Development and Life Cycle of the Indian Monsoon: Effect of the 30–50 Day Oscillation

Abstract

The velocity-potential fields generated from the FGGE III-b horizontal winds of the European Centre for Medium Range Weather Forecasts were subjected to an empirical orthogonal function (EOF) analysis to extract the annual cycle and the 30–50 day mode of the divergent circulations. We found that the Indian monsoon circulation is portrayed by the annual cycle of the divergent circulation and develops as a classical, giant sea-breeze model. On the other hand, this monsoon system is modulated by the planetary-scale 30–50 day low-frequency mode to establish an onset-active-break-revival-retreat life cycle. This modulation is accomplished through the following interaction process. The northeastward propagation of the planetary-scale 30–50 day mode over the Indian monsoon region induces transient local Hadley circulation. Through this type of circulation, the planetary-scale 30–50 day mode couples with and steers northward the low-level, 30-50 day monsoon troughs and ridges that originated around the equator. The northward migration of these low-level transient troughs and ridges cause, respectively, the deepening and filling of the monsoon trough over central India. The evolution of this monsoon trough results in the intensification and weakening of the Indian monsoon and its life cycle.