Dynamic Amplification of Subtropical Extreme Precipitation in a Warming Climate

Abstract

Projected precipitation changes in a warming climate vary considerably, spatially, and between intensities. The changes can be greater or less than the ∼7% K⁻¹ Clausius-Clapeyron (CC) prediction, owing to dynamic effects. Using two global-climate-model large ensembles, we quantify the dynamically induced changes to precipitation extremes from the present (1996–2005) to late-21st-century (2071–2080) climates, as a function of recurrence interval, focusing on the subtropics. We separate non-CC changes into a term proportional to the present-day vertical-velocity spatial pattern (i.e., an amplification or damping thereof by a constant factor) and a residual. The amplitude term varies with recurrence interval, approximately canceling (doubling) CC for moderate (large) extremes, increasing precipitation variability. Contrastingly, the residual is quasi-uniform across recurrence intervals but spatially heterogeneous, weakening extremes over dry zones. This residual may be related to Hadley cell expansion, although this explanation is insufficient to explain many features, and other possible mechanisms are discussed.

Plain Language Summary

In a warming climate, the most extreme rainfall is projected to intensify, including in the subtropical dry regions, for example, Southern California and the Mediterranean. This will occur partly because a warmer atmosphere can hold more moisture, which implies more intense rainfall, but also because the vertical motion in the atmosphere that generates rainfall may undergo changes. We study the component relating to vertical motion in the subtropics and find that this results partly from a weakening/strengthening of vertical motion (weakening for moderate rainfall but strengthening for the most extreme rainfall). This effect will lead to a stronger contrast between the moderate rainfall events and the most extreme as the 21st century progresses. Thus, water resources in subtropical regions will become increasingly challenging to manage. In addition to this vertical-motion effect, a poleward shift of the circulation will have a drying effect in certain dry regions on both moderate rainfall and the most extreme rainfall, which may offset the effect of strengthening vertical motion.