Influence of the Eastern Indian Ocean Warm Pool Variability on the Spring Precipitation in China

The relationship between the variability of the Eastern India Ocean Warm Pool (EIWP) and the spring precipitation in China is studied in the paper based on an analysis of the Simple Ocean Data Assimilation (SODA) Sea Surface Temperature (SST) data, the reanalysis data of monthly grid wind field at 925 hPa with a resolution of 2.5° latitude and longitude from the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR),and the monthly mean rainfall data from 160 observational stations in China. The results show that there is a strong correlation between the EIWP variability and the spring precipitation in China. The area, volume and intensity indices of the EIWP are negatively correlated with the spring precipitation in southwestern China, while they are positively correlated with the spring precipitation in the rest of China, especially in the northeast. For this correlation between the EIWP variability and the spring precipitation in China, it is found that the correlative relationship is mainly connected with the variations of the moisture transport by the warm air flow, which is under the influence of the EIWP variability, into the inland of China in spring. Two causative factors may influence this transport. One is the variation of the moisture transport carried by the warm air flow from the Arabian Sea influenced by the EIWP variability. The other is the variation of the equator-crossing flow (70°-90°E) influenced by the EIWP anomaly in the previous winter which exerts its effect on the moist warm air transported from the Southern Hemisphere. The position and intensity of the Western North Pacific Subtropical High (WNPSH)variability caused by EIWP variation also influence the spring precipitation in China.

Contrast between the Climatic States of the Warm Pool in the Indian Ocean and in the Pacific Ocean

Based on the analysis of Levitus data, the climatic states of the warm pool in the Indian Ocean (WPIO) and in the Pacific Ocean (WPPO) are studied. It is found that WPIO has a relatively smaller area, a shallower bottom and a slightly lower seawater temperature than those of WPPO. The horizontal area at different depths, volumes, central positions, and bottom depths of both WPIO and WPPO show quite apparent signals of seasonal variation. The maximum amplitude of WPIO surface area’s seasonal variation is 58% larger over the annual mean value. WPIO’s maximum volume variation amplitude is 66% larger over the annual mean value. The maximum variation amplitudes of the surface area and volume of WPPO are 20. 9% and 20.6% larger over the annual mean value respectively. WPIO and WPPO show different temporal and spatial characteristics mainly due to the different wind fields and restriction of ocean basin geometry. For instance, seasonal northern displacement of WPIO is, to some extent, constrained by the basin of the Indian Ocean, while WPPO moves relatively freely in the longitudinal direction. The influence of WPIO and WPPO over the atmospheric motion must be quite different.

北半球夏季Hadley环流的主导模态及其变率

本文分析了1948~2007年北半球夏季Hadley环流的主导模态及其变率,结果表明:北半球夏季Hadley环流变率的主导模态包括两个赤道非对称模态,其主体分别位于北半球(简称为AMN)和南半球(简称AMS)和一个赤道准对称的模态(简称QSM),AMN和AMS主要表征Hadley环流的年代际变率部分,而QSM主要表征Hadley环流的年际变率部分。AMN的时间系数呈现明显的减弱趋势,AMS的时间系数则表现为明显的增强趋势,两个模态的年代际变率表明:北半球夏季Hadley环流发生了显著的年代际转型,在1970年代以前呈现"北强南弱"型,之后转变为"南强北弱"型。印度洋—西太平洋暖池和热带大西洋赤道带海温的异常增暖以及由热带大西洋和印度洋海温非均匀增暖形成的减弱的北半球大尺度经向海温梯度和加强的南半球大尺度经向海温梯度可能是导致上述Hadley环流变率的重要影响因子。不同于两个非对称模态,QSM模态的变率主要与热带东太平洋的海温以及Ni^no3.4指数有明显的线性关系,说明ENSO对夏季Hadley环流的影响主要是在年际尺度上。对Hadley环流年代际转型的进一步分析发现,其越赤道部分的减弱与东半球热带季风区经向越赤道环流的减弱有密切联系。相关和合成分析的结果显示,南海季风、南亚东区季风以及西非季风的强弱与越赤道环流异常有显著相关,热带季风在这些区域的减弱趋势很可能共同受到北半球夏季Hadley环流年代际转型中越赤道环流减弱的影响。然而,南亚西区季风与经向环流没有明显相关,同时也未呈现显著的年代际趋势,这一结果从环流的角度验证了Li and Zeng(2002)将南亚季风区划分为东区和西区的合理性。

东印度洋-西太平洋暖池的年代际变化特征研究

采用1958～2002年海洋同化资料SODA（Simple Ocean Data Assimilation）的海温场，定义了东印度洋。西太平洋永久性暖池（简称印．太暖池）指数，即不随季节变化的27．5℃等温面所包含的〉27．5℃的暖水体积或强度，并采用功率谱和小波分析的方法研究了其周期变化特征。结果表明，印度洋暖池和西太平洋暖池均具有显著的准10a的周期振荡和1976～1986年前后的年代际突变特征，暖池由1976年前的“冷”暖池转变为1986年后的“热”暖池；暖池指数的季节循环也存在显著的年代际突变特征，特别是西太平洋暖池在异常暖年代其季节变化还呈现出明显的增暖趋势；暖池三维结构的年代际变化主要表现为在暖年代热带南印度洋暖水的向西向南扩张和西太平洋暖池东边界的向东及北边界的向北扩张，暖异常主要分布在60m以浅的上混合层中暖池的东边界区域，而其下面的温跃层内则为更强的异常降温，垂向上表现出上暖下冷的斜压模态结构，而温跃层和混合层深度的变化在不同暖池区则有不同的特点，表明东印度洋暖池和西太平洋暖池的年代际变化可能由不同的机制引起，尚需进一步分析其海洋动力学和热力学过程。

太平洋和印度洋在南海夏季风爆发年代际变化中的作用

利用1951～1998年多种大气和海洋资料,研究了太平洋和印度洋在南海夏季风爆发中的作用。结果表明,影响南海夏季风爆发早晚的因素存在着年代际变化:1951～1970年,印度洋起主要作用;1970～1998年西太平洋起主要作用。该年代际变化主要是1970年前后北极涛动(AO)的跃变以及西太平洋副高强度变化的结果。1951～1970年间,AO指数为负值,北印度洋出现西风异常,同时西太平洋副高强度偏弱,有利于南海夏季风早爆发。此时,北印度洋纬向风成为控制南海夏季风爆发的主要因素。同时,南印度洋副热带偶极子(IOSD)对南海夏季风的爆发也具有一定影响。当IOSD为正偶极子[西南印度洋海表面温度异常(SSTA)为正,印度洋的其它区域为负]时,北印度洋盛行西风异常,南海夏季风爆发偏早;反之偏晚。1970～1998年间,AO指数为正值,北印度洋盛行东风异常,同时西太平洋副高强度偏强,不利于南海夏季风爆发。在此期间,西太平洋暖池热含量成为控制南海夏季风爆发的主要因素:当西太平洋暖池热含量为正异常时,南海夏季风爆发早,反之偏晚。

南印度洋海温偶极子型振荡及其气候影响

印度洋海表温度(Sea Surface Temperature,简称SST)的方差分析和相关分析表明南印度洋也存在一个海温偶极子型振荡,并定义了一个南印度洋海表温度异常偶极子指数.夏、秋季(南半球冬、春)的南印度洋偶极子指数与后期热带500hPa和100hPa高度场异常有显著而持续的相关,在冬、春达到最大,并可以持续到次年夏、秋.前期夏、秋季节的南印度洋偶极模对次年我国大陆东部夏季降水异常有显著的影响,对应偶极子正位相,次年夏季印度洋、南海(东亚)夏季风偏弱;副高加强且南撤、西伸,南亚高压偏强且位置偏东,易形成我国长江流域降水偏多,华南降水偏少;负位相年反之.后期冬季西太平洋暖池是联系南印度洋偶极子与次年我国夏季降水异常关系的一条重要途径.南印度洋偶极子表现出了明显的独立于ENSO(El Ni o-Southern Oscillation,简称ENSO)的特征.

Temporal and Spatial Characteristics of Summer Extreme Precipitation in Eastern China and Possible Causalities

In the past decades, with the increasing frequency of extreme weather and climate events, the world has suffered huge losses. Based on NCEP/NCAR reanalysis data and China regional precipitation data provided by China Meteorological Administration, the extreme precipitation events in eastern China are defined by relative threshold method, and the temporal and spatial characteristics of summer extreme precipitation in eastern China from 1961 to 2016 are analyzed by empirical orthogonal function (EOF), and the reverse distribution of extreme precipitation in the middle and lower reaches of the Yangtze River and south China by Indian Ocean warm pool is revealed influence. The results show that the total amount and frequency of extreme precipitation in summer are concentrated in the Yangtze River Basin and south China. EOF1 decomposition of extreme precipitation reflects the interannual oscillation characteristics of reverse spatial distribution in the Yangtze River Basin and south China. The time series corresponding to EOF1 has significant interannual characteristics. The Pacific-Japan (PJ) teleconnection pattern is a circulation system that significantly affects the spatial-temporal pattern of extreme precipitation in southern China. When the PJ pattern is in the positive phase, the anticyclone controls the south China region, and restrains the convective activity, which results in the decrease of extreme precipitation. The anomalous southwest wind to the south of 30°N and the anomalous northerly wind to the north of 30°N converge in the middle and lower reaches of the Yangtze River. Combining with the sufficient water vapor carried by the anomalous southwest airflow at the edge of anticyclone, it is more conducive to the formation of extreme precipitation. The east propagating Kelvin wave in the warm pool of the Indian Ocean is an important reason for the formation of the PJ pattern and finally the formation of extreme precipitation anomalies in China.

印度洋暖池热含量变化对低纬高原汛期降水的影响

利用美国Scripps海洋研究所提供的1961—2003年的海洋热含量再分析资料、低纬高原148站降水资料和NCEP/NCAR环流再分资料,采用EOF分析、相关分析、合成分析等方法研究了印度洋暖池热含量变化,及其对低纬高原6—8月降水的影响及其可能原因.结果表明,印度洋暖池6—8月热含量变化E0F分析第1模态为全场一致型,解释方差为28%.印度洋暖池热含量与中国低纬高原6—8月降水的关系主要体现为与云南北部和东部等地区的显著正相关,这种相关关系源于前期2—4月,且随时间的推移其影响范围不断扩大,至同期时达到最好.在印度洋暖池热含量偏高年,暖池区持续的加热异常在东侧对流层低层激发出反气旋式环流异常,造成副高西伸,从而在副高外围形成一条自孟加拉湾向低纬高原区域的经向水汽输送带,为低纬高原区域输送大量的水汽,从而造成低纬高原区域降水增多.相反,在印度洋暖池热含量异常偏低时,西南风水汽输送带较弱,水汽输送无法穿越山脉输送到低纬高原区,造成低纬高原汛期降水偏少.

为什么2014年没有发展成强El Nio

利用ICOADS重建的月平均海表温度（SST）、GODAS的月平均次表层海温（SOT）、向外长波辐射（OLR）以及NCEP/NCAR的再分析资料等数据,通过与1997/1998年El Nino的发展对比,分析了2013年下半年至2014年热带太平洋的海气演变特征,探讨了2014年未能发展出强El Nino的原因.主要结论如下：1）2013年9月~2014年2月,暖水在西太平洋暖池堆积,虽然满足了El Nino爆发的热力条件,但由于西风异常爆发较晚,且维持时间较短,并在2014年4月以后转为东风异常,抑制东传暖的Kelvin波,使得暖水缺少进一步东移的条件;2）从西风异常发生发展来看,2013年东亚冬季风较弱并且缺少赤道印度洋西风异常的支持,导致赤道西太平洋西风异常爆发较晚;3）2014年4月以后,缺少南北半球的经向风辐合和质量输送,导致赤道西风异常西退而后转为东风异常.此外,赤道印度洋也有着显著不同.在1997/1998强El Nino发展之前的1996年冬季,IOD位相和太平洋东风异常减弱消失,Walker环流上升支东移,暖水在西风异常的驱动下沿着温跃层向东移动,为El Nino的爆发提供条件.2013/2014年冬季缺乏这个条件。