Marine Heatwave Trends in Eastern North America Waters Using a High‐Resolution (5‐km) <scp>SST</scp> Dataset

ABSTRACT Marine heatwaves (MHWs) are periods of persistent extreme sea surface temperatures (SSTs) that extend over vast distances and have enormous impacts on marine ecosystems and economies. This study examines SST and MHW trends (1985–2024) in the subtropical and tropical waters adjacent to eastern North America, using the high‐resolution (5 km) NOAA Coral Reef Watch (CRW) dataset, which can elucidate trends both in the open ocean and more shallow coastal waters. Results show widespread large and statistically significant increases in MHW frequency (&gt; 2.0 MHWs per decade), duration (&gt; 2.0 days per decade), and spatial extent (25%–45% larger since 1985) in all warm‐season months (May–October). Large frequency and duration trends are especially widespread in September and October; intensity trends are less pronounced but are notable near Florida in July and August and in October across the Caribbean. While frequency and duration trend patterns are strongly correlated, neither resembles intensity trend patterns, especially before October. Applying an existing MHW intensity scale to analyse trends, the frequency of Category I (‘Moderate’; one to two times the difference between the climatological daily mean SST and the daily 90th percentile SST) MHWs has increased most dramatically. However, Category II (‘Strong’; two to three times the difference) and III (‘Severe’; three to four times the difference) MHWs have also become more common during the past decade. For example, the frequency of Category II MHWs in 2023–2024 exceeds all but 1 year (2005) of Category I frequency before 2023. MHW frequency and intensity are modulated by ENSO, with peaks preferentially occurring during warm seasons that follow (December–March) El Niño events. Overall, results support model projections of a near‐perpetual warm‐season MHW state by the end of the 21st century.