Instability of Baroclinic Currents That Are Locally Nonzonal

Abstract

The zonally localized instability of a zonal flow that has an added meridional component in the zonal interval |x| < a is examined. The flow occurs on the infinite β plane and is analyzed in a two-layer model. The basic flow in the lower layer is zero. The zonal flow velocity is small enough so that it is subcritical with regard to baroclinic instability. The flow is rendered unstable only by its horizontal change of direction, which introduces the meridional flow. The stability problem is solved by matching simple plane wave solutions in the regions upstream, downstream, and within the region of meridional flow. Localized instability is found for all values of the unstable interval a. The growth rate diminishes as a shrinks, but no critical value of a ≠ 0 needs to be exceeded for instability. For small values of a, the disturbance extends well beyond a as a wake of slowly damped downstream radiating waves. However, the heat fluxes driving the instability are sharply confined to |x| < a. Although the instability is possible only because the meridional component of the flow is different from zero, the baroclinic energy conversion of the available potential energy associated with the zonal flow is at least as large as, and usually larger than, that associated with the meridional flow. The authors suggest that the process described in this simple model may be present whenever zonally accelerated flows are locally unstable since the region of zonal acceleration of the flow must be accompanied by meridional flow.