Contextualizing Southwest US Precipitation in Past and Future Mean Climates with Marine Heatwaves

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 localized warming of the California (CA) margin sea surface temperature (SST) enhanced the North American Monsoon. Marine heatwaves (MHWs) in the eastern Pacific have been suggested as an analog for this mechanism in past and future mean climates. However, the differing timescales complicate this analog: paleoclimate proxies average over thousands of years and represent an equilibrated climate, whereas MHWs are short-lived extreme events. Using millennial-long climate simulations, we test whether the drivers of SWUS precipitation during MHWs are comparable to those governing long-term climate change. Under increased CO2 forcing, we find no model consensus on the response of equilibrated SWUS precipitation to CA margin warming. In contrast, models agree on an increase in SWUS precipitation when CA margin SST is warmer during MHWs. We use constructed circulation analogs to decompose these responses into the dynamic and thermodynamic contributions. The circulation anomaly during both MHWs and in equilibrium climates enhances SWUS precipitation but through different processes. The thermodynamic response to mean state warming drives strong drying in the SWUS that offsets dynamic wetting, whereas the thermodynamic response to MHWs is negligible. Our findings have different implications for the Pliocene and future mean climates and highlight an important caveat in contextualizing past and future mean climate states with extreme events.