The mixed layer of the western equatorial Pacific Ocean

Abstract

The mixed layer of the western equatorial Pacific and its thermodynamics are poorly known because of a general lack of data. Conductivity‐temperature‐depth (CTD) profiles from the recent Western Equatorial Pacific Ocean Circulation Study (WEPOCS) cruises have been analyzed for various measures of the upper layer and mixed layer thickness, using criteria which depend on vertical gradients of temperature, salinity, and density. From 434 profiles, the average mixed layer depth in the western equatorial Pacific during the two WEPOCS cruises was 29 m, which is about a factor of 3 shallower than had previously been thought. The mean depth of the top of the thermocline was found to be 64 m, so there is a nearly isothermal layer that is deeper than the mixed layer. This discrepancy is attributable to salinity stratification. It is hypothesized that the waters in this “barrier” layer between the bottom of the mixed layer and the top of the thermocline are formed to the east of the WEPOCS region, and subducted below the shallow and lighter mixed layer waters found in the west. Under light wind conditions, there was a tendency for warm and thin layers to form at the sea surface as a result of diurnal heating; however, there did not appear to be any nighttime maximum to the mixed layer depth associated with convective overturn due to cooling. This contrast with the central Pacific may be caused by the influence of salinity on the thermodynamics of the mixed layer. A strong westerly wind burst was observed during WEPOCS II, and apparently the mixed layer nearly doubled in depth while cooling by more than 1°C. Evidence of downwelling near the equator, and upwelling off the equator, was seen in the distribution of temperature, salinity, and density in the meridional section along 143°E, which was occupied immediately following the wind event. This event was apparently strong enough to erode through the salinity‐stratified layer and into the thermocline, resulting in the observed cooling. The results of this study suggest that except during strong wind events, entrainment cooling may not be an important component of the heat budget of the western Pacific warm pool. This has potentially important implications for the El Niño/Southern Oscillation (ENSO) phenomenon.