the depth integrated value of the meridional velocity usually does not vanish the suggestion of Montgomery and Palmen 1940 does not represent a proper explanation for the North Equatorial Countercurrent Sverdrup 1947 suggested that the North Equatorial Countercurrent was not only a consequence of the zonal wind stress but also was related to the curl of the wind stress and continuity requirements He assumed in his model that the currents were steady and vanished at a deep level The equations were integrated from this depth to the sea surface hence the solution gave no information about the vertical structure of the motion field The apparent success of Sverdrup s theory in the eastern Pacific is often cited as observational endorsement of its widespread use in large scale oceancirculation theory In light of the earlier discussion about the large variability in the position and amplitude of the NECC and the considerable monthly variation in the winds in this region it is surprising that this steady theory should be applicable there In fact Sverdrup 1947 and Reid 1948 instead of using the mean wind stress in N 110 70 30 0o 30 70 110 S 6 4 2 0 2 4 6 dyn cm 2 Figure 6 3 Profiles of zonal wind stress from 100 to 120 W illustrating the annual cycle To emphlasize the meridional tasize the meridional structure the weak monthly mean vallue of the equatorial stress the average of figures 6 i and 6 l was subtractedfrom the monthly mean value at each latitud e 1ovir rr rnoa hT ho L 1evLI rrterfein 3 tneir cumputatluns useu lilc uil oier lNUVllln values They also used October November oceanographic data to compute vertically integrated pressure Implicit in their theory then was the assumption that the oceanic response to the fluctuating winds was sufficiently rapid that the ocean was almost always in equilibrium with the instantaneous winds i e quasisteady 6 3 Theories 6 3 1 IntegratedTheories The earliest theoretical attempts sought an explanation for the North Equatorial Countercurrent This current defies intuition since it flows opposite to the prevailing winds Montgomery and Palm6n 1940 suggested that the easterly wind stress in the equatorial zone is balanced by the vertically integrated zonal pressure gradient They presented supporting observational evidence in the Atlantic One interesting result was their demonstration that the baroclinic pressure gradients generally were confined to the top few hundred meters of the water column As an explanation for the North Equatorial Countercurrent they hypothesized that in the doldrums i e in the vicinity of the ITCZ where the magnitude of the zonal stress is greatly reduced the pressure gradient would maintain the value that it had on either side of this region and hence no longer be balanced by the wind stress As a result an eastward flow would develop which they suggested would be retarded by lateral friction Charts of dynamic topography Tsuchiya 1968 indicate that the zonal pressure gradient in this region is in fact less than it is farther to the north and south Furthermore since 1 or 2 off the equator Coriolis terms cannot be neglected and Therefore it is of some interest to redo the computations of Sverdrup and Reid using the values for the monthly mean stress field from Wyrtki and Meyers 1975 and to compare the results with oceanographic data from different times of the year to see whether Sverdrup balance really is quasi steady The monthly values of the zonal Sverdrup transport are shown in figure 6 4 The October and November curves look quite similar to those of Sverdrup The overall spatial and temporal evolution of the currents throughout the year resembles the picture that has been derived for the surface flow field from ship drift observations It is possible to check to see whether the historical integrated geostrophic transports follow a similar pattern of change During 1967 and the first part of 1968 hydrographic sections at four different longitudes between 100 and 120 W were occupied across the equator at seven different times The geostrophic velocity computations relative to 500 db for these sections are given by Love 1972 Using these figures the geostrophic transport for 2 bands of latitude from 2 to 12 N was computed by a planimetric integration The minimum contour interval for the velocities was 5 cms 1 this interval introduces an uncertainty of about 3 x 109kgs into I88 A Leetmaa J P McCreary Jr and D W Moore I 10 N O0 10 N 0 10 N 0 Figure 6 4 Upper panel depicts monthly Sverdrup transport Lower two panels show geostrophic transports observed dur ing the EASTROPAC expedition each transport computation 2 5 cms x 500 m x 2 x 1 gcm 3 The results of these integrations for the sections at 119 W and 112 W are shown in figure 6 4 The tendency at both sections is for the transport in the south equatorial current to be strongest during the summer The NECC lies close to the equator early in the year and moves northward during the summer and southward again during late fall The transport patterns at both sections during the first part of 1967 and 1968 are quite similar This agreement could be coincidence or indicate that these fluctuations perhaps have a regular annual cycle The observed geostrophic transport patterns are visually similar to the theoretical pattern there clearly are quantitative differences however The important conclusion from this study is that it cannot be said whether the Sverdrup relation provides an accurate description of the currents in the eastern tropical Pacific Sverdrup and Reid were probably fortunate in that their results seemed to agree so well with observations The real problem with testing Sverdrup theory in light of what we now know about barotropic and baroclinic adjustment is associated with the assumed level of no motion For an ocean with a free slip flat bottom Sverdrup balance should hold for the integrated flows and pressures as long as the integral goes all the way to the bottom and barotropic adjustment has had time to occur approximately a few days If the integration extends only over the surface layers upper 500 or 1000 m say however then the integrated quantities depend on both the barotropic and baroclinic components and a quasi steady theory will apply only if the dominant baroclinic modes have also reached equilibrium In general for the annual cycle there is no reason this should be true since baro clinic adjustment even near the equator takes at least a few months Clearly what is
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