Lengthening of Warm Periods Increased the Intensity of Warm-Season Marine Heatwaves over the Past Four Decades

Marine heatwaves (MHWs), periods of anomalously warm sea surface temperature (SST), have increased in frequency and severity over recent decades. Many coastal systems (e.g. coral reefs) are particularly vulnerable to warm-season heat stress when temperature can exceed organisms thermal thresholds and lead to mass mortality. While many studies have examined the change of the warm-season heat stress occurrence over time, e.g., for coral reefs, there has been less analysis of the thermal structure of heat stress events. Here we examined the trend in the characteristics of warm-season heat stress (referred to as warm-season MHWs), such as duration, peak intensity, accumulated heat stress, heating rates and level of intensity, at the global-scale from 1985 to 2019 using a global daily satellite SST dataset gridded at 0.05 0.05 (i.e. CoralTemp v3.1) from the NOAA Coral Reef Watch program. The decomposition of the trends in warm-season MHWs structure showed that the increased accumulated heat stress was predominantly driven by the increased duration rather than the increased intensity, especially in the western and central equatorial Pacific. We also compared the global patterns of the observational changes in these characteristics with the CMIP6 model-simulated changes and identified the significant ensemble model errors. The results contribute to improving the understanding of warm-season MHWs, which may help inform the prediction of their impacts on marine ecosystems and marine conservation and management under climate change.