Upper equatorial Pacific Ocean current and salinity variability during the 1996-1998 El Nino-La Nina cycle

The recent El Nino-La Nina 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 8 degrees S to 8 degrees N between 165 degrees E and 95 degrees W, 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 +/- 32 x 10(6) 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 El Nino and was then disrupted by upwelled salty water with the onset of La Nina, a very fresh mixed layer observed in the eastern equatorial Pacific late in the El Nino, and a reduction in the strength of the meridional equatorial salinity gradient within the pycnocline to one third of the usual value during the El Nino. Finally, the zonal transports above the thermocline from 5 degrees S to 5 degrees N were well correlated with the rate of change of warm-water volume west of the individual CTD/ADCP sections.