Predictability of European Winters 2017/2018 and 2018/2019: Contrasting influences from the Tropics and stratosphere

Abstract

The European winters of 2017-18 and 2018-19 were not climatically extreme, but both winters had a major sudden stratospheric warming (SSW). In February 2018, an SSW led to an intense cold outbreak across Europe and further spells of cold weather in March. The SSW of January 2019, although well predicted and expected to increase the chance of a cold end to winter, apparently produced little impact. In this study, we examine the performance of the Met Office seasonal prediction system in these winters, and the influences that led to these outcomes. To achieve this latter objective, sets of numerical experiments are performed in which the tropical troposphere and the extratropical stratosphere are relaxed towards their observed state, allowing the influence of each on the North Atlantic-European atmospheric circulation to be identified. Using these experiments, we show that the SSWs had similar impacts in each case, creating a signal of easterly surface wind anomalies in the weeks following the event. In contrast, tropical influences were opposite in the two winters, acting to strengthen the easterly signal at the end of February 2018 and opposing it in January 2019. The different apparent responses to the two events therefore came about largely through tropical tropospheric variability. Furthermore, we highlight the importance of a very strong cycle of the Madden-Julian Oscillation (MJO) in late January and early February 2018 as an important driver for the February 2018 SSW. MJO teleconnections appear to have been critical in creating the large mid-latitude wave 2 amplitude that has been identified as the immediate cause of this event.

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

“…The MJO (Figure 3b) is relatively inactive until 21st January 2021, when it enters an extended period of Phase 6 or 7. Similar to El Niño, the MJO in these phases can weaken the polar vortex through tropospheric wave forcing (e.g., Jiang et al, 2017), and it has been considered an important influence on SSWs in previous winters (e.g., February 2018; Knight et al, 2021). For winter 2020/2021, although the MJO was not the SSW trigger, it may have been a factor in the prolonging the polar vortex disruption in early 2021.…”

“…The North Atlantic Oscillation (NAO) is the dominant pattern of atmospheric circulation variability in the North Atlantic (Wallace & Gutzler, 1981), with positive phases associated with stormy, mild and wet conditions over north‐west Europe and eastern United States, and negative phases associated with dry, cold conditions (and vice versa for southwest Europe and eastern Canada). Recently, the surface winter mean (December–February [DJF]) NAO has been demonstrated to be predictable 1 month ahead (Athanasiadis et al, 2017; Scaife et al, 2014), and this has been followed by real time operational predictions from the Met Office's seasonal prediction system, GloSea5, where the model ensemble mean has consistently predicted the correct sign of the anomaly (Dunstone et al, 2018; Hardiman et al, 2020; Knight et al, 2021; Scaife et al, 2017), aiding decision makers in sectors such as transport (Palin et al, 2016), energy (Clark et al, 2017; Thornton et al, 2019) and water management (Stringer et al, 2020; Svensson et al, 2015).…”

Diverse Surface Signatures of Stratospheric Polar Vortex Anomalies

“…Moreover, the vortex has been shown to influence surface climate in more diverse ways than is revealed by considering solely the NAO (Beerli & Grams, 2019; Domeisen et al., 2020), particularly over North America (Cohen et al., 2021; Kretschmer et al., 2018; Lee et al., 2022). There is also a complex relationship between the stratospheric vortex and tropical variability, including the Madden–Julian Oscillation and El Niño–Southern Oscillation (ENSO), which can influence the state of the vortex through tropical‐extratropical teleconnections (Barnes et al., 2019; Domeisen et al., 2019; Green & Furtado, 2019) and can also directly modulate the tropospheric response to ENSO (Jiménez‐Esteve & Domeisen, 2018; Knight et al., 2021). Furthermore, tropospheric precursors to extreme stratospheric states, such as blocking, may induce systematic tropospheric temperature anomaly patterns before and during the onset of anomalous vortex states, independent of the downward propagation of the stratospheric vortex anomaly (e.g., Kolstad & Charlton‐Perez, 2011).…”

A Minimal Model to Diagnose the Contribution of the Stratosphere to Tropospheric Forecast Skill

“…On sub‐seasonal and seasonal timescales, coupling between the stratosphere and troposphere is a significant part of the available tropospheric forecast skill (e.g., Domeisen, Butler, et al., 2020; Scaife et al., 2016). The contrasting impact of stratospheric variability during recent winter seasons with similar stratospheric disturbances (Knight et al., 2021) has, however, brought into sharp relief the lack of a full quantitative understanding of the stratospheric contribution to tropospheric prediction skill. The Sudden Stratospheric Warming (SSW) which occurred in February 2018 has been clearly linked to enhanced sub‐seasonal predictive skill of cold conditions in Europe during late‐winter and spring (Karpechko et al., 2018; Kautz et al., 2020).…”

“…Related work notes a difference in the tropospheric response to the morphology of SSW, vortex displacement or vortex split, with enhanced coupling following splitting events (White et al., 2020). Other recent work suggests tropospheric drivers of sub‐seasonal skill, unconnected to the stratosphere, significantly influence the sign and predictability of the tropospheric response (e.g., Afargan‐Gerstman & Domeisen, 2020; Knight et al., 2021). It has also been proposed that coupling between the stratosphere and troposphere is strongly dependent on the tropospheric state at the time of the stratospheric perturbation (Charlton‐Perez et al., 2018; Domeisen, Grams, & Papritz, 2020; Maycock et al., 2020).…”