Simulations of linear and nonlinear disturbances in the stratosphere

Abstract

Abstract The aim is to study and contrast the dynamics of linear and nonlinear, large‐scale disturbances generated in the stratosphere by a localized perturbation in the troposphere. The perturbation is applied at the lower boundary of a primitive equation model of the stratosphere and mesosphere. The experiments are diagnosed using a range of dynamical quantities; in particular isentropic maps of Ertel's potential vorticity reveal where and how the stratospheric disturbance becomes nonlinear. The simulations are contrasted with an idealized theoretical model of a nonlinear critical layer, and the relevance for our experiments of ‘wave breaking’ and irreversible mixing of potential vorticity is discussed. Apparent polar focusing of Eliassen‐Palm fluxes is related to localized changes in a widening ‘buckling zone’. Wave amplitudes are far too large to apply simple ideas of wave propagation. The notion of pre‐conditioning as it has been applied to the stratosphere is challenged; there are occasions when it is an oversimplification to regard the troposphere as a wavemaker forcing changes in the stratosphere according to the tenets of wave, mean‐flow theory. Preceding a major warming, the flow is asymmetric about any axis (even in the much‐studied case of February 1979), and strong warmings develop through the nonlinear interaction of large, deep vortices associated with a tropospheric asymmetry. The nonlinearity of the flow is also manifested in an anticorrelation in time between zonal harmonics one and two over a deep layer, and by a phase shift of harmonics as amplitudes change. Associated changes in zonal mean winds give the appearance of distinct minor and major warmings, though a synoptic view allows systematic changes to be seen which caution against dividing the evolution into separate events.