Interannual meridional displacement of the East Asian upper-tropospheric jet stream in summer

On the interannual timescale, the meridional displacement of the East Asian upper-tropospheric jet stream (EAJS) is significantly associated with the rainfall anomalies in East Asia in summer. In this study, using the data from the National Centers for Environmental Prediction-Department of Energy (NCEP/DOE) reanalysis-2 from 1979 to 2002, the authors investigate the interannual variations of the EAJS‘s meridional displacement in summer and their associations with the variations of the South Asian high (SAH) and the western North Pacific subtropical high (WNPSH), which are dominant circulation features in the upper and lower troposhere, respectively. The result from an EOF analysis shows that the meridional displacement is the most remarkable feature of the interannual variations of the EAJS in each month of summer and in summer as a whole. A composite analysis indicates that the summer (June-July-August, JJA) EAJS index, which is intended to depict the interannual meridional displacement of the EAJS, is not appropriate because the anomalies of the zonal wind at 200 hPa (U200) in July and August only, rather than in June, significantly contribute to the summer EAJS index. Thus, the index for each month in summer is defined according to the location of the EAJS core in each month. Composite analyses based on the monthly indexes show that corresponding to the monthly equatorward displacement of the EAJS, the South Asian high (SAH) extends southeastward clearly in July and August, and the western North Pacific subtropical high (WNPSH) withdraws southward in June and August.     

Predictability of the summer East Asian upper-tropospheric westerly jet in ENSEMBLES multi-model forecasts

The interannual variation of the East Asian upper-tropospheric westerly jet(EAJ) significantly affects East Asian climate in summer. Identifying its performance in model prediction may provide us another viewpoint,from the perspective of uppertropospheric circulation,to understand the predictability of summer climate anomalies in East Asia. This study presents a comprehensive assessment of year-to-year variability of the EAJ based on retrospective seasonal forecasts,initiated from1 May,in the five state-of-the-art coupled models from ENSEMBLES during 1960–2005. It is found that the coupled models show certain capability in describing the interannual meridional displacement of the EAJ,which reflects the models' performance in the first leading empirical orthogonal function(EOF) mode. This capability is mainly shown over the region south of the EAJ axis. Additionally,the models generally capture well the main features of atmospheric circulation and SST anomalies related to the interannual meridional displacement of the EAJ. Further analysis suggests that the predicted warm SST anomalies in the concurrent summer over the tropical eastern Pacific and northern Indian Ocean are the two main sources of the potential prediction skill of the southward shift of the EAJ. In contrast,the models are powerless in describing the variation over the region north of the EAJ axis,associated with the meridional displacement,and interannual intensity change of the EAJ,the second leading EOF mode,meaning it still remains a challenge to better predict the EAJ and,subsequently,summer climate in East Asia,using current coupled models.     

2021年盛夏中国东部极端降水月际演变成因及可预测性

2021年7—8月中国东部雨带演变特征与气候平均季风北推进程存在显著差异。其中,7月降水正异常中心位于江淮-华北地区,8月则南移至华中地区。2021年中国东部降水异常偏多且存在月际差异主要与7(8)月西北太平洋副热带高压(西太副高)偏北偏东(偏南偏西)、东亚副热带西风急流偏北(偏南)以及南亚高压持续东伸相关联。进一步研究表明,热带对流的活跃位置和北大西洋的增暖加强是影响其降水中心南移的主要原因。2021年7月热带大气低频振荡(MJO)在海洋性大陆地区活跃对应其热带海洋性大陆对流异常偏强,激发北传的类太平洋-日本(PJ)型遥相关波列,使得西太副高偏北偏东,有利于西北太平洋水汽在江淮-华北地区辐合,导致其降水偏多。8月,新发展MJO在热带印度洋上空对流异常持续偏强,加强局地经向环流,使得中国35°N以南至西北太平洋地区出现异常下沉运动,有利于西太副高南移西伸。此外,2021年8月北大西洋海温(SST)异常偏暖激发对流层高层向东南传播的Rossby波,有利于南亚高压加强和东亚副热带西风急流加强南移。因此,8月降水中心南移至华中地区。CFSv2预测系统(6月起报)结果能预测7月江淮-华北大部分地区降水偏多,但预测的8月华中南部地区降水偏少与实况相反。这可能是由于模式能够较好再现7月海洋性大陆热带对流活动影响江淮-华北地区降水的过程,但不能预测2021年8月热带印度洋对流活动和北大西洋海温异常偏暖对华中地区降水的影响。     

Subseasonal Reversal of East Asian Surface Temperature Variability in Winter 2014/15

Although there has been a considerable amount of research conducted on the East Asian winter-mean climate, subseasonal surface air temperature(SAT) variability reversals in the early and late winter remain poorly understood. In this study,we focused on the recent winter of 2014/15, in which warmer anomalies dominated in January and February but colder conditions prevailed in December. Moreover, Arctic sea-ice cover(ASIC) in September–October 2014 was lower than normal,and warmer sea surface temperature(SST) anomalies occurred in the Ni ?no4 region in winter, together with a positive Pacific Decadal Oscillation(PDO|+) phase. Using observational data and CMIP5 historical simulations, we investigated the PDO|+ phase modulation upon the winter warm Ni ?no4 phase(autumn ASIC reduction) influence on the subseasonal SAT variability of East Asian winter. The results show that, under a PDO|+ phase modulation, warm Ni ?no4 SST anomalies are associated with a subseasonal delay of tropical surface heating and subsequent Hadley cell and Ferrel cell intensification in January–February, linking the tropical and midlatitude regions. Consistently, the East Asian jet stream(EAJS) is significantly decelerated in January–February and hence promotes the warm anomalies over East Asia. Under the PDO|+ phase,the decrease in ASIC is related to cold SST anomalies in the western North Pacific, which increase the meridional temperature gradient and generate an accelerated and westward-shifted EAJS in December. The westward extension of the EAJS is responsible for the eastward-propagating Rossby waves triggered by declining ASIC and thereby favors the connection between ASIC and cold conditions over East Asia.     

冬季东亚中纬度西风急流对我国气候的影响

利用1957—2001年欧洲中期数值天气预报中心再分析资料及地面台站观测资料，分析了冬季东亚西风急流与我国气候的关系。首先定义了冬季东亚西风急流强度指数(区域30°~35°N，127.5°~155°E冬季200 hPa纬向风u₂₀₀平均值的标准化值)和切变指数(区域15°~25°N，100°~115°E与区域30°~40°N，100°~115°E的平均u₂₀₀之差的标准化值)，这两个指数能较好地反映冬季东亚西风急流的强度变化和位置的南北移动，二者相关系数为-0.48，通过99%信度检验。西风急流强度与亚洲和西太平洋大范围的大气环流有密切关系，而西风急流位置移动则与印度洋、中东太平洋的大气环流有密切关系，并分析了冬季急流强度指数和切变指数与我国温度和降水的关系。结果表明:当西风急流强度偏强时，西风急流位置偏北，此时在急流入口区左侧由于气流辐合造成低层气压上升，在出口区左侧则由于气流发生强烈辐散，引起低层气压下降，所以西伯利亚地区上空从对流层低层到中层高度值升高，北太平洋高度值降低，东西向气压差加大的形势，同时东亚大槽偏强，海陆气压差加大和东亚大槽偏强，导致冬季风强度偏强，引起我国从北到南的陆面降温，同时30°~40°N低层有下沉气流，使得华北、华中和长江中下游地区降水偏少；当西风急流强度偏弱时，西风急流位置偏南，整个东亚地区存在南风异常，东亚冬季风较弱，在25°N附近有上升气流，此时华南和内蒙古、华北降水偏多，内蒙古地表温度偏高。     

The ENSO’s Effect on Eastern China Rainfall in the Following Early Summer

ENSO’s effect on the rainfall in eastern China in the following early summer is investigated by using station precipitation data and the ERA-40 reanalysis data from 1958 to 2002. In June, after the El Nino peak, the precipitation is significantly enhanced in the Yangtze River valley while suppressed in the Huaihe River-Yellow River valleys. This relationship between ENSO and the rainfall in eastern China is established possibly through two teleconnections: One is related to the western North Pacific (WNP) ...     

东亚中纬度干旱/半干旱区降水年际变化及其可能成因

基于NOAA的全球陆地降水资料(PREC/L)1948～2003年56年的月平均降水资料、NCEP/NCAR月平均再分析资料以及英国气象局哈德莱中心的海温(Sea Surface Temperature,SST)资料,并根据多年降水平均图选定了东亚中纬度干旱/半干旱区,对该区域夏季(6～8月)降水进行了经验正交分解(Empirical Orthogonal Function,EOF)。EOF第一模态呈现出全区一致的变化类型,第二模态则呈现出以100°E为界东西相反的分布类型。通过分析干旱/半干旱区以及以100°E为界的东西两部分降水异常年的环流形势和海温并加以对比,结果表明:在环流场上,对应于东亚中纬度干旱/半干旱区降水偏多年,对流层中下层环流异常在中高纬度呈现为一个东西向波列,乌拉尔山东侧为正的高度异常,贝加尔湖附近乃至以东地区为低压槽所控制;不同的是,对应于100°E以西的干旱/半干旱区夏季降水偏多年,波列有所东移,并且西太平洋副热带高压有显著北抬;而对应于100°E以东干旱/半干旱区夏季降水偏多年,环流形势异常基本与整个干旱/半干旱区降水偏多年一致,只是在里海附近有一高度负异常。在200hPa纬向风场上可以看到,当西亚副热带急流偏南加强时,对应于100°E以西的干旱/半干旱区降水偏多;而当东亚、西亚风急流都有显著北抬且加强时,对应于100°E以东干旱/半干旱区的夏季降水偏多,这可能与急流所激发的次级环流有关。进一步对SST的分析表明,海温与100°E以东或以西干旱/半干旱区降水异常的关系也不一样。当前冬、前春赤道中东太平洋都有正的海温异常,而到夏季转换为负的海温异常,且南太平洋在前冬和前春呈现显著负海温异常时,整个干旱/半干旱区夏季降水偏多;当赤道中东太平洋海温在前冬、前春有正的海温异常并一直减弱,但能维持到夏季,并且北印度洋海温也存在类似的海温异常时,100°E以西的干旱/半干旱区夏季降水偏多;而当前冬中东太平洋海温较暖但其南部海域偏冷,到了前春这些异常维持,并发展到同期为大范围弱的异常冷海温时,有利于100°E以东的干旱/半干旱区夏季降水偏多。比较的结果还揭示出,对应于干旱/半干旱区以及100°E以东干旱/半干旱区的降水异常年,海温异常分布大致是一致的;而对应于100°E以西干旱/半干旱区的降水异常年,海温异常分布及时间演变则有较大差异。     

El Nino事件对其衰减阶段夏季中国降水季节内演变的影响及其机理

利用1979—2007年NOAA重建海温逐月资料和中国160站夏季降水资料,使用扩展奇异值分解（extended singular value decomposition,ESVD）方法,研究了冬季热带太平洋海温异常与次年夏季中国降水异常季节内演变型之间的关系,指出前冬El Nino事件是与次年夏季中国降水季节内变化相联系的最重要的热带太平洋海温异常模态。相应的降水异常季节内变化情况为：6月在长江以南为正异常,江淮流域有负异常;7月在华南沿海有负降水异常,而正异常北进到长江流域,华北地区也出现正降水异常;8月在长江南北分别为少雨和多雨。进一步研究前冬El Nino事件与次年春夏印度洋、太平洋海温异常、对流层低层风场异常以及副热带高压等的联系,结果表明：El Nio事件发生的次年春夏,热带西太平洋周边存在东负西正的海温异常分布;西太平洋反气旋异常较强;副高在6月、7月偏西偏北,但在8月迅速南退。虽然与El Nino事件相联系的6月与7月、8月的降水型不同,但是西太平洋反气旋异常带来的充沛水汽造成7月长江流域雨季多雨,8月副高迅速南退带来的又一次长江流域降水,造成了El Nino事件发生次年夏季长江流域涝而华南沿海旱的夏季平均降水异常型。     

Impacts of the leading modes of tropical Indian Ocean sea surface temperature anomaly on sub-seasonal evolution of the circulation and rainfall over East Asia during boreal spring and summer

The two leading modes of the interannual variability of the tropical Indian Ocean (TIO) sea surface temperature (SST) anomaly are the Indian Ocean basin mode (IOBM) and the Indian Ocean dipole mode (IODM) from March to August. In this paper, the relationship between the TIO SST anomaly and the sub-seasonal evolution of the circulation and rainfall over East Asia during boreal spring and summer is investigated by using correlation analysis and composite analysis based on multi-source observation data from 1979 to 2013, together with numerical simulations from an atmospheric general circulation model. The results indicate that the impacts of the IOBM on the circulation and rainfall over East Asia vary remarkably from spring to summer. The anomalous anticyclone over the tropical Northwest Pacific induced by the warm IOBM is closely linked with the Pacific–Japan or East Asia–Pacific teleconnection pattern, which persists from March to August. In the upper troposphere over East Asia, the warm phase of the IOBM generates a significant anticyclonic response from March to May. In June and July, however, the circulation response is characterized by enhanced subtropical westerly flow. A distinct anomalous cyclone is found in August. Overall, the IOBM can exert significant influence on the western North Pacific subtropical high, the South Asian high, and the East Asian jet, which collectively modulate the precipitation anomaly over East Asia. In contrast, the effects of the IODM on the climate anomaly over East Asia are relatively weak in boreal spring and summer. Therefore, studying the impacts of the TIO SST anomaly on the climate anomaly in East Asia should take full account of the different sub-seasonal response during boreal spring and summer.     

东亚夏季风强弱年大气环流和热源异常对比分析

根据黄刚等定义的东亚夏季风指数, 对强、弱东亚夏季风年大气环流、大气热源和外强迫源SST的差异进行分析, 结果表明:强 (弱) 东亚夏季风年前期冬季到夏季, 太平洋SSTA为La Ni?a (El Ni?o) 型分布, 西太平洋暖池SST暖 (冷), 使得暖池附近对流活动较强 (较弱)。与此同时, 南亚大陆从印度半岛、青藏高原南部、中南半岛至华南大气异常加热 (变冷), 并且海陆热力对比加强 (减弱), 有利于出现强 (弱) 的东亚夏季风。此外, 由于暖池附近对流活动强 (弱), 该地区上升气流较强 (弱), Walker环流增强 (减弱), 当强 (弱) 的东亚夏季风向北推进时, 副热带西风急流北撤位置偏北 (南), 副热带高压位置也偏北 (南), 7月至8月华北 (江淮流域) 位于副热带西风急流南侧, 降水偏多, 江淮流域 (华北) 降水偏少。并给出与东亚夏季风年际变异有关的大气环流和SST异常的物理图像。