Air-sea interaction on a seasonal scale over north Indian Ocean -Part II. : Monthly mean atmospheric and oceanic parameters during 1972 and 1973

Abstract

Monsoon-Indian Ocean interaction is examined in detail using 5-dcgree square monthly mean data of ocean and atmosphere of two consecutive years of contrasting monsoon rainfall, 1972 and 1973. There was a major monsoon failure (drought) in 1972; in 1973 India had excess rainfall during the monsoon season.

It is found that north Indian Ocean is colder than normal (negative SST anomaly) during the months prior to monsoon of 1972" During this –period upper tropospheric sub-tropical westerlies (monthly mean) intruded equatorwards over soJ1tlfAsia, farthest south over the Arabian Sea longitudes. The weak monsoon of June to Septemb~72 produced a warm SST anomaly over tropical Indian Ocean on account of decreased cooling of the ocean surface layer during the monsoon season due to decreased upwelling, particularly off the coasts of Somalia and Arabia, decreased wind mixing, decreased evaporation, and decreased clo1.lding. The positive SST anomaly of large spatial extent thus created persisted from October 1972 to May 19-73. The upper tropospheric circulation over south Asia during this period had equatorial easterlies (instead of westerlies) and increased strength of the sub-tropical westerly jet stream over north India. These results agree with observations and GCM simulations over Pacific Ocean with warm SST anomalies. Monsoon of 1973 was good and cold SST anomalies again appeared over north Indian Ocean from October 1973.