Duration and decay of Arctic stratospheric vortex events in the ECMWF seasonal forecast model

Abstract

Previous studies have documented extreme stratospheric polar vortex events, but the factors governing their duration have received little attention. Here, we investigate weak and strong stratospheric polar vortex events simulated in ensemble reforecasts for the Northern Hemisphere winter with the seasonal forecast model (System 4) of the European Centre for Medium‐Range Weather Forecasting, in comparison with the European Reanalysis Interim dataset. The individual strong and weak vortex events are classified into short and long events. A composite analysis is conducted for each of the four categories of events with respect to their initial or end days. We examine the characteristics of each category in the forecast model and reanalyses, including anomalous zonal‐mean zonal winds, heat and wave‐activity fluxes, lower‐tropospheric temperature and surface pressure, as well as the synoptic evolution of upper‐tropospheric anomalies during the precursory and decaying stages. The main findings are: (a) the forecast System 4 is found to produce both short and long anomalous vortex events in comparable amplitude to the reanalyses, but the polar zonal wind anomalies are more persistent, for each of the four categories considered; (b) short events penetrate as deep as the long events right at their onset, but they rapidly decay under a wave forcing that is less sustained than in the long events; (c) subtle differences in the synoptic evolution of blocking Highs and Lows over both oceanic basins or over Northern Eurasia or America appear to condition the duration of the forcing; and (d) during such short events, characteristic surface or lower‐tropospheric signatures of the annular mode paradigm arising from the downward stratospheric influence are not allowed sufficient time to develop.

Seasonal prediction of the boreal winter stratosphere

“…Dobrynin et al (submitted) and Fig. 3], corroborating the idea that the stratospheric skill feeds on the prediction of wave injection, in particular that generated by the Eurasian tropospheric flux (Orsolini et al 2018;Peings 2019;Schlichtholz 2019). Just as the representation of the stratosphere is known to impact the ability to forecast the mid-latitude tropospheric flow (Nie et al 2019;Stockdale et al 2015;O'Reilly et al 2019), here the prediction of tropospheric variability appears to regulate the seasonal forecast skill of the SPV.…”

The Remarkably Strong Arctic Stratospheric Polar Vortex of Winter 2020: Links to Record‐Breaking Arctic Oscillation and Ozone Loss

“…These relationships are commonly expressed using metrics that describe phases of the “Northern Annular Mode” (NAM), a pattern that characterizes meridional shifts of mass into or out of the polar cap throughout the atmospheric column (note that the NAM and AO are often used interchangeably; Baldwin, 2001; Thompson & Wallace, 2000). Anomalously strong or weak polar vortex states correspond to positive or negative phases of the stratospheric NAM, respectively, and these tend to be followed in the troposphere by positive or negative AO events, which may last for weeks to months and alter patterns of surface temperatures and precipitation (Baldwin & Dunkerton, 2001; Domeisen, 2019; Dunn‐Sigouin & Shaw, 2015; Kidston et al, 2015; King et al, 2019; Limpasuvan et al, 2005; Orsolini et al, 2018; Polvani & Kushner, 2002; Tripathi et al, 2015). Downward wave coupling events can not only strengthen the polar vortex but also directly induce tropospheric circulation patterns consistent with a positive AO on short time scales (Dunn‐Sigouin & Shaw, 2015; Shaw & Perlwitz, 2013).…”

The Remarkably Strong Arctic Stratospheric Polar Vortex of Winter 2020: Links to Record-Breaking Arctic Oscillation and Ozone Loss

“…These relationships are commonly expressed using metrics that describe phases of the "Northern Annular Mode" (NAM), a pattern that characterizes meridional shifts of mass into or out of the polar cap throughout the atmospheric column (note that the NAM and AO are often used interchangeably; Thompson & Wallace, 2000;Baldwin, 2001). Anomalously strong or weak polar vortex states correspond to positive or negative phases of the stratospheric NAM, respectively, and these tend to be followed in the troposphere by positive or negative AO events, which may last for weeks to months and alter patterns of surface temperatures and precipitation (Baldwin & Dunkerton, 2001;ston et al, 2015;Orsolini et al, 2018;Domeisen, 2019;King et al, 2019). Downward wave coupling events can not only strengthen the polar vortex, but also directly induce tropospheric circulation patterns consistent with a positive AO on short timescales (Shaw & Perlwitz, 2013;Dunn-Sigouin & Shaw, 2015).…”