A Global, Multiproduct Analysis of Coastal Marine Heatwaves: Distribution, Characteristics, and Long-Term Trends

Major marine heatwave (MHW) events have caused catastrophic impacts on coastal marine ecosystems. However, to date there has not been a global assessment of MHWs in coastal areas where rich marine ecosystems are at risk. Here, we combine four satellite Sea Surface Temperature (SST) products to quantify the distribution, characteristics, and decadal trend of coastal MHWs, using an ensemble approach. Hotspots of MHW stress, defined as yearly cumulative intensity, were found to be concentrated in mid latitude coasts like the Mediterranean Sea, Japan Sea, and Tasman Sea, as well as the north-eastern coast of the United States. We found a global increase in coastal MHW frequency and duration during the past 25 years by 1-2 events per decade and 5-10 days per decade, respectively, with regional distribution closely related to decadal climate variability. Increases in frequency and duration of MHWs has led to large increases of cumulative intensity and yearly cumulative intensity, particularly in mid-latitudes and hotspot regions. Long-term changes in mean SST were the main driver of the observed trends of coastal MHWs, while internal variability was important for explaining local decreases in MHW metrics such as along the south-eastern Pacific coast. MHW average metrics and trends were consistent across all four products used in this analysis, giving high confidence in the results. However, important differences between products were observed for MHW mean intensity, which was well correlated to SST variability, suggesting sensitivity of this metric to the specific SST data set and demonstrating a need for an ensemble approach to MHW analysis.

Automated summary

A global assessment of coastal MHWs using an ensemble approach reveals an increase in frequency and duration over the past 25 years, with main driver being long-term changes in mean SST. Differences in MHW mean intensity between products suggest sensitivity to specific SST data sets and emphasize the need for an ensemble approach in MHW analysis.