Abstract
Abstract. In the beginning of February 2018 a rapid deceleration of the westerly circulation in the polar Northern Hemisphere stratosphere took place, and on 12 February the zonal-mean zonal wind at 60∘ N and 10 hPa reversed to easterly in a sudden stratospheric warming (SSW) event. We
investigate the role of the tropospheric forcing in the occurrence of the
SSW, its predictability and teleconnection with the Madden–Julian
oscillation (MJO) by analysing the European Centre for Medium-Range Weather
Forecasts (ECMWF) ensemble forecast. The SSW was preceded by significant
synoptic wave activity over the Pacific and Atlantic basins, which led to
the upward propagation of wave packets and resulted in the amplification of
a stratospheric wavenumber 2 planetary wave. The dynamical and statistical
analyses indicate that the main tropospheric forcing resulted from an
anticyclonic Rossby wave breaking, subsequent blocking and upward wave
propagation in the Ural Mountains region, in agreement with some previous
studies. The ensemble members which predicted the wind reversal also
reasonably reproduced this chain of events, from the horizontal propagation
of individual wave packets to upward wave-activity fluxes and the
amplification of wavenumber 2. On the other hand, the ensemble members which failed to predict the wind reversal also failed to properly capture the blocking event in the key region of the Urals and the associated
intensification of upward-propagating wave activity. Finally, a composite
analysis suggests that teleconnections associated with the record-breaking
MJO phase 6 observed in late January 2018 likely played a role in
triggering this SSW event.
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