The 10–20-Day Mode of the 1979 Indian Monsoon: Its Relation with the Time Variation of Monsoon Rainfall

Abstract

The synoptic structure of the 10?20-day monsoon mode and this intraseasonal monsoon mode's relationship with the Indian monsoon rainfall are examined with the 1979 summer First GARP Global Experiment IIIb data of the European Centre for Medium-Range Weather Forecasts and the daily 1° ? 1° rainfall estimates retrieved from the satellite data by the Goddard Laboratory for Atmospheres. The major findings of this study are as follows. 1) The 10?20-day monsoon mode exhibits a double-cell (either double-high or double-low) structure; one cell is centered at about 15°?20°N and the other at the equator. 2) Both cells of the 10?20-day monsoon mode propagate coherently westward along the Indian monsoon trough and along the equator, respectively. 3) Based upon the zonal wind and local Hadley circulation, the vertical structure of the 10?20-day monsoon mode does not exhibit a phase change. 4) A significant rainfall occurs around low centers of the 10?20-day monsoon mode through the modulation of this monsoon mode on the Indian monsoon rainfall. 5) The northern-cell lows are initiated from the redevelopment of the westward-propagating residual lows over the Bay of Bengal when the 30?60-day transient monsoon trough is present there. In addition to these findings, the possible genesis and westward-propagation mechanisms of the 10?20-day monsoon mode are also discussed in this study.