The Relationship between Oscillating Subtropical Wind Stress and Equatorial Temperature*

An earlier study showed that an atmosphere–ocean model of the Pacific develops a midlatitude oscillation that produces decadal sea surface temperature (SST) variability on the equator. The authors use the ocean component of this model to understand better how subtropical wind stress oscillations can cause such SST variability. The model ocean consists of three active layers that correspond to the mixed layer, the thermocline, and intermediate water, all lying above a motionless abyss. For a steady wind, the model develops a subtropical cell (STC) in which northward surface Ekman transport subducts, flows equatorward within the thermocline, and returns to the surface at the equator. Analytic results predict the model’s equatorial temperature, given some knowledge of the circulation and external forcing. A prescribed subtropical wind stress anomaly perturbs the strength of the STC, which in turn modifies equatorial upwelling and equatorial SST. The transient response to a switched-on wind perturbation is used to predict the ocean response to an oscillating wind. This method correctly predicts the results of several numerical experiments, and extends these results to a wide range of forcing periods. For an oscillating wind, there is a more complicated relationship between perturbations to equatorial SST and the various branches of the STC. The thermocline-branch anomalies are generally weaker than those in the surface and equatorial-upwelling branches. Equatorial SST anomalies lead, follow, and are roughly coincident with, variations in the thermocline, surface, and upwelling branches, respectively. Thus, while recent studies have suggested using the subsurface branch variations as a predictor of tropical–subtropical interactions, the surface branch may be a better predictor.