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.