The number of mid-latitude cyclones and windstorms in northern Europe has no long-term changes between 1980 and 2019
The inter-annual variability of mid-latitude cyclones and windstorms was large in 1980-2019. The study showed the well-known annual cycle; windstorms occur mostly during winter. Photo: Pauli Jokinen.The study investigated climatological characteristics of mid-latitude cyclones and windstorms in northern Europe using ERA5 reanalysis. The study classified all mid-latitude cyclones and windstorms separately. A windstorm means a mid-latitude cyclone with strong winds – therefore, this study does not consider thunderstorms. The inter-annual variability of mid-latitude cyclones and windstorms was large in 1980-2019. The study showed the well-known annual cycle; windstorms occur mostly during winter. “More surprising result was that there was no annual cycle in the number of all mid-latitude cyclones,” says Terhi Laurila, research scientist at the Finnish Meteorological Institute. Every year there are on average 149 mid-latitudes cyclones in northern Europe region and of those 23% have strong winds i.e. are windstorms. On monthly scale, there are on average 5-6 windstorms per month in January and December but no windstorms at all from May to July.
Windstorms are more predictable during the cold season
The study also examined the structure of windstorms during their evolution and so-called precursors which are factors that precede the windstorms. During the development of a windstorm, the minimum pressure decreases, the horizontal pressure and temperature gradients increase, and the winds get stronger. The location of the highest winds moves and extends during the development from the warm sector to behind the cold front. “Cold-season windstorms have stronger winds and pressure and temperature gradients than warm-season windstorms, and the extent of strong winds is larger in cold season,” Laurila says. The precursors to windstorms were investigated using an ensemble sensitivity analysis where higher sensitivity means stronger correlation. The sensitivity was higher in cold-season windstorms than in warm-season windstorms which indicates that windstorms are potentially better predictable during the cold season than the warm season. From the examined precursors, the highest impact on the windstorm intensity was found with the low-level horizontal temperature gradient. Therefore, the low-level temperature gradient is an important variable when predicting the windstorm intensity.
Further information:
Research scientist Terhi Laurila, Finnish Meteorological Institute, tel. +358 50 4648812, terhi.laurila@fmi.fi
Laurila, T. K., Gregow, H., Cornér, J., and Sinclair, V. A.: Characteristics of extratropical cyclones and precursors to windstorms in northern Europe, Weather and Climate Dynamics, 2, 1111-1130, https://doi.org/10.5194/wcd-2-1111-2021, 2021.