annual and interannual variabilities of East Asian Monsoon revealed by spacebased observations

A significant portion of the world's population is under the influence of monsoon changes. Better understanding of physical processes governing the evolution of Asian monsoon is not only important for regional climate variation, but also crucial to global climate variation. Spaceborne microwave scatterometers and radiometers observe ocean surface with high spatial and temporal resolution, which provides an unprecedented opportunity to explore variability at the sea surface on various time scales. Spacebased measurement including surface wind vectors, sea surface temperature (SST), integrated water vapor, cloud liquid water, and precipitation rate are analyzed to study the intraseasonal to interannual fluctuations of the East Asian monsoon and the South China Sea (SCS) monsoon.

Both subseasonal variation during boreal summer and seasonal variation over the SCS basin are consistent with the Ekman pumping scenario. Cyclonic winds drive surface divergence and upwelling in the ocean, inducing cooler SST. Whereas anticyclonic winds cause surface convergence, ocean downwelling, and warmer SST. On the other hand, significant interannual variabilities of East Asian monsoon and SCS monsoon are linked to the tropical Pacific SST. Strong SCS winter monsoon is associated with Pacific cold events, and weak monsoon linked to Pacific warm events. The SCS summer monsoon is very weak after strong Pacific warming events, whereas strong SCS summer monsoon is not always followed by Pacific warming in the winter.

During the winters of Pacific warming events, the anticyclone over Philippine Sea persists to the following spring or summer. The wind anomalies bring warm and moist air to land, affecting the east Asian winter and pre-monsoon rainfall. Winter and spring become anomalously wet in southeastern/eastern China and southern Japan. Onset of the Meiyu-baiu is earlier than non-ENSO years, and more intense rainfall is expected during the pre-monsoon season. The precipitation anomalies associated with wind convergence bear similar spatial pattern with the climatological Meiyu-Baiu rain band.