Predicting future changes in regional climate and extreme weather events is becoming a main goal for climate science and a topic of great societal interest. For example, a key question for the mid-latitudes countries is how extreme extratropical storm activity, and their associated winds and precipitation, may change in the future. As the recent stormy British winter reminds us, extreme storminess can lead to substantial infrastructure losses, coastal damage and widespread floods. On the other hand, “non extreme” storms are themselves vital to societies: the passage of each storm brings a variable amount of rainfall and winds which can impact on societal needs such as fresh water availability and wind energy production.
Work I have done (1, 2) within the TEMPEST project has been aimed at obtaining new evidence on the impact that climate change may have on the behaviour of storms (extratropical cyclones) in the North Atlantic and European regions. For doing this work, computer software (a cyclone tracking algorithm) is applied to identify the tracks of the individual extratropical cyclones in simulations of the present day climate and of the climate conditions that may occur by the end of the 21st century. This approach is applied to 19 different climate models available from the CMIP5 archive. A strength of this approach is that comparing different climate models in a consistent manner allows us to identify the future changes in storminess which are consistent between the different models. Crucially, identifying a consistent response is a first necessary ingredient for having confidence in the future projections and changes in cyclone properties.
The results show that the number of cyclone tracks passing close to the UK is projected to increase slightly in winter, but to decrease in summer. However, the number of extratropical cyclones associated with extreme rainfall is expected to increase across the whole North Atlantic region in both seasons (Figure 1). This behaviour is common to all the different climate models and it is related to the higher amount of moisture content available in the atmosphere in a warmer climate. For an intermediate level of mitigation of future greenhouse gas emissions, the average of the model projections shows that the amount of rainfall generated by cyclones over the UK increases by about 9% in winter and 8% in summer by 2100.
FIGURE 1. Future changes in the number of winter (December-February) storms associated with intense rainfall, from an average of 19 simulations. The number of storms with the heaviest rainfall increases across the whole North Atlantic region. Units are in number density of storms per month per unit area.
Not only the UK may experience future changes in the behaviour of extratropical cyclones. A large reduction in the number of extratropical cyclones is projected to occur in the Mediterranean region, with a 15% decline by 2100 estimated from the average of the simulations. Given that water scarcity already affects some Mediterranean countries, the projected reduction in the number of Mediterranean cyclones may have some severe consequences for precipitation and fresh water availability of the region. However, the physical drivers of the reduction in the number of Mediterranean cyclones are still not entirely clear and they will have to be understood better to increase the confidence in the Mediterranean climate change projections.
1. G. Zappa et al, 2013. A Multimodel Assessment of Future Projections of North Atlantic and European Extratropical Cyclones in the CMIP5 Climate Models. Journal of Climate, http://dx.doi.org/10.1175/JCLI-D-12-00573.1
2. P.G. Sansom et al, 2013. Simple Uncertainty Frameworks for Selecting Weighting Schemes and Interpreting Multimodel Ensemble Climate Change Experiments. Journal of Climate, Vol 26, http://dx.doi.org/10.1175/JCLI-D-12-00462.1