Abstract

The intensity and position of the Southern Hemisphere stratospheric polar vortex edge is evaluated as a function of equivalent latitude over the 1979–2020 period on three isentropic levels (475 K, 550 K and 675 K) from ECMWF ERA-Interim reanalysis. The study also includes an analysis of the onset and breakup dates of the polar vortex, which are determined from wind thresholds (e.g. 15.2 m.s−1, 20 m.s−1and 25 m.s−1) along the vortex edge. The vortex edge is stronger in late winter, over September–October – November with the period of strongest intensity occurring later at the lowermost level. A lower variability of the edge position is observed during the same period. Long-term increase of the vortex edge intensity and break-up date is observed over the 1979–1999 period, linked to the increase of the ozone hole. Long-term decrease of the vortex onset date related to the 25 m.s−1wind threshold is also observed at 475 K during this period. The solar cycle and to a lower extent the quasi-biennal oscillation (QBO) and El Niño Southern Oscillation (ENSO) modulate the inter-annual evolution of the strength of the vortex edge and the vortex breakup dates. Stronger vortex edge and longer vortex duration is observed in solar minimum (minSC) years, with the QBO and ENSO further modulating the solar cycle influence, especially at 475 K and 550 K: during West QBO (wQBO) phases, the difference between vortex edge intensity for minSC and maxSC years is smaller than during East QBO (eQBO) phases. The polar vortex edge is stronger and lasts longer for maxSC/wQBO years than for maxSC/eQBO years. ENSO has a weaker impact but the vortex edge is somewhat stronger during cold ENSO phases for both minSC and maxSC years.

Highlights

  • The stratospheric polar vortex is a seasonal low-pressure system characterized by a strong wind belt that isolates polar air from lower latitudes

  • We have analyzed the seasonal evolution of the stratospheric polar vortex edge intensity and position as a function of equivalent latitude in the Southern hemisphere at three isentropic levels, using European Centre for Medium-Range Weather Forecast (ECMWF) data over the 1979 - 2020 period

  • The vortex edge intensity corresponds to the gradient of the potential vorticity weighted by the wind module as a function of equivalent latitude (Nash et al, 1996)

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Summary

Introduction

The stratospheric polar vortex is a seasonal low-pressure system characterized by a strong wind belt that isolates polar air from lower latitudes. As the incident solar energy decreases, and the gradient of temperature between the pole and the tropics becomes stronger, the 20 strength of the stratospheric westerly winds increases. When the winds reach a critical value, a large-scale vortex is formed, which extends from the lowermost stratosphere to the stratopause. An altitude of about 14 km, the vortex edge region is stable and constitutes a powerful barrier, preventing mixing of cold polar air with warmer air masses from lower latitudes. Over Antarctica, the polar vortex is generally present from May until the end of November. The less 25 stable Arctic polar vortex forms in November and lasts until the end of February or early April, depending on the year.

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