Surface layer variations observed in multiyear time series measurements from the western equatorial

Abstract. Mooring measurements at 0 ø, 165øE, for the period November 1988 to August 1991 indicate that surface layer structure was characterized by two distinct climatic regimes associated with dramatic differences in large-scale atmospheric and oceanic conditions. La Nifia conditions existed from November 1988 to November 1989, during which time the easterly trades were strong, Ekman divergence and upwelling were pronounced, surface velocity was strongly westward, and rainfall was low. The surface layer was cold, salty and well mixed down to 100-m depth, with density variations controlled primarily by temperature. In contrast, from November 1989 to August 1991, the zonal winds were on average westerly and punctuated by frequent westerly wind bursts, the surface currents reversed and flowed eastward in the upper 50 m, and rainfall was high. Compared to the La Nifia period, the surface layer was warmer and fresher, and the density mixed layer was shallower than the isothermal layer owing to the presence of a 30-m-thick mean halocline (or barrier layer) between 55- and 85-m depth. Moreover, density variations in the mixed layer were determined primarily by salinity. During the November 1988 to November 1989 La Nifia period, variability in sea surface temperature was influenced by local upwelling and zonal advection. However, during November 1989 to August 1991, the presence of the barrier layer effectively prevented the entrainment of cooler, saltier water from the thermocline into the surface layer. Local air-sea heat fluxes were therefore more likely to be prominent in the surface layer temperature balance. The barrier layer thickness, which varied with a dominant time scale of 12-25 days, appears to have been affected by variations in zonal advection of low-salinity water past the mooring. 1