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.

Abstract. The recent E1 Nifio-La Nifia cycle exhibited striking patterns of current and salinity variability in the upper equatorial Pacific Ocean. This evolution is described from mid-1996 through 1998 using a remarkable data set of 35 meridional conductivity-temperature-depth (CTD)/acoustic Doppler current profiler (ADCP) sections along with buoy data. The sections, nominally from 8øS to 8øN between 165øE and 95øW, were occupied over the course of 27 months. A wide range of current variability was sampled with currents that appeared or disappeared some time during the cycle, including an equatorially trapped eastward surface current, the Equatorial Undercurrent, the northern branch of the South Equatorial Current, and the North Equatorial Countercurrent. Basin-wide, interannual changes in upper ocean and pycnocline zonal transports were as large as 64 4-32 x 106 m 3 s -1. Changes in the salinity structure included a deep and fresh mixed layer in the central equatorial Pacific that built up during the E1 Nifio and was then disrupted by upwelled salty water with the onset of La Nifia, a very fresh mixed layer observed in the eastern equatorial Pacific late in the E1 Nifio, and a reduction in the strength of the meridional equatorial salinity gradient within the pycnocline to one third of the usual value during the E1 Nifio. Finally, the zonal transports above the thermocline from 5øS to 5øN were well correlated with the rate of change of warm-water volume west of the individual CTD/ADCP sections.

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An isopycnal stream-coordinate analysis of velocity, transport, and potential vorticity (PV), recently applied to observations of the subsurface countercurrents (SCCs) in the equatorial Pacific Ocean, is applied here to the SCCs in a numerical general ocean circulation model, run by the Japan Marine Science and Technology Center (JAMSTEC). Each observed SCC core separates regions of nearly uniform potential vorticity: low on the equatorward side, high on the poleward side. Similar low-PV pools are found in the model, but the high-PV region poleward of the southern SCC is missing. The potential vorticity gradient in each core is weaker in the model than in observations, and relative vorticity plays only a minor role in the model. Its unusually high vertical resolution, with 55 levels, together with its weak lateral dissipation may be key factors in the JAMSTEC model's ability to simulate SCCs.