A Baroclinic Mechanism for the Eddy Feedback on the Zonal Index

Abstract

A baroclinic mechanism for the positive eddy feedback on the zonal index is proposed. The author considers the quasigeostrophic response of the zonally averaged flow to forcing by baroclinic eddies. The transient response to eddy forcing is a largely barotropic acceleration of the westerly flow and a reduction of the baroclinicity at the latitudes of eddy generation, but over time, the action of surface drag leads to enhanced baroclinicity at these latitudes. The steady-state response has positive baroclinicity at the latitude of eddy generation, if the eddies propagate away from this latitude before they dissipate. This reinforcement of the low-level baroclinicity provides a positive feedback, if it is assumed that baroclinic eddies are generated more vigorously in regions of stronger low-level baroclinicity. The proposed mechanism explains observed and modeled features of zonal index variations: the frequency and drag dependence of eddy feedback, the bandedness in latitude of zonal wind variations, and the tendency for anomalies in the zonally averaged zonal wind to drift poleward over time.