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 impacts^1-5. Despite extensive research on surface marine heatwaves (MHWs) and marine cold spells (MCSs) based on satellite observations^6,7, our knowledge of these extreme events and their drivers in the subsurface ocean-home to the majority of marine organisms-is very limited^8,9. 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.