A Simple Model for the Pacific Cold Tongue and ENSO*

A conceptual model is constructed based upon the Bjerknes hypothesis of tropical atmosphere–ocean interaction. It is shown that strong feedbacks among the trade winds, equatorial zonal sea surface temperature contrast, and upper-ocean heat content occur in the tropical Pacific basin. Coupled atmosphere–ocean dynamics produce both the strong Pacific cold-tongue climate state and the El Niño–Southern Oscillation (ENSO) phenomenon. The cold-tongue climate state is unstable and gives rise to the self-sustained ENSO, which can be understood as an equatorial ocean recharge oscillator. The small basin size and the influence of a wind system resulting from heating sources of its adjacent landmasses are responsible for a weak and stable Atlantic cold-tongue state that cannot support ENSO-like interannual variability. The presence of westerly wind associated with the Walker circulation ascending at the western Pacific warm pool disables the dynamical coupling processes in the equatorial Indian Ocean. As a result, the equatorial Indian Ocean maintains a stable warm climate state. The conceptual coupled model reproduces the basic features of the climate states of the tropical Pacific, Atlantic, and Indian Ocean basins and the dominant interannual climate variability of the tropical climate system.