Understanding Projected Changes in Heavy Precipitation in CMIP5 Climate Model Simulations

Coupled atmosphere-ocean climate models from the Coupled Model Intercomparison Project Phase V (CMIP5) are used to study projected changes in heavy to extreme daily precipitation globally and regionally between the late 20th century and late 21st century under the RCP8.5 emissions scenario. Results show that annual heavy precipitation increases over most land areas between the late 20th and 21st centuries, with the largest increases in the tropics and at middle to high latitudes. However, there is seasonal variability in the change in heavy precipitation, with some land regions in tropical and subtropical latitudes showing a drying trend during certain times of the year. Small changes or decreases in heavy precipitation are consistent year round in subtropical oceanic dry regions. We attempt to connect projected changes in local extreme precipitation events with changes in atmospheric circulation by developing a composite analysis of the atmospheric circulation associated with extreme precipitation events at individual grid cells. The goal of this analysis is to better understand the physical mechanisms responsible for projected changes in regional heavy precipitation that are not constrained by atmospheric water vapor increases or other thermodynamic constraints, such as changes in the vertical moisture profile. Preliminary results show a weakening of the atmospheric circulation associated with extreme precipitation over many subtropical to middle latitude regions characterized by smaller increases (or decreases) in heavy precipitation than is predicted with thermodynamic constraints. A poleward shift in storm tracks and changes in the tropical circulation associated with changes in sea surface temperatures appear to be playing a role in the relationship between changes in atmospheric circulation and heavy precipitation events in a warmer climate.