.SECT. 3] EQUATORIAL CURRENT SYSTEMS 241 



may be a real difference in the two oceans ; however, considering the scatter of 

 the individual values in Fig. 4, it might mean that a similar feature would be 

 found in the Atlantic if there were sufficient data to be averaged. 



The stability values from the Indian Ocean are of the same order as those of 

 the Atlantic and Pacific. 



B. Slope of Sea Surface 



If one assumes a level isobaric surface at some depth like 1000 m, the sea 

 surface slopes up to the west along the equator in both the Atlantic and the 

 Pacific. 1 The slope in the Atlantic is about 4 x 10~ 8 (Montgomery and Palmen, 

 1940) ; in the Pacific it is about 5 x 10~ 8 (Austin, 1958). The slope is completely 

 accounted for by the depth of the mixed layer and the distribution of density 

 within the thermocline (the mixed layer becomes deeper toward the west). 

 Hence, there is little or no isobaric gradient along the equator at 500 m. It is 

 believed that the slope of the sea surface toward the west results from the mean 

 wind stress of the easterly trades, and Montgomery and Palmen have shown 

 that the mean slope is approximately that which would be expected for the 

 mean wind stress. The slope of the sea surface is not constant across the Pacific : 

 and, as noted previously, neither are the winds. It can be shown that the zone 

 of greatest slope (6.5 xlO -8 between 120° and 170°W) is also the zone of 

 maximum easterly wind stress, using the data of Hidaka (1958) (Fig. 5). Austin 

 (1958) has suggested that there is a real difference in the slope between January 

 to June and July to December ; however, this difference is not apparent in 

 Fig. 5. 



As might be expected, the slope in the Indian Ocean is quite different from 

 the Atlantic and Pacific. The summer monsoon winds so dominate the circula- 

 tion pattern that the mean wind stress along the equator, averaged over the 

 year, is westerly rather than easterly (Hidaka, 1958). Likewise, the sea surface 

 slopes to the west rather than to the east (Fig. 6). There might be expected to 

 be marked seasonal variation in the slope of the sea surface, and the limited 

 data available suggest that this is the case although the variation appears to be 

 limited to eastern sections of the Indian Ocean. The fact that the Indian Ocean 

 is divided down the center to 1°S along 73°E by the Maldive Islands perhaps 

 complicates the slope as well as the circulation. 



C. Oxygen Minimum 



Because the thermocline is a zone of high stability, reduced mixing takes 

 place across it. The water in the mixed layer above the thermocline is high in 

 oxygen and low in phosphate and silicate. Below the thermocline the water is 

 low in oxygen and rich in phosphate and silicate (Fig. 7) (Montgomery, 1954; 



1 Because the meridional pressure gradients are very small near the equator, the zonal 

 gradient along the equator is usually considered to be a good estimate of the mean slope 

 of the equatorial waters. All data on which the statements in this section are based came 

 from stations within 80 miles of the equator. 



9— s. ii 



