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Abstract

We have investigated the modal structure of climate variability in the tropical Pacific by analyzing zonal surface wind stress, sea surface temperature, and upper ocean heat content during the period 1967 to 1986. Three principal climate modes could be identified: The annual cycle, a quasi—biennial (QB) mode, and a low—frequency (LF) mode with a time scale of about 3 years. The annual cycle is mostly governed by the movement of the sun, local air-sea heat exchange and mixing processes. In the eastern equatorial Pacific, the annual cycle involves a westward propagating coupled mode which is caused by processes within the surface mixed layer. The quasi—biennial mode near the equator shows aspects of both mixed layer physics and shallow water wave dynamics and is therefore best desrcibed as a "mixed surface/subsurface dynamics" mode. Poleward of 10 degrees local air—sea heat exchange and mixing processes become also important. The low—frequency mode is the traditional ENSO mode, which is best described as the oceanic response to low—frequency atmospheric forcing. Equatorial wave dynamics is crucial for the low—frequency mode. The quasi—biennial mode shows some evidence for a phase—locking to the annual cycle. However, the period of the quasi-biennual mode is not steady, ranging from about 20 to 30 months during the analysed period. Evidence was found that the annual cycle and the quasi—biennual mode together influence the low—frequency mode such that the, low-frequency mode attains maximum amplitude several months after the annual cycle and the quasi-biennual mode were in phase. Our study confirms also the existence of nonlinear interactions involving the QB and LF modes of interannual variability only.