Understanding Hydroclimate in the Contiguous United States: Roles of Temperature vs Precipitation and Subtropical Pacific Warming

This study investigates the physical drivers and mechanisms of hydroclimate variability across the Contiguous United States (U.S.), using a combination of observable data, reanalysis data, land modeling, and coupled climate model experiments. One of the major findings is that the historic 2021 drought event across the Western U.S. was primarily driven by a severe lack of precipitation rather than increased temperatures. The study further finds that anthropogenic forcing has substantially increased the risk of this extreme drought by approximately 26 times relative to a world without the influence. Additionally, the study reveals that the Pacific Meridional Mode (PMM) characterized by sea surface temperature (SST) anomalies in the subtropical Pacific plays a key role in shaping the historically observed "Dry-West, Wet-East" summertime precipitation pattern over the Contiguous United States (CONUS). A robust zonal dipole pattern was identified by the association between the PMM index and precipitation, with negative correlations over the Western U.S. and positive correlations from the Coupled Model Intercomparison Project Phase 6 (CMIP6), indicating that the PMM may serve as a useful predictor of summertime precipitation variability in the U.S. Chapter 4 explores how global climate model simulations from the Community Earth System Model (CESM) version 1.0 respond to changes in Pacific SSTs, including those tied to the PMM. Results show that SST anomalies confined to the PMM region successfully reproduce the canonical PMM structure and trigger large-scale atmospheric circulation. However, the CESM simulations also exhibit a shift in PMM-related moisture transport from the subtropical Pacific into the Western U.S. and precipitation, relative to the observations. These findings highlight the importance of improving climate models to more accurately represent tropical rainfall and atmospheric dynamics, which are essential to advancing drought predictions and water resource planning.