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.
Highlights
Sudden stratospheric warmings (SSWs) are the most prominent phenomena taking place in the wintertime polar stratosphere, representing the dynamical linkage between troposphere and stratosphere
We focused on the identification of the involved dynamical processes and studied the role of the tropospheric forcing leading to the polar vortex split
Comparison of the ENS+ and ENS− forecast composites reveals that the ENS+ forecasts correctly captured the whole chain of the observed events: from downstream propagation of individual wave packets to the upward propagation of wave activity, amplification of stratospheric PW2 and breaking down of the stratospheric polar vortex
Summary
Sudden stratospheric warmings (SSWs) are the most prominent phenomena taking place in the wintertime polar stratosphere, representing the dynamical linkage between troposphere and stratosphere. SSWs have been shown to be related to the enhancement of tropospheric forced planetary wave packets that propagate upward into the stratosphere and interact with the mean flow (Charney and Drazin, 1961; Matsuno, 1971; McIntyre, 1982; Limpasuvan et al, 2004). These upward-propagating planetary waves amplify with height, approaching the critical level where they irreversibly break and deposit westward angular momentum (quantified as a convergence of the Eliassen–Palm flux), which leads to the deceleration and breaking down of the polar night jet (Polvani and Saravanan, 2000). The 2013 and 2018 events were followed by cold and snowy weather in Europe
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