Distinguishing the Mechanisms of the Zonal Mean Atmospheric Circulation Response to Global Warming versus El Niño---Role of Irreversible PV Mixing

Although El Niño and global warming are both characterized by a warming in the tropical upper troposphere, the latitudinal changes of the Hadley cell boundary and mid-latitude eddy-driven jet location are opposite in sign between these two phenomena. The mechanisms of the zonal mean circulation response to tropical heat sources mimicking El Niño and global warming are investigated using an idealized dry atmospheric model.

It is surprising to find that the traditional WKBJ approach, as employed by previous studies to interpret the extratropical circulation response to El Nino, fails to account for the actual circulation response simulated by the full model. The upshot is that the linear-theoretic approach is limited in explanatory power, and sometimes even misleading in predicting how the extratropical circulation should respond to a thermal perturbation. To understand the poleward shift of the eddy-driven jet and the expansion of the Hadley cell under global warming, theoretical framework more faithfully representing the nonlinear nature of the midlatitude eddy-mean flow interaction must be developed.

In order to delineate the key processes that lead to the final poleward shift of the eddy-driven jet under global warming-like tropical heating, a hybrid Euler-Lagrangian diagnostic framework was developed based on the novel concept of finite-amplitude wave activity of N. Nakamura. It is found that the tropical upper-tropospheric warming enhances the irreversible mixing of potential vorticity (PV) hence the dissipation of the wave activity in the subtropical upper troposphere and reduces the mixing and the associated wave activity dissipation poleward of jet. As a result, an equatorward wave activity flux ensues between the regions of weakened and enhanced mixing, giving rise to a net acceleration at the poleward side and a deceleration at the equatorward side of the jet and hence a poleward displacement of the eddy-driven circulation. It is interesting to note that the diagnosed change in the effective diffusivity for PV mixing is in conformity with the increased (decreased) frequency of the anticyclonic (cyclonic) wave breaking at the equatorward (poleward) flank of the jet, as found in previous studies.