Large-Scale Controls of Landfalling North Pacific Atmospheric Rivers across a CESM2 Hierarchy

Atmospheric rivers (ARs), filamentary plumes of intense horizontal transport of water vapor, can result in a host of socio-economic hazards and yet can deliver up to half of the annual precipitation to regions critically reliant on water resources such as the western United States. However, model predictive skill of ARs suffers due to our inadequate understanding of the large-scale weather patterns that precede AR events and exert control over these storms. AR prediction is further complicated by the fact that these antecedent patterns are dependent on the latitude of AR landfall. To investigate these issues, we identify robust AR events and their related precursor weather patterns within a reanalysis data set and across a CESM version 2 hierarchy. The model hierarchy includes an Earthlike setup as well as aquaplanet configurations in which sea-surface temperature (SST) and aqua-topography are incrementally varied to isolate influences of tropical thermal forcing and Asian topographical forcing on North Pacific ARs, respectively. Results from the Earthlike CESM2 simulation are qualitatively similar to the reanalysis data set and indicate that high-latitude atmospheric blocking strongly influences North Pacific AR landfall statistics. The suite of aquaplanet simulations reveals that (a) aquaplanets produce ARs just as in Earthlike simulations, (b) the large-scale precursor weather patterns produced by the aquaplanets are strikingly similar to those in both the Earthilike simulations and within the reanalysis data set, and (c) tropical SST and subtropical topographical forcing impact AR statistics in unique ways. Our results highlight the important roles that high-latitude atmospheric blocking and tropical thermal forcing play in modulating northeast Pacific AR landfalls.