亚澳季风异常与ENSO准四年变化的联系分析

分析了赤道地区纬向风的年际变化特征，以及亚澳季风与ENSO在各个位相的联系。结果表明：赤道纬向风变化与中东太平洋海温变化在准四年周期上是强烈耦合的；在El Eino期间东亚冬季风弱，夏季风强，而南亚夏季风弱，反之，在La Nina期间东亚冬季风强，夏季风弱，而南亚夏季风强；东亚地区的异常北风有利于西太平洋西风异常爆发，使得东太平洋海温升高，但只有随后在中东太平洋出现持续性西风异常，El Nino才能发展，其中来自太平洋中部的异常北风（并不是来自东亚大陆地区）和南太平洋中部的异常南风的辐合对中东太平洋出现持续性西风异常起重要的作用，尤其是澳大利亚东北部的季风异常的影响更为显。     

大样本初始化十年际预测试验（CESM-DPLE）对东亚夏季气候预测的评估

基于气温和降水观测资料以及美国国家环境预报中心/国家大气研究中心（NCEP/NCAR）大气再分析资料,系统评估了大样本初始化十年际预测试验（CESM-DPLE）对1959—2016年东亚夏季气候预测的能力。结果表明,CESM-DPLE能较好地模拟东亚夏季气候以及相关主要大气环流系统的基本态特征,在年际尺度上对东亚气温有很高的预测技巧但对降水几乎没有预测能力。CESM-DPLE再现了北大西洋多年代际振荡（AMO）经由激发遥相关波列所引起的中高纬度大气环流、东亚夏季风和气候的异常。20世纪90年代末之后,北大西洋多年代际振荡由冷位相转为暖位相,遥相关波列位相调整,东亚受异常低压控制,东亚夏季风偏强,夏季气温偏高、降水偏多。总体上,尽管还存在着不足,但CESM-DPLE对东亚夏季温度年际变化以及与20世纪90年代末北大西洋多年代际振荡位相转变相联的东亚夏季气候年代际变化具备一定的预测能力,是目前研究和预测东亚气候变化的一套较好试验数据。      

青藏高原大气热源年际变率及其驱动因子

青藏高原（以下简称高原）大气热源对亚洲夏季风爆发、演变、推进,乃至全球气候系统都有重要影响,因此近年来高原大气热源变异机理也日益受到关注。本文在回顾已有关于不同季节高原热源变异原因的研究基础上,利用1980～2018年日本气象厅再分析数据JRA55（Japanese 55-year Reanalysis）,对逐月高原大气总热源的年际变率进行分类,并进一步探究了影响不同类别高原大气总热源的异常大尺度环流系统及海温驱动因子。除了传统上受关注的“冬季型”和“夏季型”以外,本文还提出了“早春型”和“过渡型”两种高原大气热源变率模态。总体而言,高原大气总热源年际变率以降水引起的凝结潜热异常为主,其中“冬季型”及“早春型”高原大气热源异常中心位于高原西部,主要受到中高纬遥相关波列的影响。此外,“冬季型”还受到厄尔尼诺—南方涛动（El Ni?o-Southern Oscillation, ENSO）及印度洋偶极子（Indian Ocean Dipole, IOD）的影响。“夏季型”高原大气热源呈东西偶极型反相变化,最大异常中心位于高原东南部,主要受北大西洋涛动（North Atlantic Oscillation, NAO）的影响;“过渡型”高原大气热源呈南北偶极型反相变化,受热带太平洋—印度洋海表温度异常的共同影响。因此,不同背景环流下高原热源年际变率的驱动因子存在明显差异。     

The Summer Snow Cover Anomaly over the Tibetan Plateau and Its Association with Simultaneous Precipitation over the Mei-yu–Baiu region

The summer snow anomalies over the Tibetan Plateau （TP） and their effects on climate variability are often overlooked,possibly due to the fact that some datasets cannot properly capture summer snow cover over high terrain.The satellite-derived Equal-Area Scalable Earth grid （EASE-grid） dataset shows that snow still exists in summer in the western part and along the southem flank of the TP.Analysis demonstrates that the summer snow cover area proportion （SCAP） over the TP has a significant positive correlation with simultaneous precipitation over the mei-yu-baiu （MB） region on the interannual time scale.The close relationship between the summer SCAP and summer precipitation over the MB region could not be simply considered as a simultaneous response to the Silk Road pattern and the SST anomalies in the tropical Indian Ocean and tropical central-eastern Pacific.The SCAP anomaly has an independent effect and may directly modulate the land surface heating and,consequently,vertical motion over the western TP,and concurrently induce anomalous vertical motion over the North Indian Ocean via a meridional vertical circulation.Through a zonal vertical circulation over the tropics and a Kelvin wave-type response,anomalous vertical motion over the North Indian Ocean may result in an anomalous high over the western North Pacific and modulate the convective activity in the western Pacific warm pool,which stimulates the East Asia-Pacific （EAP） pattern and eventually affects summer precipitation over the MB region.     

Summertime atmospheric teleconnection pattern associated with a warming over the eastern Tibetan Plateau

By using a surface air temperature index (SATI) averaged over the eastern Tibetan Plateau (TP), investigation is conducted on the short-term climate variation associated with the interannual air warming (or cooling) over the TP in each summer month. Evidence suggests that the SATI is associated with a consistent teleconnection pattern extending from the TP to central-western Asia and southeastern Europe. Associated rainfall changes include, for a warming case, a drought in northern India in May and June, and a stronger mei-yu front in June. The latter is due to an intensified upper-level northeasterly in eastern China and a wetter and warmer condition over the eastern TP. In the East Asian regions, the time-space distributions of the correlation patterns between SATI and rainfall are more complex and exhibit large differences from month to month. Some studies have revealed a close relationship between the anomalous heating over the TP and the rainfall anomaly along the Yangtze River valley appearing in the summer on a seasonal mean time-scale, whereas in the present study, this relationship only appears in June and the signal's significance becomes weaker after the long-term trend in the data was excluded. Close correlations between SATI and the convection activity and SST also occur in the western Pacific in July and August: A zonally-elongated warm tone in the SST in the northwestern Pacific seems to be a passive response of the associated circulation related to a warm SATI. The SATI-associated teleconnection pattern provides a scenario consistently linking the broad summer rainfall anomalies in Europe, central-western Asia, India, and East Asia.     

Interannual Variability of Snow Depth over the Tibetan Plateau and Its Associated Atmospheric Circulation Anomalies

The interannual variability of wintertime snow depth over the Tibetan Plateau(TP) and related atmospheric circulation anomalies were investigated based on observed snow depth measurements and NCEP/NCAR reanalysis data.Empirical orthogonal function(EOF) analysis was applied to identify the spatio-temporal variability of wintertime TP snow depth.Snow depth anomalies were dominated by a monopole pattern over the TP and a dipole structure with opposite anomalies over the southeastern and northwestern TP.The atmospheric circulation conditions responsible for the interannual variability of TP snow depth were examined via regression analyses against the principal component of the most dominant EOF mode.In the upper troposphere,negative zonal wind anomalies over the TP with extensively positive anomalies to the south indicated that the southwestward shift of the westerly jet may favor the development of surface cyclones over the TP.An anomalous cyclone centered over the southeastern TP was associated with the anomalous westerly jet,which is conducive to heavier snowfall and results in positive snow depth anomalies.An anomalous cyclone was observed at 500 hPa over the TP,with an anomalous anticyclone immediately to the north,suggesting that the TP is frequently affected by surface cyclones.Regression analyses revealed that significant negative thickness anomalies exist around the TP from March to May,with a meridional dipole anomaly in March.The persistent negative anomalies due to more winter TP snow are not conducive to earlier reversal of the meridional temperature gradient,leading to a possible delay in the onset of the Asian summer monsoon.     

西风与季风在青藏高原的耦合模态及其季节性变化特征

青藏高原是我国的水塔,西风与季风及其相互作用是导致亚洲天气和气候变化最重要的环流系统。本文基于1981～2020年大气再分析资料,采用经验正交函数分解方法（Empirical Orthogonal Function,EOF）提取了西风与季风季节循环分量在青藏高原的耦合模态,并对其季节变化特征进行分析。研究发现,第一主模态方差贡献率高达78.39%,主要反映的是东亚季风、南亚季风和对流层高层中纬度西风的季节循环特征及各个季节的年际变化特征。夏季在对流层高层高原及其南侧主要为东风气流,范围从北纬5°至35°,对流层低层则表现为典型的绕高原气旋式季风环流系统,热带和副热带地区为西南季风控制,冬季的环流结构刚好相反。耦合模态的冬、夏季节转换节点与东亚季风和南亚季风的季节转换时间基本一致。从年际变化的角度来看,各个季节耦合模态的强度偏强时,东亚季风和南亚季风均偏强,西风带位置偏北;反之,季风偏弱,西风带位置偏南。厄尔尼诺—南方涛动（El Ni?o–Southern Oscillation,ENSO）是影响西风与季风耦合模态年际变化的关键外强迫,拉尼娜（La Ni?a）事件发生的前夏、前秋和次年夏季耦合模态的强度均增强,冬季至次年春季耦合模态的强度均减弱。西风与季风耦合的第二主模态主要表现为对流层高层高原上的东风及其南侧西风,以及低层南亚季风区的西南季风和西北太平洋反气旋的协同变化特征。该模态的方差贡献率为4.68%,表现出明显年际差异的同时还呈现显著减弱的长期趋势,尤其是在冬季。     

多因子协同作用对1992年和1998年黄淮地区夏季降水异常的影响

1991年5月和1997年4月赤道中东太平洋均发生了El Ni?o事件,但是1992年夏季黄淮地区降水异常偏少,而1998年夏季却异常偏多。分析结果显示,1992年夏季西北太平洋副热带高压（以下简称副高）偏东,中高纬阻塞高压偏弱,黄淮地区降水异常偏少;而1998年夏季,副高偏西,中高纬阻塞高压活动频繁,黄淮地区降水异常偏多。对海温外强迫信号的诊断和数值模式试验显示:当西太平洋对流活动偏弱时,有利于副高西伸;鄂霍茨克海及以东海温偏高时,其上空的阻塞高压增强;北大西洋中纬度地区海温偏高时,有利于后期乌拉尔山高压脊明显增强。即在赤道中东太平洋发生El Ni?o事件的背景下,西太平洋对流、鄂霍茨克海附近亲潮区域和北大西洋中纬度区域海温异常可能是导致黄淮区域1992年夏季和1998年夏季降水差异大的主要原因。该工作显示仅根据El Ni?o事件的发生时间和强度无法完全预测黄淮地区夏季降水变化,需要综合考虑西太平洋对流、鄂霍茨克海附近海域和北大西洋中纬度区域海温异常对季风环流的影响,从多因子协同作用的角度诊断和预测黄淮地区夏季降水异常趋势,提高预测能力。     

夏季逐月东亚高空西风急流经向偏移与热带中、东太平洋海温的关联

Previous studies have shown that meridional displacement of the East Asian upper-tropospheric jet stream (EAJS) dominates interannual variability of the EAJS in the summer months. This study investigates the tropical Pacific sea surface temperature (SST) anomalies associated with meridional displacement of the monthly EAJS during the summer. The meridional displacement of the EAJS in June is significantly associated with the tropical central Pacific SST anomaly in the winter of previous years, while displacements in July and August are related to tropical eastern Pacific SST anomalies in the late spring and concurrent summer. The EAJS tends to shift southward in the following June (July and August) corresponding to a warm SST anomaly in the central (eastern) Pacific in the winter (late spring-summer). The westerly anomaly south of the Asian jet stream is a result of tropical central Pacific warm SST anomaly-related warming in the tropical troposphere, which is proposed as a possible reason for southward displacement of the EAJS in June. The late spring-summer warm SST anomaly in the tropical eastern Pacific, however, may be linked to southward displacement of the EAJS in July and August through a meridional teleconnection over the western North Pacific (WNP) and East Asia.     

Links between tropical SST anomalies and precursory signalsassociated with the interannual variability of Asian summer monsoon

Summary ?The interannual variability of broad-scale Asian summer monsoon was studied using a general circulation model (GCM) and NCEP (National Center for Environmental Prediction) data set during 1979–95. In the GCM experiment, the main emphasis was given to isolate the individual role of surface boundary conditions on the existence of winter-spring time circulation anomalies associated with the interannual variability of Asian summer monsoon. In order to understand the role of sea-surface temperatures (SSTs) alone on the existence of precursory signals, we have conducted 17 years numerical integration with a GCM forced with the real-time monthly averaged SSTs of 1979 to 1995. In this experiment, among the many surface boundary conditions only SSTs are varying interannually. The composite circulation anomalies simulated by the GCM have good resemblance with the NCEP circulation anomalies over subtropical Asia. This suggests that the root cause of the existence of winter-spring time circulation anomalies associated with the interannual variability of Asian summer monsoon is the interannual variability of SST. Empirical Orthogonal Functions (EOFs) of 200-mb winds and OLR were constructed to study the dynamic coupling between SST anomalies and winter-spring time circulation anomalies. It is found that the convective heating anomalies associated with SST anomalies and stationary eddies undergo systematic and coherent interannual variations prior to summer season. We have identified Matsuno-Gill type mode in the velocity potential and stream function fields. This suggests the existence of dynamic links between the SST anomalies and the precursory signals of Asian summer monsoon. Received June 9, 1999/Revised April 7, 2000     

Cause of Extreme Heavy and Persistent Rainfall over Yangtze River in Summer 2020

Record-breaking heavy and persistent precipitation occurred over the Yangtze River Valley (YRV) in June-July (JJ) 2020. An observational data analysis has indicated that the strong and persistent rainfall arose from the confluence of southerly wind anomalies to the south associated with an extremely strong anomalous anticyclone over the western North Pacific (WNPAC) and northeasterly anomalies to the north associated with a high-pressure anomaly over Northeast Asia. A further observational and modeling study has shown that the extremely strong WNPAC was caused by both La Ni?a-like SST anomaly (SSTA) forcing in the equatorial Pacific and warm SSTA forcing in the tropical Indian Ocean (IO). Different from conventional central Pacific (CP) El Ni?os that decay slowly, a CP El Ni?o in early 2020 decayed quickly and became a La Ni?a by early summer. This quick transition had a critical impact on the WNPAC. Meanwhile, an unusually large area of SST warming occurred in the tropical IO because a moderate interannual SSTA over the IO associated with the CP El Ni?o was superposed by an interdecadal/long-term trend component. Numerical sensitivity experiments have demonstrated that both the heating anomaly in the IO and the heating anomaly in the tropical Pacific contributed to the formation and maintenance of the WNPAC. The persistent high-pressure anomaly in Northeast Asia was part of a stationary Rossby wave train in the midlatitudes, driven by combined heating anomalies over India, the tropical eastern Pacific, and the tropical Atlantic.     

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.     

Toward Understanding the Extreme Floods over Yangtze River Valley in June?July 2020: Role of Tropical Oceans※

The extreme floods in the Middle/Lower Yangtze River Valley (MLYRV) during June?July 2020 caused more than 170 billion Chinese Yuan direct economic losses. Here, we examine the key features related to this extreme event and explore relative contributions of SST anomalies in different tropical oceans. Our results reveal that the extreme floods over the MLYRV were tightly related to a strong anomalous anticyclone persisting over the western North Pacific, which brought tropical warm moisture northward that converged over the MLYRV. In addition, despite the absence of a strong El Ni?o in 2019/2020 winter, the mean SST anomaly in the tropical Indian Ocean during June?July 2020 reached its highest value over the last 40 years, and 43％ (57％) of it is attributed to the multi-decadal warming trend (interannual variability). Based on the NUIST CFS1.0 model that successfully predicted the wet conditions over the MLYRV in summer 2020 initiated from 1 March 2020 (albeit the magnitude of the predicted precipitation was only about one-seventh of the observed), sensitivity experiment results suggest that the warm SST condition in the Indian Ocean played a dominant role in generating the extreme floods, compared to the contributions of SST anomalies in the Maritime Continent, central and eastern equatorial Pacific, and North Atlantic. Furthermore, both the multi-decadal warming trend and the interannual variability of the Indian Ocean SSTs had positive impacts on the extreme floods. Our results imply that the strong multi-decadal warming trend in the Indian Ocean needs to be taken into consideration for the prediction/projection of summer extreme floods over the MLYRV in the future.     

Toward Understanding the Extreme Floods over Yangtze River Valley in June?July 2020: Role of Tropical Oceans※

The extreme floods in the Middle/Lower Yangtze River Valley (MLYRV) during June?July 2020 caused more than 170 billion Chinese Yuan direct economic losses. Here, we examine the key features related to this extreme event and explore relative contributions of SST anomalies in different tropical oceans. Our results reveal that the extreme floods over the MLYRV were tightly related to a strong anomalous anticyclone persisting over the western North Pacific, which brought tropical warm moisture northward that converged over the MLYRV. In addition, despite the absence of a strong El Ni?o in 2019/2020 winter, the mean SST anomaly in the tropical Indian Ocean during June?July 2020 reached its highest value over the last 40 years, and 43％ (57％) of it is attributed to the multi-decadal warming trend (interannual variability). Based on the NUIST CFS1.0 model that successfully predicted the wet conditions over the MLYRV in summer 2020 initiated from 1 March 2020 (albeit the magnitude of the predicted precipitation was only about one-seventh of the observed), sensitivity experiment results suggest that the warm SST condition in the Indian Ocean played a dominant role in generating the extreme floods, compared to the contributions of SST anomalies in the Maritime Continent, central and eastern equatorial Pacific, and North Atlantic. Furthermore, both the multi-decadal warming trend and the interannual variability of the Indian Ocean SSTs had positive impacts on the extreme floods. Our results imply that the strong multi-decadal warming trend in the Indian Ocean needs to be taken into consideration for the prediction/projection of summer extreme floods over the MLYRV in the future.     

Interdecadal Change in the Interannual Variation of the Western Edge of theWestern North Pacific Subtropical High During Early Summer and the Influenceof Tropical Sea Surface Temperature

This study reveals that the interannual variability of the western edge of the western North Pacific (WNP) subtropical high (WNPSH) in early summer experienced an interdecadal decrease around 1990. Correspondingly, the zonal movement of the WNPSH and the zonal extension of the high-pressure anomaly over the WNP (WNPHA) in abnormal years possess smaller ranges after 1990. The different influences of the tropical SSTAs are important for this interdecadal change, which exhibit slow El Ni?o decaying pattern before 1990 while rapid transformation from El Ni?o to La Ni?a after 1990. The early summer tropical SSTAs and the relevant atmospheric circulation anomalies present obvious interdecadal differences. Before 1990, the warm SSTAs over the northern Indian Ocean and southern South China Sea favor the WNPHA through eastward-propagating Kelvin wave and meridional-vertical circulation, respectively. Meanwhile, the warm SSTA over the tropical central Pacific induces anomalous ascent to its northwest through the Gill response, which could strengthen the anomalous descent over the WNP through meridional-vertical circulation and further favor the eastward extension of the WNPHA to central Pacific. After 1990, the warm SSTAs over the Maritime Continent and northern Indian Ocean cause the WNPHA through meridional-vertical and zonal-vertical circulation, respectively. Overall, the anomalous warm SSTs and ascent and the resultant anomalous descent over the WNP are located more westward and southward after 1990 than before 1990. Consequently, the WNPHA features narrower zonal range and less eastward extension after 1990, corresponding to the interdecadal decease in the interannual variability of the western edge of the WNPSH. On the other hand, the dominant oscillation period of ENSO experienced an interdecadal reduction around 1990, contributing to the change of the El Ni?o SSTA associated with the anomalous WNPSH from slow decaying type to rapid transformation type.     

Numerical study for characteristic change of Asian summer monsoon circulation and its influence mechanism during the El Nino period

In this paper, the relation between Asian summer monsoon circulation and sea surface temperature anomalies over equatorial central-eastern Pacific is investigated by using a global spectral model. This model has nine layers in the vertical and the model variables are represented in the horizontal as truncated expansions of the surface spherical harmonics with rhomboidal truncation at wave number 15. The model involves comparatively complete physical pro-cesses and parameterizations with mountains.Using the above model, two experimental schemes are designed, namely control case and anomalous sea surface temperature case. The above two schemes are respectively integrated for forty days and the simulated results are ob-tained from the last 30-day averaged simulations.The simulations show that positive SST anomalies over equatorial central-eastern Pacific weakens Indian mon-soon circulation, decreases precipitation in Indian sub-continent whereas it intensifies East Asian monsoon circula-tion and increases precipitation in East Asian area. All these results reflect the characteristics of Asian summer mon-soon during the El Nino period. In this paper, SST anomalies over equatorial central-eastern Pacific have a direct in-fluence on the intensity and position of subtropical high via the wave train over Northern Hemisphere, which is simi-lar to that suggested by Nitta (1987) and the wave train over Southern Hemisphere has an influence on the intensity of Mascarene high and Australia high resulting in affecting cross equatorial flow. As a result, atmospheric interior heat sources and sinks are redistributed because of the change of cross equatorial flow. And the response of atmos-phere to the new heat source and sink has a significant influence on Asian summer monsoon.     

中国华南春季季风及其与大尺度环流特征的关系

定义了中国华南春季季风，并用NCEP／NCAR再分析资料研究了春季风的气候特征以及春季风降水和大尺度环流在年际变化上的关系。结果表明，从降水和大气环流的变化来看，华南春季风在气候上发生于4月和5月；与华南春季风相联系的大气环流特征与夏季风和冬季风所对应的大气环流特征完全不同。华南春季风降水的年际变化主要与太平洋北部的异常环流相关联，而这种异常环流又与亚洲北部的西风急流和极地涡旋有联系；华南春季风降水的年际变化还与太平洋的海表温度异常有关；而亚洲热带大气环流的年际变化与华南春季风降水的变化关系不大。     

The impact of warm pool SST and general circulation on increased temperature over the Tibetan Plateau

In this paper, the possible reason of Tibetan Plateau (TP) temperature increasing was investigated. An increase in T min (minimum temperature) plays a robust role in increased TP temperature, which is strongly related to SST over the warm pool of the western Pacific Ocean, the subtropical westerly jet stream (SWJ), and the tropical easterly upper jet stream (TEJ), and the 200-hPa zonal wind in East Asia. Composite analysis of the effects of SST, SWJ, and TEJ on pre- and post-abrupt changes in T a (annual temperature) and T min over the TP shows remarkable differences in SST, SWJ, and TEJ. A lag correlation between T a /T min , SST, and SWJ/TEJ shows that changes in SST occur ahead of changes in T a /T min by approximately one to three seasons. Partial correlations between T a /T min , SST, and SWJ/TEJ show that the effect of SWJ on T a /T min is more significant than the effect of SST. Furthermore, simulations with a community atmospheric model (CAM3.0) were performed, showing a remarkable increase in T a over the TP when the SST increased by 0.5 ? C. The main increase in T a and T min in the TP can be attributed to changes in SWJ. A possible mechanism is that changes in SST force the TEJ to weaken, move south, and lead to increased SWJ and movement of SWJ northward. Finally, changes in the intensity and location of the SWJ cause an increase in T a /T min . It appears that TP warming is governed primarily by coherent TEJ and SWJ variations that act as the atmospheric bridges to remote SSTs in warm-pool forcing.     

Influence of Intraseasonal Oscillation on the Asymmetric Decays of El Ni?o and La Ni?a

Warm and cold phases of El Nino–Southern Oscillation (ENSO) exhibit a significant asymmetry in their decay speed. To explore the physical mechanism responsible for this asymmetric decay speed, the asymmetric features of anomalous sea surface temperature (SST) and atmospheric circulation over the tropical Western Pacific (WP) in El Nino and La Nina mature-to-decay phases are analyzed. It is found that the interannual standard deviations of outgoing longwave radiation and 850 hPa zonal wind anomalies over the equatorial WP during El Nino (La Nina) mature-to-decay phases are much stronger (weaker) than the intraseasonal standard deviations. It seems that the weakened (enhanced) intraseasonal oscillation during El Nino (La Nina) tends to favor a stronger (weaker) interannual variation of the atmospheric wind, resulting in asymmetric equatorial WP zonal wind anomalies in El Nino and La Nina decay phases. Numerical experiments demonstrate that such asymmetric zonal wind stress anomalies during El Nino and La Nina decay phases can lead to an asymmetric decay speed of SST anomalies in the central-eastern equatorial Pacific through stimulating di erent equatorial Kelvin waves. The largest negative anomaly over the Nino3 region caused by the zonal wind stress anomalies during El Nino can be threefold greater than the positive Nino3 SSTA anomalies during La Nina, indicating that the stronger zonal wind stress anomalies over the equatorial WP play an important role in the faster decay speed during El Nino.     

Abnormal Warming of the Summer Surface Air Temperature in Central Asia from 1980 to 2019北大核心CSCD

This work analyzes the abrupt change in summer surface air temperature (SAT) in Central Asia (CA) and its relationship with sea surface temperature (SST) in the North Atlantic and snow cover in the Qinghai Tibet Plateau between 1980 and 2019 based on NCEP/NCAR reanalysis data, CRU SAT, and snow cover and global SST data. The results reveal a significant summer SAT change in CA in 2005. The standardized regional average temperature index in CA shifts from the previous negative phase to the subsequent positive phase, indicating a significant summer SAT increase in CA. Analysis of the anomalous atmospheric circulations related to interdecadal changes in summer SAT in CA shows the abnormally enhanced anticyclonic circulation system in the west of CA after 2005. The atmospheric subsidence associated with the anomalous anticyclone can cause warming. On the other hand, the reduction in the amount of cloud caused by this anticyclone anomaly enhancement results in the increase in downward short-wave radiation and thus is favorable for the increased summer temperature in CA. Furthermore, the interdecadal summer SAT changes in CA in 2005 are closely related to SST warming in the middle and high latitudes of the North Atlantic and the reduction in snow cover in the west of the Tibet Plateau (TP). The SST increase in the middle and high latitudes of the North Atlantic can stimulate a Rossby wave propagating downstream. The reduction in snow cover in the west of the TP can cause warming to the above atmosphere through the snow albedo effect. The changes in both the North Atlantic SST and the TP snow can strengthen the anticyclone over CA, leading to an abnormally high summer SAT over there. © 2023 Science Press. All rights reserved.