The Monsoon over the Indian Ocean During Summer 1975. Part II: Break and Active Monsoons

Abstract

In the second part of this paper we examine the active and break cycle of the monsoon during summer 1975. The time evolution of the flow over the Arabian Sea shocks that three main accelerations of the circulation occurred during the 1975 monsoon. The circulation over the Arabian Sea responds more to the pressure difference between the Mascarene High and the monsoon trough than to pressure changes over central India alone. The intensity of the trade winds in the south Indian Ocean is also significantly positively correlated to the interhemispheric pressure gradient. The establishment of the strong circulation associated with the onset is followed with a lag of 2?3 days by a temperature drop [air temperature and sea surface temperature (SST)] of about 2°C. There is a significant positive correlation between the intensity of the wind over the Arabian Sea and monsoon activity over central India. Break conditions are associated with a weak interhemispheric pressure gradient, decrease of the wind intensity, and increase of SST over the Arabian Sea. During the 1975 summer monsoon, these variations of the monsoon activity related to the break and active cycles occur about every 30 days and affect a large part of the Indian Ocean. Time-longitude, time-latitude diagrams, cross-spectra of the pressure and of the components of the wind show that, in this period range, there are westward propagating perturbations in the Northern Hemisphere and northward propagating perturbations from the equator in the eastern Indian Ocean. When a ridge originating from the equator reaches India, break conditions prevail with weaker westerlies over the Arabian Sea, high pressure over India, and intense activity including the formation of vortices in the Southern Hemisphere equatorial trough. The reverse occurs when positive anomalies prevail over the northern Indian area. The sequence of events suggests that the westward propagating waves initiate the northward motion of the meridional component of the wind.