Quantifying the Barotropic and Baroclinic Mechanisms in the Persistence of Annular Modes
Here we introduce a method to quantify the strength of barotropic and baroclinic feedback mechanisms through the use of finite-amplitude wave activity formalism. Particularly, the barotropic mechanism is quantified by the variability of wave activity and the eddy mixing of potential vorticity in the upper troposphere and the baroclinic mechanism by the upward wave activity flux from the lower troposphere to the upper troposphere. It is shown that while the wave activity provides a negative feedback to the annular mode variability, eddy mixing and upward wave activity flux provide a positive feedback. These relationships are further investigated in an idealized dry atmospheric model in which varying temperature profiles are used to shift the climatological jet to various latitudes. As the latitude of the jet shifts poleward with tropical warming, there is a substantial reduction in the strength of the baroclinic feedback, while a modest decrease in the strength of the barotropic feedback occurs. This suggests that a combination of both mechanisms is responsible for the decrease in annular mode timescales with increasing latitude in this model, which may have implications for a similar relationship in the CMIP models.