Extreme rainfall over land exacerbated by marine heatwaves.

Marine heatwaves (MHWs), characterized by multiple days of exceptionally elevated sea surface temperature (SST), have profound marine ecological impacts, but their effect on precipitation, particularly extreme rainfall over coastal regions, remains unknown. Using multi-platform observational data since 2000, here we show that SST gradients of MHW intensify surface wind speeds and drive downwind surface wind convergence and upward motions by enhancing vertical turbulent flux over the warm water. The induced anomalies lead to substantially increased local precipitation with spatial scale several hundreds of kilometers and temporally peaking one-day after the MHW. Furthermore, in global coastal regions, about 5%-25% of extreme rainfall over land (>99% wet-day) occurs in the downwind direction of nearby MHWs. Averaged land precipitation of the extreme rainfall events in the downwind direction of a strong MHW increases by 20%-30%, or 4-8 mm/day, from the amount without an influence from MHWs, exacerbating flood-related fatalities. Our finding identifies an impact of MHWs on coastal extreme events with important implications for affected communities, particularly given the projected increase in MHW intensity and frequency under greenhouse warming.