Abstract
The erosion of a model stratospheric polar vortex in response to bottom boundary forcing is investigated numerically. Stripping of filaments of air from the polar vortex has been implicated in the occurrence of stratospheric sudden warmings (SSWs) but it is not understood in detail what factors determine the rate and amount of stripping. Here a shallow water vortex forced by topography is used to investigate the factors initiating stripping and whether this leads the vortex to undergo an SSW. It is found that the amplitude of topographic forcing must exceed some threshold (of order 200–450m) in order for significant stripping to occur. For larger forcing amplitudes significant stripping occurs, but not as an instantaneous response to the forcing; rather, the forcing appears to initiate a process that ultimately results in stripping several tens of days later. There appears to be no simple quantitative relationship between the amount of mass stripped and the topography amplitude. However, at least over the early stages of the experiments, there is a good correlation between the amount of mass stripped and the global integral of wave activity, which may be interpreted as a measure of the accumulated topographic forcing. Finally there does not appear to be a simple correspondence between amount of mass stripped and the occurrence of an SSW.
Highlights
The stratospheric polar vortex is a dominant feature of the winter stratosphere
The goal of our model experiments is to investigate vortex erosion by strip180 ping of filaments of air as a function of three main parameters: the forcing amplitude given as the maximum mountain height Hmax, the timescale of increase in topography τ and the zonal wavenumber m of the forcing
The two potential vorticity (PV) contours are highlighted in red in figure 5, and as they lie around the vortex edge the transports across them can be interpreted as changes in mass 205 of the vortex
Summary
The stratospheric polar vortex is a dominant feature of the winter stratosphere. Variations in the polar vortex, such as stratospheric sudden warmings (SSWs) can have effects that influence surface climate [1]. The other main approach is to embed a vortex in an externally imposed barotropic flow [e.g. 5, 8, 6] This simulates the effects that other vortices have 35 on the erosion of the polar vortex. The first two regimes show that when the externally imposed strain growth is slow enough the vortex goes through a series of equilibrium states This gives rise to the question in the third bullet point below, which will be investigated using a bottom boundary forcing rather than the externally imposed flow of Legras et al [6]. The topography Φs consists of a zonally symmetric part, Φ, corresponding to the initial condition plus a time-varying mountain, Φpert, which imposes a wave-like forcing on the flow. The zonal mean part of the topography Φ needed to balance the zonal velocity is found by integrating equation (A.1)
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