A model of tropical ocean-atmosphere interaction is used to study the El Niño–Southern Oscillation phenomenon. The model ocean consists of the single baroclinic mode of a two-layer ocean. The thermodynamics of the upper layer are highly parameterized; sea-surface temperature is assigned one of two values, warm or cool, according to whether the interface is shallower or deeper than an externally specified depth. The model atmosphere consists of two wind patches of zonal stress that are idealizations of the annual cycle of the equatorial trades, τs, and of Bjerknes' Walker circulation, τw. When the eastern ocean is in its cool state both patches drive the ocean; when it is warm τw is switched off. Solutions compare favorably with observations in several ways. Most importantly, for reasonable choices of parameters solutions oscillate at the long time scales associated with the Southern Oscillation. The response of the ocean to τw introduces positive feedback into the system, with the result that the... Abstract A model of tropical ocean-atmosphere interaction is used to study the El Niño–Southern Oscillation phenomenon. The model ocean consists of the single baroclinic mode of a two-layer ocean. The thermodynamics of the upper layer are highly parameterized; sea-surface temperature is assigned one of two values, warm or cool, according to whether the interface is shallower or deeper than an externally specified depth. The model atmosphere consists of two wind patches of zonal stress that are idealizations of the annual cycle of the equatorial trades, τs, and of Bjerknes' Walker circulation, τw. When the eastern ocean is in its cool state both patches drive the ocean; when it is warm τw is switched off. Solutions compare favorably with observations in several ways. Most importantly, for reasonable choices of parameters solutions oscillate at the long time scales associated with the Southern Oscillation. The response of the ocean to τw introduces positive feedback into the system, with the result that the...