西南地区冬季异常冷湿/暖干的气候背景分析

Analysis on Climate Background of the Abnormal Cold and Wet/Warm and Dry Winters in Southwest China

中文关键词:  西南地区冬季气候，冷湿/暖干气候配置，气候背景，青藏高原高压，贝加尔湖高压

英文关键词:Winter climate in SWC, cold-wet/warm-dry climate configuration, climate background, Qinghai Tibet Plateau High, Baikal Lake High

中文摘要:

      本文通过历史诊断分析和个例分析探寻西南冬季异常冷湿/暖干气候配置的关键环流系统和重要影响因子。结果表明,全区大部冷湿/暖干是西南地区最常出现的气候配置,是降水和气温多变量经验正交分析(MVEOF)第一模态,方差贡献率达34.4%。该模态PC系数呈显著下降趋势。青藏高原高压和贝加尔湖附近高压反向变化构成的“北正南负”(“南正北负”)偶极型环流形势,有(不)利于增强的偏北风将中高纬冷空气输送至西南地区,同时加深(减弱)南支槽,有(不)利于水汽从印度洋输送到西南大部地区,是西南冷湿(暖干)气候配置的关键环流形势场。两个环流系统彼此独立,影响来源不同。秋季阿留申群岛南侧海温偏高(低)通过激发太平洋北美型(PNA)波列,增强(减弱)冬季北大西洋涛动(NAO),激发(抑制)Rossby波辐散东传,进而使得下游贝加尔湖高压增强(减弱)。秋季澳大利亚西侧海温偏低(高)有利于马斯克林高压增强(减弱),进而增强(减弱)索马里和菲律宾越赤道气流,使得赤道东印度洋和海洋性大陆对流偏强(弱),通过局地Hadley环流增强(减弱)青藏高原高压,是冬季青藏高原高压强弱重要的前兆外强迫因子。上述外强迫因子以及环流系统是2021年冬季西南异常冷湿的关键原因。

英文摘要:

      This article explores the key circulation systems and important influencing factors of the abnormal cold- wet/warm-dry climate pattern of winter in Southwestern China (SWC) by historical diagnostic analysis and case studies. As the results shown, cold-wet or warm-dry is the most common winter climate configuration of SWC, which is the first mode of precipitation and temperature by multivariate empirical orthogonal function (MVEOF), contributing 34.4% of the total variance. While temporal coefficient of this mode decreases with time dramatically. Reverse variation of the Qinghai Tibet Plateau High and the High nearby Baikal Lake at the middle layers forms a circulation dipole pattern of ‘positive in north while negative in south’ (‘positive in south while negative in north’), which enhances (weakens) the northerly winds to transport cold air from high latitudes to SWC. Meanwhile, the southern branch trough is intensified (abated), which is beneficial (unfavorable) for water vapor transportation from Indian Ocean to the most parts of SWC. Such patterns are key to cold-wet or warm-dry climate configuration of winter in SWC. In addition, the two circulation systems are independent from each other and have different influence sources. In autumn, the higher (lower) sea surface temperature to the south of the Aleutian Islands (NPS) could stimulate the Pacific North American type (PNA) wave train and then lead the enhanced (weakened) North Atlantic Oscillation (NAO) in winter, which stimulates (suppresses) the eastward propagation of Rossby wave and thereby intensifies (reduces) the downstream Lake Baikal high-pressure. While the lower (higher) sea surface temperature to the west of Australia (AWS) in autumn is conducive to the strengthening (weakening) of the Mascarene High, and enhancing (weakening) the cross equatorial flow in Somalia and the Philippines. That would result in a stronger (weaker) convection in the equatorial East Indian Ocean and oceanic continents, and then strengthens (weakens) the Qinghai Tibet Plateau High through local Hadley circulation. The above external forcing factors and circulation systems are the key reasons for the abnormal cold and wet winter of SWC in 2021.