Exceptionally persistent Eurasian cold events and their stratospheric link

Finke, Kathrin; Hannachi, Abdel; Hirooka, Toshihiko

doi:10.1007/s13143-022-00308-y

Cited by 6 publications

(6 citation statements)

References 46 publications

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“…On the other hand, when the forecasts are initialized during strong polar vortex conditions, most of the models predict low probabilities (<0.2) of cold events over the study region (Figure 9f,h), which aligns with the expectation that a strong polar vortex is followed by warm anomalies (e.g., Tripathi et al, 2015). Finke et al (2023) suggested that cold Eurasian events following strong polar vortex may be related to downward wave reflection conditions. In this scenario, we considered 140 forecasts for ECMWF, 97 for UKMO, 127 for BoM, 104 for CNRM and 53 for JMA.…”

Section: Reliabilitysupporting

confidence: 74%

Factors influencing subseasonal predictability of northern Eurasian cold spells

Statnaia,

Karpechko

2024

Quart J Royal Meteoro Soc

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Cold‐air outbreaks have significant impacts on human health, energy consumption, agriculture, and overall well‐being. This study aims to evaluate the effectiveness of Subseasonal‐to‐Seasonal (S2S) models in predicting cold conditions over northern Eurasia, defined here as the lower tercile of weekly mean 2‐metre temperature anomalies. To assess the predictability of these events we use ensemble hindcasts from five prediction systems from the S2S database. Our analysis focuses on identifying the conditions under which the models confidently predict cold temperatures with a high (>0.5) probability 3–4 weeks ahead, which potentially can represent windows of forecast opportunity. We compare the group of forecasts that correctly predicted the events to the group that forecasted events that did not occur in practice (false alarms). Most of the confident forecasts of cold spells, both correct and false alarms, have cold anomalies already in the initial conditions, often in conjunction with either a negative phase of the North Atlantic Oscillation, or Scandinavian Blocking. We find that S2S models tend to overpredict cold temperatures, with false alarms occurring more likely when the forecasts are initialized during a weak polar vortex. Furthermore, most of the confident false alarms receive the signal from the stratosphere rather than following internal tropospheric dynamics. False alarms initialized during the weak polar vortex conditions are more common when the vortex is in a recovery stage and, subsequently, the downward ‐propagating signal is short‐lived in the troposphere. The analysis of forecasts during different Madden–Julian Oscillation (MJO) phases shows that nearly half of all confident correct cold‐temperature forecasts are initialized during an active MJO in phases 6–8. On the other hand, most false alarms occur during phase 3, which we suggest is due to the presence of the Scandinavian Blocking regime in the initial conditions for this phase.

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Section: Reliabilitysupporting

confidence: 74%

Factors influencing subseasonal predictability of northern Eurasian cold spells

Statnaia,

Karpechko

2024

Quart J Royal Meteoro Soc

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“…However, most of the results provided in this study are from a zonally‐integrated perspective, and we have not discussed the local and zonally asymmetric downward impacts from the stratosphere on the troposphere, which are more closely related to SSW events of displacement and split types (e.g., Charlton & Polvani, 2007; Lehtonen & Karpechko, 2016; Mitchell et al., 2013). In that case, waves would play an important role (Cohen et al., 2021; Davis et al., 2022; Finke et al., 2023; Gong et al., 2019; Kretschmer et al., 2018; Messori et al., 2022; Perlwitz & Harnik, 2004; Rupp et al., 2022; Shen et al., 2023). Future studies on the relationship between regional cold air transports and the related continental CAOs and the SSW type are needed.…”

Section: Conclusion and Discussionmentioning

confidence: 99%

“…Note that our concern here is the zonally integral meridional mass transport across the polar circle. The wave‐contributed weakening of the equatorward cold airmass transport after the anomalous WB‐ST and that during and after negative NAM events may not necessarily represent the local downward wave reflection processes that are known to be favorable for the CAOs occurring in North America (Cohen et al., 2021; Davis et al., 2022; Finke et al., 2023; Gong et al., 2019; Kretschmer et al., 2018; Messori et al., 2022; Rupp et al., 2022; Shen et al., 2023).…”

Section: Stratospheric Downward Impacts Exist Mainly/barely At the Mo...mentioning

confidence: 99%

When and How Can the Stratosphere Modify the Midlatitude Cold Air Outbreaks in Northern Winter: An Isentropic Meridional Mass Circulation View

Ren

Liu

et al. 2023

JGR Atmospheres

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Cold air outbreaks (CAOs) in northern winter can be attributed to the variations in the lower‐tropospheric equatorward cold branch (CB) of the isentropic meridional mass circulation (IMMC) across the subpolar latitudes. Understanding when and how the variations of CB are related to the stratosphere is crutial for improving the extended‐range forecast skills of CAOs in winter. Investigation on the vertical coupling of the stratospheric and tropospheric branches of IMMC suggests that the CB (or CAOs) exhibits two major timescales: sub‐monthly (10–30 days) and monthly‐to‐seasonal (>30 days). The CB can be modified by the stratospheric variability that is represented by the stratospheric poleward warm branch (WB‐ST) of the IMMC at monthly‐to‐seasonal timescales via the two‐way stratosphere‐troposphere coupling. At submonthly timescales, however, the upward one‐way coupling from the troposphere to the stratosphere dominates. While changes of the CB always precede those of the WB‐ST at both timescales, indicating the upward tropospheric wave forcing into the stratosphere, the downward impacts of the WB‐ST on the CB (particularly CAOs in North America) are observed only at monthly‐to‐seasonal timescales. Specifically, the accumulated effect of the monthly‐to‐seasonal mass transport by the poleward WB‐ST can be strong enough to dominate the Arctic total column airmass changes, producing not only a barotropic structure of the Northern Annular Mode but also direct changes to the low‐level ageostrophic zonal‐mean meridional flow. The anomalous low‐level ageostrophic flow, in turn, plays a greater role in intensifying (weakening) the equatorward CB following a stronger (weaker) WB‐ST than tropospheric wave activities.

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“…The anomalous flow associated with these opposite, extreme stratospheric states can affect the tropospheric circulation and surface weather (Baldwin and Dunkerton, 2001;Hinssen et al, 2011;Kodera et al, 2013;Davini et al, 2014;Afargan-Gerstman et al, 2020;Domeisen et al, 2020c;White et al, 2020). Previous research has particularly focused on SSW events because of their linkage to extreme weather events (e.g., Domeisen and Butler, 2020;Finke et al, 2023) and periods of higher than usual extended-range and seasonal predictability (e.g., Domeisen et al, 2020b;Zhang et al, 2022), while SPVs have received comparatively less attention, although these events have also been linked to increased surface predictability (Tripathi et al, 2015).…”

Section: Introductionmentioning

confidence: 99%

Opposite spectral properties of Rossby waves during weak and strong stratospheric polar vortex events

Schutte,

Domeisen,

Riboldi

2023

Preprint

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Abstract. In this study we provide a systematic characterization of Rossby wave activity during the 25 sudden stratospheric warming (SSW) and 31 strong polar vortex (SPV) events that occurred in the period 1979–2021, identifying the specific tropospheric and stratospheric waves displaying anomalous behaviour during such events. Space-time spectral analysis is applied to ERA5 data for this purpose, so that both the wavenumber and the zonal phase speed of the waves can be assessed. We find that SSW events are associated with a reduction in the phase speed of Rossby waves, first in the stratosphere and then in the troposphere; SPV events are tied to a concomitant increase of phase speed across vertical levels. Phase speed anomalies become significant around the event and persist for 2–3 weeks afterwards. Changes of Rossby wave properties in the stratosphere during SSW and SPV events are dominated by changes in the background flow, with a systematic reduction or increase, respectively, in eastward propagation of the waves across most wavenumbers. In the troposphere, on the other hand, the effect of the background flow is also complemented by changes in wave properties, with a shift towards higher wavenumbers during SSW events and towards lower wavenumbers for SPV events. The opposite response between SSW and SPV events is also visible in the meridional heat and momentum flux co-spectra, which highlight from a novel perspective the connection between stratospheric Rossby waves and upward propagation of waves.

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Exceptionally persistent Eurasian cold events and their stratospheric link

Cited by 6 publications

References 46 publications

Factors influencing subseasonal predictability of northern Eurasian cold spells

Factors influencing subseasonal predictability of northern Eurasian cold spells

When and How Can the Stratosphere Modify the Midlatitude Cold Air Outbreaks in Northern Winter: An Isentropic Meridional Mass Circulation View

Opposite spectral properties of Rossby waves during weak and strong stratospheric polar vortex events

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