A Global, Multiproduct Analysis of Coastal Marine Heatwaves: Distribution, Characteristics, and Long‐Term Trends. Issue 2 (7th February 2021) - Marin, Maxime
Abstract: 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. Plain Language Summary: Despite marine ecosystems having recently been severely impacted by marine heatwave (MHW) events, there is still a lack of MHW studies in coastal areas. Here, we use a combination of four Sea Surface Temperature (SST) global products derived from satellite measurements, to provide the best estimation of MHW characteristics and long‐term changes during the last 25 years. We found that hotspots were concentrated along the Mediterranean Sea, Japan Sea, south‐eastern Australia and the north‐eastern coast of the United States. We also found that the frequency of MHW events and their duration globally increased by 1–2 events and 5–20 days per decade, respectively. Most of the MHW hotspots identified were associated with high trends. The main driver of long‐term MHW increase was long‐term changes in SST which, in the context of climate change, has been steadily increasing. In some regions like the south‐eastern Pacific coast, long‐term changes of MHWs were driven by the local variability of the climate system. All SST products used in this analysis agreed well, except for MHW mean intensity, which was found to be well correlated to short‐term SST variations, reminding us of the specificities of SST products and the importance of combining them to reduce uncertainty. Key Points: Global assessment of coastal MHWs using an ensemble of four daily Sea Surface Temperature (SST) products Coastal MHW hotspots in terms of simple and cumulative metrics were concentrated in the mid‐latitudes Along most coastlines, increasing MHW exposure during the last 25 years is driven by long‐term warming of SST