Rossby Wave Breaking in the Antarctic Stratosphere during the Southern Spring

The present work aims to a better understanding of the mechanisms responsible for air transport inside, outside, and across the periphery of the stratospheric polar vortex (SPV) during the southern spring. SPV in this season defines a complex flow structure; air inside is primarily isolated from the outside (Chen 1994; Beron-Vera et al. 2012). The vortex edge acts as an effective barrier to transport. However, episodes of air crossing develop sporadically (Bowman 1993). The associated horizontal transport and mixing during those episodes may affect the ozone distribution. A recent study by de la Camara et al. (2013; hereafter DLC2013) examined the Lagrangian structure of the flow in the Antarctic lower stratosphere during the southern spring of 2005. During this period a field campaign released super-pressure balloons that provided an approximate view of the air parcel trajectories. Using a Lagrangian analysis, DLC2013 found hyperbolic trajectories both outside and inside the SPV. Hyperbolic trajectories are Lagrangian structures that trace out the movement of material hyperbolic points, and provide a relevant framework to study the (Lagrangian) transport in general time-dependent flows. DLC2013 conjectured that hyperbolic trajectories signify Rossby wave breaking; hence identifying such trajectories imply transport in and out of the SPV. The findings of DLC2013 motivated us to numerically investigate the processes leading to wave breaking and tracer transport inside and around the SPV during southern spring. Our methodology for research is based on idealizing the flow in the lower stratosphere as a one-layer, shallow-water system on a sphere. These model equations are horizontally discretized using a numerical scheme that conserves potential enstrophy. The initial flow correspond to a zonal-wind which mimics the zonally averaged flow on 1st September 2005 on the 475K isentropic surface. The average shallow water height is 8,000m, with the height at the South Pole being 5,800m. The model resolution is 144x90, time-step is 1.5 seconds, and runs are 60-day long The flow is perturbed by varying the height of the lower boundary as follows: a) Stationary forcing: This is represented by a topography distribution that is spatially localized, but whose amplitude increases in time from 0 to 1500m during the first 10 days of integration. In the north-south direction the topography profile is a Gaussian centred at 45S and with with 15-degrees width; in the west-east direction the profile corresponds to zonal wavenumber 1. This forces a quasi-stationary Rossby wave of the same wavenumber, which finally breaks outside the SPV. Using a Lagrangian analysis on the computed data, we have successfully captured hyperbolic trajectories in the surf-zone. Numerical experiments with this stationary forcing, however, have not produced evidence of Rossby wave breaking inside the vortex. b) Eastward travelling forcing: According to linear theory, a critical layer develops when the phase-speed of the wave matches the zonal flow speed. Hence Rossby waves should ideally break at critical latitudes. Our working hypothesis is that hyperbolic trajectories such as those observed by DLC2013 within the SPV are associated with Rossby wave breaking; hence our goal here is to reproduce this phenomenon. The transient forcing is such that it creates an eastward traveling wave of wavenumber 1 centred at 70S. The phase-speed of this wave corresponds to the initial zonal mean velocity at 70S. Using Lagrangian analysis on the computed data, we show that these simulations capture hyperbolic trajectory inside the SPV. This verifies the conjecture of DLC2013 regarding Rossby wave breaking inside the SPV. Results on tracer transport occurring due to the wave breaking will be presented at the conference.