SURFACE-LAYER VARIATIONS OBSERVED IN MULTIYEAR TIME-SERIES MEASUREMENTS FROM THE WESTERN EQUATORIAL PACIFIC

Mooring measurements at 0 degrees, 165 degrees E, for the period Novem ber 1988 to August 1991 indicate that surface layer structure was char acterized by two distinct climatic regimes associated with dramatic di fferences in large-scale atmospheric and oceanic conditions. La Nina c onditions existed from November 1988 to November 1989, during which ti me the easterly trades were strong, Ekman divergence and upwelling wer e pronounced, surface velocity was strongly westward, and rainfall was low. The surface layer was cold, salty and well mixed down to 100-m d epth, with density variations controlled primarily by temperature. In contrast, from November 1989 to August 1991, the zonal winds were on a verage westerly and punctuated by frequent westerly wind bursts, the s urface currents reversed and flowed eastward in the upper 50 m, and ra infall was high. Compared to the La Nina period, the surface layer was warmer and fresher, and the density mixed layer was shallower than th e isothermal layer owing to the presence of a 30-m-thick mean haloclin e (or barrier layer) between 55- and 85-m depth. Moreover, density var iations in the mixed layer were determined primarily by salinity. Duri ng the November 1988 to November 1989 La Nina period, variability in s ea surface temperature was influenced by local upwelling and zonal adv ection. However, during November 1989 to August 1991, the presence of the barrier layer effectively prevented the entrainment of cooler, sal tier 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 w ith a dominant time scale of 12-25 days, appears to have been affected by variations in zonal advection of low-salinity water past the moori ng.