The Sensitivity of the Hadley Circulation to Past and Future Forcings in Two Climate Models

Abstract

A comparison is made of the Hadley cell response to altered external forcing and climatic boundary conditions in two different climate models. The experiments performed cover past and future climatic forcings that range from the last glacial maximum (LGM) to a doubling in atmospheric CO2. Both models have a consistent response to all forcings in the depth and meridional extent of the Hadley cell in both solstitial seasons. The strength of the December-January-February (DJF) cell varies consistently between the two models, increasing in colder climates and weakening in warmer climates. For June-July-August (JJA), however, the sign of the response is opposite between models for the range of forcing. It is suggested that the origin of this different response in the JJA cell is related to the type of ocean model used. Nevertheless, the results from the LGM and doubled CO2 experiments show a consistent correlation between the surface temperature gradient between the winter and summer hemisphere tropics and Hadley cell strength: When the temperature contrast is larger (smaller), the Hadley cell is stronger (weaker). The Holocene, with the response to the forcing peaking over the subtropical and mid-latitude continents, has temperature gradients and Hadley cell responses that deviate from this relationship. The implications of these results to paleoclimate studies are discussed.