Optimally coherent sets in geophysical flows: A new approach to delimiting the stratospheric polar vortex

TL;DR

This paper introduces a transfer operator-based method to objectively identify the polar vortex boundary in the stratosphere, improving the understanding of its structure and transport barriers in atmospheric flows.

Contribution

A novel transfer operator technique is developed to accurately delineate the polar vortex boundary in three dimensions, enhancing previous observational and computational methods.

Findings

Improved 3D estimates of the vortex location in the upper stratosphere.

The method effectively identifies regions with minimal external transport.

Potential applications in various fluid dynamical systems.

Abstract

The "edge" of the Antarctic polar vortex is known to behave as a barrier to the meridional (poleward) transport of ozone during the austral winter. This chemical isolation of the polar vortex from the middle and low latitudes produces an ozone minimum in the vortex region, intensifying the ozone hole relative to that which would be produced by photochemical processes alone. Observational determination of the vortex edge remains an active field of research. In this letter, we obtain objective estimates of the structure of the polar vortex by introducing a new technique based on transfer operators that aims to find regions with minimal external transport. Applying this new technique to European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-40 three-dimensional velocity data we produce an improved three-dimensional estimate of the vortex location in the upper stratosphere where the vortex is most pronounced. This novel computational approach has wide potential application in detecting and analysing mixing structures in a variety of atmospheric, oceanographic, and general fluid dynamical settings.