Mechanisms and predictability of sudden stratospheric warming in winter 2018

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.

The Predictability of the 2021 SSW Event Controlled by the Zonal‐Mean State in the Upper Troposphere and Lower Stratosphere

“…This result suggests that wave propagation conditions in the upper troposphere and the lower stratosphere, rather than tropospheric precursors, play a more important role in determining the predictability of the 2021 SSW in the ECMWF and JMA forecasts. This contrasts with previous studies that have emphasized the role of tropospheric precursors in SSW prediction (Karpechko et al., 2018; Statnaia et al., 2020; Tripathi et al., 2016). This aligns with the earlier studies showing that the enhanced wave flux in the lower stratosphere is common in successful SSW prediction (Rao et al., 2019; Taguchi, 2018), while strong heat flux in the troposphere is not always necessary for successful SSW prediction (Noguchi et al., 2016).…”

The dynamical evolution of Sudden Stratospheric Warmings of the Arctic winters in the past decade 2011–2021

“…It has been observed from previous studies that the MW in 2012/13, 2017/18 and 2018/19 was favoured by the easterly phase of QBO (weak polar vortex), although the phase was not easterly in 2020/21. The winters 2012/13, 2017/18, 2018/19 and 2020/21 were preceded by a neutral ENSO, La Nina, El Nino and La Nina phases, respectively, as reported in the studies of Dai and Tan [64], Statnaia et al [65], Lu et al [42] and others. The weak polar vortex in the easterly phase of QBO, and the El Nino and La Nina conditions supported the occurrence of MWs in these years.…”

The January 2021 Sudden Stratospheric Warming and Its Prediction in Subseasonal to Seasonal Models

“…Since the World Climate Research Program (WCRP) and World Weather Research Program initiated the S2S prediction project in November 2013, several extreme stratospheric polar vortex events have been observed in both the NH and SH. Specifically, two major sudden stratospheric warmings (SSWs) were observed over the Arctic stratosphere in February 2018 (Ayarzagüena et al, 2018;Karpechko et al, 2018;Kautz et al, 2020;Statnaia et al, 2020) and January 2019 (Butler et al, 2020;, respectively. The February 2018 SSW was preceded by blocking over the Urals and Alaska in phase with the local climatological ridges, and thus amplified the climatological wavenumber 2 (Lee et al, 2019;Lü et al, 2020;Overland et al, 2020;.…”