598 The Tropospheric Circulation 



currents at the sea surface derived from it (see ''Meteor''' Report VI §2, supplement 22) 

 with current charts derived from observations shows that the trade winds are the main 

 cause of the currents in the uppermost layer of the sea. These give rise to a total water 

 transport at right angles cum sole of the wind direction. In the Northern Hemisphere 

 the water '^flows towards west-north-west and in the Southern Hemisphere towards 

 west-south-west. Along the east coasts of continents and also at the eastern boundary 

 of the strong water displacements, which are directed from north to south along 

 the coast lines, water is accumulated and piled up and thus a pressure gradient is 

 created to the south-east in the Northern Hemisphere and to the north-east in the 

 Southern Hemisphere. This is shown clearly by the topographies of the pressure sur- 

 faces and of the sea surface, respectively. In the trade-vv-ind region the resultant ocean 

 current is then no longer solely due to the effect of the permanent air currents charac- 

 teristic for these latitudes, but is also affected decisively by the mass distributions in 

 the uppermost layers. A diagram of forces for the central part of the South Equatorial 

 Current according to the "Meteor" observations, has already been discussed (Fig. 180, 

 p. 424). It allows an estimate to be made of the effect of the individual forces in the 

 formation of this major current. It is of particular interest that the water masses in the 

 equatorial currents^ow against the slope of the physical sea level and the pressure surfaces, 

 that is to say, uphill. Part of the force transferred to the water by the winds is used 

 in overcoming this gradient, so that the velocities of the water displacement are 

 correspondingly somewhat reduced. 



The pressure field associated with the Equatorial Counter Current is clearly shown 

 in the topography of the physical sea level (Fig. 271) and in the topography of the 

 isobaric surfaces (Fig. 272). This current is undoubtedly an essential feature necessary 

 for the stability of the tropical current system. Its asymmetry about the equator is a 

 consequence of the displacement of the thermal equator into the Northern Hemi- 

 sphere and of the accompanying asymmetry of the atmospheric circulation (see p.463). 

 The main contributions to the theoretical explanation of the mode of formation of an 

 Equatorial Counter Current have been primarily due to Sverdrup (1932); Defant 

 (1935, 1941); Thorade (1941) and Palmen and Montgomery (1940). For an atmos- 

 pheric circulation assumed symmetrically about the equator, the Equatorial Counter 

 Current can be readily explained as a compensation current produced by the distur- 

 bances of the pressure field by a meridional continent opposing the wind drifts 

 corresponding to the North and South Equatorial Currents. It flows eastwards as a 

 gradient current in the direction of downward sloping sea level and is retarded only by 

 friction at the lower boundary surface and at both sides of the current. Stockman 

 {\9A6a-d) has attempted to consider also the baroclinic mass field, though without 

 taking into account the dependence of the Coriolis parameter on latitude. According 

 to this explanation the accumulation of water carried westwards and piled up by the 

 equatorial currents is the most important factor in the formation of the counter 

 current. The asymmetry of the counter current about the equator would then be due 

 to the asymmetry of the atmospheric circulation. Presumably for the Atlantic this 

 explanation of the counter current can be considered as an adequate one, but for the 

 considerably more extended Pacific it is doubtful whether the effect of the water accumu- 

 lation piled up in the west is sufficient in order to give rise to a counter current as a 

 very narrow band over such a great distance. 



