Abstract
This chapter presents a comprehensive review of the seasonal march of the East Asian summer monsoon. First, the earliest onset of the summer monsoon over the South China Sea (SCS) and the Indo-China Peninsula occurring around mid-May or even earlier, is discussed in terms of low-level and upper-level wind, OLR and rainfall fields. A sudden change in these fields before and after the onset is revealed. This is characterized by a wind switch from low-level easterlies and high-level westerlies to low-level westerlies and high-level easterlies, the rapid growth of convection and increase in rainfall, and the eastward retreat of the subtropical high. The onset of the summer monsoon over the Indo-China Peninsula and the SCS is preceded by the development of circulation features and convective activity in the tropical East Indian Ocean and the Bay of Bengal. These are characterized by the development of a twin cyclone crossing the equator, the rapid acceleration of low-level westerlies, and a significant increase in both the areal extent and the intensity of convection.
Climatologically, the seasonal march of the summer monsoon displays distinct stepwise northward and northeastward advances. Over East Asia, two abrupt northward jumps and three stationary periods have been identified while over the western North Pacific three stages in the onset and advance of the summer monsoon have been identified. The noted East Asian monsoon rainy seasons, including the pre-summer rainy season over South China, Meiyu/Baiu over eastern China and Japan, and Changma over Korea occur normally during the stationary periods that are imbedded in the northward advance of the summer monsoon.
Major physical processes and mechanisms responsible for the onset and seasonal progress are discussed with special emphasis on three controlling factors and related physical mechanisms. The land-sea thermal contrast and the effect of the elevated heat source of the Tibetan Plateau are identified as pre-conditions for the abrupt onset of the Asian summer monsoon over the SCS and Indo-China Peninsula through the rapid reversal of the meridional temperature gradient. The arrival of the intra-seasonal oscillations (ISO) provides a triggering mechanism, along with the several phase-locking wet ISO phases. The intrusion of mid-latitude troughs into the northern SCS and central and northern Indo-China Peninsula is also seen to be another triggering mechanism that induces the convective activity through the release of potential instability, thus enhancing the monsoon trough there through the feedback process of meso-scale convective systems.
Description
SEASONAL MARCH OF THE EAST-ASIAN SUMMER MONSOON | East Asian Monsoon