Pacific Equatorial Subsurface Countercurrent Velocity, Transport, and Potential Vorticity

Abstract

Concurrent shipboard ADCP and CTD sections are used to calculate the mean velocity, transport, and potential vorticity fields associated with the Pacific equatorial subsurface countercurrents (SCCs). Averaged in stream coordinates, the core eastward velocity of the SCCs is a factor of 2 higher than previously reported means, but the estimated transports are unchanged. The meridional profile of zonal velocity along isopycnals is sharply peaked, and nearly linear on each flank of the current. The sharp and strong reversal of relative vorticity over a distance of 20 km at the jet core sharpens the coincident potential vorticity front. This front separates a region of very low homogeneous potential vorticity on the equatorward side from a homogeneous high potential vorticity region on the poleward side. Each potential vorticity pool extends well beyond the edges of the SCC into regions of westward flow. On the equatorward sides of the SCCs this westward flow is the Equatorial Intermediate Current in the western Pacific, but in the central and eastern parts of the basin it is off the equator in deep extensions of the South Equatorial Current. In the central and eastern regions the net westward transport in an isopycnal layer between the SCCs exceeds the combined eastward SCC transport in that layer. The net zonal transport between the SCC cores is highly divergent in the east and convergent in the west. This pattern, together with downstream changes in SCC density, indicates that neither they nor the westward return flows are simple inertial recirculations; strong diapycnal processes must be involved.