Abstract
Atmospheric rivers (ARs) play a key role in California's water supply and are responsible for most of the extreme precipitation and major flooding along the west coast of North America. Given the high societal impact, it is critical to improve our understanding and prediction of ARs. This study uses a regional coupled ocean-atmosphere modeling system to make hindcasts of ARs up to 14 days. Two groups of coupled runs are highlighted in the comparison: (1) ARs occurring during times with strong sea surface temperature (SST) cooling and (2) ARs occurring during times with weak SST cooling. During the events with strong SST cooling, the coupled model simulates strong upward air-sea heat fluxes associated with ARs; on the other hand, when the SST cooling is weak, the coupled model simulates downward air-sea heat fluxes in the AR region. Validation data shows that the coupled model skillfully reproduces the evolving SST, as well as the surface turbulent heat transfers between the ocean and atmosphere. The roles of air-sea interactions in AR events are investigated by comparing coupled model hindcasts to hindcasts made using persistent SST. To evaluate the influence of the ocean on ARs we analyze two representative variables of AR intensity, the vertically integrated water vapor (IWV) and integrated vapor transport (IVT). During strong SST cooling AR events the simulated IWV is improved by about 12% in the coupled run at lead times greater than one week. For IVT, which is about twice more variable, the improvement in the coupled run is about 5%.
Plain Language Summary Atmospheric rivers (ARs) play a key role in extreme precipitation along the west coast of North America. Because of their important societal impact, an improved understanding of ARs is critical. In the present work, we use a coupled ocean-atmosphere modeling system to investigate the role of air-sea interactions in simulating ARs. We highlight two groups in our simulations for which the ocean's response to ARs differs. One group is associated with strong ocean cooling, where the ocean cools everywhere. The other group is associated with weak ocean cooling, where the ARs can warm part of the ocean. We investigate the AR water vapor content and transport to evaluate the ocean's impact on ARs. We find that the coupled model better simulates the air-sea exchanges and AR water vapor content. The improvements are more significant during the AR events associated with strong ocean cooling.