Dynamical sensitivity of the stratospheric circulation and downward influence of upper level perturbations

Abstract

The response of the stratospheric circulation to a perturbation to the zonal mean flow applied above some level zc in the extratropical middle or upper stratosphere is considered, focusing in particular on the possibility that there is a significant response in the lower stratosphere (and hence, the troposphere, though the potentially important feedbacks in the troposphere between baroclinic eddies and zonal mean flow are not considered). The response is considered first in one‐dimensional wave‐mean models of Holton‐Mass type, with a steady Rossby wave forcing of amplitude Φ0 applied at the lower boundary, which for small values of Φ0 have a stable steady state and for larger values of Φ0 have stable regular vacillations. The response of the zonally symmetric system with Φ0 = 0 is well known. The results here show that when Φ0 takes moderate values, the response to a perturbation is similar to that in the zonally symmetric case, i.e., downward propagation but with the amplitude of the response decaying exponentially downward. As Φ0 increases, the amplitude of the response at a given level below zc tends to increase, but a more spectacular change occurs when the value of Φ0 is sufficiently large for there to be regular vacillation rather than a steady stage. The response below zc is then significantly larger than for small Φ0. The response is then considered in three‐dimensional models, one with geopotential imposed on an artificial lower boundary and the other with a topographic lower boundary condition. In these models too the response to an imposed upper level perturbation is shown to depend on the amplitude of the wave forcing in the underlying state, but here the existence of steady states and vacillating states is not as straightforward as in the 1‐D case, and it seems clearer to distinguish between dynamically insensitive cases (e.g., small wave forcing) and dynamically sensitive cases (e.g., large wave forcing), with a much stronger low level response to upper level forcing in the latter case.