Removing ENSO’s Influence from Global SST Variability, with Insights into the Record-Setting Marine Heat Waves of 2023–24

Abstract El Niño–Southern Oscillation (ENSO) is a dominant driver of seasonal–interannual climate variability and has been linked to record-setting extremes such as marine heat waves (MHWs). However, quantifying the effects of ENSO on MHW characteristics remains a challenge due to data limitations. Here, we used an ensemble of tropical Pacific “pacemaker” simulations with a fully coupled Earth system model as a testbed for assessing the skill of four empirical methods aimed at isolating ENSO’s contribution to monthly SST anomalies including MHW extremes. We then applied the most skillful method to the observational record to determine ENSO’s impact on the spatial coverage, intensity, and duration of MHWs since 1960 (after removing the background warming trend). We found that El Niño of 2023–24 contributed to about half of the global coverage of record-setting MHWs, with the tropical Indian and tropical Atlantic Oceans being most clearly impacted. Our results shed light on the critical role ENSO plays in driving the most severe MHW conditions in the historical record. Significance Statement In 2023 and early 2024, global surface ocean temperatures reached a record-breaking peak, resulting in marine heat waves that covered large areas of the global ocean. They coincided with an El Niño event, leading us to investigate the degree to which El Niño events drive global marine heat waves. To separate out El Niño’s contribution to these heat waves, we compared the effectiveness of several methods at removing the influence of El Niño from ocean surface temperature data. Using the best of these methods, we found that the 2023–24 El Niño was responsible for about half of the spatial coverage of the concurrent marine heat waves, with the remainder due to unrelated climate variability.