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

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 140oE. 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 15oN in the South China Sea, and large amounts of water vapor are consumed by deep cumulus convection.