Air and water vapour transport across the equator in Western Indian Ocean during Northern Summer

Abstract

Magnitudes of net total northward fluxes of air and water vapour into the Arabian Sea across an equatorial section extending from 42° E to 75° E and from sea surface to the 450 mb level are computed for September 1963 and June through September 1964, a period during which a large number of ships' reports in the equatorial West Indian Ocean and Seychelles (04°36' S, 55°30' E) aerological data were specially available as part of the International Indian Ocean Expedition meteorology programme in addition to routine meteorological data. The computed values exhibit seasonal variation from 4.11 × 1012 tons/day for air and 3.34 × 1010 tons/day for water vapour during June to 3.99 × 1012 tons/day for air and 3.04 × 1010 tons/day for water vapour during September with a maximum of 5.02 × 1012 tons/day for air and 4.43 × 1010 tons/day for water vapour during July, in 1964. The transports during September 1963 are 2.52 × 1012 tons/day for air and 2.35 × 1010 tons/day for water vapour. The above equatorial fluxes for both air and water vapour are seen to constitute about 60–80% of the net total eastward fluxes across a meridional section of the same vertical extent as the equatorial section but extending from the equator to about 26° N roughly along the West coast of India. The computed value of the net total northward flux of water vapour during July 1964 is shown to be nearly double that computed earlier by Pisharoty (1965), which is 2.2 × 1010 tons/day. The difference is largely due to a stronger meridional component of wind used in the present investigation for the middle and western sectors of the equatorial section as suggested by Seychelles aerological data and ships' reports along the equator. Occurrence of very strong meridional components of winds in equatorial East Africa and Somalia during the northern summer has been reported by Findlater (1969). Strong evidence in support of large-scale cross-equatorial flow in the west Indian Ocean is found from radio-activity and satellite data.