2019年4～6月云南持续性高温天气的大气环流异常成因

2019年4～6月云南省发生了历史罕见的持续性极端高温天气,并引发了严重气象干旱。本文利用1961～2019年逐日温度和大气再分析等资料以及CESM-LE计划（Community Earth System Model Large Ensemble Project）模式模拟结果,分析了历史同期云南极端高温天气发生的环流特征,探讨了2019年云南破纪录持续性高温的成因。历史极端高温日的合成分析表明,云南地区对流层上层显著异常反气旋伴随的强下沉异常和到达地表太阳辐射增加,是引发该区域极端高温天气的主要成因。该异常反气旋的形成主要源自北大西洋经东欧平原、西西伯利亚平原向东亚传播的高纬度罗斯贝波和经北非、黑海、伊朗高原向东亚传播的中纬度罗斯贝波之间的相互作用。2019年极端高温的强度和与之相应异常反气旋出现自1961年以来的最强。外强迫导致的增暖对2019年极端暖异常强度的贡献约为37.51%,同时对类似2019年以及更强极端暖事件发生概率的贡献为56.32%,内部变率对该事件也具有重要贡献。2019年4～6月北极涛动（Arctic Oscillation,AO）和ENSO事件分别处于历史极端负位相和暖位相。一方面,在AO强负位相影响下,极地上空深厚的位势高度正异常向南伸至东欧平原,有利于高纬度波列和云南上空的反气旋异常增强。另一方面,ENSO事件暖位相加强了西北太平洋异常反气旋环流,令西北太平洋副热带高压增强西伸至我国内陆地区,维持了云南上空反气旋异常。两者的共同作用,造成了2019年4～6月云南上空持续的深厚异常反气旋,云南地区继而出现持续性极端高温事件。     

华北地区干旱的气候背景及其前兆强信号

将华北地区春、夏季出现异常干旱和多雨典型年份的 5 0 0hPa高度和北太平洋海温的距平进行合成 ,讨论了干旱期和多雨期大气环流和海洋变化的气候背景特征。结果表明 ,华北地区的干旱期和多雨期间的大气环流和海温变化存在趋势性差异。以 3个特旱年为例 ,使用信噪比的方法识别发生特别干旱的前期大气、海洋的强信号。虽然 3个特旱年的前期大气和海洋表现出的强信号的位置和强度并不完全一致 ,但是乌拉尔山脉附近和贝加尔湖附近高度的异常变化及黑潮、暖池及Nino 4区东部附近海温的异常变化是值得注意的前兆信号     

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.     

Fluctuations of the relationship between ENSO and northeast Australian rainfall

It is well known that during an El Niño-Southern Oscillation (ENSO) warm event, drought occurs in regions of northeastern (NE) Australia, leading to anomalously low annual rainfall. The present study explores fluctuations of this ENSO-rainfall relationship. It is found that the relationship tends to weaken when the linearly detrended global mean temperature is rising or particularly high, as in the period of 1931–45 period and since the late 1970s. Prior to a weakening, a correlation pattern of increased rainfall during El Niño events is seen first in northwestern Australia, then in eastern and southeastern Australia, and eventually in NE Australia. The 1931–45 period was particularly intriguing, when in terms of rainfall variability over NE Australia, the interannual ENSO-rainfall relationship went through a process of weakening, reversal, and rapid recovery. Features associated with the reversal are therefore examined and these features are: (1) the global background anomaly pattern (upon which internnal ENSO events operate) is ENSO-like; (2) ENSO sea surface temperature (SST) anomalies in tropical Pacific are weaker compared with those averaged over all ENSO events, whereas SST anomalies in the mid- to-high latitude Pacific (which have opposing polarity to those in tropical Pacific) are larger; (3) there is strong coherence between ENSO and variability in northern mid- to high-latitudes; and (4) the relationship that an El Niño event contributes to a warming anomaly of global mean SST weakens. Possible interrelationship among these features are discussed.     

Triggering of El Niño by westerly wind events in a coupled general circulation model

Two ten-members ensemble experiments using a coupled ocean-atmosphere general circulation model are performed to study the dynamical response to a strong westerly wind event (WWE) when the tropical Pacific has initial conditions favourable to the development of a warm event. In the reference ensemble (CREF), no wind perturbation is introduced, whereas a strong westerly wind event anomaly is introduced in boreal winter over the western Pacific in the perturbed ensemble (CWWE). Our results demonstrate that an intense WWE is capable of establishing the conditions under which a strong El Niño event can occur. First, it generates a strong downwelling Kelvin wave that generates a positive sea surface temperature (SST) anomaly in the central-eastern Pacific amplified through a coupled ocean-atmosphere interaction. This anomaly can be as large as 2.5°C 60 days after the WWE. Secondly, this WWE also initiates an eastward displacement of the warm-pool that promotes the occurrence of subsequent WWEs in the following months. These events reinforce the initial warming through the generation of additional Kelvin waves and generate intense surface jets at the eastern edge of the warm-pool that act to further shift warm waters eastward. The use of a ten-members ensemble however reveals substantial differences in the coupled response to a WWE. Whereas four members of CWWE ensemble develop into intense El Niño warming as described above, four others display a moderate warming and two remains in neutral conditions. This diversity between the members appears to be due to the internal atmospheric variability during and following the inserted WWE. In the four moderate warm cases, the warm-pool is initially shifted eastward following the inserted WWE, but the subsequent weak WWE activity (when compared to the strong warming cases) prevents to further shift the warm-pool eastwards. The seasonal strengthening of trade winds in June–July can therefore act to shift warm waters back into the western Pacific, reducing the central-eastern Pacific warming. This strong sensitivity of the coupled response to WWEs may therefore limit the predictability of El Niño events, as the high frequency wind variability over the warm pool region remains largely unpredictable even at short time lead.     

北大西洋年际变率的海气耦合模式模拟II：热带太平洋强迫

利用一个全球海气耦合模式(BCM),结合观测资料,讨论了热带太平洋强迫对北大西洋年际气候变率的影响。研究表明,BCM能够相对合理地模拟赤道太平洋的年际变率模态及相应的海温距平型和大气遥相关型,尽管其准3年的振荡周期过于规则。来自数值模式和观测上的证据都表明,北大西洋冬季海温的主导性变率模态,即自北而南出现的“- -”的海温距平型,受到来自热带太平洋强迫的显著影响,其正位相与赤道中东太平洋冷事件相对应。换言之,赤道太平洋暖事件的发生,在太平洋-北美沿岸激发出PNA遥相关型,进而通过在北大西洋产生类似NAO负位相的气压距平型,削弱本来与NAO正位相直接联系的三核型海温距平。北大西洋三核型海温距平对热带太平洋强迫的响应,要滞后2—3个月的时间。     

热带太平洋西风异常对ENSO事件发生的作用

本文从观测资料对80年代两次ENSO事件产生过程中，热带太平洋西风异常及其对赤道中、东太平洋表层海温增暖的作用进行了分析和比较。分析结果表明:在这两次ENSO事件的产生过程中，赤道西太平洋上空均有较大的西风异常，并且它由赤道西太平洋向赤道中、东太平洋传播，随着西风异常从西向东传播，赤道中、东太平洋的表层相继增温。分析还表明，1982／1983年ENSO事件发生过程中，热带太平洋西风异常的强度要比1986／1987年热带太平洋西风异常强得多，这使得1982／1983 ENSO事件的强度比1986／1987_ENSO事件强得多。为了说明热带西太平洋西风异常对赤道中、东太平洋ENSO事件发生的作用，本文还利用IAP太平洋环流模式对西风异常在ENSO事件产生过程中的作用进行了数值模拟。模拟的结果说明了热带太平洋的西风异常对赤道太平洋暖水的向东传播和赤道中、东太平洋的增温起了很重要作用，这与观测事实分析一致。     

江淮流域梅雨期降水的空间非均匀分布与前期海温的关系

利用中国气象局提供的1978-2007年全国753站逐日降水资料、NECP/NCAR提供的逐日再分析资料和NOAA提供的第2套扩展重建海温资料,从区域整体角度确定了近30 a（1978-2007年）江淮流域梅雨期.采用EOF（empirical orthogonal function,经验正交函数）分析,讨论了江淮流域梅雨期降水空间非均匀分布特征,着重研究了影响江淮梅雨空间非均匀分布的前期海温关键区及关键时段.结果表明：全区一致梅雨旱涝与前期冬季北太平洋鄂霍次克海附近的海温异常有密切的联系.当前期冬季该海域海温偏高时,冬季风偏弱,对应后期梅雨一致偏涝,反之则偏旱.5月南海至台湾和菲律宾以东附近海温偏低,江淮流域梅雨量偏多,反之则偏少.梅雨的南北反相分布与前期秋冬季中印度洋的海温有非常密切的关系,当前一年10月至当年1月中印度洋海温偏高时,梅雨期850 hPa江淮之间易形成切变线,有利于梅雨区“南旱北涝”,反之则“南涝北旱”.梅雨的东西反相分布与前期秋、冬季热带中东太平洋的海温关系密切,ENSO事件有可能通过影响西太平洋副热带高压的东西位置,从而引起东亚大气环流异常,导致梅雨东西分布反相.前期秋季和冬季热带中东太平洋海温偏高年（对应ENSO暖事件）,西太副高位置偏西,有利于梅雨区“东旱西涝”,反之则“东涝西旱”.     

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.     

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.     

两类厄尔尼诺事件发展年秋季印度洋海温异常特征对比

基于1951—2010年逐月海气多要素观测资料,对比分析了两类厄尔尼诺事件发展年秋季印度洋的海温异常及大气响应特征,探讨了印度洋偶极子的发生与两类厄尔尼诺事件特征的可能联系。结果表明,两类厄尔尼诺事件的发展年均会出现印度洋偶极子,但出现的概率不同:大多数东部型厄尔尼诺事件都会伴有正位相印度洋偶极子发生;而仅一半的中部型厄尔尼诺事件期间会出现正位相印度洋偶极子的异常海温型,且强度较弱。从印度洋偶极子与两类厄尔尼诺事件的物理联系上看,东部型厄尔尼诺事件期间,印度洋偶极子的发生与其强度联系密切:印度洋偶极子发生在东部型厄尔尼诺事件较强期间,两者通过海洋大陆的异常强下沉运动及大范围负异常降水相联系;东部型厄尔尼诺事件偏弱时并无印度洋偶极子出现,海洋大陆异常下沉运动及负异常降水很弱。然而,中部型厄尔尼诺事件期间印度洋偶极子的发生与其强度并无显著的关系,而与太平洋高海温区的位置存在一定的可能联系:在有印度洋偶极子发生的中部型厄尔尼诺事件发展年秋季,热带太平洋异常高海温区的位置相对偏东,海洋大陆出现显著下沉运动和大范围负异常降水,热带东印度洋为大范围强异常东风控制;但无印度洋偶极子发生的中部型厄尔尼诺事件时,热带太平洋高海温区位置相对偏西,极弱的海洋大陆下沉支对热带印度洋异常海温作用非常有限。     

Nonlinear precipitation response to El Niño and global warming in the Indo-Pacific

Precipitation changes over the Indo-Pacific during El Niño events are studied using an Atmospheric General Circulation Model forced with sea-surface temperature (SST) anomalies and changes in atmospheric CO₂ concentrations. Linear increases in the amplitude of the El Niño SST anomaly pattern trigger nonlinear changes in precipitation amounts, resulting in shifts in the location and orientation of the Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ). In particular, the maximum precipitation anomaly along the ITCZ and SPCZ shifts eastwards, the ITCZ shifts south towards the equator, and the SPCZ becomes more zonal. Precipitation in the equatorial Pacific also increases nonlinearly. The effect of increasing CO₂ levels and warming SSTs is also investigated. Global warming generally enhances the tropical Pacific precipitation response to El Niño. The precipitation response to El Niño is found to be dominated by changes in the atmospheric mean circulation dynamics, whereas the response to global warming is a balance between dynamic and thermodynamic changes. While the dependence of projected climate change impacts on seasonal variability is well-established, this study reveals that the impact of global warming on Pacific precipitation also depends strongly on the magnitude of the El Niño event. The magnitude and structure of the precipitation changes are also sensitive to the spatial structure of the global warming SST pattern.     

强El Niño衰减年东亚夏季风的季节内变化:1998年和2016年的对比分析

本文对比分析了1998年和2016年这两个强El Ni?o衰减年东亚夏季风的季节内变化。结果表明,在6~7月期间,由于热带印度洋海温偏高、对流偏强,造成西太平洋暖池对流偏弱,西太平洋副热带高压（副高）偏西偏强,长江流域降水偏多,华南偏少,东亚夏季风异常具有典型的El Ni?o衰减年特征。但两年的8月份有很大差异,虽然1998年8月与6~7月相似,但2016年8月份则完全不同。受乌拉尔地区异常反气旋的影响,源自西伯利亚东部的北风异常穿越东亚并直抵暖池地区,造成副高分裂并减弱东退,同时激发暖池对流发展,而对流的发展则进一步促使副高减弱。因此,2016年8月东亚夏季风异常与1998年8月相反,中国北方夏季降水异常也呈现很大差异。另外,1998年热带大西洋偏暖,并通过热带环流变化影响到东亚夏季风异常,其强迫作用与热带印度洋类似。而2016年大西洋海温异常较弱,对东亚夏季风影响也较弱。因此,El Ni?o对东亚夏季风的影响不仅与其强度有关,还与El Ni?o衰减之后造成的印度洋和大西洋海温异常有关。本文的分析结果表明,即使在强El Ni?o衰减年夏季,由于El Ni?o之间的个性差异以及其他因子的影响,东亚夏季风季节内变化仍然能呈现出显著差异,特别是在8月份。因此,在预测东亚夏季风异常时,宜将6~7月和8月分别考虑。此外,为进一步提高东亚夏季风预测水平,除传统的季度预测外,还需要进一步加强季节内尺度的预测。     

海温异常对台风形成的影响

本文利用地球流体力学实验室(GFDL)的低分辨气候模式进行数值试验,以研究海温异常对台风形成的影响.试验采用恒定8月气候条件和海表温度(SST).海温异常(SSTA)被置于北太平洋不同区域.结果表明,台风生成频率在暖SSTA区明显增加.这是由于暖SSTA区低层辐合的增强一方面使低空气旋式环流和高空反气旋式环流加大,另一方面导致低层水汽向该区辐合,使潜热释放加强,对流加剧所致.此一机制被用于解释台风频率和ENSO事件的相关.在冷ENSO年份,西北和西南太平洋台风增多不仅是由于赤道东太平洋SST异常冷,还与西太平洋SST异常暖有关.     

On the Second-Year Warming in Late 2019 over the Tropical Pacific and Its Attribution to an Indian Ocean Dipole Event※

After its maturity, El Ni?o usually decays rapidly in the following summer and evolves into a La Ni?a pattern. However, this was not the case for the 2018/19 El Ni?o event. Based on multiple reanalysis data sets, the space-time evolution and triggering mechanism for the unusual second-year warming in late 2019, after the 2018/19 El Ni?o event, are investigated in the tropical Pacific. After a short decaying period associated with the 2018/19 El Ni?o condition, positive sea surface temperature anomalies (SSTAs) re-intensified in the eastern equatorial Pacific in late 2019. Compared with the composite pattern of El Ni?o in the following year, two key differences are evident in the evolution of SSTAs in 2019. First, is the persistence of the surface warming over the central equatorial Pacific in May, and second, is the re-intensification of the positive SSTAs over the eastern equatorial Pacific in September. Observational results suggest that the re-intensification of anomalous westerly winds over the western and central Pacific, induced remotely by an extreme Indian Ocean Dipole (IOD) event, acted as a triggering mechanism for the second-year warming in late 2019. That is, the IOD-related cold SSTAs in the eastern Indian Ocean established and sustained anomalous surface westerly winds over the western equatorial Pacific, which induced downwelling Kelvin waves propagating eastward along the equator. At the same time, the subsurface ocean provided plenty of warm water in the western and central equatorial Pacific. Mixed-layer heat budget analyses further confirm that positive zonal advection, induced by the anomalous westerly winds, and thermocline feedback played important roles in leading to the second-year warming in late 2019. This study provides new insights into the processes responsible for the diversity of El Ni?o evolution, which is important for improving the physical understanding and seasonal prediction of El Ni?o events.     

Recent Increased Warming of the Alaskan Marine Arctic Due to Midlatitude Linkages

Alaskan Arctic waters have participated in hemispheric-wide Arctic warming over the last two decades at over two times the rate of global warming. During 2008–13, this relative warming occurred only north of the Bering Strait and the atmospheric Arctic front that forms a north–south thermal barrier. This front separates the southeastern Bering Sea temperatures from Arctic air masses. Model projections show that future temperatures in the Chukchi and Beaufort seas continue to warm at a rate greater than the global rate, reaching a change of +4℃ by 2040 relative to the 1981–2010 mean. Offshore at 74°N, climate models project the open water duration season to increase from a current average of three months to five months by 2040. These rates are occasionally enhanced by midlatitude connections. Beginning in August 2014, additional Arctic warming was initiated due to increased SST anomalies in the North Pacific and associated shifts to southerly winds over Alaska, especially in winter 2015–16. While global warming and equatorial teleconnections are implicated in North Pacific SSTs, the ending of the 2014–16 North Pacific warm event demonstrates the importance of internal, chaotic atmospheric natural variability on weather conditions in any given year. Impacts from global warming on Alaskan Arctic temperature increases and sea-ice and snow loss, with occasional North Pacific support, are projected to continue to propagate through the marine ecosystem in the foreseeable future. The ecological and societal consequences of such changes show a radical departure from the current Arctic environment.     

The Response of Anomalous Vertically Integrated Moisture Flux Patterns Related to Drought and Flood in Southern China to Sea Surface Temperature Anomaly

With the extreme drought (flood) event in southern China from July to August in 2022 (1999) as the research object, based on the comprehensive diagnosis and composite analysis on the anomalous drought and flood years from July to August in 1961-2022, it is found that there are significant differences in the characteristics of the vertically integrated moisture flux (VIMF) anomaly circulation pattern and the VIMF convergence (VIMFC) anomaly in southern China in drought and flood years, and the VIMFC, a physical quantity, can be regarded as an indicative physical factor for the "strong signal" of drought and flood in southern China. Specifically, in drought years, the VIMF anomaly in southern China is an anticyclonic circulation pattern and the divergence characteristics of the VIMFC are prominent, while those are opposite in flood years. Based on the SST anomaly in the typical draught year of 2022 in southern China and the SST deviation distribution characteristics of abnormal draught and flood years from 1961 to 2022, five SST high impact areas (i.e., the North Pacific Ocean, Northwest Pacific Ocean, Southwest Pacific Ocean, Indian Ocean, and East Pacific Ocean) are selected via the correlation analysis of VIMFC and the global SST in the preceding months (May and June) and in the study period (July and August) in 1961-2022, and their contributions to drought and flood in southern China are quantified. Our study reveals not only the persistent anomalous variation of SST in the Pacific and the Indian Ocean but also its impact on the pattern of moisture transport. Furthermore, it can be discovered from the positive and negative phase fitting of SST that the SST composite flow field in high impact areas can exhibit two types of anomalous moisture transport structures that are opposite to each other, namely an anticyclonic (cyclonic) circulation pattern anomaly in southern China and the coastal areas of east China. These two types of opposite anomalous moisture transport structures can not only drive the formation of drought (flood) in southern China but also exert its influence on the persistent development of the extreme weather.     

Modulation of the Pacific Decadal Oscillation on the summer precipitation over East China: a comparison of observations to 600-years control run of Bergen Climate Model

Observations show that the summer precipitation over East China often goes through decadal variations of opposite sign over North China and the Yangtze River valley (YRV), such as the “southern flood and northern drought” pattern that occurred during the late 1970s–1990s. In this study it is shown that a modulation of the Pacific Decadal Oscillation (PDO) on the summer precipitation pattern over East China during the last century is partly responsible for this characteristic precipitation pattern. During positive PDO phases, the warm winter sea surface temperatures (SSTs) in the eastern subtropical Pacific along the western coast of North American propagate to the tropics in the following summer due to weakened oceanic meridional circulation and the existence of a coupled wind–evaporation–SST feedback mechanism, resulting in a warming in the eastern tropical Pacific Ocean (5°N–20°N, 160°W–120°W) in summer. This in turn causes a zonal anomalous circulation over the subtropical–tropical Pacific Ocean that induces a strengthened western Pacific subtropical high (WPSH) and thus more moisture over the YRV region. The end result of these events is that the summer precipitation is increased over the YRV region while it is decreased over North China. The suggested mechanism is found both in the observations and in a 600-years fully coupled pre-industrial multi-century control simulations with Bergen Climate Model. The intensification of the WPSH due to the warming in the eastern tropical Pacific Ocean was also examined in idealized SSTA-forced AGCM experiments.     

与广东持续性干旱事件有关的两类海温异常型

利用1961\_2005年广东省86个常规地面观测站降水资料、 ERSST全球月平均海温资料以及NCEP/NCAR 大气多要素再分析资料, 分析了广东跨季节持续性干旱事件(下称广东干旱)与海温外强迫之间的关系及其物理过程。结果表明, 与广东干旱相关的海温异常有赤道东太平洋海温负异常型（即La Nia型）、 赤道中太平洋海温正异常型两种类型。约在1975年之前, 广东干旱基本上属于La Nia型; 在1975年之后, 除了La Nia型外, 赤道中太平洋海温正异常型干旱出现概率明显增加。可见, 在全球气候变暖的背景下, 广东干旱成因更复杂。近几年来广东干旱频繁发生, 与赤道中太平洋海温正异常型出现概率增加有关。La Nia型、 赤道中太平洋海温正异常型与广东干旱之间都存在物理上的联系, 不同的海温异常型对广东干旱影响的物理过程有所不同, 即使同一种海温异常型对不同季节的影响过程也有所不同, 然而所有物理过程最终都是通过改变和削弱降水所需的动力条件或水汽条件, 从而造成局地干旱的发生。