A "Heat Pump" Picture for the ENSO System

Understanding the role of ENSO in the global heat balance is a prerequisite for addressing the response of ENSO to global warming and fundamental for a better understanding of the observed decadal variability in the magnitude of El Nino warming. An analysis of the heat sinks and sources of the 1986-87 El Nino suggests that the ENSO system behaves like a heat pump-it sucks heat into the equatorial upper ocean during La Nina and pushes the accumulated heat in the upper ocean to the higher latitudes during El Nino. The surface warming of El Nino derives its heat from the heat sucked into the equatorial upper ocean during the cold phase and is responsible for the large poleward heat transport during the warm phase. Thus while the accumulated heat in the upper ocean provides the fuel for the development of the surface warming, the increased poleward heat transport accompanying the surface warming ensures the termination of El Nino. The new "heat pump" picture extends the delayed oscillator hypothesis and highlights a fundamental role of the surface heat flux into the equatorial ocean and the associated equator-to-pole heat transport in driving ENSO. To check whether the "heat pump" picture also applies to other El Ninos, the heat balance of the tropical ocean over the entire period of 1980-1999 was then examined using NCEP reanalysis for the atmosphere and NCEP assimilated data for the Pacific ocean. All El Ninos during this period were proceeded by accumulation of heat in the equatorial upper ocean-the heat content was increasing prior to the onset of each El Nino. The occurrence of El Nino reverses the positive trend in the heat content by increasing the poleward transport of heat in the ocean. The accumulated heat in the equator comes from the surface heating as the ocean heat transport remains poleward throughout the entire period. The strongest El Nino-1997-98 El Nino is found to be proceeded by the highest heat content in the equatorial Pacific. The 1997-98 El Nino is also accompanied with the largest poleward heat transport. These findings thus support the inference from the heat budget of the 1986-87 El Nino that ENSO acts as a heat pump of the coupled climate system. An analytical model for the coupled ocean-atmosphere system was developed to further highlight the relationship among the magnitude of El Nino warming, the surface heating, the upper ocean heat content, and the equator-to-pole heat transport. The implication of the findings for the origin of the decaldal variability of El Nino and the response of El Nino to global warming is discussed