Abstract

Changes in atmospheric compositions (e.g., the rising greenhouse gases, changes in air pollution or Antarctic ozone hole or recovery) can alter the latitudes of midlatitude jet streams, and consequently impact the hydrological cycles. The mechanisms of the circulation changes are studied with both idealized and comprehensive GCMs. Particularly, although El Nino and global warming are both characterized by warming in the tropical upper troposphere, the latitudinal changes of mid-latitude eddy-driven jet are opposite in sign. By comparing the circulation changes to several patterns of tropical warmings in a dry atmospheric model, we will illustrate the important distinctions between the processes associated with upper-tropospheric baroclinicity and lower-tropospheric baroclinicity.

With an initial value approach, the circulation changes in idealized models are compared with classical paradigms of baroclinic eddy life cycles. It is found that while the change in the lower-level baroclinicity is important for the (El Nino-like) equatorward contraction in circulation under narrow tropical warming, the change in the upper-level baroclinicity dominates the (global warming-like) poleward shift in circulation in response to broad tropical warming. Additionally, it is found that the circulation change be better understood by a hybrid Eulerian-Lagrangian diagnostic that quantifies the eddy diffusivity change associated with wave breaking in the upper troposphere. The eddy diffusivities in quasi-geostrophic flow and isentropic flow will be also briefly compared.