By Ben Harvey
Strong synoptic-scale low pressure systems, known as as extratropical cyclones, are one of the major weather risks in the UK. This was apparent during the period of extreme storminess experienced last winter. High winds and precipitation, caused by an unusually-prolonged series of extratropical cyclones tracking near to the UK, resulted in widespread flooding, wind damage and, perhaps most dramatically, numerous scenes of extraordinary coastal damage.
Understanding the potential impacts of climate change on extratropical cyclones is therefore critical to assessing future weather risk. Will the frequency of the most intense wintertime extratropical cyclones alter in a warmer world? Might their associated precipitation increase in intensity? Or perhaps the most common paths taken by extratropical cyclones as they advance across the North Atlantic Ocean will change?
These questions are currently being addressed in Reading by the (very aptly-named) TEMPEST project (Testing and Evaluating Model Predictions of European Storms). This three year project, now reaching its conclusion, has provided a detailed assessment of how intense extratropical cyclones are predicted to change in the future in a number of state-of-the-art climate models from around the world (see references 1, 2 and 3 below).
There is some uncertainty in how much different climate models predict storminess will change. To make progress, it is therefore important to understand the reasons for the differences between the predictions made by the models. This has formed a second focus of the TEMPEST project: what are the physical mechanisms that cause extratropical cyclones to respond differently to climate change in the different models? Which mechanisms do we have most confidence in, and where would further research help to constrain the current range of projections?
As an example of one such mechanism, extratropical cyclones in the northern hemisphere owe their existence to the temperature contrast between the Tropics and the Arctic. The Arctic is expected to warm more rapidly than the rest of the world as climate change progresses (a process known as polar amplification), thus reducing this temperature contrast. However, the magnitude of this effect varies greatly between different climate models. We have shown (see reference 4 below) how some of the range in future projections of storminess made with multiple climate models is linked to how much the Arctic warms in comparison to the warming in tropical regions: those models with the most warming in the Arctic have relatively less storm activity in some areas (see Figure 1). Of course, this is not to say that storm activity will necessarily decrease in these regions under climate change, just that the polar amplification mechanism in isolation will likely have a negative impact.
Figure 1. Regions where warming in the Arctic is linked to decreased storminess (the warmer the colour, the larger the impact). The variable shown is the slope of a linear regression between the impact of climate change on a measure of storminess (the standard deviation of the 2-6 day bandpass filtered MSLP during winter) and the corresponding changes in the lower-tropospheric Tropics-Arctic temperature contrast in a large number of different climate models – see reference 4 for details.
1. Harvey, B. J., et al., 2012. How large are projected 21st century storm track changes? Geophysical Research Letters
2. Zappa, G., et al., 2013. A multimodel assessment of future projections of North Atlantic and European extratropical cyclones in the CMIP5 climate models. Journal of Climate
3. Sansom, P. G., et al., 2013. Simple uncertainty frameworks for selecting weighting schemes and interpreting multimodel ensemble climate change experiments. Journal of Climate
4. Harvey, B. J., et al., 2013. Equator-to-pole temperature differences and the extra-tropical storm track responses of the CMIP5 climate models. Climate Dynamics