pairs examined. During the summer of 1964, Swallow and Bruce (1966), using 

 both directly measured current meter observations and geostrophic calculations, 

 found a reversal of the northeasterly near surface currents at depth between 

 300 m and 400 m. Below this depth they found southwesterly currents. 



VI. GEOSTROPHIC CURRENT AND SALINITY CROSS-SECTIONS 



Figure 36 shows geostrophic currents along section 1 (cm sec"') calculated 

 relative to 1500 dbar. Station 1 (00° 03.0'N, 50° 54.rE) was omitted from 

 the current section because of its proximity to the equator where geostrophy 

 is indeterminate. Surface geostrophic currents setting northeastward in 

 excess of 300 cm sec"l were measured between stations 5 and 6. In the upper 

 100 m between station 5 and 6 currents are in excess of 200 cm sec"', yet 

 decrease rapidly with depth; at 400 m the current is only 29 cm sec"'. The 

 strong current within 40 nmi of the coast results from cold, dense water 

 adjacent to the coast and the attendant low steric anomalies. Because the 

 zone of rapid northeasterly setting currents is confined to a narrow strip 

 about 80 nmi in extent immediately adjacent to the Somali Coast, it is reason- 

 able to attribute this coastal circulation to upwelling induced by prevailing 

 southwesterly monsoon winds paralleling the coast. The TIROS-N Satellite 

 infrared image for 18 August 1979 (figure 2), which is contemporaneous with 

 Section 1, reveals a patch of cold upwelled water in the vicinity of station 6 

 (03° 56.7'N, 48° 07.6'E). Further offshore the current reverses and becomes 

 southwesterly between stations 3 (2° 13.0'N, 49° lO.l'E) and 4 (3° 0.5'N, 48° 

 33.3'E). This southwesterly flow is associated with the southern edge of the 

 Great Whirl and the northern edge of the Southern Eddy. Between station 2 (00° 

 56.8'N, 50° 02.0'E) and 3 the flow is again northeasterly-setting below 100 m 

 indicating the northern edge of the Southern Eddy. It should be remembered 

 that this section was taken when the Great Whirl and the Southern Eddy were 

 merging. 



The salinity analysis made from this section (figure 37) reveals a rather 

 irregular and chaotic salinity field. Of particular interest is the shallow 

 salinity maximum observed at stations 1 and 2 between 60 m and 120 m depth. 

 Although a similar salinity maximum observed in the equatorial Atlantic is 

 associated with the eastward-setting Equatorial Undercurrent, Bruce (1973a) 

 found the shallow salinity maximum straddling the Western Indian Ocean equator 

 during the southwest monsoon season to be coincident with westward-setting 

 currents. This salinity maximum is seen to be associated with weak (10-20 cm 

 sec"') southwesterly-setting geostrophic currents. This saline (S>35.50 °/oo) 

 water centered about the 24.0 o^ surface has its origin in the Arabian Sea 

 which had been subjected to continuous strong winds for about three months. 

 The southwest monsoon season is a season of cooling rather than heating at the 

 sea surface owing to strong evaporative cooling induced by persistent south- 

 westerly monsoon winds. Large areas of the Arabian Sea are subject to a net 

 sea-air heat flux in excess of 400 cal cm~2 day"^ which exceeds the net 

 radiation balance in the area. Colon (1964) calculated values for the net 

 sea-air heat flux at 11° 30'N, 58° 30'E during June to be as high as 690 cal 

 cm-2 day-'. The salinity maximum (S>35.40 °/oo) at station 6 at 130 m depth 

 probably results from entrainment of near-surface Arabian Sea water along the 

 southern edge of the Great Whirl into the northeastward-setting coastal 

 current. 



14 



