Diabatic Amplification of Atmospheric River Intensity by Marine Heatwaves: Multi-Scale Air-Sea Interaction and Implications for Marine Heatwave Dissipation

The climate along the US West Coast is profoundly affected by the extratropical ocean and air-sea interaction near the coast, influencing moisture transport and valuable precipitation that play an important role in agricultural and water resource management efforts. On a basin scale, seasonal to interannual anomalies in the atmospheric circulation can create persistent upper-ocean temperature anomalies known as marine heatwaves (MHWs). These anomalous SST conditions have direct impact on air-sea fluxes, thereby influencing diabatic processes associated with synoptic-scale weather patterns, such as atmospheric rivers (ARs). Given the heat and moisture pickup by the ARs from the oceans, these multi-scale MHW-AR interactions may also represent a potential mechanism for dissipation of MHWs. This study examines diabatic multi-scale coupled air-sea interaction processes between persistent MHWs and synoptic-scale ARs, and evaluate their downstream effects on the coastal and inland climate.

Here, we present a comprehensive analysis based on observations and high-resolution, large-ensemble regional coupled model simulations targeting a series of landfalling ARs that interacted with warm SST anomalies during the Northeast Pacific MHW event in winter 2014/2015. Sensitivity simulations are conducted where various aspects of the observed MHW feature are removed from the ocean component of the coupled model to quantify the diabatic modification of the AR moisture and energy budgets. Our results show that MHWs exert diabatic forcing of the lower troposphere via enhanced latent heat flux from the ocean to the atmosphere and an associated increase in evaporation. This ultimately represents a nontrivial moisture source leading to an amplification of ARs indicated by a robust increase in rainfall intensity. Furthermore, the model results suggest noticeable shifts in the precise landfalling locations of the AR, the statistical significance of which is being assessed via ongoing ensemble simulations. The implications of MHW dissipation arising from the diabatic interaction between ARs and MHW will be discussed.