The heat sources and sinks of the 1986-87 El Nino

The question of whether El Nino will become more energetic in response to an increase in the greenhouse effect is of high societal concern. Addressing this question requires a clear understanding of what constitutes the thermal forcing of El Nino. One elementary way to discern the thermal forcing of El Nino from observations is to examine where El Nino derives its heat and where El Nino eventually deposits or loses the heat. To this end, the heat balance of the coupled tropical ocean-atmosphere system during the ERBE period (1985-1989) was thoroughly analyzed. The analysis involves the use of radiation data from ERBE, circulation statistics from NCEP reanalysis, and the assimilated data for the Pacific ocean.

Accumulation of heat in the equatorial upper ocean is found prior to the onset and after the termination of the 1986-87 El Nino. The accumulation of heat in the equatorial upper ocean is due to the accumulative effect of surface heating. The heat transport in the ocean was poleward when the heat was being accumulated. The trend in the upper ocean heat content was reversed by the 1986-87 El Nino which resulted in a substantial increase in the equator-to-pole heat transport in the equatorial ocean. (The increase is larger than the radiative perturbation of a doubling of CO2 by an order of magnitude). The accompanying reduction in the net radiative flux at the top of the atmosphere (TOA) in the subtropics during El Nino is small compared to the changes in the net divergence of energy in the atmosphere or changes in the net surface heat flux into the ocean. The changes in the net surface heat flux are in turn smaller than the corresponding increases in the poleward heat transport in the ocean. Most of the heat removed from the deep tropics by El Nino is used to increase the heat content of the subtropical ocean, implying that the subtropical ocean acts as a buffer reducing the immediate thermodynamic impact of El Nino upon the extratropical region.

The results reveal that El Nino is a basic process by which ocean transports heat from the equatorial ocean to the higher latitudes, supporting the theoretical finding that El Nino is thermally driven by the surface heat flux into the equatorial ocean. The implication is that whether El Nino will become more energetic to an increase in the greenhouse effect depends crucially on whether the enhanced trapping of the outgoing long-wave radiation results in an increase in the surface heat flux into the equatorial ocean. The analysis of the energy balance of the equatorial atmosphere over the ERBE period suggests that the response of the surface heat flux into the equatorial ocean to global warming will depend critically on the response of the poleward energy transport in the equatorial atmosphere to global warming.