Relationship of the 850-500 mb thickness patterns over northern and southern hemispheres vis-à-vis the activity of the Indian summer monsoon

Abstract

The thickness between 850 and 500 mb over the Indian Ocean area have been determined from the SIRS data available on a real time basis. The five-day averages of thickness between the southern and northern hemisphere sub-tropical ridges, which give an idea of the main driving force for the Indian summer monsoon circulation were worked out for the monsoon period in 1975 and 1976 and their relationship with the monsoon activity examined.

The rainfall anomaly (percentage) of daily rainfall from 285 reporting stations in India having normals, the thickness difference between the locations (i) Lat. 17½ ⁰S, Long. 55⁰E and (ii) Lat. 32½ ^oN, Long. 80°E and the actual thickness at the latter point were plotted. There is a close correspondence between the three curves. A time lag of one to two days has been noted between the variations in the thickness curves and the rainfall curve. Thus it can be said that the fluctuations in the rainfall activity of the southwest monsoon over India as a whole could be correlated with the thickness difference in the lower troposphere between the sub-tropical ridges in the two hemispheres.