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INTRODUCTION 



This report deals with the low level thennal stratification of the 

 atmosphere over the Arabian Sea during the summer monsoon season, its 

 relation to the water temperature field, and to the general atmospheric 

 circulation in the area. In order to understand better the problems involved, 

 we should review briefly the general characteristics of the Indian monsoon 

 circulation, although these are fairly well known to most readers . 



The mean circulation near the surface over the Arabian Sea, once the 

 southwest monsoon current is well established, can be illustrated by the 

 mean chart for August (Figure l). The monsoon circulation is usually 

 established over the northern Indian Ocean by early June. The flow pattern 

 shown in Figure 1 persists with little variation from June until late 

 September. The surface flow over the Indian Ocean northward from about 

 latitude 20 S consists essentially of a current formed by the southeast trades 

 of the southern Indian Ocean turning in a clockwise direction near the equator 

 to continue as a broad southwest current over the Arabian Sea into the Indian 

 subcontinent and southeast Asia. 



During the I963 season we had the opportunity to analyze and study the 

 Indian monsoon weather while stationed in Bombay i)articipating in the 

 activities of the International Indian Ocean Expedition. One of the features 

 noted with great interest was the persistent and steady character of the 

 circulation over the Arabian Sea which showed very little interdiumal varia- 

 tions in a picture that day after day differed little from what appears in 

 Figure 1. The daily charts showed important variations in wind speed, but 

 only small variations in the direction of flow. 



The establishment of the southwest monsoon current over the Arabian Sea 

 brings about significant change in the oceanic circulation and distribution 

 of water temperatures. There is as a result a large variety of air-sea 

 interactions going on over this oceanic area during the monsoon, with signifi- 

 cant effects on both the properties of the oceanic surface layers and of the 

 atmospheric layer above . Some of these effects were studied in an earlier 

 report (Colon, 196^) . The Arabian Sea during the summer season presents as 

 varied and striking evidence of air-sea interactions as can be found anywhere 

 else in the globe . Some of these developments assume even greater importance 

 when considered in the light of the vast production of rain which normally 

 takes place a little farther downstream over India. 



Among the activities carried out during I963 in Bombay were a series 

 of aircraft flights over the Arabian Sea by the U- S. Weather Bureau-Research 

 Flight Facility and by the Woods Hole aircraft operated by Dr. Andrew Bunker 

 to investigate the monsoon flow over the ocean. One important reconnaissance 

 mission consisted of a flight upstream following the surface streamlines, 

 which in a period of 2 days covered the path all the way from Bombay to the 



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