Editorial Type:
Article
Article Type:
Research Article

The Splitting of the Stratospheric Polar Vortex in the Southern Hemisphere, September 2002: Dynamical Evolution

Andrew J. Charlton Department of Meteorology, University of Reading, Reading, United Kingdom

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Alan O’Neill Department of Meteorology, University of Reading, Reading, United Kingdom

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William A. Lahoz Department of Meteorology, University of Reading, Reading, United Kingdom

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Paul Berrisford Department of Meteorology, University of Reading, Reading, United Kingdom

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Print Publication:
01 Mar 2005
DOI:
https://doi.org/10.1175/JAS-3318.1
Page(s):
590–602
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Abstract

The polar vortex of the Southern Hemisphere (SH) split dramatically during September 2002. The large-scale dynamical effects were manifest throughout the stratosphere and upper troposphere, corresponding to two distinct cyclonic centers in the upper troposphere–stratosphere system. High-resolution (T511) ECMWF analyses, supplemented by analyses from the Met Office, are used to present a detailed dynamical analysis of the event. First, the anomalous evolution of the SH polar vortex is placed in the context of the evolution that is usually witnessed during spring. Then high-resolution fields of potential vorticity (PV) from ECMWF are used to reveal several dynamical features of the split. Vortex fragments are rapidly sheared out into sheets of high (modulus) PV, which subsequently roll up into distinct synoptic-scale vortices. It is proposed that the stratospheric circulation becomes hydrodynamically unstable through a significant depth of the troposphere–stratosphere system as the polar vortex elongates.

Corresponding author address: Prof. Alan O’Neill, Department of Meteorology, University of Reading, Whiteknights, Reading, RG6 6BB, United Kingdom. Email: alan@met.reading.ac.uk

Abstract

The polar vortex of the Southern Hemisphere (SH) split dramatically during September 2002. The large-scale dynamical effects were manifest throughout the stratosphere and upper troposphere, corresponding to two distinct cyclonic centers in the upper troposphere–stratosphere system. High-resolution (T511) ECMWF analyses, supplemented by analyses from the Met Office, are used to present a detailed dynamical analysis of the event. First, the anomalous evolution of the SH polar vortex is placed in the context of the evolution that is usually witnessed during spring. Then high-resolution fields of potential vorticity (PV) from ECMWF are used to reveal several dynamical features of the split. Vortex fragments are rapidly sheared out into sheets of high (modulus) PV, which subsequently roll up into distinct synoptic-scale vortices. It is proposed that the stratospheric circulation becomes hydrodynamically unstable through a significant depth of the troposphere–stratosphere system as the polar vortex elongates.

Corresponding author address: Prof. Alan O’Neill, Department of Meteorology, University of Reading, Whiteknights, Reading, RG6 6BB, United Kingdom. Email: alan@met.reading.ac.uk

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