SECT. 3] EQUATORIAL CURRENT SYSTEMS 249 



currents would converge toward the equator. This slope is maintained by the 

 wind stress, and, because of the importance of the vertical stress term in the 

 surface layer, there may not be a geostrophic balance in the region of the mixed 

 layer ; but immediately below the mixed layer, in the upper part of the thermo- 

 cline, it seems possible that geostrophic balance might be achieved. In other 

 words, the high salinity tongues (Fig. 7) which converge toward the equator 

 along the top of the thermocline from both sides of the Atlantic and Pacific 

 might be explained by geostrophic currents resulting from the zonal tilt of the 

 sea surface caused by the easterly trades. The fact that these tongues are 

 absent in the Indian Ocean in the vicinity of the equator (Tchernia et al., 1958) 

 would be consistent with this idea since the Indian Ocean appears to slope up- 

 ward toward the east (Fig. 6). In these circumstances, there should be a di- 

 vergence at the top of the thermocline. An alternate explanation of the observed 

 salinity distribution, based on an eastward zonal flow of less saline water from 

 the Timor Sea, has been suggested by Rochford (1958). 



However, both Defant and Cromwell suggest that this high salinity tongue 

 in the South Atlantic and Pacific crosses the equator and can be found in the 

 Northern Hemisphere. Unless this is accomplished completely by horizontal 

 mixing, this transport is inconsistent with geostrophic flow. Fofonoff and 

 Montgomery (1955) in their theory of the Pacific Equatorial Undercurrent 

 (the Cromwell Current) considered such a geostrophic current. They calculated 

 an equatorward flow of 4 cm/sec at 3° latitude. 



B. Oceanic Fronts 



One of the arguments in Cromwell's picture of the meridional circulation is 

 the occasional appearance of a well-marked convergence zone north of the 

 equator — an oceanic front. Although such fronts, or convergence zones, have 

 been reported in other areas, they are a particularly striking phenomenon in 

 the equatorial Pacific where horizontal temperature gradients are usually so 

 small. So far as this writer is aware, they have only been reported in the Pacific 

 in the zone 5°S-5°N. 1 They are an intermittent phenomenon which makes an 

 observational program difficult, but of the many unsolved problems concerning 

 the dynamics of the equatorial circulation, they represent one of the most 

 interesting. 



Fronts in the equatorial Pacific have been described by Beebe (1926, chap. 2), 

 Cromwell (1953), Cromwell and Reid (1956) and Knauss (1957). In the Northern 

 Hemisphere they are characterized by a layer of cold water to the south in- 

 fringing on warmer water to the north. At the surface the front is usually 

 marked by a foam line, a color contrast on either side, a sharp increase in the 

 fauna and, apparently, a marked difference in the fauna on either side of the 

 front. The water appears to be sinking on the south, or cold, side of the front. 



On the one occasion where surface current observations have been made, 



1 The records of fronts between the equator and 5°S are apparently not very well 

 substantiated (T. S. Austin, in lift.). 



