FY 1992

The response of the western equatorial Pacific Ocean to westerly wind bursts during November 1989 to January 1990

McPhaden, M.J., F. Bahr, Y. du Penhoat, E. Firing, S.P. Hayes, P.P. Niiler, P.L. Richardson, and J.M. Toole

J. Geophys. Res., 97(C9), 14,289–14,303, doi: 10.1029/92JC01197 (1992)

Several 5 to 10 m s^-1 westerly wind bursts of 10–15 days' duration occurred in the western equatorial Pacific during November 1989 to January 1990. The response to these wind bursts was characterized by a 400- to 600- km-wide eastward jet in the upper 100–150 m along the equator between 135°E and the date line. Flow in this jet accelerated to speeds of over 100 cm s^-1 within 1 week after the onset of westerly winds in November 1989 in association with supertyphoon Irma. In addition, a 20 to 40 cm s^-1 westward counterflow developed between 2°N and 2°S below the surface jet separating it from the eastward flow of the Equatorial Undercurrent in the thermocline. Changes in surface layer zonal volume transports in the western Pacific due to westerly wind bursts were 25–56 Sv based on comparison of three shipboard velocity transects in November and December 1989. Although fluctuations in current speeds in the thermocline were generally smaller and less directly related to local wind forcing than those in the surface layer, the Equatorial Undercurrent decelerated to less than 20 cm s^-1 (i.e., less than half its speed before the onset of westerlies) by early December 1989. Westerly winds also produced a strong surface convergence in both the meridional and zonal directions west of the date line. In late November 1989, this convergence was associated with downwelling of 2–3 m d^-1 along the equator, and a 20- to 30-m depression of the thermocline. In addition, sea level rose by 10 dyn cm from mid-November to mid-December between 1°S and 3°N, 165°E. Sea surface temperature dropped over a large region by 0.2°–0.5°C during episodes of high westerly winds, and some episodes of high winds were associated with 0.2°–0.3°C sea surface temperature inversions supported by salt-stratified buoyancy gradients in the upper 100 m. The near-surface salinity balance was dominated by lateral advection when relatively fresh water from north of a salinity front centered near 3°N was advected southward by Ekman drift in late November 1989. Displacement of this front past a current meter mooring at 0°, 165°E appeared as a local drop of 1 psu in surface layer salinity in 10 days. These results are discussed in the context of a swing in the climate system towards El Niño/Southern Oscillation-like conditions in early 1990.