Sensitivity of the Tropospheric Circulation to Changes in the Strength of the Stratospheric Polar Vortex

Abstract

The sensitivity of the wintertime tropospheric circulation to changes in the strength of the Northern Hemisphere stratospheric polar vortex is studied using one of the latest versions of the ECMWF model. Three sets of experiments were carried out: one control integration and two integrations in which the strength of the stratospheric polar vortex has been gradually reduced and increased, respectively, during the course of the integration. The strength of the polar vortex is changed by applying a forcing to the model tendencies in the stratosphere only. The forcing has been obtained using the adjoint technique. It is shown that, in the ECMWF model, changes in the strength of the polar vortex in the middle and lower stratosphere have a significant and slightly delayed (on the order of days) impact on the tropospheric circulation. The tropospheric response shows some resemblance to the North Atlantic Oscillation (NAO), though the centers of action are slightly shifted toward the east compared to those of the NAO. Furthermore, a separate comparison of the response to a weak and strong vortex forcing suggests that to first order the tropospheric response is linear within a range of realistic stratospheric perturbations. From the results presented, it is argued that extended-range forecasts in the European area particularly benefit from the stratosphere?troposphere link.