The Roles of Barotropic and Baroclinic Processes in the Atmospheric Response to the Lower Tropospheric Thermal Forcing

Both observations and climate model simulations have shown that the eddy-driven jet exhibits significant meridional shift in response to the lower boundary thermal forcing, such as the recent Arctic amplified warming induced by the sea ice loss, El Nino-like oceanic warming and extratropical sea surface temperature anomalies. Understanding the dynamical mechanisms of the atmospheric response to such lower boundary thermal forcing is central for the prediction of the mid-latitude climate and the evaluation of the climate sensitivity.

Previous studies suggest that, in addition to the direct thermal wind adjustment, the eddy feedbacks, including the changes in the baroclinic processes such as the lower-level eddy generation and the barotropic processes such as the upper-level wave propagation and breaking could play important roles in the atmospheric response to the thermal forcing. In this study, using a nonlinear beta plane quasi-geostrophic channel model, the mechanism through which lower level thermal forcing affecting the jet shift is investigated. Further more, by diagnosing the finite amplitude wave activity budget and setting up overriding experiments, the relative roles of baroclinic and barotropic processes in the eddy feedbacks to the lower-level thermal forcing are estimated and explicitly compared. With the two methodologies, our results show that the lower-level thermal forcing affects the eddy-driven jet rapidly by modifying the upper-level zonal thermal wind distribution, as well as the associated meridional wave propagation and breaking. Unlike the traditional baroclinic viewpoint, our results suggest that the barotropic eddy feedback dominates the total atmospheric response to the lower boundary thermal forcing.