No “Wet Gets Wetter” in Kilometer-Scale Mock-Walker Circulations

Many climate model simulations and limited observations indicate that regions of tropical ascent and precipitation contract in response to surface warming. This response has well-studied implications for the width of the zonal- and annual-mean Intertropical Convergence Zone, but its applicability to zonally asymmetric circulations such as the Pacific Walker circulation remains unknown. Here, we investigate the impact of warming on the area of large-scale ascent in kilometer-scale, mock-Walker simulations with both fixed and interactive surface temperatures. Contrary to the “wet-gets-wetter” and “upped-ante” paradigms of precipitation change, the simulations show a “wet-gets-drier” response to warming in which the ascent region becomes larger and, on average, drier. We attribute these changes to rapid circulation weakening, which limits the transport of moisture into the ascent region. To meet the growing moisture demand for precipitation, local evaporation within the ascent region must increase rapidly, and the ascent region expands to draw moisture from a larger surface area. We link the slowdown of the circulation to increases in gross moist stability driven by a previously unknown mechanism. Central to this mechanism are changes in the vertical structure of the circulation, which features two vertically stacked overturning cells reminiscent of some tropical convergence zones. These results challenge long-held paradigms of tropical precipitation change and show that the vertical structure of tropical circulations can play a critical role in the hydrological response to warming.