The role of coastal ocean surface fluxes during landfalling Atmospheric Rivers along the US West Coast

Atmospheric Rivers (ARs) landfalling along the US West Coast have attracted much scholarly and public attention due to their unique importance to the region. As extreme precipitation events, ARs provide up to half of the West Coast’s water resources but are also responsible for nearly all of its most devastating floods. Despite extensive research into both the large-scale atmospheric dynamics and the land-based hydrological impacts associated with ARs, the role of coastal sea-surface temperature (SST) and air-sea fluxes under landfalling ARs remains mostly unexplored. In this study we analyze in-situ measurements of near-surface meteorological variables and SST from 138 buoys along the US West Coast (NDBC/IOOS/NOS-COOPS) for the period 1979–2017, supplemented by the new high-resolution ERA5 reanalysis, in order to assess the importance of coastal SSTs and air-sea fluxes to observed AR characteristics. From the investigation of an extreme AR in winter 2015 as well as all ARs from 1979–2017, we find robust upward latent heat fluxes, acting to destabilize the lower atmosphere, throughout AR landfalling lifetimes. We also compare surface observations and AR characteristics between El Niño and La Niña years, since coastal SSTs are strongly modulated by ENSO via coastally-trapped Kelvin waves. We find that El Niño (La Niña) winters are associated with enhanced (reduced) upward latent heat flux 1-2 days before and after AR peak intensities along the southern (northern) West Coast, coincident with increased (subdued) strength of ARs. A Reynolds decomposition of the bulk flux formula into the individual contributions of SST, near-surface stability, relative humidity, and wind speed anomalies suggests that ENSO-related coastal SST anomalies make a dominant contribution to the latent heat flux variability under ARs.