Interdecadal change of South China Sea summer monsoon intensity in the mid-1990s

摘要

This study analysed the interdecadal variation of the South China Sea summer (June-September) monsoon (SCSSM) in recent 32 years (1979-2010). The SCSSM showed a strengthening interdecadal variability starting from 1995, which was also verified in time series of precipitation and precipitable water averaged over the SCSSM domain. Thus, 32 years of total analysis period was divided into two: first half of past 16 years (1979-1994, hereafter 7994) and recent 16 years (1995-2010, hereafter 9510) thereby analysing a mean difference between 9510 and 7994 periods to determine causes of the strengthened SCSSM since the mid-1990s. The spatial distribution of difference in outgoing longwave radiation, precipitable water, and precipitation between the two periods showed a typical La Nina pattern. The pattern which occurred during La Nina event has been verified through analysis on differences in 850- and 200-hPa stream flows between the two periods. At 850 hPa, anomalous huge anticyclones were strengthened in the both hemispheres so that anomalous easterlies were strengthened in the equatorial central and western Pacific. As a result, the SCS was affected by anomalous southeasterlies. On the contrary, anomalous huge cyclones were strengthened in the both hemispheres at 200 hPa so that anomalous westerlies were strengthened in the equatorial western and central Pacific. This means that rising air from the tropical western Pacific since the mid-1990s was moved to the east and then Walker circulation, which was descended at the tropical central and eastern Pacific, was strengthened. Due to the strengthened Walker circulation, La Nina events have been more dominant than before in recent years and as a result of this, SCSSM was more strengthened than before.