Abstract

Major features of the southwestern tropical Pacific (SWTP), defined between 160E–140W and 24S–10S, are brought to light through analysis of surface water samples (23000) and temperature/depth observations (8500), both collected by ship of opportunity programs during the 1979–85 period. The mean vertical thermal structure (and its related parameters), mean sea surface temperature (SST) and salinity (SSS) are first portrayed, to further quantify the 1979–85 variability. It is demonstrated that the observed seasonal and interannual variabilities, the latter being associated with the strong 1982–83 EI Niño Southern Oscillation (ENSO) event, are mostly governed by specific mechanisms involving varying wind field and rainfall regimes. During the non-ENSO period (1979–81 + 1984–85), SST annual cycle and 0–100 m thermal structure changes are tied to the seasonal variations of the sun's position (minimum SST in August). At the mean South Pacific Convergence Zone (SPCZ) position, there is a marked seasonal SSS cycle, with minimum SSS in March. This minimum occurs two to three months after maximum precipitation whose variations suffice to explain SSS changes, assuming a 28 ± 7 m mixed layer depth. It suggests that the seasonal SSS cycle is mostly governed by the rainfall regime associated with the SPCZ migration and intensity. During a “normal” year, the seasonal SPCZ migration causes local alternation of cyclonic and anticyclonic wind stress curls, so that west of 175W and between 13S–17S, the thermocline depth seasonal variations are governed by local Ekman pumping. During the ENSO period (1982–83), notable changes in the vertical temperature distribution were mostly perceivable within 10S–15S and below 100 m, in response to an anomalous wind stress field that strongly raises the thermocline through local Ekman pumping (as much as 70 m in May 1983). The resulting thermocline shoaling intensified the southern branch of the south equatorial current, and induced a southward shift of the subtropical gyre center, as already observed during the 1957 and 1972 ENSO. In addition, it modified the whole water column all the way to the surface, and was thus responsible for the SST cooling anomaly (–0.5°C to –1°C) observed in the northern SWTP (i.e. in the warm pool area). In the southern SWTP, similar SST cooling was concomitant with a positive latent heat flux anomaly (>20W ˙ m–2) mostly resulting from an increase in the northward wind component. At the mean SPCZ position, the SSS augmented to as much as +1 in early 1983, mainly in response to a rainfall deficit resulting from the equatorward shift of the SPCZ, and also to unusual zonal salt advection and vertical mixing.

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