A Critical Role of the North Pacific Bomb Cyclones in the Onset of the 2021 Sudden Stratospheric Warming

Cho, Hyeong‐Oh; Kang, Min‐Jee; Son, Seok‐Woo; Hong, Dong‐Chan; Kang, Joonsuk M.

doi:10.1029/2022gl099245

Cited by 8 publications

(9 citation statements)

References 37 publications

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“…This analysis therefore does not reveal any obvious discrepancies between model and observations in the ENSO‐Aleutian Low‐SSW relationship, but further work is needed to understand the observed La Niña teleconnection and more sensitive tests may yet reveal an error. Unpredictable variability in the Aleutian Low region (Cho et al, 2022), along with variability in other SSW precursor regions such as northern Eurasia (Lu et al, 2021; Rao et al, 2021), resulted in an increased likelihood of an SSW through tropospheric wave forcing. This appears to be unpredictable 1 month in advance with the current seasonal forecast system.…”

Section: Resultsmentioning

confidence: 99%

Predictability of European winter 2020/2021: Influence of a mid‐winter sudden stratospheric warming

Lockwood

Stringer

Thornton

et al. 2022

Atmospheric Science Letters

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Boreal winter (December-February) 2020/2021 in the North Atlantic/European region was characterised by a negative North Atlantic Oscillation (NAO) index.Although this was captured within the ensemble spread of predictions from the Met Office Global Seasonal forecast system (GloSea5), with 17% of ensemble members predicting an NAO less than zero, the forecast ensemble mean was shifted towards a positive NAO phase. The observed monthly NAO anomalies were particularly negative in January and February, following an early January sudden stratospheric warming (SSW), and a prolonged period of Phase 6 or 7 of the Madden Julian Oscillation (MJO) in late January/early February. In contrast, predictions showed the expected teleconnection from the observed La Niña, with a positive NAO signal resulting from a weakening of the Aleutian Low leading to a reduction in tropospheric wave activity, an increase in polar vortex strength and a reduced chance of an SSW. Forecasts initialised later in the winter season successfully predicted the negative NAO in January and February once the SSW and MJO were within the medium range timescale. GloSea5 likely over-predicted the strength of the La Niña which we estimate caused a small negative bias in the SSW probability. However, this error is smaller than the uncertainty in SSW probability from the finite forecast ensemble size, emphasising the need for large forecast ensembles. This case study also demonstrates the advantage of continuously updated lagged ensemble forecasts over a 'burst' ensemble started on a fixed date, since a change in forecast signal due to events within the season can be detected early and promptly communicated to users.

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Section: Resultsmentioning

confidence: 99%

Predictability of European winter 2020/2021: Influence of a mid‐winter sudden stratospheric warming

Lockwood

Stringer

Thornton

et al. 2022

Atmospheric Science Letters

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“…It is found that the Ural blocking and Aleutian cyclones, which have been proposed as synoptic precursors of the 2021 SSW (Cho et al., 2022; Lu et al., 2021), do not have a significant impact on the SSW prediction in both models. Despite the small differences between ySSW and nSSW ensembles in the tropospheric precursors, the ySSW ensembles show much stronger wave fluxes at 100 hPa prior to the SSW onset than the nSSW ensembles.…”

Section: Summary and Discussionmentioning

confidence: 89%

“…In the weeks prior to the SSW onset, planetary‐scale eddy heat flux at 100 hPa, averaged over 45–75°N, sharply increased and its magnitude was maintained for longer than a week (red solid line). This strong upward flux of planetary‐scale waves was reported to be the major contributor to the 2021 SSW as it converged in the stratosphere and weakened the polar vortex (Cho et al., 2022; Lu et al., 2021). The zonal wavenumber one ( k = 1) and two ( k = 2) waves alternatively contributed to this strong eddy heat flux (green and blue dotted lines, respectively).…”

Section: Resultsmentioning

confidence: 99%

“…In midlatitudes, however, no systematic differences are found in both the ECMWF and JMA forecasts. Most importantly, differences in the precursor regions, that is, the Ural high and Aleutian low regions (boxed in each figure; Cho et al., 2022; Garfinkel et al., 2010; Lu et al., 2021), are relatively small. Although the positive eddy geopotential height over the Ural high region (i.e., blocking) and the negative eddy geopotential height over the Aleutian low region (i.e., bomb cyclone) are slightly stronger in the ySSW ensembles than in the nSSW ensembles, they do not likely change the vertically propagating planetary‐scale waves into the stratosphere in a major way.…”

Section: Resultsmentioning

confidence: 99%

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

Cho,

Kang,

Son

2023

JGR Atmospheres

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Sudden stratospheric warming (SSW) describes a disruption of the stratospheric polar vortex in the winter hemisphere. It affects not only the stratospheric circulation but also the surface climate for up to 2 months, serving as an important source of subseasonal‐to‐seasonal (S2S) predictability in midlatitudes. This study evaluates the predictability of the 2021 SSW and investigates the crucial factors that determine its predictability in the ECMWF and JMA S2S real‐time forecasts. In both models, only a subset of the ensemble members predicted the SSW at the lead time of about 2 weeks before the onset. By comparing the 10 ensembles with successful SSW predictions and those with failed predictions, we found that the ensembles predicting the SSW have relatively stronger wave fluxes from the upper troposphere to the stratosphere than the others. Stronger wave fluxes, particularly those of zonal wavenumber one, are not the result of the tropospheric precursors such as the Ural blocking and Aleutian cyclones but they result from the modulation of the wave propagation by the background state. In particular, the ensembles with failed SSW predictions tend to have a negative potential vorticity gradient in the upper troposphere and lower stratosphere, which limits the upward wave propagation into the stratosphere and provides an unfavorable condition for the SSW. This result suggests that not only the wave sources in the troposphere but also the background state in the upper troposphere and lower stratosphere can modulate the predictability of SSW in S2S prediction models.

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“…SSWs are driven by the rapid amplification and breaking of upward propagating Rossby waves (Baldwin et al., 2021; Matsuno, 1971) associated with both tropospheric wave activity (Bao et al., 2017; Garfinkel et al., 2010; Martius et al., 2009; Matsuno, 1971) and internal stratospheric conditions (Christiansen, 1999; Holton & Mass, 1976; Matthewman & Esler, 2011; Scaife & James, 2000). There can be a high sensitivity to stratospheric conditions when the tropospheric flow is constrained (de la Cámara et al., 2017), as well as a critical role from wave generation by extratropical cyclones (Cho et al., 2022). Historical analysis suggest at least one third of SSWs immediately follow anomalous tropospheric wave activity, with the longer‐term accumulation of upward wave activity also important (Birner & Albers, 2017; de la Cámara et al., 2019; Polvani & Waugh, 2004).…”

Section: Introductionmentioning

confidence: 99%

Identifying Perturbations That Tipped the Stratosphere Into a Sudden Warming During January 2013

Kent,

Scaife,

Seviour

et al. 2023

Geophysical Research Letters

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We present a new methodology demonstrating that specific small‐magnitude regional perturbations can cause large systematic responses in subseasonal predictions. We show this with ensemble forecasts of the January 2013 Sudden Stratospheric Warming (SSW) from an operational long‐range global prediction system. In forecast members which predict the SSW, tropospheric ridging over the North Atlantic is strengthened 10 days prior to the event. This subsequently enhances planetary wave forcing and weakens the stratospheric polar vortex. Transplanting tropospheric conditions in this region from “correct” to “incorrect” forecasts (and vice versa) significantly alters the SSW forecast probability. The occurrence of this SSW is therefore strongly controlled by the troposphere several days prior. Tipping more members into a sudden warming also significantly affects surface predictions the following month. Despite chaotic behavior of the climate system, small‐magnitude synoptic‐scale perturbations can drive different dynamical states and systematically impact medium and long‐range predictions.

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A Critical Role of the North Pacific Bomb Cyclones in the Onset of the 2021 Sudden Stratospheric Warming

Cited by 8 publications

References 37 publications

Predictability of European winter 2020/2021: Influence of a mid‐winter sudden stratospheric warming

Predictability of European winter 2020/2021: Influence of a mid‐winter sudden stratospheric warming

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

Identifying Perturbations That Tipped the Stratosphere Into a Sudden Warming During January 2013

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