印度洋偶极子对中国南海夏季西南季风水汽输送的影响

Influence of the Indian Ocean Dipole on Water Vapor Transport from Southwesterly Monsoon over the South China Sea in the Summer

利用NCEP/NCAR再分析资料和中科院大气物理研究所PIAP3大气环流模式,分析了印度洋偶极子对夏季中国南海西南季风水汽输送的影响。结果表明,印度洋偶极子正位相期间夏季中国南海西南水汽输送较强,负位相期间则较弱。原因可归结为以下:正位相期间,MJO(Madden-Julian Oscillation)多活动于热带西印度洋,其向东传播受到阻碍,但经向传播明显,通常可传播至孟加拉湾地区,同时PIAP3显示印度洋季风槽位置偏北,且印尼以西过赤道气流较强,从而使得这一地区气旋性环流得到建立与加强。孟加拉湾地区对应着较强的对流活动以及深厚积云对流加热,从而通过对流加热的二级热力响应使西太平洋副热带高压位置向北推进,进而使得南海地区西南季风水汽输送得到建立与加强。在此期间孟加拉湾、中南半岛至南海地区对流活动较强,而苏门答腊沿岸对流活动受到抑制,由此增强了Reverse-Hadley环流,使低层经向风较强,进而增强了南海西南季风的水汽输送,PIAP3大气环流模式证实了Reverse-Hadley环流的增强。负位相期间,MJO多活动于热带东印度洋,在东传过程中受到Walker环流配置影响,在140°E赤道附近形成东西向非对称积云对流加热热源,其东侧Kelvin波响应加强了东风异常并配合副热带高压南缘东风压制了中国南海的西南季风水汽输送。在此期间,MJO在南海地区的经向传播较强,但经向传播常止步于南海地区15°N附近,虽携带大量水汽,但深厚积云对流强烈地消耗水汽使大气中水汽含量降低,PIAP3大气环流模式证实负位相期间深厚积云对流对水汽消耗加大,从而使得负位相期间南海地区水汽含量与正位相期间大体相近,但由于经向风不足使水汽向北输送较弱。

Based on the NCEP/NCAR reanalysis and the Atmospheric Circulation Model(ACM) PIAP3 model developed at the Institute of Atmospheric Physics(IAP), the influence of the Indian Ocean Diploe(IOD) on southwesterly monsoon moisture transport over the South China Sea in the summer is analyzed. The results show that in the positive phase of IOD, the Indian monsoon can pass through the Indochina Peninsula and enter the South China Sea, strengthening the southwesterly moisture transport in the summer. In the negative phase of IOD, the opposite is true. During the positive phase of IOD, the Madden-Julian Oscillation(MJO) activity is concentrated in the tropical western Indian Ocean, and its eastward propagation is blocked while its meridional propagation to the north is significant. It can propagate to the Bay of Bengal, leading to the establishment and strengthening of cyclonic circulation there. Meanwhile, the PIAP3 simulation shows that the monsoon trough in the Indian Ocean is located much more northward and the cross-equatorial flow to the west of Indonesia is stronger than normal, which correspond to active convection and strong heating in deep cumulus convection. The subtropical high shifts northward due to the secondary thermal response of deep cumulus convection, which is favorable for the strengthening of southwesterly water vapor transport over the South China Sea. Meanwhile, convective activities intensify in the Bay of Bengal, the Indochina Peninsula and the South China Sea but weaken in the coastal regiothajb,jqwwjqwwwlw n of Sumatra, which strengthens the Reverse-Hadley circulation and low-level meridional wind. As a result, the southwesterly water vapor transport over the South China Sea increases. During the negative phase of IOD, the MJO activity is found in the tropical eastern Indian Ocean, and its eastward propagation is significantly affected by the Walker circulation. The heat sources are unsymmetrical in the east-west direction about 140 oE. The Kelvin wave response in the east strengthens the easterly wind anomaly, which combines with easterly wind in the southern flank of the subtropical high to strongly suppress the Indian monsoon water vapor transport over the Indochina Peninsula and the South China Sea. Although the meridional propagation of MJO over the South China Sea is robust, it is blocked at around 15 oN in the South China Sea, and large amounts of water vapor are consumed by deep cumulus convection.