Extreme Marine Heatwaves in The Global Oceans During The Past Decade Bulletin of the American Meteorological Society

This study assesses the sensitivity of marine heatwave (MHW) intensity to different MHW thresholds, and examines the contributions of linear sea surface temperature (SST) warming and record-breaking SSTs to extreme MHWs (xMHW) over 2012–2024. The xMHW is defined by the high threshold of the 97.5th percentile and is compared to the super MHWs (sMHW) and MHWs due to the SST warming trend (wMHW). The sMHW is identified when and where the daily SST exceeds the highest historic value during the previous years on a given calendar day. The wMHW is approximated as the difference between xMHW and dMHW. The dMHW is defined using detrended SSTs at the threshold of the 97.5th percentile. Averaged over 2012–2024, sMHW and wMHW account for approximately 52% and 43% of xMHW, indicating a more important role of record-breaking SST than that of SST warming trend. Spatially, sMHW dominates over wMHW in high-latitude oceans and during the period of 2012–2018; in contrast, wMHW dominates over sMHW mostly in the tropical oceans and during the period of 2019–2024. On global average, sMHW and wMHW increase with time at a rate of 1.1, and 3.3 degree-days per year. However, their relative contributions to xMHWs diverge: the sMHW ratio decreases at −3% per year, but the wMHW ratio increases at 3.6% per year, indicating a growing role of long-term warming in xMHW events.