal winter: year-to-year variability,influence factors and associated winter climate anomalies

Compared to the zonal-mean Hadley cell (HC), our knowledge of the characteristics, influence factors and associated climate anomalies of the regional HC remains quite limited. Here, we examine interannual variability of the northern poleward HC edge over western Pacific (WPHCE) during boreal winter. Results suggest that interannual variability of the WPHCE is impacted by the El Niño-Southern Oscillation (ENSO) Modoki, North Pacific Oscillation (NPO) and North Atlantic Oscillation (NAO). The WPHCE tends to shift poleward during negative phase of the ENSO Modoki, and positive phases of the NPO and NAO, which highlights not merely the tropical forcing but also the extratropical signals that modulate the WPHCE. ENSO modoki, NPO and NAO modulate the WPHCE via inducing atmospheric anomalies over the western North Pacific. We further investigate the climatic impacts of the WPHCE on East Asia. The poleward shift of the northern descending branch of the WPHC results in anomalous upward (downward) motions and upper-level divergence (convergence) anomalies over south-central China (northern East-Asia), leading to increased (decreased) rainfall there. Moreover, pronounced cold surface air temperature anomalies appear over south-central China when the sinking branch of the WPHC moves poleward. Based on the temperature diagnostic analysis, negative surface temperature tendency anomalies over central China are mostly attributable to the cold zonal temperature advection and ascent-induced adiabatic cooling, while the negative anomalies over South China are largely due to the cold meridional temperature advection. These findings could improve our knowledge of the WPHCE variability and enrich the knowledge of forcing factors for East Asian winter climate.

     

全球变暖背景下西北太平洋冬季热带气旋的活动

Using tropical cyclone （TC） observations over a 58-yr period （1949-2006） from the China Meteorological Administration, the 40-year ECMWF Reanalysis （ERA-40）, NCEP-NCAR reanalysis, and the Hadley Centre sea ice and sea surface temperature （HadISST） datasets, the authors have examined the behaviors of tropical cyclones （TCs） in the western north Pacific （WNP） in boreal winter （November-December-January-February）. The results demonstrate that the occurrences of wintertime TCs, including super typhoons, have decreased over the 58 years. More TCs are found to move westward than northeastward, and the annual total number of parabolic-track-type TCs is found to be decreasing. It is shown that negative sea surface temperature anomalies （SSTAs） related to La Nifia events in the equatorial central Pacific facilitate more TC genesis in the WNP region. Large-scale anomalous cyclonic circulations in the tropical WNP in the lower troposphere are observed to be favorable for cyclogenesis in this area. On the contrary, the positive SSTAs and anomalous anticyclonic circulations that related to E1 Nifio events responsible for fewer TC genesis. Under the background of global warming, the western Pacific subtropical high tends to intensify and to expand more westward in the WNP, and the SSTAs display an increasing trend in the equatorial eastern-central Pacific. These climate trends of both atmospheric circulation and SSTAs affect wintertime TCs, inducing fewer TC occurrences and causing more TCs to move westward.     

Spatial and temporal analysis of climate change in Hispañola

Climate change in Hispañola is studied since 1900 using a variety of datasets. The longer station-observed temperature record has a significant trend of 0.012 °C/year, while the shorter reanalysis datasets exhibit faster warming, suggesting accelerating greenhouse radiative absorption and Hadley circulation. Rainfall trends are insignificant in the observed period, but a CMIP5 model simulation predicts a significant drying trend. The spatial pattern of climate trends was mapped with reanalysis fields and indicates a faster rate of warming over the eastern half of the island, where observations are dense and the drying trend is greatest. Northeasterly trade winds strengthen on the Atlantic side of the island. While trends intensify in the satellite era compared to the earlier 20th century, part of that effect is ascribed to an upturn in the Atlantic Multi-decadal Oscillation.     

Uncertainty estimation of the global temperature trends for multiple radiosondes, reanalyses, and CMIP3/IPCC climate model simulations

Based on three groups of datasets that include radiosondes, reanalyses, and climate model simulations (e.g., Coupled Model Intercomparison Project, CMIP3) from 1979 to 2008, the interannual variability, global temperature trends, and their uncertainty using ensemble spread among intra-group and inter-group datasets have been discussed. The results show that the interannual temperature variability increased from the troposphere to stratosphere, and the maximum occurs around 50?hPa. The CMIP3 climate models have the largest discrepancy in the stratosphere. The intra-group correlations at 500?hPa generally show high similarity within each data group while the inter-group correlations between reanalyses and the CMIP3 climate model simulations indicate lesser similarity. In contrast, the inter-group correlation at 50?hPa is improved except with the Japanese 25-year Reanalysis Project (JRA-25) dataset, and the Twentieth Century Reanalysis (20CR) reanalysis shows a weak cross correlation. The global temperature trends are highly dependent on the individual data sources. Compared to the radiosondes, the reanalyses show a large ensemble spread of trends in the stratosphere, and the CMIP3 climate model simulations have a large ensemble spread in the height of the crossover point where tropospheric warming changes into stratospheric cooling. The largest ensemble spread among the reanalyses in the stratosphere is mainly from the large discrepancy in the JRA-25 reanalysis after 1998 and a relatively weak anomaly in the 20CR before 1986. The largest ensemble spread among the CMIP3 climate models in the troposphere is related to the influence of both volcanic eruptions and El Ni?o/La Ni?a–Southern Oscillation events. The strong anomalies corresponding to the volcanic eruptions of El Chichon in 1982 and Mt Pinatubo in 1991 are clearly identified in the stratosphere. These volcanic eruptions reduced the warming in the troposphere and strengthened the cooling in the stratosphere during the most recent 30?years.     

Internal variability in multidecadal trends of surface air temperature over antarctica in austral winter in model simulations

The surface air temperature (SAT) exhibits pronounced warming over West Antarctica in recent decades, especially in austral spring and winter. Using a 30-member ensemble of simulations by Community Earth System Model (CESM), two reanalysis datasets, and observed station data, this study investigates the relative contributions of internally generated low-frequency climate variability and externally forced climate change to the austral winter SAT trend in Antarctica. Although these simulations share the same external forcing, the SAT trends during 1979–2005 show large diversity among the individual members in the CESM ensemble simulations, suggesting that internally generated variability contributes a considerable part to the multidecadal SAT change in Antarctica. Quantitatively, the total forced contribution to the SAT (1979–2005) change is about 0.53 k/27 yr, and the internal variability can be strong enough to double or cancel the externally forced warming trend. A method called “dynamical adjustment” is utilized to further divide the forced response. We find both the forced thermodynamically-induced and the forced dynamically-induced SAT trends are positive over all the regions in Antarctica, with the regional mean values of 0.20 k /27 yr and 0.33 k/27 yr, respectively. The diversity of SAT trends among the simulations is closely linked to a Southern hemisphere Annular Mode (SAM)-like atmospheric circulation multidecadal change in the Southern Hemisphere. When there exists a positive–negative seesaw of pressure trend between Antarctica and the mid-latitudes, the SAT trend is positive over most of Antarctica but negative over the Antarctic Peninsula, and vice versa. The SAM-like atmospheric circulation multidecadal change mainly arises from atmospheric internal variability rather than remote tropical Sea Surface Temperature (SST).

     

Hadley环流强度与中国东部气温的相关分析

利用NCEP / NCAR再分析月平均风场资料和国家气候中心提供的中国160个气象站的月平均气温资料,选用质量流函数计算方法描述平均经圈环流,用环流中心值大小表征其强度,分析1951-2010年Hadley环流强度趋势变化及其与中国东部气温的关系。结果表明：质量流函数能较好地表征平均经圈环流特征,且环流中心值大小能形象地反映环流强度。近60 a来,冬季北半球Hadley环流强度呈线性增强的趋势;夏季南半球Hadley环流强度在20世纪80年代初发生了由减弱趋势转变为显著增强趋势的年代际转折。中国东部大部分地区气温的年代际变化能很好地被Hadley环流强度变化解释,二者呈正相关关系,但显著相关区域随季节变化有所不同。     

Long-term trend in potential vorticity intrusion events over the Pacific Ocean: Role of global mean temperature rise

In this study, we examine a long-term increasing trend in subtropical potential vorticity (PV) intrusion events over the Pacific Ocean in relation to the global mean temperature rise, based on multiple reanalysis datasets. The frequency of the PV intrusions is closely related to the upper-tropospheric equatorial westerly duct and the subtropical jet (STJ). An overall strengthening of the westerly duct and weakening of the STJ are found to be driven by the warming-induced strengthening of Walker circulation and regional changes in Hadley circulation on multi-decadal timescale, leading to an increase in the PV intrusion frequency over the tropics. The results are robust in all datasets. The multi-decadal strengthening in the Pacific Walker circulation is consistent with the global mean temperature rise. In this way, the PV intrusions are correlated with the warming related global mean temperuate rise. When the interannual variability of ENSO is removed from the intrusion time series, the long-term trend in PV intrusions due to external forcing associated with anthropogenic warming (global mean temperature rise) becomes clearer. The link between the global mean temperature rise and intrusion frequency is further verified by performing a correlation analysis between the two. The significant (> 95%) correlation coefficient is 0.85, 0.94, 0.84, 0.83, and 0.84 for ERA-40, ERA-Interim, NCEP-NCAR, JRA-55, and JRA-25, respectively. This unequivocally indicates that the global mean temperature rise can explain around 69%–88% of the variance related to the long-term increase in PV intrusion frequency over the Pacific Ocean.     

Structure and variances of equatorial zonal circulation in a multimodel ensemble

The structure and variance of the equatorial zonal circulation, as characterized by the atmospheric mass flux in the equatorial zonal plane, is examined and inter-compared in simulations from 9 CMIP3 coupled climate models with multiple ensemble members and the NCEP-NCAR and ERA-40 reanalyses. The climate model simulations analyzed here include twentieth century (20C3M) and twenty-first century (SRES A1B) simulations. We evaluate the 20C3M modeled zonal circulations by comparing them with those in the reanalyses. We then examine the variability of the circulation, its changes with global warming, and the associated thermodynamic maintenance. The tropical zonal circulation involves three major components situated over the Pacific, Indian, and Atlantic oceans. The three cells are supported by the corresponding diabatic heating extending deeply throughout the troposphere, with heating centers apparent in the mid-troposphere. Seasonal features appear in the zonal circulation, including variations in its intensity and longitudinal migration. Most models, and hence the multi-model mean, represent the annual and seasonal features of the circulation and the associated heating reasonably well. The multi-model mean reproduces the observed climatology better than any individual model, as indicated by the spatial pattern correlation and mean square difference of the mass flux and the diabatic heating compared to the reanalysis based values. Projected changes in the zonal circulation under A1B forcing are dominated by mass flux changes over the Pacific and Indian oceans. An eastward shift of the Pacific Walker circulation is clearly evident with global warming, with anomalous rising motion apparent over the equatorial central Pacific and anomalous sinking motions in the west and east, which favors an overall strengthening of the Walker circulation. The zonal circulation weakens and shifts westwards over the Indian Ocean under external forcing, whereas it strengthens and shifts slightly westwards over the Atlantic Ocean. The forced circulation changes are associated with broad SST and atmospheric diabatic heating changes in the tropics. Linear trends of these forced circulation changes, as characterized by regional spatial maximum amplitudes of mass fluxes and their longitudes over the three oceans, are statistically significant at the 5?% level for 2000–2099 for the multi-model mean. However, wide differences of the trends are apparent across the models, because of both deficiencies in the simulation of the circulations in different models and the high internal variability of the circulations.     

An assessment and interpretation of the observed warming of West Antarctica in the austral spring

We synthesize variability and trends in multiple analyses of Antarctic near-surface temperature representing several independent source datasets and spatially complete reconstructions, and place these into the broader context of the behavior of other components of the climate system during the past 30–50?years. Along with an annual-mean trend during the past 50?years of about 0.1°C/decade averaged over Antarctica, there is a distinct seasonality to the trends, with insignificant change (and even some cooling) in austral summer and autumn in East Antarctica, contrasting with warming in austral winter and spring. Apart from the Peninsula, the seasonal warming is largest and most significant in West Antarctica in the austral spring since the late 1970s. Concurrent trends in sea ice are independent evidence of the observed warming over West Antarctic, with the decrease in sea ice area in the Amundsen and Bellingshausen Seas congruent with at least 50% of the inland warming of West Antarctica. Trends in near surface winds and geopotential heights over the high-latitude South Pacific are consistent with a role for atmospheric forcing of the sea ice and air temperature anomalies. Most of the circulation trend projects onto the two Pacific South American (PSA) modes of atmospheric circulation variability, while the Southern Annular Mode lacks a positive trend in spring that would otherwise cause a cooling tendency. The largest circulation trend is associated with the PSA-1 mode, a wave-train extending from the tropics to the high Southern latitudes. The PSA-1 mode is significantly correlated with SSTs in the southwestern tropical and subtropical Pacific. The increased SSTs in this region, together with the observed increase in rainfall, suggest that anomalous deep convection has strengthened or increased the occurrence of the Rossby wave-train associated with PSA-1. This hypothesis is supported by results from two ensembles of SST-forced atmospheric general circulation model simulations. Finally, the implications of the seasonality, timing, and spatial patterns of Antarctic temperature trends with respect to interpreting the relative roles of stratospheric ozone depletion, SSTs and increased atmospheric concentrations of greenhouse gasses are discussed.     

VARIABILITY IN THE WESTERN PACIFIC SUBTROPICAL HIGH AND ITS RELATIONSHIP WITH SEA TEMPERATURE VARIATION CONSIDERING THE BACKGROUND OF CLIMATE WARMING OVER THE PAST 60 YEARS

By adopting characteristic index data for the Western Pacific Subtropical High (WPSH) from the National Climate Center of China, U.S. National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP/NCAR) reanalysis data, and the National Oceanic and Atmospheric Administration (NOAA) sea surface temperature (SST) data, we studied the WPSH variability considering the background of climate warming by using a Gaussian filter, moving averages, correlation analysis, and synthetic analysis. Our results show that with climate warming over the past 60 years, significant changes in the WPSH include its enlarged area, strengthened intensity, westward extended ridge point and southward expanded southern boundary, as well as enhanced interannual fluctuations in all these indices. The western ridge point of the WPSH consistently varies with temperature changes in the Northern Hemisphere, but the location of the ridgeline varies independently. The intensity and area of the WPSH were both significantly increased in the late 1980s. Specifically, the western ridge point started to significantly extend westward in the early 1990s, and the associated interannual variability had a significant increase in the late 1990s; in addition, the ridgeline was swaying along the north-south-north direction, and the corresponding variability was also greatly enhanced in the late 1990s. With climate warming, the SST increase becomes more weakly correlated with the WPSH intensity enhancement but more strongly correlated with the westward extension of the ridge point in the equatorial central and eastern Pacific Ocean in winter, corresponding to an expanding WPSH in space. In the northern Pacific in winter, the SST decrease has a weaker correlation with the southerly location of the ridgeline but also a stronger correlation with the westward extension of the ridge point. In the tropical western Pacific in winter, the correlations of the SST decrease with the WPSH intensity enhancement, and the westward extension of the ridge point is strengthened. These observations can be explained by strengthened Hadley circulations, the dominant effects of the southward shift, and additional effects of the weakened ascending branch of the Walker circulation during warm climatological periods, which consequently lead to strengthened intensities, increased areas, and southward expansions of the WPSH in summer.     

春季热带南大西洋海表温度异常与夏季亚澳季风区降水异常的联系

利用1979—2019年Hadley中心的海表温度资料、GPCP的降水资料以及NCEP-DOE的再分析资料等,分析了北半球春季热带南大西洋海表温度异常与北半球夏季亚澳季风区降水异常的联系。研究表明,北半球春季热带南大西洋海表温度异常与随后夏季热带西太平洋到南海（澳大利亚东侧海域到热带东印度洋）地区的降水异常为显著负相关（正相关）关系。北半球春季热带南大西洋的海表温度正异常可以引起热带大西洋和热带太平洋间的异常垂直环流,其中异常上升支（下沉支）位于热带大西洋（热带中太平洋）。热带中太平洋的异常下沉气流和低层辐散气流引起热带中西太平洋低层的异常东风,后者有利于热带中东太平洋海表温度出现负异常。通过Bjerknes正反馈机制,热带中东太平洋海表温度异常从北半球春季到夏季得到发展。热带中东太平洋海表温度负异常激发的Rossby波使得北半球夏季热带西太平洋低层出现一对异常反气旋。此时,850 hPa上热带西太平洋到海洋性大陆地区为显著的异常东风,有利于热带西太平洋到南海（澳大利亚东侧海域到热带东印度洋）地区出现异常的水汽辐散（辐合）,导致该地区降水减少（增加）。     

Tropical intraseasonal rainfall variability in the CFSR

While large-scale circulation fields from atmospheric reanalyses have been widely used to study the tropical intraseasonal variability, rainfall variations from the reanalyses are less focused. Because of the sparseness of in situ observations available in the tropics and strong coupling between convection and large-scale circulation, the accuracy of tropical rainfall from the reanalyses not only measures the quality of reanalysis rainfall but is also to some extent indicative of the accuracy of the circulations fields. This study analyzes tropical intraseasonal rainfall variability in the recently completed NCEP Climate Forecast System Reanalysis (CFSR) and its comparison with the widely used NCEP/NCAR reanalysis (R1) and NCEP/DOE reanalysis (R2). The R1 produces too weak rainfall variability while the R2 generates too strong westward propagation. Compared with the R1 and R2, the CFSR produces greatly improved tropical intraseasonal rainfall variability with the dominance of eastward propagation and more realistic amplitude. An analysis of the relationship between rainfall and large-scale fields using composites based on Madden-Julian Oscillation (MJO) events shows that, in all three NCEP reanalyses, the moisture convergence leading the rainfall maximum is near the surface in the western Pacific but is above 925?hPa in the eastern Indian Ocean. However, the CFSR produces the strongest large-scale convergence and the rainfall from CFSR lags the column integrated precipitable water by 1 or 2?days while R1 and R2 rainfall tends to lead the respective precipitable water. Diabatic heating related to the MJO variability in the CFSR is analyzed and compared with that derived from large-scale fields. It is found that the amplitude of CFSR-produced total heating anomalies is smaller than that of the derived. Rainfall variability from the other two recently produced reanalyses, the ECMWF Re-Analysis Interim (ERAI), and the Modern Era Retrospective-analysis for Research and Applications (MERRA), is also analyzed. It is shown that both the ERAI and MERRA generate stronger rainfall spectra than the R1 and more realistic dominance of eastward propagating variance than R2. The intraseasonal variability in the MERRA is stronger than that in the ERAI but weaker than that in the CFSR and CMORPH.     

秋季亚洲-太平洋涛动与中国近海热带气旋活动的关系

采用联合台风警报中心的台风最佳路径资料和NCEP/NCAR再分析资料,分析了秋季(9—10月)亚洲-太平洋涛动(APO)强度的年际变化与东亚-太平洋大气环流的关系,探讨了APO与西北太平洋和中国近海热带气旋(TC)活动的关系。结果表明:秋季APO年际变化与同期西北太平洋和中国近海TC活动关系紧密,即当APO偏强(弱),西北太平洋TC活跃区明显偏西(东),中国近海TC偏多(少);APO可以通过影响中国近海对流层纬向风垂直切变、低层辐合和对流层中层引导气流等,从而影响西北太平洋和中国近海TC活动;当APO偏强(弱)时,东亚大槽偏弱(强),东亚冬季风偏弱(强),使得侵入中国近海和热带西北太平洋的冷空气活动偏弱(强),有(不)利于这些海域TC的生成和发展;此外,在APO偏强时,西太平洋副热带高压脊偏西,其南侧偏东气流加强,有利于TC在偏强的偏东气流引导下向西移动或者其转向点偏西;而在APO弱年,副热带高压脊偏弱和偏东,偏东引导气流减弱,不利于TC西行或有利于其转向点偏东。     

冬夏东亚季风环流对太平洋热状况的响应

冬夏隔季韵律关系一直是我国长期天气预报和短期气候预测的一个重要依据,然而迄今为止对它们之间的物理过程及成因机理并不十分清楚。利用NCEP/NCAR全球2.5°×2.5°网格月平均再分析资料,研究1951~2000年冬夏东亚季风环流异常变化与太平洋海面温度(SST)的关系及对关键海温区响应机理。研究指出:冬夏东亚季风环流隔季韵律关系及其年际变化与赤道东太平洋海面温度异常(SSTA)变化密切相关,冬季赤道东太平洋出现La Ni~na(El Ni~no)型的SST分布,有利冬、夏东亚季风环流加强(减弱),其影响过程通过赤道Walker环流强(弱)以及东亚地区Hadley环流强(弱)过程完成。冬季赤道东太平洋海温变化是冬、夏东亚环流季节以及年际变化的一个重要外强迫因子。     

Trends and low frequency variability of extra-tropical cyclone activity in the ensemble of twentieth century reanalysis

An objective cyclone tracking algorithm is applied to twentieth century reanalysis (20CR) 6-hourly mean sea level pressure fields for the period 1871–2010 to infer historical trends and variability in extra-tropical cyclone activity. The tracking algorithm is applied both to the ensemble-mean analyses and to each of the 56 ensemble members individually. The ensemble-mean analyses are found to be unsuitable for accurately determining cyclone statistics. However, pooled cyclone statistics obtained by averaging statistics from individual members generally agree well with statistics from the NCEP-NCAR reanalyses for 1951–2010, although 20CR shows somewhat weaker cyclone activity over land and stronger activity over oceans. Both reanalyses show similar cyclone trend patterns in the northern hemisphere (NH) over 1951–2010. Homogenized pooled cyclone statistics are analyzed for trends and variability. Conclusions account for identified inhomogeneities, which occurred before 1949 in the NH and between 1951 and 1985 in the southern hemisphere (SH). Cyclone activity is estimated to have increased slightly over the period 1871–2010 in the NH. More substantial increases are seen in the SH. Notable regional and seasonal variations in trends are evident, as is profound decadal or longer scale variability. For example, the NH increases occur mainly in the mid-latitude Pacific and high-latitude Atlantic regions. For the North Atlantic-European region and southeast Australia, the 20CR cyclone trends are in agreement with trends in geostrophic wind extremes derived from in-situ surface pressure observations. European trends are also consistent with trends in the mean duration of wet spells derived from rain gauge data in Europe.     

Simulation and Projection of the Western Pacific Subtropical High in CMIP5 Models

This work examined the performance of 26 coupled climate models participating in the Coupled Model Intercomparison Project Phase 5 （CMIP5） in the simulation of the present-day temporal variability and spatial pattern of the western Pacifi c subtropical high （WPSH）. The results show that most models are able to capture the spatial distribution and variability of the 500-hPa geopotential height and zonal wind fi elds in the western subtropical Pacifi c, but with underestimated mean intensity of the WPSH. The underestimation may be associated with the cold bias of sea surface temperature in the tropical Indian and western Pacifi c oceans in the models. To eliminate the impact of the climatology biases, the climatology of these models is replaced by that of the NCEP/NCAR reanalysis in the verifi cation, and the models reproduce the WPSH’s enhancement and westward extension after the late 1970s. According to assessment of the simulated WPSH indices, it is found that some models （CNRM-CM5, FGOALS-g2, FIO-ESM, MIROC-ESM, and MPI-ESM-P） are better than others in simulating WPSH. Then, the ensemble mean of these better models is used to pro ject the future changes of WPSH under three representative concentration pathway scenarios （RCP8.5, RCP4.5, and RCP2.6）. The WPSH enlarges, strengthens, and extends westward under all the scenarios, with the largest linear growth trend projected in RCP8.5, smallest in RCP2.6, and in between in RCP4.5;while the ridge line of WPSH shows no obvious long-term trend. These results may have implications for the attribution and prediction of climate variations and changes in East Asia.     

Asymmetric Variations in the Tropical Ascending Branches of Hadley Circulations and the Associated Mechanisms and Effects

This study investigates the variations in the tropical ascending branches(TABs) of Hadley circulations(HCs) during past decades, using a variety of reanalysis datasets. The northern tropical ascending branch(NTAB) and the southern tropical ascending branch(STAB), which are defined as the ascending branches of the Northern Hemisphere HC and Southern Hemisphere HC, respectively, are identified and analyzed regarding their trends and variability. The reanalysis datasets consistently show a persistent increase in STAB during past decades, whereas they show less consistency in NTAB regarding its decadalto multidecadal variability, which generally features a decreasing trend. These asymmetric trends in STAB and NTAB are attributed to asymmetric trends in the tropical SSTs. The relationship between STAB/NTAB and tropical SSTs is further examined regarding their interannual and decadal-to multidecadal variability. On the interannual time scale, the STAB and NTAB are essentially modulated by the eastern-Pacific type of ENSO, with a strengthened(weakened) STAB(NTAB) under an El Ni ?no condition. On the decadal-to multidecadal time scale, the variability of STAB and NTAB is closely related to the southern tropical SSTs and the meridional asymmetry of global tropical SSTs, respectively. The tropical eastern Pacific SSTs(southern tropical SSTs) dominate the tropical SST–NTAB/STAB relationship on the interannual(decadal-to multidecadal)scale, whereas the NTAB is a passive factor in this relationship. Moreover, a cross-hemispheric relationship between the NTAB/STAB and the HC upper-level meridional winds is revealed.