Observed variations of upper ocean zonal current in the western equatorial Pacific and their relation to the local windiJournal of Geophysical Research-OceanA2002j
Observed variations of upper-ocean zonal currents on the equator at 142 degree E and 147degree E from November 1994 through October 1996, when the surface warm water layer was well developed, are described in relation to the wind forcing and the stratification. The variations of zonal currents were strongly correlated with the variations of local zonal wind stress in two distinct frequency bands separated by very low coherence for periods exceeding 10 days. One band encompasses the annual period, and the other band brackets approximately the 30- to 80-day period. At annual period, high coherence with almost zero phase was limited to a region of very shallow depth, with high coherence at almost 180 degrees phase seen at about 100m. An eastward current above the Equatorial Undercurrent (EUC), which developed only during boreal summer with the seasonal upward displacement of the upper thermohaline, was responsible for this subsurface coherence maximum. At 30- to 80-day period, a high positive cross correlation between zonal current speed and zonal wind stress was seen from the shallowest observation depth to about the zero-crossing depth between the South Equatorial Current and the EUC (ZCD), and a high negative correlation was seen below. The ZCD was located near the 29 degrees isotherm, and the vertical extent of high positive cross correlation in this band was found to vary together with the inter-annual and annual variations of the ZCD. There were temperature variations in this band, possibly caused by meridional or vertical advection, that were coherent with those of the zonal current.