Long-Term Regional Hydroclimate Modeling for Communities and Decision-makers Across Alaska and Northwestern Canada

Abstract A novel, high-resolution land-atmosphere regional climate model (RCM) configuration for Alaska and northwestern Canada is presented. Key model decisions were informed by Indigenous survey respondents who improved model and data usefulness and usability. First, to enable climate change assessments at Arctic community scales, the Regional Arctic System Model (RASM) was modified from its 50 km grid-spacing to 4 km. Next, to advance RASM land-modeling, the Community Terrestrial Systems Model (CTSM) was coupled to RASM with hydrology specific optimization; this was a novel effort for coupled land-atmosphere RCM streamflow performance. The resultant configuration, forced by ERA5 reanalysis, has resulted in high quality simulations of the historical hydroclimate (Water Year, WY1990-2021). An ensemble of six future simulations for the mid-21st century, WY2035-2065, was then developed using two approaches that address community needs for a range of plausible futures to inform decision-making. The two future climate simulation approaches were 1) the pseudo-global warming (PGW) method, using two deltas applied to historical weather, and 2) direct downscaling of four CESM2 Large Ensemble members. Overall, the unique RASM model was used to produce 330 simulation years intended for use within interdisciplinary climate change research, and which serve Arctic community needs. Highlighted features include the co-design process, model developments, dataset characteristics, and examples of projected future regional hydrometeorological change across the future ensemble including contrasts across the PGW and direct downscaled futures are highlighted.