The Impact of Soil Preconditioning on the Evolution of Heatwaves Under Constrained Circulation: A Case Study of the 2021 Pacific Northwest Heatwave

Abstract

The Pacific Northwest (PNW) experienced a record-breaking heatwave in late June 2021. Previous studies showed that an anomalous upper-level anticyclone and associated subsidence heating, fueled by upwind latent heat release, were the main drivers. Land-atmosphere interactions have generally been found to play a secondary but important role; however their temporal evolution and state dependence on prior soil moisture (SM) and evaporative regimes remain largely unexplored. To assess this, we run 100 ensemble members of the heatwave with varying initial land surface conditions in the Community Earth System Model version 2 (CESM2). The circulation outside the PNW is constrained to observations, ensuring that the large-scale dynamical drivers are reproduced while local land-atmosphere interactions are free to evolve in the PNW region under differing soil-moisture states. While circulation largely dictates the heatwave's magnitude (peak day temperatures about 17$°$C above the climatological mean), perturbations of the SM preconditioning across a realistic range for the time of year lead to about 3$°$C spread in CESM2. We demonstrate how the land-atmosphere interactions evolve as ensemble members fall below a critical SM threshold where evapotranspiration reduces substantially. We also investigate how an antecedent rain event might have affected this heatwave event. Finally, we simulate the same circulation induced heatwave but in a future climate state with higher greenhouse gases and drier soils. Beyond mean warming effects, drier soils increase the probability of shifting from a wet regime into a transitional regime, exacerbating and elongating the heatwave.