Extra-tropical cyclones: The science

Severe extra-tropical cyclones usually form in the North Atlantic basin and represent the dominant feature of the mid-latitudes (Ulbrich et al, 2009). The graphic below from Risk Management Solutions (RMS, 2006)  documents key historical European windstorm events with their associated economic losses. It is striking that developed, heavily populated countries appear so exposed.  The UK, France, Germany, Poland and other significant European countries have suffered from significant losses in the past, and seem sure to be struck again. Cluster events can be particularly damaging, and the events of 1990 and 1999 provide a reminder of these dangers. But what exactly are extra-tropical cyclones? 

The mechanics of extra-tropical cyclones take a very different form to, for example, tropical cyclones, during the processes of genesis, intensification and propagation. Extratropical windstorms tend to be fast moving and broad reaching storms which result in low level of damage at individual sites. The breath of the storms, which can reach 2,000km in diameter, mean losses have the potential to accumulate to significant sums.  In addition to strong winds, heavy precipitation and sharp temperature changes are associated with these large frontal systems. The extra-tropical transition at the midlatitudes is usually a product of low pressure systems where energy is gained from meridional temperature gradients created by polar air masses and subtropical air masses (Harr et al, 1999; Jones et al, 2003). The gradient is greatest during winter and most sever storms tend to develop over this period. The track of the storms is controlled by the position of the Polar Jet, which has the capacity to shift the track north or south.  Ulbrich et al  (2009) suggest that many studies show overall cyclone frequencies to have decreased over the period ~1980-2000. In contrast, however, the average intensity of such storms is shown to have increased with some illustrating an eastwards extension of deep lows in wintertime over the last 50 years. 

Of chief importance in producing conditions favourable to the development of extra-tropical cyclones is the North Atlantic Oscillation (NAO). The NAO is a 'hemispheric meridional oscillation in atmospheric mass with centers of action near Iceland and over the subtropical Atlantic' (Visbeck et al, 2001). The figure below shows its changes in index between 1860 and 2000. The index is defined as the difference between the polar low and subtropical high from December to March. A positive NAO index reflects a stronger than normal pressure gradient and hence more and stronger winds over the North Atlantic ocean basin. Over the last 30 years the NAO has seen an unprecedented trend towards a more positive phase. Incidentally the big cluster storms of 1990 and 1999 occurred over this period. There is an increasing argument that anthropogenic climate change might be influencing this natural mode of atmospheric variability. GCMs do not yet give an unequivocal answer on this viewpoint however. 

Ulbrich et al (2009) support the view that anthropogenic climate change is taking hold, stating that a major result from an ensemble of different models forced with different greenhouse gas concentrations is that extreme extratropical cyclones increase in number in the winter months (when NAO has been at its peak), whilst the total number is slightly reduced in both the norther and southern Hemispheres. 

Bonazzi et al (2012) comment that the damages associated with the European Windstorm peril are amongst the costliest natural perils in Europe. Knowledge on storm development, spatial structure and future frequency and intensity will be crucial in minimising the risk. 

Now you know a little more about the Windstorm peril, the next post will take a look at the impact they can have on humans and our preparedness in a little more detail.