the downstream velocity, v, and the horizontal pressure 

 gradient caused by the cross- stream slope, i, of the sea 

 surface: 



(2n sin ip ) V = ig (1) 



where n is the earth's rate of rotation, (p is latitude, 

 and g is gravity. The term in parenthesis is the Coriolis 

 parameter, and has a magnitude lO''^ sec" •'■ at 43°. In a 

 strong steady current, it is believed that the geostrophic 

 relation, equation (1), is in error by no more than 10%. 

 In the major currents it is observed that the surface currents, 

 as determined from ship drift, are in essential agreement 

 with the currents inferred from maps of geopotential . 



There are some areas, however, where it is obvious 

 that the observed surface current is not parallel to the 

 isobars shown in Figure 1; the outstanding example is the 

 1.0 contour in the Atlantic Ocean which goes straight into 

 the coast. Recent work by Anati (1971), based on the IGY 

 data in the Atlantic, gives the same result as shown on 

 the old map. The reason for this apparent conflict is that 

 in this part of the ocean the currents are weak and the 

 slope of the sea surface is small. As a result, the effect 

 of the wind stress on the sea surface (not included in the 

 geostrophic approximation, eqn. 1) is large enough to contribute 

 an effect that can be seen on these maps. Anati has shown, 

 for this region, that the differences between the observed 

 surface currents and the direction of the isobars on this 

 map can be quite well reconciled by taking the effects of 

 wind stress into consideration. Reid (1961) has also shown 

 a similar correspondence between the "geostrophic failures" 

 in the Pacific Ocean and the mean wind stress. 



In other words, the comparison with observed currents 

 shows that the maps are correct in regard to the physical 

 shape of the sea surface. Adjustments for the effects of 

 wind stress are required only for the purpose of inferring 

 currents through the geostrophic relation. We conclude 

 that, qualitatively, the general features shown on these 

 maps are correct. 



24-4 



