92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Tuesday, 24 January 2012: 12:00 AM
Applications of Satellite Observations of Outgoing Longwave Radiation and Sea Surface Temperature to Studies on the East Asian-Western North Pacific Summer Monsoon Circulation and Rapid Phase Transition
Room 245 (New Orleans Convention Center )
Ming-Dah Chou, National Central University, Chung-Li, Taiwan; and C. H. Wu

The subtropical western North Pacific (WNP) during May through mid-July is under the influence of the NP high pressure system, and the sea surface temperature (SST) is high but the atmosphere is stable and clear. In late July, the East Asian (EA) - WNP monsoon undergoes rapid transition when the western stretch of the NP high shifts northward by ~ 10 latitude to the south of Japan. Prior to the rapid monsoon phase transition, high-level clouds were detected from satellite observations of the outgoing longwave radiation and the reflected solar radiation in the subtropical WNP. During the monsoon phase transition, the mid-oceanic clouds/convections descended from the upper troposphere to the lower troposphere and expanded westward to cover the entire subtropical WNP. Reanalysis data showed that the mid-oceanic convections were induced by a weakened upper-level jet stream in mid-latitudes. The weakening of the jet stream was related to the reduced gradient of the geopotential height on the northern flank of the Asian high. Prior to the monsoon transition, the Asian high weakened along the ridge but strengthened in the mid-latitudes. The former was related to a weakening of the South Asian monsoon circulation, while the latter was related to the maximum heating of land in late July, which has a one-month lag of the sun's position.

The satellite-inferred SST surrounding South and East Asia was used to investigate interannual variations of Asian summer monsoon circulation. Taiwan is located at the western stretch of the NP high pressure ridge, and its climate is highly sensitive to the NP high. Summer precipitation in Taiwan was used as an index for the monsoon circulation in the EA-WNP. Satellite observations of SST showed that when summer precipitation in Taiwan was anomalously high, the SST surrounding South and East Asia was anomalously low and the land-sea temperature contrast was large, leading to a strengthened summer monsoon circulation surrounding South and East Asia. In association with the strengthened summer monsoon circulation when summer precipitation in Taiwan was high, the western stretch of the NP High weakened. Weakening of the western stretch of the NP High induced strengthened southerly wind and enhanced vertical motion in East Asia and the western NP (EA-WNP) region. Corresponding to the invigorated circulation, precipitation increased in the southern section of the EA-WNP but decreases in the mid-latitude section of the EA-WNP. It was further found that in those wet years of Taiwan, the westerly wind in the tropical Indian Ocean and the southerly wind in the South China Sea and the subtropical East Asia all strengthened, which is an accelerated cyclonic circulation surrounding South and Southeast Asia.

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