Upper ocean variability on the equator in the Pacific at 170°W

Abstract

The monitoring of upper ocean velocity, temperature, and surface winds at 0°, 170°W was initiated in May 1988 as part of the Tropical Ocean‐Global Atmosphere program. Located between regions of warm (cold) sea surface temperature in the western (eastern) equatorial Pacific, this west central region exhibits large interannual variations in surface wind stress associated with the El Niño‐Southern Oscillation. Here we report on the first 3 years of data collected at 0°, 170°W, exclusive of an El Niño event. The west central Pacific is a region through which the Equatorial Undercurrent (EUC) accelerates downstream. The vertical position of the EUC core, the vertical penetration of the South Equatorial Current (SEC), and the vertically integrated zonal volume transport all show large annual cycles. Annual and higher‐frequency transport variations are due primarily to fluctuations occurring above the thermocline, as opposed to within the EUC itself. Unlike the EUC position, the EUC core speed remains relatively steady annually, except during boreal fall and winter, when short duration, zonal momentum pulses generated as Kelvin waves to the west reduce the EUC to minimal values. Interannually, the EUC core speed was highest in 1988, and it has decreased, on average, since then. Also during 1988, tropical instability waves, generally observed farther east, were well developed at 170°W, while not in subsequent years. This suggests the same role interannually as annually for these waves: to smooth out heat and momentum gradients that form in response to the wind stress. (The winds in 1988, following the 1986–1987 El Niño, were stronger than in subsequent years.) At higher frequencies, evidence exists for an inertial‐gravity wave mode previously identified in nearby Canton Island sea level records.