Reconciling Hadley Cell Expansion Trend Estimates in Reanalyses

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. Plain Language Summary Reanalyses are weather forecast models that use observations to constrain hindcasts of the past evolution of Earth's atmosphere. They are one tool used to study the Hadley cells, two large circulation cells in the tropics with far-reaching impacts on Earth's climate. The amount the Hadley cells expanded in older reanalysis products was much larger than what was simulated in climate models. However, some of this expansion is due to the violation of a basic physical law. When this is corrected, the expansion in older reanalyses is closer to the expansion in more modern reanalyses. It is possible that some of the supposed discrepancy between models and reanalyses was due to these sorts of problems in the older reanalyses.