The SST of the western Pacific warm-pool has been anomalously warm over the last two decades. Strongest El Nino events in the instrumental record have also occurred during this period. Are the anomalously warm warm-pool SST and the exceptional strength of the 1982-83 and 1997-98 El Nino linked? Observational and modeling results are presented to argue for a positive answer to this question. The observational results come from an analysis of the heat balance of the tropical Pacific over the last two decades. The analysis confirms an earlier notion that El Nino represents a basic mechanism by which the equatorial Pacific transports heat poleward. Moreover, the analysis reveals a systematic relationship between the heat content in the western Pacific and the magnitude of El Nino warming--the higher the heat content in the western Pacific, the stronger the subsequent El Nino warming. Furthermore, the analysis shows that a higher heat content in the western Pacific is achieved through a deepening of the thermocline, thus linking the heat content in the western Pacific to the potential energy of the ocean and thereby with the stability of the coupled ocean-atmosphere system. These empirical results thus suggest a positive relationship between the amplitude of ENSO and the warm-pool SST. An increase in the warm-pool SST initially increases the zonal SST contrast. A stronger zonal SST contrast then strengthens the surface winds. Because of the stronger winds and the resulting steeper tilt of the equatorial thermocline, the coupled system is potentially unstable and is poised to release its energy through a stronger El Niņo warming.

The hypothesis is then tested using a coupled model. The atmospheric model is statistical in which the equatorial surface winds are proportional to the zonal SST gradients. The ocean component is a primitive equation model and therefore explicitly calculates the heat budget of the entire equatorial upper ocean. The model produces ENSO-like variations. The evolution of the subsurface ocean temperature over the life cycle of the model El Niņo resembles that from observations. In response to an increase in the tropical maximum SST--the SST of the western Pacific warm-pool, the model has a stronger ENSO. As hypothesized based on observational results, an increase in the tropical maximum SST strengthens the zonal SST contrast during the cold phase which sucks more heat down to the subsurface ocean. Stronger El Niņo warming then develops and transports more heat poleward to prevent buildup of heat content in the western Pacific and thereby to stablize the coupled system.

To the extent the increases in the warm-pool SST can be attributed to anthropogenic forcing, the results suggest that global warming may have already contributed to the observed strengthening of the ENSO cycle.