Regional Attribution of Tropical Expansion

The width of the tropical Hadley circulation (HC) has garnered intense interest in recent decades, owing to the emerging evidence for its expansion in observations and models, and to the anticipated impacts on surface climate in its descending branches. Recent work by the US CLIVAR Tropical Width Working Group and the ISSI Tropical Width Diagnostic Intercomparison Project has highlighted the need to understand the regional structure and impacts of tropical widening. To this end, we generalize the zonal mean HC to the regional level by defining meridional overturning cells (RC) using the horizontally divergent wind. We find that the edges of the RC are more closely connected to surface hydroclimate than more traditional metrics of regional tropical width (such as the sea level pressure ridge), or even than the zonal mean HC. We find that the weakening of the RC is a more robust response to greenhouse gas increases than the widening at a regional level. For example, simulated widening of the zonal mean HC in the Southern Hemisphere appears to arise in large part from regional overturning anomalies over the Eastern Pacific, where there is no clear RC. Unforced interannual variability in the position of the zonal mean HC edge is associated with a more general regional widening. These distinct regional signatures suggest that the RC may be well suited for the attribution of observed circulation trends. We find that the spatial pattern of regional meridional overturning trends in reanalyses corresponds more closely to the pattern associated with unforced interannual variability than to the pattern associated with CO2 forcing, suggesting a large contribution of internal atmospheric variability to the recent observed tropical widening trends. Methods for diagnosing local regional widening due to eddies and mean flow are proposed.