J. Sprintall et Mj. Mcphaden, SURFACE-LAYER VARIATIONS OBSERVED IN MULTIYEAR TIME-SERIES MEASUREMENTS FROM THE WESTERN EQUATORIAL PACIFIC, J GEO RES-O, 99(C1), 1994, pp. 963-979
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.