Distinguishing the mechanisms of the zonal mean atmospheric circulation response to global warming versus El Niño---role of the irreversible PV mixing

ABSTRACT

Although El Nino and global warming are both characterized by a warming in the tropical upper troposphere, the latitudinal changes of the Hadley cell edges and mid-latitude eddy-driven jets 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 under strong tropical temperature gradient or narrow tropical warming, similar to the changes associated with El Nino events. In contrast, the zonal mean characters of the circulation expand or shift poleward in response to upper tropospheric warming or broad tropical warming, resembling the changes under future global warming.

The mechanisms of the opposite changes in circulation are investigated by comparing between the dry dynamical response to a narrow tropical warming and a broad warming as analogues for El Niño and global warming. It turns out that both the narrow and wide forcing give rise to an equatorward shift of the jet when the eddy feedback is disabled by running the model under an axis-symmetric configuration. With the eddy feedback turned back on, the wide forcing drives a poleward shift of the eddy-driven jet and poleward expansion of the Hadley cell, underlining the key role of the eddy adjustment in determining the destination of the final circulation response.

The processes of the eddy adjustment are further examined using sets of large ensembles of experiment under a sudden switch-on of the forcing. For both narrow and broad tropical warming, the jets move equatorward initially. In the subsequent adjustment, the initial equatorward shift is further enhanced and sustained by the low level baroclinicity and the accompanying upward wave activity fluxes at the equatorward flank of the jet under the narrow tropical warming, whereas the initial equatorward shift transitions to a poleward shift under the broad warming. This equatorward-to-poleward transition is further analyzed with a novel finite-amplitude wave activity diagnostics. It is found that the broad tropical warming enhances the irreversible mixing of PV hence the dissipation of the wave activity in the subtropical upper troposphere and reduces the diffusivity and associated wave activity dissipation poleward of it. As a result, an equatorward wave activity propagation ensues between the anomalous source and sink, resulting in a net acceleration at the poleward side and a deceleration at the equatorward side of the climatological jet and an poleward displacement of the eddy-driven circulation.