Increasing Future Precipitation in the Southwestern US in the Summer and Its Contrasting Mechanism With Decreasing Precipitation in the Spring.

It has been known that the southwestern United States (US) has experienced a drying trend during spring as climate warms. Less is known about the projected increase of summer precipitation in the region and its physical mechanisms. By using climate model simulations and moisture budget analysis, we show that in contrast to the spring, the changes of moisture gradient and wind convergence both cause increasing precipitation during summer. The change of the moisture gradient is featured by larger increase of water vapor along the western coast than inland, thus causing enhanced moisture advection in the future climate. The change of the wind convergence is caused by the maximum jet weakening over the continental US near the latitudes of the region, which leads to enhanced moisture convergence. Plain Language Summary: Precipitation in the southwestern United States (US) is projected to decrease in the spring but increase in the summer under climate warming. It has been known that the spring dry is caused by enhanced zonal drying advection. Here, we performed moisture budget analysis using climate model simulations and found that in contrast to the spring, the moisture gradient change and the wind convergence change both increase the precipitation in the summer. There is more moisture increase along the western coast than inland in the summer. Hence the westerly brings more moisture into the southwestern US and increases precipitation. There is more wind convergence with warming during summer in the southwestern US, which induces more moisture convergence and hence more precipitation. Such wind convergence increase is associated with the maximum jet weakening over land near the southwestern US. Spring drying and summer wetting in the southwestern US demonstrate how different changes of atmospheric dynamics and thermodynamics in different seasons determine the precipitation change. Key Points: Precipitation decreases during spring but increases during summer in the southwestern United States in response to warmingEnhanced zonal moisture advection causes moisture convergence in the summerStronger wind convergence associated with jet weakening is mainly responsible for the summer precipitation increase [ABSTRACT FROM AUTHOR]