Further Probing the Mechanisms Driving Projected Subtropical Decreases of Extreme Precipitation Intensity.

Abstract Regional projections of extreme precipitation intensity (EPI) are strongly influenced by regional projections of “extreme ascent,” i.e. ascending air during periods of extreme precipitation. Earlier studies have performed analysis suggesting that long-term changes in eddy length scale and vertical stability are key factors influencing extreme ascent projections, but these mechanisms have yet to be confirmed with controlled model experiments. In this study, we perform such controlled experiments using a cloud-resolving model (CRM). The selected CRM domains are two locations over the subtropical South Atlantic Ocean where global climate models consistently project weakening of extreme ascent with accordingly decreased EPI. At each study location, three pairs of 20-year maximum precipitation events are simulated with the CRM, with each pair consisting of an event during the historical period (1981-2000) and an event during the future period (2081-2100), with large-scale forcings for the three pairs derived from three different members of an initial condition ensemble of the Canadian Earth System Model version 2 (CanESM2). These experiments reveal that, in both study locations, weakening of differential cyclonic vorticity advection (dCVA) is a key driver of projected decreases in extreme ascent and EPI. Weakening of dCVA is expected in accordance with hydrostatic balance because, as temperatures warm, the pressure spacing between geopotential surfaces increases. Although there is evidence that the CRM is more sensitive to dCVA changes than CanESM2, such a dCVA mechanism may nonetheless be important to consider for EPI changes in the real world.