Future MJO Change and Its Impact on Extreme Precipitation and Temperature Over the Western US in CMIP6

The Madden-Julian oscillation (MJO) has remarkable impacts on global weather and climate systems. This study examines the future changes in MJO projected by 23 Coupled Model Intercomparison Project Phase 6 (CMIP6) models that produce a realistic MJO propagation in their historical runs. Results from the multi-model mean show a ∼17% increase in MJO precipitation amplitude, a ∼11%–14% increase in MJO circulation amplitude, a ∼9% increase in propagation speed, a ∼2-day decrease in MJO period, and a ∼5° eastward extension. Analysis of the lower tropospheric moisture budget suggests the dominant role of an increased meridional advection of mean moisture caused by the steepening of mean moisture gradient over the Indo-Pacific warm pool in a warming climate in the majority of models. The enhanced zonal moisture advection plays a secondary or comparable role in some models. The stronger anticyclonic gyres to the east of the MJO convection center along with the enhanced moisture gradient favor an enhanced export of moisture away from the Equator and a local moistening at the flanks of the MJO convection center. This leads to a stronger positive moisture tendency to the east of MJO convection and hence an enhanced eastward MJO propagation with strengthened amplitude and faster speed. As the MJO changes in the future, its relationships with the precipitation and temperature extremes over the western United States (US) are found to change accordingly. The MJO impacts on precipitation tend to be stronger with the stronger MJO amplitude, whereas its impacts on temperature are generally weaker.