Collaborative role of warm pool edge and ocean heat content in El Niño development: implications for the 1982/83 extreme El Niño

El Niño can cause global-scale climatic and socioeconomic impacts, but its forecast remains challenging. Ocean heat content (OHC) and warm pool eastern edge (WPE) in the equatorial Pacific are regarded as two important precursors for the development of El Niño-Southern Oscillation (ENSO) events. In this study, we analyze historical observations and coupled model simulations to show that combining OHC and WPE is critical to predicting ENSO events, especially in boreal spring-summer when the two metrics are largely independent. This collaborative role of WPE and OHC in determining the amplitude of ENSO events is particularly evident for El Niño as compared to La Niña, because WPE is more effective in modulating air-sea coupling when the background sea surface temperature (SST) is relatively warm. Our observational and CMIP6 model analyses further suggest that, for an extreme El Niño to develop, it would require either a larger-than-normal OHC or an eastward displaced WPE in early summer or a combination of the two. These results have direct implications for understanding the 1982/83 extreme El Niño, which was preceded by near-normal conditions of both OHC and WPE in early summer based on existing observations. As the WPE estimated from satellite-based SSTs was likely accurate, we speculate that the equatorial Pacific OHC was underestimated in the early summer of 1982. The underestimation of OHC at that time could be due potentially to the absence of in-situ buoy measurements in the western equatorial Pacific, where anomalously large OHC was recorded by station observations of sea level.