A linear theory is proposed that can explain both the period and the westward propagation of the equatorial annual cycles in the SST and zonal wind. A coupled model linearized about a mean state of the air-sea system is used. This model allows the surface winds to directly change the SST through surface evaporation and vertical mixing, in contrast to the formulation of the conventional air-sea coupling models that emphasize the effects of winds on ocean dynamics. It is demonstrated that the characteristics of the equatorial seasonal cycle, including its period, phase, and amplitude, are determined to a large extent by the mean state of the air-sea coupling system. The meridional wind, which is specified here, is the most important forcing for the annual cycle in SST, suggesting that the equatorial annual cycle is the response to an annual solar forcing in the off-equatorial regions.

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