The Response of the Hadley Circulation to Climate Changes, Past and Future

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Abstract

A suite of altered climate experiments for the Paleocene, the last glacial maximum (LGM), and a 2 × CO2 climate were compared to assess the factors responsible for producing variations in Hadley cell intensity and extent. The climate simulations used best-guess topography and marine surface fields, as well as feasible alternative sea surface temperature (SST) patterns. The individual contributions to the circulations were quantified, and compared among the different simulations. The results show that the Hadley cell intensity is associated with the gradient in latent heat release from the tropics to the subtropics, driven in the model by the gradient in sea surface temperature. It is not related to the absolute warmth of the climate, or of the tropical sea surface temperatures. Eddy forcing, primarily through transient eddy heat transport, amplified the subtropical portion of the cell, as well as the mid-latitude Ferrel cell. The poleward extent of the Hadley cell is affected by numerous processes, including the influence of topography in the extratropics. It also does not vary systematically with global mean temperature. Only the strongest Hadley cell changes are longitudinally homogeneous; there is little relationship between the change in Hadley cell intensity and the change in strength of the Walker cell, and the Pacific Ocean is the most important basin for the zonal average Hadley cell response. Although the latitudinal average precipitation does respond interactively with Hadley cell intensity and extent, the soil moisture variations are less correlated, due to differing seasonal effects and the influence of temperature/evaporation changes. The importance of the Hadley cell variations for assessing past and future water availability changes should not be overestimated, although it is a contributing factor.