Reconciling Hadley Cell Expansion Trend Estimates in Reanalyses

Abstract

Numerous studies have concluded that historical Hadley cell expansion simulated in reanalyses is much larger than the future expansion predicted by climate model simulations. Is Hadley cell expansion too weak in climate models, or are the trends in reanalyses spuriously large? This study shows that the mean meridional circulation in reanalyses generally does not conserve mass. The mass imbalance projects onto trends in the Hadley cell edge latitudes by modifying both the mean and anomalous circulation. In correcting for the imbalance, the majority of Hadley cell expansion trends in early-generation reanalyses in both hemispheres are revised to be smaller in magnitude, bringing them into closer agreement with the trends in modern reanalyses and climate models. While the methodology presented here is statistical in nature, it produces quantitatively similar results to a more sophisticated mass budget correction method.