1. INTRODUCTION 



A program of XBT measurements in the northwestern and equatorial Indian 

 Ocean has continued from the end of the southwest monsoon in 1975 through 

 1979. A series of temperature sections has been accumulated which, combined 

 with other surveys in the area, has given a considerable amount of information 

 about this region. The long-range scientific objectives have been to attempt 

 an understanding of the circulation characteristics particularly in the Somali 

 Basin and off the Arabian coast, in order to examine the time variations and 

 horizontal scale of the eddy circulation associated with the commencement of 

 the dominant southwest monsoon, the variation during an entire season and also 

 that from year to year. It is important to know whether certain preferred 

 modes exist in the current patterns, the decline of the flow upon cessation of 

 the monsoon winds, the changes in the heat content of the mixed layer, the 

 variations in the region of strong upwelling off the Somali and Arabian coasts, 

 and the changes occurring in the near-equatorial dynamic topography as a 

 result of the Somali circulation. 



These studies in the western Indian Ocean are important to the Navy 

 because considerable information about the unusually strong horizontal tempera- 

 ture gradients which develop during northern summer off the Somali and Arabian 

 coasts within a relatively short period (two to three months for full strength) 

 has been obtained. The surface currents associated with these gradients are 

 on the order of twice those found in the Gulf Stream, and volume transports in 

 the upper 200 m exceed that of the Gulf Stream. Variation in acoustic patterns 

 should be expected in this area. 



This program was set up for monitoring the thermal structure in the 

 northwestern Indian Ocean by means of XBT observations from tankers and other 

 available ships. It is a cooperative effort originated and managed by the 

 Woods Hole Oceanographic Institution and shared during 1979 by the U. S. Naval 

 Oceanographic Office. The University of Cape Town, South Africa, has coopera- 

 ted with and aided the observational program. This study has been part of 

 INDEX (Indian Ocean Experiment), a program designed for examining the circula- 

 tion dynamics by a series of oceanographic studies associated with the First 

 GARP Global Experiment (FGGE). 



2. BACKGROUND AND RESULTS 



A series of temperature sections along the tanker sea lane (figure la) off 

 the East African and Arabian coasts has allowed the observqtion of the seasonal 

 development of large eddies which occur during the period of strong southwest 

 monsoon winds in the northwestern Indian Ocean. For five consecutive years 

 (1975-1979), a large eddy described by Bruce (1968) was formed in the northern 

 Somali Basin between approximately 4°N and 12'-'N. 



This eddy appears to be the first to form in the region upon commencement 

 of the southwest monsoon; it is considerably larger and more energetic than 

 other eddies formed there during the year; and it has been observed to remain 

 in this location at least three months after cessation of the southwest monsoon, 

 It is first discernable from sections in late May or early June in the near 

 surface waters (0-100 m) and continues to intensify until late September or 

 early October at the end of the southwest monsoon. The fully developed eddy 



