Surface heating, upper ocean heat content, and the magnitude of El Nino warming

The observational and theoretical basis for the response of global mean surface temperature to an increase in the greenhouse effect has been well established. Less certain is whether the increase in the greenhouse effect also influences the magnitude of El Niño warming. A major contributor to this uncertainty is the lack of understanding of whether and how the magnitude of El Niño warming is constrained by the heat balance of the tropical Pacific. To understand the role of El Nino in the heat balance of the tropical Pacific, the heat budget of the tropical Pacific over the last 20 years was calculated using data from NCEP assimilation system for the Pacific ocean. The results reveal that the heat removal from the equatorial Pacific to the higher latitudes is achieved episodically and those transporting episodes correspond well with the occurrences of El Ninos. Moreover, the two strongest El Ninos--the 1982-83 El Nino and the 1997-98 El Nino--are accompanied with the largest poleward heat transport. The two strongest El Ninos are also preceded with the largest build up of heat content in the equatorial Pacific and result in the largest drop in the heat content of the equatorial Pacific. Thus El Nino appears to act as a regulator of the equatorial upper ocean heat content through its effect on the poleward heat transport. Furthermore, it is noted that the equatorial zonal SST contrast represents a mechanism to suck heat into the equatorial upper ocean. Based on these empirical evidences, it is suggested that an increase in the equatorial surface heating results in stronger zonal SST gradient and higher equatorial upper ocean heat content and thereby more energetic El Ninos. The more energetic El Ninos in return transport more heat poleward and satisfy the heat balance of the tropical Pacific. Numerical results with a primitive equation ocean model coupled with a simple atmosphere support this suggestion. The implication of these findings for the response of ENSO to increases in the greenhouse effect is discussed.