Mechanisms of the zonal mean atmospheric circulation responses to tropical warming

Although El Nino and global warming are both characterized by atmospheric temperature increase in the tropics, the changes of the latitudes of the Hadley cell widths and the eddy-driven jets are opposite. Using an idealized atmospheric model, the different changes in circulation are attributed to, at least partially, different patterns of tropical warming. In particular, the circulation change under narrow tropical warming is similar to that of observed El Nino event, while the circulation change with broad warming resembles that of global warming. Running the same model without any changes in eddies produces equatorward circulation shifts in spite of the width of warming, implicative of the critical role of midlatitude eddy feedbacks. The mechanisms of eddy feedbacks are studied by large ensembles of transient experiments with a sudden switch-on of tropical warming. The jets move equatorward initially for both narrow and broad tropical warmings, consistent with the circulation change without eddy feedback. The initial equatorward jet drift under narrow warming is sustained by the change in the surface baroclinicity. However, the initial equatorward shift with broad warming is overwhelmed by the change in the upper tropospheric wave breaking. Broad tropical warming enhances irreversible eddy mixing (defined formally by the concept of effective diffusivity) in the subtropics and displaces the eddy-driven circulation poleward, followed by the poleward shift in the surface baroclinicity. Overall, these results point to the limitation of small-amplitude linear theories in explaining the circulation change and highlight the important role of the upper-level irreversible mixing in driving the latitudinal shift of the midlatitude jet and the Hadley cell under climate change.