On the dynamics of equatorial currents, with application to the Indian Ocean
The dynamics of equatorial currents are considered in the light of the wind and current measurement at Gan (0.5°S, 73°E) reported by Knox (Deep-Sea Research, 23, 211–221, 1976). For westerly and meridional winds the response of equatorial currents is predominantly local and rapid. It follows that the current reversals observed at Gan are most likely due to changes in the local winds and not to a reflection from the eastern boundary, as was previously suggested. The winds at Gan are predominantly westerly, resulting in downwelling at the equator. The eastward momentum put in by the wind at the surface is advected downward giving subsurface eastward flow. With persistent easterlies (as in the Atlantic and Pacific) there is an undercurrent driven by the eastward pressure gradient force, both because of the direct, down gradient flow and because of the vorticity transported by the associated meridional circulation. It is suggested that the undercurrent that has been observed in the Indian Ocean in the early spring is similarly driven by an eastward pressure force. This pressure gradient is a non-local transient feature generated by the zonal readjustment of mass induced by the relaxation of the winds at the fall monsoon transition. This idea is consistent with the presence of the undercurrent in 1973 and its absence in 1974.
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- Deep Sea Research Part A. Oceanographic Research Papers