The representation of the winter and summer extratropical storm tracks in both hemispheres is evaluated in detail for the available models in phase 6 of the Coupled Model intercomparison Project (CMIP6). The state of the storm tracks from 1979 to 2014 is compared to that in ERA5 using a Lagrangian objective cyclone tracking algorithm. It is found that the main biases present in the previous generation of models (CMIP5) still persist, albeit to a lesser extent. The equatorward bias around the SH is much reduced and there appears to be some improvement in mean biases with the higher-resolution models, such as the zonal tilt of the North Atlantic storm track. Low-resolution models have a tendency to underestimate the frequency of high-intensity cyclones with all models simulating a peak intensity that is too low for cyclones in the SH. Explosively developing cyclones are underestimated across all ocean basins and in both hemispheres. In particular the models struggle to capture the rapid deepening required for these cyclones. For all measures, the CMIP6 models exhibit an overall improvement compared to the previous generation of CMIP5 models. In the NH most improvements can be attributed to increased horizontal resolution, whereas in the SH the impact of resolution is less apparent and any improvements are likely a result of improved model physics.

Abstract. Future changes in extratropical cyclones and the associated storm tracks are uncertain. Using the new CMIP6 models, we investigate changes to seasonal mean storm tracks and composite wind speeds at different levels of the troposphere for the winter and summer seasons in both the Northern Hemisphere (NH) and Southern Hemisphere (SH). Changes are assessed across four different climate scenarios. The seasonal mean storm tracks are predicted to shift polewards in the SH and also in the North Pacific, with an extension into Europe for the North Atlantic storm track. Overall, the number of cyclones will decrease by ∼5 % by the end of the 21st century, although the number of extreme cyclones will increase by 4 % in NH winter. Cyclone wind speeds are projected to strengthen throughout the troposphere in the winter seasons and also summer in the SH, with a weakening projected in NH summer, although there are minimal changes in the maximum wind speed in the lower troposphere. Changes in wind speeds are concentrated in the warm sector of cyclones, and the area of extreme winds may be up to 40 % larger by the end of the century. The largest changes are seen for the SSP5-85 scenario, although a large amount of change can be mitigated by restricting warming to that seen in the SSP1-26 and 2-45 scenarios. Extreme cyclones show larger increases in wind speed and peak vorticity than the average-strength cyclones, with the extreme cyclones showing a larger increase in wind speed in the warm sector.

The winter season of 2013/2014 was the stormiest on record for the British Isles. In this article we show that there was an unprecedented amount of cyclone clustering during this season, corresponding to an average of one intense cyclone affecting the country every 2.5 days. An intensely clustered period from 6 to 13 February 2014 that was associated with one specific cyclone family is analysed in detail. This cyclone family is shown to be associated with a strong and straight upper level jet that is flanked by Rossby wave breaking on both its northern and southern sides for the duration of the clustering event. This mechanism is also identified for other periods in this season. The persistence of these conditions resulted in the clustered cyclone activity, and it was accompanied by record‐breaking rainfall, widespread flooding and large socio‐economic losses.

Winter 2013/14 was the stormiest on record for the UK and was characterized by recurrent clustering of extratropical cyclones. This clustering was associated with a strong, straight and persistent North Atlantic jet and was also associated with Rossby wave breaking (RWB) on both flanks, pinning the jet in place. The occurrence of RWB and cyclone clustering is further studied in 36 years of the ERA‐Interim Reanalysis. Clustering at 55°N is associated with an extended and anomalously strong eddy‐driven jet flanked on both sides by RWB. However, clustering at 65(45)°N has a dominance of RWB to the south (north) of the jet, deflecting the jet northward (southward). A positive correlation was found between clustering and RWB occurrence to the north and south of the jet. However, there is considerable spread in these relationships.