Abstracted/indexed

The Circumglobal Teleconnection Pattern (CTP) is a wavy pattern of wintertime midlatitude subseasonal flow. It is also linked to various extreme weather events. The CTP is predicted to play a prominent role in the future climate. We find that for future projections, most CMIP5 models predict that the CTP will develop a “preferred” phase. Our work establishes that the CTP-like climate change signature is in fact comprised of several regional effects, partly due to shifts in phase distributions.

The causal effect of latent heating on atmospheric blocking is investigated using numerical sensitivity experiments. The modification of latent heating in the upstream cyclone has substantial effects on the upper-tropospheric circulation, demonstrating that some blocking systems do not develop at all without upstream latent heating. The results highlight the importance of moist-diabatic processes for the dynamics of prolonged anticyclonic circulation anomalies.

We perform 50-year-long time-slice experiments using the MetOffice HadGEM3 global climate model in order to decompose the Brewer–Dobson circulation (BDC) response to an abrupt quadrupling of CO₂ in three distinct components, (a) the rapid adjustment, associated with CO₂ radiative effects, (b) a global uniform sea-surface temperature warming and (c) sea-surface temperature patterns. We further demonstrate a potential for a fast and slow timescale of the response of the BDC to greenhouse gas forcing.

This paper investigates the waveguidability of midlatitude jets, i.e., their ability to duct Rossby waves in the zonal direction. Waveguidability is quantified in the framework of an idealized numerical model and compared with the predictions from two complementary theoretical concepts. It is found that the concept of ray tracing, which has been used extensively in the past, does not provide a satisfactory explanation for the numerical results. Further analysis uncovers the underlying reasons.

This work investigates the influence of topography on atmospheric blocking dynamics, spatial frequency, duration and displacement. Using an idealized model, a landless integration, and integrations with topography are analyzed. Topography is found to significantly increases blocking and anchors where blocks most likely occur (i.e just upstream from mountains and near storm track exits). Blocks forced by topography are found to behave similarly to those that are not, but tend to last longer.

We investigated an exceptional thunderstorms episode in 2018. In this context, atmospheric blocking provided the large-scale environmental conditions favouring convection in its vicinity. In addition, blocking was accompanied by high cut-off frequency on its upstream side (10 times higher than climatology), which often served as trigger for the thunderstorms. The exceptional persistence of low stability combined with weak wind speed in the mid-troposphere has never been observed during the past.

We analyse the formation of upper-tropospheric anticyclones connected to European surface heat waves. Tracing air masses backwards from these anticyclones, we found out that trajectories are diabatically heated either by North Atlantic cyclones or by convection closer to the heat wave anticyclone. The first branch primarily affects the onset of the anticyclone, while the second branch is more relevant for the maintenance. Our results are relevant for heat wave predictions in numerical models.

We cannot currently predict the weather over Europe beyond 2 weeks. The stratosphere provides a promising opportunity to go beyond that limit by providing a change in probability of certain weather regimes at the surface. However, not all stratospheric extreme events are followed by the same surface weather evolution. We show that this weather evolution crucially depends on the tropospheric weather regime at the onset of the stratospheric extreme event, allowing for improved predictions.

Atmospheric predictability over Europe on sub-seasonal to seasonal timescales remains limited. However, the remote impact from El Niño Southern Oscillation (ENSO) can help to improve predictability. Research has suggested that the ENSO impact in the North Atlantic region is affected by nonlinearities. Here, we isolate the nonlinearities in the tropospheric pathway through the North Pacific, finding that e.g. a strong El Niño leads to a stronger and district impact than La Niña.

In blocking situations the typical westerly atmospheric flow in the midlatitudes is blocked by near-stationary high pressure systems. These blocking events can be associated with extremes at the surface such as cold spells in winter and summer heatwaves. Climate models have long been known to underestimate blocking occurrence. In this study, we assess the latest generation of models and find sizeable improvements in how blocking is represented, in part due to increases in model resolution.

Despite the global temperature rise caused by anthropogenic emissions, we still observe snowstorms that cause casualties, transport disruptions as well as energy supply problems. The goal of this paper is to investigate recent trends in snowfalls from both ERA5 reanalysis and observational (EOBSv19.0) datasets. The analysis shows an evident discrepancy between trends in average and extreme snowfalls. The latter can only be explained by looking at the atmospheric circulation.

Tropical storms are key for harnessing midlatitude weather prediction skill 2–8 weeks into the future. Recently, stratospheric activity was shown to impact tropical storminess, and thus, may also be important for midlatitude prediction skill on these timescales. This work analyzes two forecast systems to assess whether they capture this additional skill. We find there is enhanced prediction up through week 3 when both the tropical and stratospheric phenomena are active.

In this paper, we take a second look at the development an intense storm that made landfall in Norway a few hours into the new year of 1992, focussing on the effect of the Scandinavian mountains on the storm. We find that the cyclone core evolves largely unaffected, although both the warm and the cold front decay rapidly while passing over the mountains. This result suggest that the fronts of a cyclone can become detached from their cyclone core as part of the cyclone’s occlusion process.