Interaction between the Annual and Interannual Variations in the Equatorial Pacific

Abstract

The interaction between the annual and interannual variations is investigated by contrasting a pair of experiments with a general circulation model of the tropical Pacific Ocean. The atmospheric forcing applied to the model includes both annual and interannual components The phase of the annual forcing is shifted one-half year in the two runs, which are otherwise identical. Significant differences are found in the sea surface temperature (SST) evolution between the two runs that have the same interannual forcing function. SST anomalies tend to be phase-locked to the solar calendar and to appear in the cold season. A SST variance function in response to interannual forcings with a random phase distribution is constructed, which has an annual cycle and reaches its maximum in the cold season as is observed. It is suggested that this seasonality of an ocean origin is amplified by the interaction with the atmosphere, leading to the observed phase-locking. The phase-locking of the interannual cycle and the interannual variation of the annual cycle in SST are two manifestations of the interaction between the annual and interannual cycles. A simple conceptual model is proposed to explain these two features, in which strong interaction between the annual and interannual cycles occurs through the nonlinearity associated with the thermocline depth change and upwelling.