The Key Role of Lower-Level Meridional Shear in Baroclinic Wave Life Cycles

Abstract

A series of idealized nonlinear life cycle experiments is performed to compare changes in life cycle behavior caused by upper-level and near-surface meridional shear of the initial zonal wind. It is shown that both the eddy kinetic energy and the zonal flow accelerations produced during a cycle of growth and equilibration respond primarily to the meridional shear of the zonal wind near the surface and only weakly to shear near the tropopause. Near the critical shear for transition from anticyclonic to cyclonic life cycle behavior, the zonal flow accelerations are minimized and the eddy persistence is maximized. Above this critical shear, eddy breaking on the poleward side of the jet increases and strong cyclonic zonal wind shears are generated. The influence of baroclinic shears is minimized by using dipolar wind anomalies that are zero near the center of a basic baroclinic jet and by taking advantage of the fact that the life cycle response is very sensitive to small changes in the magnitude of initial meridional shear. The small baroclinic shears contribute to the differences in the sense that a cyclonic shear decreasing with height is slightly more effective in inducing cyclonic behavior than is a barotropic cyclonic shear with the same surface value. Upper-tropospheric eddy momentum fluxes by linear normal modes are also much more sensitive to lower-tropospheric meridional shear than to upper-tropospheric meridional shear. The primary response of normal modes to lower-tropospheric meridional shear is to change the momentum flux at all levels.