The heat balance of the coupled tropical ocean-atmosphere system during the Earth Radiation Budget Experiment (ERBE) period (1985-89) is analyzed in an attempt to better understand the heat sources and sinks of the 1986-87 El Nino. The analysis involves the use of radiation data from ERBE, circulation statistics from National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis, and the assimilated data for the Pacific ocean. Accumulation of heat in the equatorial upper ocean is found prior to the onset of the 1986-87 El Nino. The accumulated heat in the equatorial upper ocean comes from the surface heating, which exceeds the poleward transport of heat in the upper ocean. The accumulated heat in the upper ocean resurfaces in the eastern Pacific and the 1986-87 El Nino warming develops. The warming results in a substantial increase in the equator-to-pole heat transport in the equatorial ocean. The ocean warming is also accompanied by a significant increase in the poleward transport of energy in the atmosphere and a significant reduction in the surface heat Aux into the equatorial ocean, though these changes are smaller than the increases in the poleward heat transport in the ocean. Because of the feedbacks from water vapor and clouds, the variations in the net radiative energy flux at the top of the atmosphere are small and the surface heat flux into the equatorial ocean is mainly modulated by the poleward transport of energy in the atmosphere, which is in turn modulated by the intensity of the cold tongue. The anomalous poleward ocean heat transport does not stop right at the time when the surface warming is terminated, and this "overshooting" pushes the equatorial ocean to a cold state-the 1988-89 La Nina-during which the poleward transport in the atmosphere and ocean is reduced and heat starts to accumulate in the upper ocean again. The coupled system is then in a situation similar to 1985 and is preparing for the onset of another El Nino. The results suggest that ENSO system behaves like a heat pump: the equatorial ocean absorbs heat during the cold phase and pushes the heat to the subtropical ocean during the warm phase. This picture for El Nino implies that the surface heat flux into the equatorial ocean may be a driving force of El Nino. The relationship between this picture for El Nino and the delayed oscillator hypothesis is explored. An explanation for the absence of El Nino in the tropical Atlantic ocean is offered by noting that the zonal width of the basin limits the amount of heat that can be accumulated in the upper ocean. The implication of the present findings for the response of El Nino to global warming is discussed.
