Common occurrences of subsurface heatwaves and cold spells in ocean eddies

Extreme ocean temperature events are becoming increasingly common due to global warming, causing catastrophic ecological and socioeconomic impacts1–5. Despite extensive research on surface marine heatwaves (MHWs) and marine cold spells (MCSs) based on satellite observations6,7, our knowledge of these extreme events and their drivers in the subsurface ocean—home to the majority of marine organisms—is very limited8,9. Here we present global observational evidence for the important role of mesoscale eddies in the occurrence and intensification of subsurface MHWs and MCSs. We found that 80% of measured MHWs and MCSs below a depth of 100 m do not concur with surface events. In contrast to the weak link between surface MHWs (MCSs) and ocean eddies, nearly one-third of subsurface MHWs (MCSs) in the global ocean, and more than half of such events in subtropical gyres and mid-latitude main current systems, occur within anticyclonic (cyclonic) eddies. These eddy-associated temperature extremes have intensified at rates greater than background level in past decades, suggesting a growing impact of ocean eddies on subsurface MHWs and MCSs with ongoing global warming.