The role of ocean–atmosphere coupling in the 2017 extreme coastal El Niño

During February-April 2017, extreme coastal warming was observed off Peru and Ecuador, causing torrential rains and floods with severe socioeconomic hardships. Unlike the 1983 and 1998 extreme coastal warming that formed after the basin-wide El Niño of the century, the 2017 warming event was confined to the coast region and preceded by a moderate La Niña. This study investigates the ocean-atmosphere dynamics underlying this highly unusual extreme event using a wide range of observations as well as comprehensive oceanic and atmospheric general circulation models. Results show that both the remotely forced intraseasonal downwelling Kelvin waves caused by strong westerly wind events over the equatorial Pacific and northerly coastal wind anomalies contribute to the 2017 extreme event. Atmospheric general circulation model experiments reveal a positive ocean-atmosphere coupling in the coastal region, between coastal warming, atmospheric deep convection, and alongshore winds. The coastal coupling implies predictability at monthly leads, which we confirm from the operational seasonal forecast. Climate model projections further show that an enhanced SST increase in the southeastern Pacific compared to the tropical mean warming gives rise to more frequent extreme coastal El Niño in the future as the climate warms.