Study of relationship between wave transport and sea surface temperature anomaly(SSTA) in the tropical Pacific

Large-scale water transport is one of the key factors that affect sea surface temperature anomaly（SSTA） in the eastern equatorial Pacific（EEP）.The relationship between the wave transport in the tropical Pacific and the SSTA in the EEP is examined by different methods,including band-pass filtering,period analysis,correlation analysis,significant analysis,and empirical orthogonal function（EOF） analysis.We have found that the eastward shift of the wave transport anomaly in the tropical Pacific,with a period of 2 a and enhancing the transport of warm waters from the western Pacific warm pool,precedes the increase of sea surface temperature（SST） in the EEP.The wave transport and the SSTA in the EEP have a maximum correlation of 0.65 with a time-lag of 6 months（transport variation precedes the temperature）.The major periods（3.7 a and 2.45 a） of the wave transport variability,as revealed by the EOF analysis,appear to be consistent with the SSTA oscillation cycle in the EEP.Based on the first occurrence of a significant SSTA in the Ni？o 3 region（5°S–5°N,90°–150°W）,two types of warm events are defined.The wave transport anomalies in two types present predominantly the west anomaly in the tropical Pacific,it is that the wave transport continues transport warm water from west to east before the onset of the warm event.The impact of wave-induced water transport on the SSTA in the EEP is confirmed by the heat flux of the wave transport.The wave transport exerts significant effect on the SSTA variability in the EEP and thus is not neglectable in the further studies.

Sea Surface Temperature Anomaly and Precipitation Distribution in the Alagoas State of the Brazilian Northeast

Precipitation data of 17 pluviometrical stations in the Alagoas State of the Brazilian Northeast and global spatial distribution of the Sea Surface Temperature Anomaly (SSTA) were analyzed for the period of 1981-2007. Techniques of constructing composite charts for SSTA fields are used to study the interrelation between the ocean thermal state with precipitation more than 50 mm/24 h, 20 mm/24 h or without precipitation for six ambient regions of the state. The student test is used for estimating statistical characteristics of the composites. Synoptic-scale pattern analyses of the composites reveal strikingly different spatial distribution of SSTA within each composite. The El Niño Southern Oscillation cycle refers to the coherent, large-scale fluctuation of ocean temperatures. At the highest ambient regions during heavy precipitation days, more intensive SSTA was observed. The lowest anomalies were observed for all types of precipitation in the semi-arid region. Quantile analyses of NCEP/NCAR indexes of SSTA distribution, such as NATL, SATL, TROP and RNASA were used too. Positive SSTA values in tropical regions are associated with the highest possibility of precipitation formation. The SST interhemispheric north-south gradient in equatorial regions of the North and South Atlantic has direct influence on the precipitation formation in the Alagoas State.

Sea surface temperature anomaly in the tropical North Atlantic during El Nino decaying years

基于1979年到2016年多种再分析资料,本文分析了El Nino衰减年热带北大西洋的海温异常.结果表明,热带北大西洋海温在此期间呈显著变暖趋势.10次El Nino事件的合成结果表明热带北大西洋海温异常在El Nino事件峰值之后的春季达到最大值,并持续到夏季.一般而言,这种异常与三个因子有关,即El Nino,北大西洋涛动和长期趋势,能分别导致局地海温上升0.4℃,0.3℃和0.35℃.1983年和2005年的对比分析表明,尽管El Nino强度对春季北大西洋海温起到决定性作用,与长期趋势密切相关的前冬海温也很重要.此外,超前-滞后相关结果表明北大西洋涛动超前海温约2-3个月.比较两个冬季相反位相北大西洋涛动的年份(即1992年和2010年),表明北大西洋涛动也能调制北大西洋海温异常.冬季负位相北大西洋涛动能显著增强El Nino的强迫影响,反之亦然.换言之,如果北大西洋涛动与El Nino位相相合,衰减年北大西洋海温异常才更为显著.因此,为全面理解热带北大西洋海温变化,除长期趋势外,还必须考虑El Nino和北大西洋涛动的综合影响.

The effects of different sea surface temperature distributions over the western Pacific on the summer monsoon properties

A modified and improved primitive equation numerical model with p-sigma incorporated vertical coordinates is used to simulate the effects of different sea surface temperature distributions over the western Pacific on the summer monsoon properties. The different sea surface temperature (SST) distributions are automatically generated in the time integrations by using two different SST models, one of which is called the deep ocean model (DOM) and the other the shallow ocean model (SOM). The SST generated by the DOM has the distribution pattern of the initial SST which is similar to the pattern in the cold water years over the western Pacific, while the SST generated by the SOM has the pattern similar to that in the warm water years. The differences between the experimental results by using DOM and SOM are analyzed in detail. The analyses indicate that the most basic and important characteristics of the summer monsoon climate can be simulated successfully in both experiments, that means the climatic properties in the monsoonal climate regions are mainly determined by the seasonal heating, the contrast between the land and the sea, the topography, and the physical properties of the underlying surfaces. However, the differences between the two experiments tell us that the climatic properties in the summer monsoon regions in the cold water year and the warm water year do differ from each other in details. In the warm water year, the thermal contrast between the land and the sea becomes weaker. Over the warm water area, the upward motions are induced and the dynamical conditions favorable for the convective activities are formed, the Somali low-level cross equatorial current is somewhat weakened, while the cross equatorial currents, east of 90°E, are strongly strengthened, the precipitation amount in the tropical regions largely increases, and the precipitation over the coastal regions increases, too. However the precipitation over the southeast China and its coastal area decreases. The precipitation amount mainly depends on the strength of the convective activity.

Sea surface temperature anomalies in the South China Sea during mature phase of ENSO

Based on the 18-year(1993–2010) National Centers for Environmental Prediction optimum interpolation sea surface temperature(SST) and simple ocean data assimilation datasets,this study investigated the patterns of the SST anomalies(SSTAs) that occurred in the South China Sea(SCS) during the mature phase of the El Ni?o/Southern Oscillation.The most dominant characteristic was that of the outof-phase variation between southwestern and northeastern parts of the SCS,which was influenced primarily by the net surface heat flux and by horizontal thermal advection.The negative SSTA in the northeastern SCS was caused mainly by the loss of heat to the atmosphere and because of the cold-water advection from the western Pacific through the Luzon Strait during El Ni?o episodes.Conversely,it was found that the anomalous large-scale atmospheric circulation and weakened western boundary current during El Ni?o episodes led to the development of the positive SSTA in the southwestern SCS.

Long-term changes in sea surface temperature(SST)within the southern Levantine Basin

Knowledge of sea surface temperature(SST)behaviour is vital for long-term climate scenarios.This study highlights essential outcomes about the distinguishable and unsurprising warming of the SST along the southern border of the Levantine Basin.The analysis is based on monthly SST data for the period 1948-2018.The southern Levantine Basin has undergone SST increase,during the last 71 years.In this study,a consistent warming trend has been found for the analysed SST data series,with a rate of 0.04℃/a,i.e.,0.4℃/(10 a).From 1975 to 1991 the mean annual SST was 17.1℃,and this increased to be 19.2℃,over the period 2002-2018.Results revealed two opposite trends of variability:a decreasing trend(−0.06℃/a)over the period 1975-1991,and an increasing trend(0.2℃/a)from 2002 to 2018.Over the period 1948-2018,positive mean annual SST anomalies had an average of 1.8℃,and negative anomalies had an average of−1.1℃.The lowest SST total increase was found from January to April,with values about 0.03℃,while the highest warming appeared from June to September.The driving mechanisms behind the SST changes need to be more investigated,to understand the future trends and impacts of climate change in the Levantine Basin.

The low frequency oscillations of the sea surface temperature in the Equatorial Eastern Pacific and El Nino formation

-In this paper the variations of the sea surface temperature anomalies (SSTA) in the Equatorial Eastern Pacific are analysed. The results show that there are two peaks in the spectrum. One is the low frequency oscillation with a period of 3 - 5 years, and the other is the quasi-biennial oscillation. The former shows a westward migration in the warm episode of SSTA and the latter has the opposite trend. The El Nino events will be formed while the two frquency bands are in phase in the warming stage of SSTA in the Equatorial Eastern Pacific

Experiments in numerical modelling of the Pacific sea surface temperature anomalies

By using the atmosphere-ocean coupled model (CGCM) which is composed of a 2-level global atmospheric general circulation model and a 4-layer Pacific oceanic general circulation model developed in the Institute of Atmospheric Physics of Chinese Academy of Sciences, and two model climatological fields got from the two independent models' numerical integrations respectively, the Pacific sea surface temperature anomalies (SSTA) from 1988 to 1989 are simulated in this paper with observed atmospheric general circulation data and sea surface temperature fields as initial conditions and monthly coupling scheme. In order to remove systematic biases of the model climatological fields, interaction variables between atmosphere and ocean are also corrected simultaneously. The experiments show that the simulation results can be improved effectively if these interaction variables are corrected in spite of the fact that there always exist systematic biases in independent numerical simulations of atmospheric part and oceanic part within CGCM. The basic characteristics of the observed Pacific SSTA in September and October 1988 have been simulated by using the correction scheme, such as the negative SSTA domain in the whole E-quatorial Pacific east to 150°E and the positive SSTA domain in the Western Pacific, the northern subtropical Pacific and nearly the whole Southern Pacific. Further numerical simulations show that the model can simulate not only the SSTA in the Pacific and its seasonal variations but also its interannual changes (for example, La Nino event in the Equatorial Pacific terminated after May 1989) to a certain degree. Furthermore, some problems existing in experiment processes and what we shoud do in the following stage are also discussed and analysed in this paper.

Forecasts of South China Sea surface temperature anomalies using the Nio indices and dipole mode index as predictors

Based on an empirical orthogonal function (EOF) analysis of the monthly NCEP Optimum Interpolation Sea Surface Temperature (OISST) data in the South China Sea (SCS) after removing the climatological mean and trends of SST,over the period of January 1982 to October 2003,the corresponding TCF correlates best with the Dipole Mode Index (DMI),Nio1+2,Nio3.4,Nio3,and Nio4 indices with time lags of 10,3,6,5,and 6 months,respectively.Thus,a statistical hindcasts in the prediction model are based on a canonical correlation analysis (CCA) model using the above indices as predictors spanning from 1993/1994 to 2003/2004 with a 1-12 month lead time after the canonical variants are calculated,using data from the training periods from January 1982 to December1992.The forecast model is successful and steady when the lead times are 1-12 months.The SCS warm event in 1998 was successfully predicted with lead times from 1-12 months irrespective of the strength or time extent.The prediction ability for SSTA is lower during weak ENSO years,in which other local factors should be also considered as local effects play a relatively important role in these years.We designed the two forecast models:one using both DMI and Nio indices and the other using only Nio indices without DMI,and compared the forecast accuracies of the two cases.The spatial distributions of forecast accuracies show different confidence areas.By turning off the DMI,the forecast accuracy is lower in the coastal areas off the Philippines in the SCS,suggesting some teleconnection may occur with the Indian Ocean in this area.The highest forecast accuracies occur when the forecast interval is five months long without using the DMI,while using both of Nio indices and DMI,the highest accuracies occur when the forecast interval time is eight months,suggesting that the Nio indices dominate the interannual variability of SST anomalies in the SCS.Meanwhile the forecast accuracy is evaluated over an independent test period of more than 11 years (1993/94 to October 2004) by comparing the model performance with a simple prediction strategy involving the persistence of sea surface temperature anomalies over a 1-12 month lead time (the persisted prediction).Predictions based on the CCA model show a significant improvement over the persisted prediction,especially with an increased lead time (longer than 3 months).The forecast model performs steadily and the forecast accuracy,i.e.,the correlation coefficients between the observed and predicted SSTA in the SCS are about 0.5 in most middle and southern SCS areas,when the thresholds are greater than the 95% confidence level.For all 1 to 12 month lead time forecasts,the root mean square errors have a standard deviation of about 0.2.The seasonal differences in the prediction performance for the 1-12 month lead time are also examined.

Characteristics of Tropical Sea Surface Temperature Anomalies and Their Influences on the Onset of South China Sea Summer Monsoon

The characteristics of sea surface temperature anomalies(SSTAs)in the tropical oceans and their influences on the onset of South China Sea summer monsoon(SCSSM)have been studied.The anomaly of SST in tropical Pacific Ocean exerts persistence impact for one to three months on atmospheric circulations.If the warm pool becomes anomalously warmer during an earlier period from February to April,the SCSSM breaks out earlier,and vice versa.Singular value decomposition(SVD)and composite analysis have shown that,in La Ni a pattern,the convection over Western Pacific will occur earlier and be stronger than normal,which favors the convergence at a lower layer over Western Pacific,as well as the strengthening of upwelling branch of Walker circulation,leading to an earlier burst of westerly in the southern South China Sea.Moreover,the convection in Sumatra appears earlier than normal and favors the westerly evolution in eastern Indian Ocean,resulting in the splitting of the subtropical high belt and an early onset of SCSSM.However,the atmospheric circulation anomaly is reversed in El Nio pattern.

Possible Causes for the Persistence Barrier of SSTA in the South China Sea and the Vicinity of Indonesia

The persistence barrier refers to the lag correlation of sea surface temperature anomalies （SSTA） showing a rapid and significant decline in a specific season, regardless of the starting month. This implies that there is a decrease in forecast skill for SSTA in this specific season. This paper investigates the possible causes for the persistence barrier of SSTA in the South China Sea （SCS） and its adjacent regions from the perspective of interannuallinterdecadal time scales. The results show that the persistence barrier of SSTA exists not only in the SCS, but also in the vicinity of Indonesia south of the equator. The SCS barrier occurs around October-November, while the occurrence of the barrier in the Indonesia region is around November-December. For these two regions, the occurrence of the persistence barrier is closely associated with the interdecadal variability of SSTA, as well as the interannual variability. The persistence barriers in the SCS and the Indonesia region do not exist alone if the interdecadal variability is not considered, because SSTA have a short memory of less than 4 months, regardless of the starting month. Moreover, the influence of the interdecadal variability of SSTA on the persistence barrier of SSTA in the SCS and the Indonesia region may be associated with SSTA in the Indian Ocean and the western Pacific, but is not closely associated with the Pacific Decadal Oscillation. However, compared with the spring persistence barrier （SPB） of ENSO, the close relationship between the persistence barriers in the SCS and the Indonesia region and the interdecadal variability is unique, since the ENSO SPB is not significantly affected by such variability. In addition, although the persistence barriers in both the SCS and the Indonesia region are quite obvious in strong ENSO cases, the interdecadal variability of SSTA also plays a non-negligible role in this relationship.

A DIAGNOSTIC ANALYSIS OF AIR TEMPERATURE ANOMALY MODE OVER CHINA IN 2009/2010 WINTER BASED ON GENERALIZED EQUILIBRIUM FEEDBACK ASSESSMENT(GEFA) METHOD

By using the observed monthly mean data over 160 stations of China and NCAR/NCEP reanalysis data, the generalized equilibrium feedback assessment(GEFA) method, combined with the methods of EOF analysis, correlation and composite analysis, is used to explore the influence of different SST modes on a wintertime air temperature pattern in which it is cold in the northeast and warm in the southwest in China. The results show that the 2009/2010 winter air temperature oscillation mode between the northern and southern part of China is closely related to the corresponding sea surface temperature anomalies(SSTA) and its associated atmospheric circulation anomalies. Exhibiting warming in Northeast China and cooling in Southwest China, the mode is significantly forced by the El Nio mode and the North Atlantic SSTA mode, which have three poles. Under the influence of SSTA modes, the surface northerly flow transported cold air to North and Northeast China, resulting in low temperatures in the regions. Meanwhile, the mid-latitude westerlies intensify and the polar cold air stays in high latitudes and cannot affect the Southwest China, resulting in the warming there.

Revisiting the Seasonal Evolution of the Indian Ocean Dipole from the Perspective of Process-Based Decomposition

The seasonal phase-locking feature of the Indian Ocean Dipole(IOD)is well documented.However,the seasonality ten-dency of sea surface temperature anomalies(SSTAs)during the development of the IOD has not been widely investigated.The SSTA tendencies over the two centers of the IOD peak in September-October-November are of different monthly amplitudes.The SSTA tendency over the west pole is small before June-July-August but dramatically increases in July-August-September.Meanwhile,the SSTA tendency over the east pole gradually increases before June-July-August and decreases since then.The growth rate attribution of the SSTAs is achieved by examining the roles of radiative and non-radiative air-sea coupled thermodynamic processes through the climate feedback-response analysis method(CFRAM).The CFRAM results indicate that oceanic dynamic processes largely contribute to the total SSTA tendency for initiating and fueling the IOD SSTAs,similar to previous studies.However,these results cannot ex-plain the monthly amplitudes of SSTA tendency.Four negative feedback processes(cloud radiative feedback,atmospheric dynamic processes,surface sensible,and latent heat flux)together play a damping role opposite to the SSTA tendency.Nevertheless,the sea surface temperature-water vapor feedback shows positive feedback.Specifically,variations in SSTAs can change water vapor con-centrations through evaporation,resulting in anomalous longwave radiation that amplifies the initial SSTAs through positive feedback.The effect of water vapor feedback is well in-phase with the monthly amplitudes of SSTA tendency,suggesting that the water vapor feedback might modulate the seasonally dependent SSTA tendency during the development of the IOD.

“23·12”山东半岛特大海效应暴雪特征及成因

采用地面气象观测站、多普勒天气雷达、闪电、积雪深度人工加密观测资料、常规观测及欧洲中期天气预报中心(European Centre for Medium-Range Weather Forecasts,ECMWF)第五代大气再分析(ECMWF Reanalysis v5,ERA5)资料,对2023年12月15—22日山东半岛特大海效应暴雪过程的降雪特征及极端性成因进行了分析。结果表明:(1)此次过程有4站积雪深度突破本站历史极值,有1站2 d的日降雪量为山东半岛海效应降雪有气象记录以来的最大值,文登积雪深度达74 cm,超过山东所有国家级地面气象观测站纪录,是一次极端海效应暴雪事件。(2)欧亚中高纬度阻塞形势下两次异常强冷空气持续影响渤海和山东半岛地区,850 hPa温度最低降至-21~-20℃,冷空气强度明显强于往年12月海效应暴雪过程,造成降雪持续时间长、累计降雪量大。异常强冷空气是此次极端暴雪过程产生的关键因素,渤海海面温度(简称“海温”)异常偏高是有利的海温背景。(3)冷空气强、海温偏高造成海气温差偏大,700 hPa以下产生对流不稳定,使得降雪强度大;强降雪发生在海气温差快速增大阶段。(4)925 hPa以下存在来自渤海的北—东北风与内陆地区的西北风构成的切变线,产生强上升运动,切变线长时间维持形成“列车效应”。(5)主要降雪时段强垂直上升运动、高相对湿度层的温度为-20~-12℃,适宜树枝状冰晶形成和维持,有利于产生大的积雪和降雪含水比;2 m气温持续低于-5℃,0 cm地温在降雪开始时即降至0℃以下,且两次强降雪过程仅间隔1 d,均有利于降雪累积产生极端积雪深度。

全球热带SSTA与中国7月降水和气温的伴随相关型分析

用伴随相关型（ＡＣＰ）分析了中国７月降水和气温与全球热带ＳＳＴＡ的ＰＯＰ（主振荡型）间的关系，得到当两个典型的传播ＰＯＰ处在ＥｌＮｉｎｏ事件发展相位时中国夏季总体呈南北旱、中间涝的形势，其中江淮流域、华中、东北东部和西北大部为降水正距平，华北、华南为负距平，降水偏多（少）时相应的气温偏低（高）；当两个传播型的典型模态处于ＬａＮｉｎａ事件发展相位时情况则相反。

1750—2020年中国东北地区极端洪涝事件重建及驱动机制分析

全球变暖背景下区域极端气候事件的发生规律与归因是气候变化研究的重点领域。利用历史文献记载和现代降水数据识别了公元1750—2020年中国东北地区发生概率10%的极端洪涝事件,并分析了其发生特征及驱动机制。结果表明:(1)1750—2020年共识别出东北极端洪涝事件29年(次),显示出波动变化的特征,1871—1930年是过去270年间东北极端洪涝发生频率最高的时段;(2)中国和东北地区的增温显著增大了东北极端洪涝的发生频率;(3)多百年尺度上东亚夏季风强度变化引起的季风雨带异常是导致东北极端洪涝事件发生的主要原因。在东亚夏季风偏强、雨带位置偏北的时段,极端洪涝事件的发生概率会显著上升;(4)在多百年尺度上,印太暖池和热带中、东太平洋海温异常通过影响东亚夏季风的位置和水汽输送,引发东北极端洪涝事件的发生。其中,La Niña事件发生时的水汽输送正异常对于极端洪涝的发生至关重要,也是东北极端洪涝事件可靠的预报因子。

2020—2022年与2010—2012年两个“二次变冷”La Nina事件的特征与机制对比

厄尔尼诺-南方涛动(EI Nino-Southern Oscillation, ENSO)是由热带太平洋海-气相互作用产生的显著年际变率,其负位相的拉尼娜(La Nina)事件对中国极端天气有巨大影响。本文利用海洋、大气观测、再分析资料集,对比研究了2010—2012年和2020—2022年两个“二次变冷”La Nina事件演变过程中各阶段的物理过程和机制。结果表明,2010—2012年La Nina事件第一个峰值强于2020—2022年La Nina事件,但第二个峰值较后者弱,且其海表温度异常(Sea surface temperature anomaly, SSTA)偏西,该SSTA的强度和位置特征可从次表层海温异常得到进一步验证。另外,赤道太平洋的东风异常也在2010年夏秋季明显比2020年偏强、偏西;而偏东且增强的东风异常使2020—2022年La Nina事件的第二个冬季峰值强于2010—2012事件。通过海洋混合层热收支诊断分析,纬向平流反馈(海表面热通量异常)是引起La Nina事件发展(衰减)的主要因素。在发展阶段,东风异常引起的纬向海流异常将冷水向西输运,成为赤道中-东太平洋冷SSTA发展的主导因子,垂直方向温跃层反馈过程的贡献也不可忽略。本研究以近十年来的两个“二次变冷”La Nina事件为例进行定性和定量的对比分析,明晰了二者生命演变史过程中的物理机制,为探究La Nina事件生消机理和ENSO不对称性在全球变暖下的可能变化提供了一定的参考。

江南5月降水与热带印-太海温及对流的联系

本文将1980-2021年江南5月降水作为研究对象,采用美国国家气候预测中心(Climate Prediction Center,CPC)降水、美国环境预测中心(National Center for Environmental Prediction,NCEP)大气环流以及英国气象局哈德莱中心(Met Office Hadley Centre)海表面温度等资料,对江南5月降水年际变化特征及其与热带印度洋、太平洋的海表面温度异常(Sea surface temperature anomaly,SSTA)以及对流异常的可能联系进行了分析。结果表明,偏涝年通常伴随有自年前秋季开始发展的热带印度洋暖SSTA,5月同期对流层850 hPa在热带东印度洋至中国南海一带出现东风异常,同时热带印度洋上空对流活动旺盛,通过Hadley环流增强了在西北太平洋的下沉气流,共同增强了西北太平洋异常反气旋(Northwest Pacific anomaly anticyclone,WNPAC),促进了水汽向江南地区输送。热带印度洋中东部的冷SSTA自偏旱年前的冬季开始不断发展,在5月同期的热带印度洋异常冷洋面上空表现为对流抑制,与偏涝年相反的异常Hadley环流促使副高减弱东退,撤出南海,不利于水汽向江南地区输送。除热带印度洋外,热带太平洋和海洋性大陆地区的SSTA及其上空的对流活动异常与WNPAC和江南5月降水异常在一些年份也存在一定的关系,但较热带印度洋复杂。

泛南海地区极端降水的历史分布和未来演变特征

泛南海地区是全球海—陆—气相互作用最敏感的区域之一,该区域极端降水释放的潜热加热可以调节局地的温度和湿度廓线对大气环流进行调整,进而影响周边地区甚至全球的天气气候。因此,泛南海地区极端降水的时空变化特征及变异机理一直是国内外学者关注的焦点。本文利用观测数据(1951~2014年)和国际耦合模式比较计划第六阶段(CMIP6)两种共享社会经济路径(SSP1-2.6和SSP5-8.5)的统计降尺度数据(2015~2100年),分析了泛南海地区年平均和季节平均的日降水的最大值(RX1day)、连续5日降水的最大值(RX5day)、极端强降水天数(R20)和非常湿润天(R95p)的时空变化特征。RX1day、RX5day、R20和R95p常用于表征极端强降水、持续性强降水、极端强降水的频率和极端累计降雨量的特征。1951~2014年泛南海地区年平均和季节平均的四个极端降水指数的较大值均分布在东南亚、中国东南部以及青藏高原南坡地区,即这些区域不仅是极端强降水发生的区域,也是持续性强降水以及高频极端降水发生的区域。季节平均的极端降水指数特征表现为:东南亚一年四季都极易发生强降水、持续性强降水和高频极端降水;南亚、青藏高原以及东亚的各个极端降水指数在夏季最大,秋季和春季次之,冬季最小。SSP1-2.6和SSP5-8.5情景下2015~2100年泛南海地区年平均和季节平均的四个极端降水指数的空间分布与历史时期相似,且对整个区域而言,各个指数均呈显著增加的趋势。由各个指数在未来三个时段(2016~2035年、2046~2065年和2080~2099年)相比于1995~2014年的百分比变化可知,南亚和青藏高原是泛南海地区未来强降水、持续性强降水以及高频极端降水变化最显著的区域。由此可知,虽然东南亚是历史时期四个极端降水指数的大值区,但该区域各个极端降水指数在未来三个时段的变化没有其他区域明显。此外,以东南亚为例,本文分析了该区域1979~2019年夏季极端降水的形成机理,发现印度洋冷海温异常、热带北大西洋暖海温异常以及热带太平洋和大西洋海温异常是造成东南亚夏季极端降水呈北湿南干、全区一致偏湿和北干南湿的关键因子。

近20年北大西洋中纬度海温与江南5月降水关系增强的可能原因

本文以1980—2021年江南5月降水为研究对象,利用美国国家气候预测中心(Climate Prediction Center, CPC)降水资料、美国环境预测中心(National Centers for Environment Prediction, NCEP)大气环流和Hadley中心的海表面温度资料,分析了江南5月降水极端性的变化及其与大气环流、北大西洋海温的联系。结果表明,自2004年江南5月降水年际差异显著增大以来,200 hPa上的Rossby波能量从北大西洋向下游传播,一部分能量沿副热带西风急流传播,形成南支波列;另一部分沿不列颠群岛向西伯利亚地区传播,上述两支波列在旱涝年的不同配置一方面可以通过影响江南地区高空200 hPa附近的辐合辐散影响江南地区的垂直运动;另一方面波列下游的中国东南沿海200 hPa的异常气旋或反气旋具有准正压结构,对应于对流层中下层西北太平洋异常气旋或反气旋在孟加拉湾、中南半岛和南海上空位置和强弱变化,影响水汽向江南地区的输送。在偏涝年形成正位相的欧亚遥相关波列,利于冷空气在贝加尔湖附近积累并向江南地区输送,它和低纬暖湿气流在江南地区附近相遇,形成降水。2004年以来,欧亚大陆上空与5月不列颠群岛附近海温异常有关的波列分布发生了改变,影响了江南地区高空200 hPa的辐合辐散。上述遥相关波列增强了和西北太平洋异常反气旋的关系,进一步与江南5月降水建立起紧密联系。2004年以来自江南5月偏涝年前的冬季开始,北大西洋中纬度通常出现冷海温异常并随时间东移,于5月降水同期东移到不列颠群岛附近,偏旱年反之,这对江南5月降水具有一定的预报意义。