Abstract Changes in atmospheric compositions (e.g., the rising greenhouse gases, Antarctic ozone hole, or changes in air pollution) can alter the Hadley cell circulations and midlatitude jet streams in the troposphere, or the Brewer-Dobson Circulation (BDC) in the stratosphere. The mechanisms of the circulation changes will be discussed with both idealized and comprehensive GCMs, particularly focusing on the effects of tropical warming on (i) the tropospheric circulation responses to El Nino versus global warming and (ii) the structure of the BDC.
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 mid-latitude eddy-driven jet are opposite in sign. By comparing the circulation changes to several patterns of tropical warmings in an idealized model, we illustrate the important distinctions between the processes associated with upper-tropospheric baroclinicity and lower-tropospheric baroclinicity. 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.
In the stratosphere, the zonal asymmetry of SST perturbations is found to have a major impact on the vertical and meridional structures of BDC in an aquaplanet model as compared with other SST characteristics. Zonally localized SST warmings tend to generate a shallow acceleration of stratospheric residual circulation and a reduction in the Age of Air (AOA) mostly within the polar vortex. By contrast, SST warmings with nearly zonal structures tend to produce a deep strengthening of stratospheric residual circulation and a decline of the AOA in the entire stratosphere. The varied depth of stratospheric residual circulation change has been linked to the wave propagation and dissipation in the subtropical lower stratosphere rather than the wave generation in the troposphere.