Relationship between the onset of the South China Sea summer monsoon and the structure of the Asian subtropical anticyclone

Abstract

The US National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data are employed to study the relationship between the variation in the structure of subtropical anticyclone and the summer monsoon onset over the South China Sea (SCS). The ridge surface of the subtropical anticyclone is defined by the boundary between westerly to the north and easterly to the south [or westerly-easterly boundary (WEB) in brief]. The WEB well represents the three-dimensional structure of the subtropical anticyclone. Under the thermal wind constraint, the WEB usually tilts toward the warmer zone in the vertical. In the Asian monsoon area, the ridgelines of the subtropical anticyclone at various levels in the troposphere in winter are continuous and exhibit a relatively zonal structure. The WEB tilts southward with increasing height and the mean position of the WEB is closer to the equator. In summer, the ridgelines of the subtropical anticyclone in the middle and lower (upper) troposphere are discontinuous (continuous) and the WEB either tilts northward or is vertical, with the mean position of the WEB being located at 25-30°N. May is the period when the structure of the subtropical anticyclone varies most significantly. During the seasonal transition, the tilt of the WEB changes from southward to northward. When the WEB becomes perpendicular to the earth's surface or tilts northward, the meridional temperature gradient vanishes or becomes positive, implying the replacement of the winter monsoon by the summer monsoon. Defining the "seasonal transition axis" (STA) as the vertical ridge axis that results from the switching of the tilt of the WEB from southward to northward, it is found that the STA first establishes over the eastern Bay of Bengal and Indochina Peninsula during the first pentad of May. The onset of the Asian summer monsoon circulation is therefore closely associated with the location of the STA. When the STA reaches the northeastern SCS in the fourth pentad of May, the SCS summer monsoon occurs. The south Asian summer monsoon onset corresponds to the establishment of the STA over central India in the first two pentads of June. Based on the close relationship between the STA and monsoon onset, the area-averaged meridional mean upper tropospheric (200-500 hPa) temperature gradient in the vicinity of the WEB is suggested to be an index to define the SCS summer monsoon onset. In comparison with the onset dates as determined by the area-averaged zonal wind at 850 hPa and OLR, the dates determined by these three indices are consistent in a large number of individual years, which indicates that the establishment of the STA, or the reversal of the meridional temperature gradient, captures the essential feature associated with the SCS summer monsoon onset. © 2004, Meteorological Society of Japan.