Sensitivities and Mechanisms of the Zonal Mean Atmospheric Circulation Response to Tropical Warming

Although El Nino and global warming are both characterized by warming in the tropical upper troposphere, the latitudinal changes of the Hadley cell edge and midlatitude eddy-driven jet are opposite in sign. Using an idealized dry atmospheric model, the zonal mean circulation changes are investigated with respect to different patterns of tropical warming. Generally speaking, an equatorward shift in circulation takes place in the presence of enhanced tropical temperature gradient or narrow tropical warming, similar to the changes associated with El Nino events. In contrast, the zonal mean atmospheric circulations expand or shift poleward in response to upper-tropospheric warming or broad tropical warming, resembling the changes under future global warming.The mechanisms underlying these opposite changes in circulation are investigated by comparing the dry dynamical responses to a narrow tropical warming and a broad warming as analogs for El Nino and global warming. When running the idealized model in a zonally symmetric configuration in which the eddy feedback is disabled, both the narrow and broad warmings give rise to an equatorward shift of the subtropical jet. The eddy adjustment is further examined using large ensembles of transient response to a sudden switch-on of the forcing. For both narrow and broad tropical warmings, the jets move equatorward initially. In the subsequent adjustment, the initial equatorward shift is further enhanced and sustained by the low-level baroclinicity under the narrow tropical warming, whereas the initial equatorward shift transitions to a poleward shift associated with altered irreversible mixing of potential vorticity in the upper troposphere in the case of broad warming.