Stratospheric modulation of the large‐scale circulation in the Atlantic–European region and its implications for surface weather events

Abstract

Extreme states of the stratospheric polar vortex can have long‐lasting impacts on extratropical circulation patterns, such as the North Atlantic Oscillation (NAO). This provides windows of subseasonal predictability beyond the typical weather forecast horizon of about 10 days. Subseasonal forecasts of surface weather are of significant interest in weather‐dependent socio‐economic sectors. For example, demand and supply for electricity and gas are weather dependent and therefore accurate forecasts are important for the energy industry and energy trading. Here we investigate the subseasonal impact of stratospheric conditions on surface weather events relevant to the energy industry in five subregions of Europe in winter. We use a definition of seven Atlantic–European weather regimes to describe the variability of the large‐scale circulation on subseasonal time scales. Results indicate that weather events are often associated with more than one preferred weather regime. In turn, some weather regimes project onto a specific NAO phase, while others are independent of the NAO. As expected, anomalous stratospheric polar vortex states predominantly modulate the occurrence of regimes related to the NAO and affect the likelihood of their associated weather events. In contrast, the occurrence of weather regimes which do not project well onto the NAO is not affected by anomalous stratospheric polar vortex states. These regimes provide pathways to unexpected weather events in extreme stratospheric polar vortex states. For example, weak stratospheric polar vortex states enhance the likelihood of negative NAO. High wind events in Central Europe predominantly occur during the zonal regime, strongly projecting onto positive NAO. However, these events also occur during the Atlantic trough regime, which is unaffected by anomalous stratospheric polar vortex states and thus provides a pathway to Central European high wind events during weak stratospheric polar vortex states. A correct NAO prediction alone is therefore not sufficient to correctly predict surface weather after extreme stratospheric polar vortex states. Moreover, weather regime life cycles independent of the NAO also need to be forecast accurately.

The role of North Atlantic-European weather regimes in the surface impact of sudden stratospheric warming events

“…In contrast, mild temperatures in large parts of Europe are found for SSWs with GL at their onset. It is important to distinguish these cases, since although EuBL and GL frequently occur at the onset of SSW events, they lead to a different subsequent evolution, with a likely transition from EuBL to GL and from GL to AT around 3-4 weeks after the SSW along with their contrasting large-scale weather impacts (Beerli and Grams, 2019). These findings corroborate that the presence of either EuBL or GL regime at SSW onset will allow us to disentangle the difference in the subsequent evolution, and hence to determine if and when a "downward impact" of the SSW is expected.…”

“…CC BY 4.0 License. This behaviour obscures the potential tropospheric impact of an SSW in a composite as AT and GL trigger contrasting largescale weather conditions (rather mild and windy for AT, cold and calm for GL) for large parts of Europe (Beerli and Grams, 2019). We now sub-divide the 26 SSW events with respect to the weather regime that dominates during the SSW onset: GL (5 cases), EuBL (6 cases), and the cyclonic regimes (ZO, ScTr, AT; 9 cases).…”

“…This expands on Charlton-Perez et al (2018), who have used four weather regimes (NAO+, NAO-, Scandinavian blocking and Atlantic ridge) and characterized changes in the transition probability between regimes in the aftermath of SSW events. Prior work based on this extended regime definition revealed important differences in the surface weather response to the state of the stratosphere, which remain hidden using the canonical four NAE regimes (Papritz and Grams, 2018;Beerli and Grams, 2019). Here, we focus on the evolution and surface impact of tropospheric anomalies as a function of the weather regime present at the onset of the SSW event.…”

The effect of interactive ozone chemistry on weak and strong stratospheric polar vortex events

“…Finally, we note that while we have only focused on winter SSWs and SPVs, stratospheric final warmings also have tropospheric effects (Black et al, 2006;Ayarzaguëna and Serrano, 2009;Wei et al, 2007;Hardiman, 2011;Thieblemont et al, 2019;Butler et al, 2019). Those effects are dependent on the timing of the final warming, with earlier final warmings resulting in surface effects more like those seen following SSWs (Ayarzaguëna and Serrano, 2009;Li et al, 2012).…”

The effect of interactive ozone chemistry on weak and strong stratospheric polar vortex events

“…To the best of our knowledge, there is no standard definition of an SPV. Different methods have been used in the literature (Limpasuvan et al, 2004;Tripathi et al, 2015;Scaife et al, 2016;Beerli and Grams, 2019). We here follow the definition used in Scaife et al (2016) and Smith et al (2018), designed to be analagous to the Charlton and Polvani (2007a) SSW definition and to result in a similar number of events in reanalysis.…”