Mechanisms that control the latitude of jet streams and surface westerlies.

摘要

Observations and climate models have shown that the extratropical zonal mean zonal winds experience a latitudinal shift with an equivalent barotropic structure from surface westerlies to upper tropospheric jets, in response to several major climate forcings including increasing greenhouse gases, stratospheric ozone depletion, volcanic forcing, and the ENSO (El Nino and Southern Oscillation) cycle. We have performed a number of idealized model experiments to study the mechanisms for these jet movements, using more generic forcings such as changes in surface friction and prescribed zonal torques. Our studies suggest that these jet movements in idealized models and possibly during climate change can be explained by quasi-linear Rossby wave propagation in the upper troposphere and wave activity absorption near the critical latitudes, where the eastward propagation speed of eddies equals the background zonal mean zonal wind.;We further explore the tropospheric jet shift to a prescribed zonal torque in a model with high stratospheric resolution. The jet moves in opposite directions for the torques on the jet's equatorward and poleward flanks in the troposphere. This can be explained by different ways of modifying the critical latitudes of wave activity absorption. However, the jet moves in the same direction for the torque in the extratropical stratosphere irrespective of the latitude of the torque. The stratospheric eddies play the key role in transferring zonal wind anomalies downwards into the troposphere. We argue that these stratospheric zonal wind anomalies can affect the tropospheric jet by altering the eastward propagation of tropospheric eddies.;The tropospheric eddies display a trend towards faster eastward phase speeds in the observations and model simulations for the late 20th century, and in the model projections for the 21st century. We argue that the increased lower stratospheric or upper tropospheric zonal winds, associated with stratospheric ozone depletion or global warming, can be sufficient to increase eddy phase speeds so as to shift the circulation polewards. The trend is very similar in structure to the internal inter-annual variability due to atmospheric eddy-mean flow interactions, rather than the SST-forced variability during the ENSO cycle. This suggests that the observed and simulated shifts of surface westerlies can be more related to the processes associated with the extratropical internal variability such as the variations in the stratospheric polar vortex, rather than those for the tropical-extratropical interactions.;We first vary the strength of surface friction in an idealized dry model of the troposphere. The midlatitude jet is displaced poleward when the surface friction is reduced. If the friction on the zonal mean flow is reduced instantaneously, the response reveals two distinctive adjustment time scales. In the fast adjustment over the first 10--20 days, there is an increase in the barotropic component of zonal winds and a substantial decrease in the eddy kinetic energy; the shift in the surface westerlies and jet latitude occurs in a slower adjustment. The space-time eddy momentum flux spectra suggest that the key to the shift is a poleward movement in the subtropical critical latitude associated with the faster eastward phase speeds in the dominant midlatitude eddies.