ALBERT

Description: The surface temperature at high latitudes — especially in the Arctic — has warmed faster than the global mean temperature since the 1990s. This process, called Polar amplification, is expected to continue this century. Concurrent with Polar amplification, upper tropospheric Tropical warming is projected in the 21st century. Low-level Polar and upper-level Tropical warming influence the equator-to-pole temperature gradient in opposite ways and hence the mid-latitude jet stream. However, the effect of modified equator-to-pole temperature gradients is not fully understood. Earlier studies argued that low-level Polar warming causes a weaker and wavier jet stream. Here, we study the influence of Polar warming on the jet stream speed, position and its waviness by increased SSTs. We performed four idealised aquaplanet simulations with the Open Integrated Forecast System. First, we compared the jet stream intensity and position of the control simulation with the warmed simulations. Second, we studied the influence on jet stream waviness quantified by a modified Sinuosity Index: we adjusted the original metric to better capture the jet on an aquaplanet. Despite strong Polar and uniform warming, the mean of the Sinuosity Index distributions of the warmed simulations changed negligibly. However, the most extreme waviness events decreased with Polar warming and increased with uniform warming. Our results contradict findings in other studies on the consequences of low-level Polar warming and its influence on jet stream waviness. We conclude that a weaker jet stream does not have to become wavier with reduced temperature gradient and discuss the implications.

Description: In February 2022, a cluster of severe extratropical cyclones hit North-Western Europe within one week and caused widespread damage by wind gusts and accumulated precipitation. In that week, extratropical cyclones Dudley, Eunice and Franklin developed over the North-Atlantic within a baroclinic environment with strong jet streams accompanied with atmospheric rivers. We hypothesise that diabatic heating through latent heat release within the extratropical cyclones created a baroclinic environment favourable for secondary cyclogenesis. We presume that latent heating on the trailing cold-front of the primary cyclone was essential for the secondary cyclogenesis. To identify the influence of latent heat release on the cyclogenesis of these storms, we performed idealised simulations with the Open Integrated Forecast System (OpenIFS). Latent heat of vaporisation was doubled, switched off and reduced with a factor of hundred. The control run captured the individual cyclones, their intensities and path reasonably well; OpenIFS is suitable to fulfil our objectives. First results showed that reduced latent heating negatively impacted the cyclogenesis of secondary cyclone Eunice — the severest extratropical cyclone during that week. Overall, the baroclinicity was strongly weakened over the North-Atlantic. Furthermore, we plan experiments that examine the importance of latent heat release along the cold-front of the primary cyclone by only reducing latent heat in a box along the trailing cold-front. With this case-study we explore diabatic heating as a pathway for cyclone clustering.

Description: Cyclone clustering, the swift succession of multiple extratropical cyclones, constitutes a large fraction of European weather extremes. To investigate the dynamical causes of cyclone clustering, it is necessary to diagnose the occurrence of cyclone clustering and to determine their characteristics. So far, most diagnostics focused either on local impact or on a statistical analysis of storm recurrence. While the first cannot be applied globally, the latter is difficult to relate to individual events. We therefore use a novel method to globally detect cyclone clustering that is closer to the original concept of Bjerknes and Solberg, where extratropical cyclones follow similar tracks within a given time period.Using this novel cyclone clustering diagnostic based on spatio-temporal distance between cyclone tracks, we analyse cyclone clustering globally in Era-Interim for the period 1979 until 2016 as well as for 10 CMIP6 models. We separate the cyclone clusters into two types: one representing the ‘classical’ bjerknes-type clusters, and one representing more stationary clusters. We find that cyclone clustering mainly occurs along the climatological storm tracks, with the bjerknes-type more common at the western side of the storm tracks, while the stationary-type of cyclone clusters occurs more downstream. In general, clustered cyclones are stronger than non-clustered cyclones. While CMIP6 models feature a slight bias towards an equatorward shift of the storm tracks, cyclone clustering in a future climate occurs more poleward. Furthermore, the average number of storms per cluster decreases in a future climate, though the mean intensity of the cyclones that are clustered increases slightly.