Simulations of an observed stratospheric warming with quasigeostrophic refractive index as a model diagnostic

Abstract

A three-dimensional primitive equation model of the stratosphere and mesosphere is described. The model was forced at its lower boundary by observed 100 mb height fields for the wavenumber 2 stratospheric warming period of February 1979, and correctly simulated the reversal of the high latitude circulation. This behaviour contrasts with earlier model simulations of wavenumber 2 warmings in which forcing of a climatological zonally symmetric initial circulation by a stationary wave perturbation led to an initial reversal of the circulation in low latitudes. In a series of idealized experiments we show that wave-wave interaction played no essential role in this simulation and that the ingredients leading to its success were firstly the non-climatological initial wind structure and secondly an imposed longitudinal phase speed for the upward propagating planetary wave at the lower boundary. These studies also demonstrate that in the prewarming period the direction of propagation of planetary wave activity, as represented by integral curves of the so-called Eliassen-Palm flux, may be qualitatively described by the WKBJ limit of the quasi-geostrophic potential vorticity equation. In this limit, the Eliassen-Palm flux is simply related to the zonal mean refractive index, leading to two complementary diagnostics for studying wave, mean-flow interaction.