Interannual variability of the 30?50 day activity in the Indian summer monsoon

Abstract

In order to study the interannual variability of the 30–50 day activity in the southwest monsoon, time series at seven levels over India were formed using the NMC global analysis for four summers (1977–1980). After 30–50 day bandpass filtering, graphs of zonal wind, relative vorticity and divergence were plotted. Vertical structure of the activity is complex between 100 kPa and 10 kPa. Activity at 85 kPa moved northward, steadily in the summer of 1979 but less so in the other three summers. At 20 kPa, propagation was northward in the summer of 1979 but southward in the summers of 1977 and 1980. Meridional propagation was not clear at 20 kPa during the summer of 1978. Monsoon onset and breaks are highly correlated with 30–50 day cyclonic and anticyclonic activity, respectively. In addition, about 90% of the depressions in the summers of 1977 and 1979, and about 70% of the depressions in the summers of 1978 and 1980 formed within regions of cyclonic vorticity on the 30–50 day time scale. Rotational circulations are much stronger than divergent circulations on this time scale in the NMC global analyses.