Baroclinic instability in high latitudes induced by polar vortex: A connection to the arctic oscillation

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 3D 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, a characteristic unstable solution is obtained that dominates in high latitudes when the polar vortex is strong. The mode is called a monopole Charney mode M₁, which is similar to an ordinary Charney mode M_c in midlatitudes. In order to understand the origin of the M₁ mode, a hypothetical zonal basic state that has only the polar jet with no subtropical jet is analyzed. It is found that the M₁ mode in high latitudes is excited by the baroclinicity associated with the polar vortex. The M_l mode in high latitudes is dynamically the same Charney mode as M_c but is excited by the baroclinicity of the polar jet instead of the subtropical jet. As the M_c mode intensifies the subtropical jet by the eddy momentum transfer, the M₁ mode transfer eddy momentum to high latitudes to intensify the polar jet. Since M₁ mode appears during the strong polar jet and feeds the westerly momentum to the polar jet, there is a positive feedback between the M_l mode and the polar vortex. This positive feedback would produce a persistent strong polar jet, which may in tern result in the occurrence of the annular mode of the Arctic oscillation.