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 (NH) and Southern Hemispheres (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. Large amounts of this change can be associated with changes in the speed of cyclones in the future. 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 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.
Highlights
Extratropical cyclones are the key driver of day-to-day weather variability in the mid-latitudes and can be associated with significant impacts from extreme winds (Browning, 2004) and precipitation (Hawcroft et al, 2012)
This paper presents the first assessment of changes to objectively identified cyclones, their intensities, and associated wind speeds in the newest generation CMIP6 models
We have considered a number of metrics to assess changes in intensity across a range of different shared socio-economic pathways and the main findings of our work are as follows:
Summary
Extratropical cyclones are the key driver of day-to-day weather variability in the mid-latitudes and can be associated with significant impacts from extreme winds (Browning, 2004) and precipitation (Hawcroft et al, 2012). Only the most extreme cyclones in the top 10% of the intensity distribution are examined as these often provide the clearest climate change signal (Catto et al, 2010; Sinclair et al, 2020; Chang, 2018, 2017; Zappa et al, 2013) and are associated with the highest impacts (Ulbrich et al, 2001, e.g.) It has been demonstrated in numerous studies that cyclones in the middle of the distribution can respond differently to the most extreme cyclones (Champion et al, 2011; Pfahl 65 et al, 2015; Michaelis et al, 2017; Sinclair et al, 2020). In this study we will utilise a cyclone compositing method (as in Bengtsson et al, 2009; Catto et al, 2010; Dacre et al, 2012) and a number of models from the new CMIP6 ensemble (Eyring et al, 2016), across a range of future climate scenarios (O’Neill et al, 2016), to attempt to reduce some of the uncertainty surrounding projected change of cyclone intensity and 70 cyclone associated wind speeds in the future. – Do extreme and moderate strength cyclones respond differently to changes in the future climate?
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