Regional Widening of Tropical Overturning: Forced Change, Natural Variability, and Recent Trends.

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. To better clarify the causes and impacts of tropical widening, this work generalizes the zonal mean HC to the regional level by defining meridional overturning cells (RC) using the horizontally divergent wind. 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. Simulations reveal a robust weakening of the RC in response to greenhouse gas increases, along with a widening of the RC in some regions. 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 RCs may be well suited for the attribution of observed circulation trends. 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 natural variability to the recent observed tropical widening trends. Plain Language Summary: The large‐scale tropical wind patterns responsible for the dry desert belts on either side of the tropics appear to be expanding. Most explanations for this widening are based on the tropics as a whole and do little to address whether the subtropics are widening from one region to the next. This study extends the definition of tropical width—specifically the Hadley cell edge—to the regional level. The regional tropical edge lines up with deserts better than another, more conventional regional indicator. The observed pattern of tropical widening more closely resembles natural changes than human‐caused widening during much of the year. Key Points: The local tropical edge diagnostic follows spatial hydroclimate variations more closely than does the subtropical ridgeObserved widening, forced widening, and interannual variations each have their own regional signatureThe pattern of observed regional widening more closely matches unforced interannual variations than forced regional widening [ABSTRACT FROM AUTHOR]