Slow Variability in the Equatorial West-Central Pacific in Relation to ENSO

Abstract

Six years of upper-ocean velocity, temperature, and surface wind data collected in the west-central Pacific at 0°, 170°W reveal a slow ocean dynamical mode associated with the El Niño?Southern Oscillation (ENSO). Latent and sensible heat flux calculations using the basin-wide Tropical Atmosphere Ocean (TAO) array data show a coincident, slow ocean?atmosphere thermodynamical mode. Beginning with the La Niña conditions in 1988 through the peak El Niño conditions in 1992 the Equatorial Undercurrent (EUC) speed decreased along with the surface zonal wind stress and the zonal pressure gradient. Simultaneous with these were increasing trends in the Richardson number above the EUC core and in sea surface temperature (SST). After peak warming was achieved, the variations in all of these quantities reversed in a movement toward their previous La Niña conditions. As this evolved within the ocean the sensible and latent heat fluxes increased with large values emanating eastward from the western Pacific. The largest interannual perturbations, then, for both the surface momentum and heat flux quantities during this recent ENSO cycle were within the west-central Pacific, the transition region between the warmest waters found in the western Pacific warm pool and the coldest waters found in the eastern Pacific cold tongue. The observed ocean and atmosphere variability represents a positive feedback. This raises a question about the origin of negative feedback that is necessary for the coupled system to oscillate. Arguing from the standpoint of a Gill atmosphere and observed SST?sea level pressure correlation patterns, the paper draws a connection between condensation heating in the equatorial west-central Pacific and easterly winds over the equatorial western Pacific during the mature phase of El Niño. The formation of such easterlies by ocean?atmosphere coupling over the western Pacific is hypothesized as providing a negative feedback for reversing the sign of anomalous SST in the equatorial central Pacific. This mechanism may complement, but it is different from, the delayed oscillator mechanism for ENSO.