Contextualizing Past and Future Mean Southwest U.S. Precipitation with Marine Heatwaves

February 04, 2026

Olivia Lee

Committee: Maria Rugenstein (Advisor); Jim Hurrell; Jeremy Rugenstein (Geosciences)

Abstract

As the Southwest U.S. (SWUS) grapples with continued drought and increased water scarcity, paleoclimate analyses suggest the SWUS was wetter during past warm climates such as the Pliocene. One hypothesis for a wetter SWUS in the Pliocene is that warmer eastern Pacific sea surface temperature (SST) during the Pliocene enhanced North American Monsoon precipitation. Similarly, warmer California (CA) margin SST during modern marine heatwaves (MHWs) has been linked to a wetter SWUS. MHWs have been suggested as a potential analog for past and future mean SWUS precipitation. However, paleoclimate proxies average over thousands of years and represent an equilibrated climate, whereas MHWs are extreme events in a transient climate. Using millennial-long climate simulations, we investigate the relationship between summer SWUS precipitation and CA margin SST, first in equilibrium climate states, and next during internal variability-driven MHWs. Under increased CO2 forcing, we find no model consensus on equilibrated SWUS precipitation changes as the CA margin warms. SWUS precipitation increases, however, when CA margin SST is warmer during MHWs. This relationship is robust across forcing simulations and several models. Using constructed circulation analogs, we show that the circulation anomaly during both MHWs and in equilibrium mean climates enhances SWUS precipitation but through different processes. The thermodynamic response to mean state warming drives strong drying in the SWUS, whereas the thermodynamic response to MHWs is negligible. Our work highlights an important caveat in contextualizing past and future mean climate states with modern extreme events.