A Model Investigation of the Role of Air–Sea Interaction in the Climatological Evolution and ENSO-Related Variability of the Summer Monsoon over the South China Sea and Western North Pacific

Abstract

The summertime northeastward march of the climatological maritime monsoon over the South China Sea (SCS) and subtropical western North Pacific (WNP) is examined using the output from a 200-yr integration of a coupled atmosphere?ocean general circulation model (GCM). Increased cloud cover and surface wind speed during monsoon onset over the SCS in May?June reduce the incoming shortwave flux and enhance the upward latent heat flux at the ocean surface, thereby cooling the local sea surface temperature (SST). The resulting east?west gradient in the SST pattern, with lower temperature in the SCS and higher temperature in the WNP, is conducive to eastward migration of the monsoon precipitation over this region. Upon arrival of the precipitation center in the WNP in July?August, the local circulation changes lead to weakening of the mei-yu?baiu rainband near 30°N. The subsequent increases in local shortwave flux and SST impart a northward tendency to the evolution of the WNP monsoon. Many of these model inferences are supported by a parallel analysis of various observational datasets. The modulation of the above climatological scenario by El Niño?Southern Oscillation (ENSO) events is investigated by diagnosing the output from the coupled GCM and from experiments based on the atmospheric component of this GCM with SST forcings being prescribed separately in the equatorial Pacific, Indian Ocean, and SCS/WNP domains. During the May period after the peak phase of ENSO, the simulated monsoon onset over the SCS occurs later (earlier) than normal in El Niño (La Niña) events. These changes are primarily remote responses to the anomalous SST forcing in the equatorial Pacific and Indian Ocean. The ENSO-related changes in the SCS/WNP are associated with above-normal (below normal) mei-yu?baiu activity during warm (cold) events. In the ensuing July period of the warm events, the simulated precipitation response over the SCS to the local warm SST anomaly tends to oppose the remote response to SST forcing in the northern Indian Ocean. In the July period of cold events, the equatorial Pacific SST anomaly retains its strength and moves still farther westward. This forcing cooperates with the cold SST anomaly in the SCS in influencing the precipitation pattern in the SCS/WNP sector.