资源环境科技发展态势分析平台

The western equatorial Pacific oceanic heat content (warm water volume in the west or WWVw) is the best El Nino-Southern Oscillation (ENSO) predictor beyond 1-year lead. Using observations and selected Coupled Model Intercomparison Project Phase 5 simulations, we show that a discharged WWVw in boreal fall is a better predictor of La Nina than a recharged WWVw for El Nino 13 months later, both in terms of occurrence and amplitude. These results are robust when considering the heat content across the entire equatorial Pacific (WWV) at shorter lead times, including all Coupled Model Intercomparison Project Phase 5 models or excluding Nino-Nina and Nina-Nino phase transitions. Suggested mechanisms for this asymmetry include (1) the negatively skewed WWVw distribution with stronger discharges related to stronger wind stress anomalies during El Nino and (2) the stronger positive Bjerknes feedback loop during El Nino. The possible role of stronger subseasonal wind variations during El Nino is also discussed.

Plain language summary El Nino and La Nina have strong societal impacts at the global scale, especially large-amplitude El Nino events like in 1982-1983, 1997-1998, and 2015-2016. It is hence important to identify early warning signals for the occurrence of El Nino/La Nina. The equatorial Pacific Ocean heat content is a well-known predictor of El Nino/La Nina several seasons ahead. In this study, we show that negative heat content anomalies lead more systematically to La Nina events than positive heat content to El Nino events. We suggest that the enhanced predictability of La Nina relative to El Nino is due to larger negative heat content anomalies ahead of La Nina events and a more unstable (and hence less predictable) ocean-atmosphere system during El Nino.