Modulation of western U.S. compound precipitation and temperature extremes by compounding MJO and ENSO interaction

Abstract Extreme weather and climate events can have substantial impacts on society and the environment. Compound extremes (two or more extreme events occurring simultaneously or successively) may exert even larger impacts than individual events. This study examines physical drivers behind variability in hydrometeorological (precipitation and temperature) compound extremes on subseasonal-to-seasonal (S2S) timescales (2 weeks – 6 months). Observational evidence presented here indicates significant modulation of western U.S. compound extreme frequency by the Madden-Julian oscillation (MJO), a unique type of organized tropical convection varying primarily on S2S timescales. For example, when the MJO is active over the western Pacific, a robust increase in wet-cold extreme frequency is found in Southern California. When the MJO is over the Maritime Continent, an overall increase in dry-hot extremes is observed across the western U.S.. The MJO influence on compound extremes is largely modulated by El Niño/Southern Oscillation (ENSO), which can be seen through different magnitudes or changes in sign of the canonical MJO-extreme relationship conditioned on ENSO state. Similarly, the MJO can interrupt the canonical ENSO-compound extreme relationship. Our results suggest a potential route to improve western U.S. S2S prediction of compound hydrometeorological extremes by considering the combined effect of both MJO and ENSO.