2019年南海季风爆发异常偏早的机制分析

Mechanisms for the abnormally early onset of the South China Sea summer monsoon in 2019

利用美国国家环境预报中心/国家大气研究中心(NCEP/NCAR)逐日再分析资料及美国国家海洋和大气管理局(NOAA)逐日向外长波辐射、海温距平等资料诊断分析2019年中国南海季风爆发异常偏早的机制。结果表明:(1)南海季风爆发于5月6日,青藏高原和中南半岛热源较常年弱,对季风爆发无明显影响。(2)中高纬度环流中期变化过程中在青藏高原及附近区域形成为期两周的偏强高压脊,来自热带的暖平流以及青藏高原东部晴空辐射强等因素使其温度偏高,起到加快海陆热力差异季节转变进程的作用,对季风爆发至关重要。(3)孟加拉湾气旋“Fani”北上及登陆后迅速减弱后的残留低压的凝结潜热释放,“Fani”凝结潜热释放和气旋性环流增强诱发孟加拉湾西南季风爆发从而引起西南季风暖平流输送,因“Fani”而加强北跳的南亚高压反气旋环流的暖平流输送,“Fani”影响结束后西南季风与东亚冷槽后部回流辐合产生降水释放凝结潜热等因素,一步接一步,形成了加剧温度正距平的“接力”,最终导致中国南海地区温度梯度增强,越赤道气流增强,南海季风爆发。(4)厄尔尼诺及赤道西太平洋实时海温阶段性正距平增大使得西北太平洋副热带高压偏西偏强,本身不利于季风爆发;但其西端的偏东风在赤道印度洋与中高纬度南下的偏北气流辐合,并在赤道印度洋和孟加拉湾海温正距平阶段性增强的背景下得以强烈发展,生成孟加拉湾气旋“Fani”,其在北上过程中发展成为台风,由此引起大气环流一系列变化,最终导致南海季风的偏早爆发。

Detailed process and mechanisms of the abnormally early onset of the South China Sea Summer Monsoon(SCSSM)in 2019 were analyzed using the NCEP/NCAR reanalysis daily data and NOAA daily OLR and SSTA data.The results are as follows:(1)The SCSSM broke out on 6 May.The heat sources of Qinghai-Tibet Plateau and Indochina Peninsula are weaker than normal,and have little influence on the onset of SCSSM.(2)During the medium-range evolution of the atmospheric circulation,a warm high-pressure ridge formed and persisted for two weeks over the Tibetan Plateau and nearby.Under the influence of strong warm advection from the tropics and strong clear sky radiation in the eastern Qinghai-Tibet Plateau,this high-pressure ridge caused an abnormally high temperature in the middle-upper levels,which played a key role in the SCSSM onset by accelerating the process of seasonal change of land sea thermal difference.(3)The latent heat of condensation released by"Fani"during it northward movement and its residual low pressure after landing at northeastern Indian,the warm advection caused by the southwest monsoon in the Bengal that was triggered by condensational latent heat and the cyclonic circulation due to"Fani",the warm advection by the anticyclone circulation of the South Asian High(SAH)which was strengthened by"Fani",the condensational latent heat caused by convergence of southwest monsoon and backflow related to the East Asian cold trough after"Fani",step by step,formed a"relay"to strengthen the positive temperature anomaly,which eventually led to the increase of temperature gradient in the SCS,and resulted in the strengthening of the cross equatorial flows and the onset of SCSSM.(4)El Niño and periodic development of positive real-time SSTA in the equatorial western Pacific made the WNSPH stronger and located more westward,which is not favorable for the SCSSM onset.However,the easterly winds at the western WNSPH converged with northerly winds from the middle-high latitudes and formed a cyclone which grew dramatically under the background of strong positive SSTA in the equatorial Indian Ocean and the Bay of Bengal and produced"Fani",which developed to a typhoon and led to a series of circulation changes,and finally resulted in the extremely early onset of SCSSM.