Simulations of linear and nonlinear disturbances in the stratosphere

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.

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