Evolution of the intensity and duration of the Southern Hemisphere stratospheric polar vortex edge for the period 1979–2020
Résumé
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 (475K, 550K and 675K) 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 −1 and 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 breakup 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 −1 wind threshold is also observed at 475K 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 475K and 550K: 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.
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