Interhemispheric Gradients and Tropical Circulation in Global Climate Models

Idealized climate model experiments have shown that extratropical interhemispheric gradients in radiative forcing can drive cross-equatorial atmospheric energy transports via Hadley circulation changes. It has been proposed that such interhemispheric forcing gradients may explain Hadley circulation changes on anthropogenic timescales due to hemispherically asymmetric aerosol forcing and greenhouse feedback mechanisms. However, several factors in the climate system may complicate the influence of extratropical gradients on the meridional tropical circulation, especially considering that there are strong anthropogenic forcings in the tropics as well. The extent that the climate system responds locally or remotely to an extratropical radiative forcing gradient depends on spatially and temporally variable atmospheric dynamics, cloud feedbacks, and ocean heat storage. In this study, we attempt to characterize the tropical circulation response to twentieth-century changes in the interhemispheric gradient of radiative forcing in a suite of fully coupled global climate models from IPCC AR4. First, we investigate the interhemispheric gradients in radiative forcing and clear-sky radiation. Second, we attempt to identify how the extratropical hemispheric gradients affect tropical circulation. We find that the models do not show a consistent tropical circulation response because of variations in local compensation and propagation to the tropics. Thus, an idealized two-hemisphere/two-box energy balance model does not capture the tropical response to asymmetric forcing. A four-box energy balance model, which separates each hemisphere into tropical and extratropical components, better simulates the tropical circulation response to interhemispheric forcing gradients.