238 KNATTSS [CHAP. 10 



is stronger than the North Equatorial Current. In both, the South Equatorial 

 Current and the Countercurrent are stronger in the northern summer than in 

 winter, and in fact there is considerable question about the existence of a 

 continuous countercurrent during some of the winter months. The Deutsche 

 Seeivarte current chart of the world oceans (from which Fig. 2 is derived ; 

 Schott, 1943) shows no eastward flow in the Atlantic east of 20°W during 

 northern winter, and the concept of a continual countercurrent in the Pacific 

 during the winter months has been questioned by Cromwell (1958) and Knauss 

 (1958). 



The Indian Ocean is quite different from the Atlantic and Pacific. The wind 

 system is dominated by the monsoons. There is always a South Equatorial 

 Current (stronger in northern summer than in winter), but the current north 

 of the equator reverses with the season. 



B. "Sverdrwp" Transport 



In the light of the observed seasonal variations in both the equatorial currents 

 and the wind stress, it is of interest to consider to what extent the observed 

 changes in the current can be accounted for by seasonal change in the winds. 

 Sverdrup's integrated mass transport equations have come closest to describing 

 the observed transport in the equatorial Pacific (Sverdrup, 1947 ; Reid, 1948). 

 In Sverdrup's theory there is a net north-south flow, proportional to the curl 

 of the wind stress, £, and inversely proportional to /3, the variation in the 

 Coriolis parameter (^ = df/dy; /=2a>sin<p; a> is the angular velocity of the 

 earth; <p is the latitude, and y is positive to the north). The great zonal trans- 

 ports, M x , are a consequence of continuity requirements resulting from the 

 meridional changes in the curl of wind stress (see Fofonoff, Vol. I, Section 

 III, page 344) : 



«--J3(SK 



Wind velocities were read from charts similar to Fig. 1 for January and July. 

 Wind stress was calculated from the data of Van Dorn (1953), and the Sverdrup 

 transport was calculated from the above equation (Fig. 3). Except in the 

 vicinity of the equator there is rather good agreement with the surface current 

 charts of Schott. Specifically, the Sverdrup equations show that the North 

 Equatorial Current is stronger in January than in July and is strongest in the 

 western Pacific in both months. The countercurrent is farther to the north in 

 July than in January, is stronger in July than in January and is inclined 

 slightly to the east-northeast. 



The lack of agreement in the region of the equator is not surprising. First of 

 all, the comparison has been between surface currents and vertically integrated 

 mass transport, and it has been tacitly assumed that the speed of the surface 

 current is an index of its transport. This assumption is probably rather good 

 for most of the equatorial zone since the currents are generally shallow. How- 

 ever, it is not a good assumption within two degrees of the equator where the 



