Intra-seasonal variability of extreme boreal stratospheric polar vortex events and their precursors
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The dynamical variability of the boreal stratospheric polar vortex has been usually analysed considering the extended winter as a whole or only focusing on December, January and February. Yet recent studies have found intra-seasonal differences in the boreal stratospheric dynamics. In this study, the intra-seasonal variability of anomalous wave activity preceding polar vortex extremes in the Northern Hemisphere is examined using ERA-Interim reanalysis data. Weak (WPV) and strong (SPV) polar vortex events are grouped into early, mid- or late winter sub-periods depending on the onset date. Overall, the strongest (weakest) wave-activity anomalies preceding polar vortex extremes are found in mid- (early) winter. Most of WPV (SPV) events in early winter occur under the influence of east (west) phase of the Quasi-Biennial Oscillation (QBO) and an enhancement (inhibition) of wavenumber-1 wave activity (WN1). Mid- and late winter WPV events are preceded by a strong vortex and an enhancement of WN1 and WN2, but the spatial structure of the anomalous wave activity and the phase of the QBO are different. Prior to mid-winter WPVs the enhancement of WN2 is related to the predominance of La Niña and linked to blockings over Siberia. Mid-winter SPV events show a negative phase of the Pacific-North America pattern that inhibits WN1 injected into the stratosphere. This study suggests that dynamical features preceding extreme polar vortex events in mid-winter should not be generalized to other winter sub-periods.
KeywordsIntra-seasonal variability Wave activity Stratospheric polar vortex extremes Tropospheric precursors Tropospheric forcing Stratospheric dynamics
The authors are grateful to two anonymous reviewers, whose comments help improve this manuscript. This work was supported by the Spanish Ministry of Economy and Competitiveness (grant number CGL2012-34997). BA is supported by the Natural Environment Research Council (grant number NE/M006123/1). MA acknowledges funding from the NASA ACMAP program. The ERA-Interim reanalysis data were obtained online at http://data-portal.ecmwf.int/data/d/interim full moda/. ENSO and QBO indices have been taken from http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears_ERSSTv3b.shtml and http://www.cpc.noaa.gov/data/indices/, respectively.
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