Roles of Upper- versus Lower-level Thermal Forcing in Shifting the Eddy-Driven Jet

One most drastic change in the global warming scenarios is the increase in air temperature over tropical upper-troposphere and polar surface. The strong warming over those two regions alters the spacial distributions of the baroclinicity in the upper-troposphere of subtropics and in the lower-level of subpolar region. Previous studies indicate that such changes in the upper and lower level baroclinicity have competing effects on the midlatitude atmospheric circulation. The final destination of the eddy-driven jet in future climate could be a tug of war between the impacts of upper- and lower-level thermal forcing, and understanding the relative roles of upper- and lower-level baroclinicity in shifting the eddy-driven jet has important implication for future climate projection.

This study approaches the question by carrying out groups of sensitivity experiments to the upper and lower level thermal forcing by using a multi-level quasi-geostrophic channel model. Our study shows that, for all the sensitivity experiments, the upper level thermal forcing is more efficient in shifting the eddy-driven jet. We argue that the dominance of the upper-level thermal forcing over the lower-level thermal forcing can be attributed to their different mechanisms that drive the jet shift. The upper level thermal forcing changes the eddy momentum flux mainly by affecting the baroclinic growth. However, the lower-level thermal forcing influences the eddy momentum flux mainly by affecting the upper level thermal wind thus the nonlinear eddy breaking and dissipation in the upper troposphere. The former mechanism turns out to be more efficient in shifting the eddy driven jet.