A study of Arctic Oscillation induced by a positive feedback between the polar vortex and baroclinic instability

In this study, baroclinic instability of the northern winter atmosphere is investigated in the context of the dynamical interpretation of the Arctic Oscillation. The unstable solutions, obtained by a method of 3-D normal mode expansion, are compared for observed zonal basic states with strong and weak polar vortices in reference to the Arctic Oscillation index.

As a result of the eigenvalue problem of the linear stability analysis, we obtain a characteristic unstable solution that dominates in high latitudes when the polar vortex is strong. The mode is called a monopole Charney mode M_1, which is similar to an ordinary Charney mode M_C in mid-latitudes. In order to understand the origin of the M_1 mode, a hypothetical zonal basic state which has only the polar jet with no subtropical jet is analyzed. We find that the M_1 mode in high latitudes is excited by the baroclinicity associated with the polar vortex. Dynamically, the M_1 mode is indistinguishable from the ordinary Charney mode M_C. But, physically, the former is excited by the baroclinicity of the polar jet and the later by that of the subtropical jet.

As the M_C mode intensifies the subtropical jet by the eddy momentum transfer, the M_1 mode transfers eddy momentum to high latitudes to intensify the polar jet. Since M_1 mode appears during the strong polar jet and feeds the westerly momentum to the polar jet, there is a positive feedback between the M_1 mode and the polar vortex. This positive feedback would produce two persistent equilibria of strong and weak polar jets, which may in tern result in the occurrence of the annular mode of the Arctic Oscillation.