Interdecadal variation of summer monsoon over the southern part of South Asia in mid‐1990s

Abstract

ABSTRACTThe time‐series of South Asian summer monsoon index for 1974–2013 showed distinct interdecadal variation thereby revealing a characteristic of reduction in the index since 1994. Thus, this study analysed mean differences between 1995–2013 period and 1974–1994 period. In the difference in outgoing longwave radiation between the two periods, convection was strengthened in the Arabian Sea in the southern part of South Asia and was weakened in Indian subcontinent and the Indochina peninsula. In the difference in 850 hPa stream flows between the two periods, anomalous anticyclonic circulations extended from the western North Pacific to the Indian subcontinent via the Indochina Peninsula. Moreover, anomalous cyclonic circulations were strengthened in the western sea of Australia and anomalous warm southeasterlies that crossed the Equator from the circulations were headed towards the Arabian Sea. In the difference in 200 hPa stream flows between the two periods, the Arabian Sea was affected by anomalous cold northwesterlies. Thus, considering the circulations at the upper and lower troposphere, convection was weakened in recent years due to the effect of anomalous anticyclone over the regions from the western North Pacific via the Indochina Peninsula to the Indian subcontinent at the lower troposphere whereas convection was strengthened in the Arabian Sea due to the formation of vertical thermal instability. After defining land and sea areas in the west of the southern part of South Asia and land and sea areas in the east of the southern part of South Asia, differences between sensible heat flux in land area and sea surface temperature in sea area were analysed. A difference between land and sea areas in the west of the southern part South Asia was increased in the west of the southern part of South Asia since the mid‐1990s whereas that in the east of the southern part of South Asia tended to be clearly decreased since the mid‐1990s.