20 Dec 2022
 | 20 Dec 2022
Status: a revised version of this preprint is currently under review for the journal NHESS.

Decadal variations of European windstorms: linking research to insurance applications

Stephen Cusack

Abstract. The insurance sector is affected by decadal-scale variations in annual European windstorm losses amounting to a few billion euros, yet has not applied recent advances in understanding and predicting this variability to their pricing of windstorm risk. This is mainly due to an unknown relation between insured wind losses and meteorological definitions of storminess used in research. This study aimed to reduce this uncertainty.

A history of windstorm insurance losses over the past 72 years was developed from winds in weather reanalyses. Then, typical storm proxies used by researchers, such as the North Atlantic Oscillation (NAO) and the Arctic Oscillation, were compared to the new windstorm loss record. The relationship between the proxies and losses has two distinct regimes: highly consistent from 1950 up to the 2000s, then a divergence in the past 10 to 15 years. The recent separation is large and robust, with high confidence that modern values of researchers’ proxies approach levels last seen 30 years ago, whereas decadal-mean losses are far lower today than in the 1980s and ‘90s.

The cause of this divergence was explored. Storm damages are most closely associated with peak gusts deriving their momentum from winds in the free troposphere, and pressure gradients at the surface used in typical climate indices can only partially describe higher level winds. Based on this reasoning, a new Hemispheric Geostrophy Index (HGI) was defined as the difference in 700 hPa heights between the tropics and the Arctic. It was found to vary coherently with decadal losses in the past, and crucially retains this consistency in the past 15 years too. Breaking down the HGI into component parts revealed that lower storminess in recent times is linked to ongoing reductions in poleward baroclinicity. Further development of loss history and climate indices would help bridge decadal research to insurance applications.

Stephen Cusack

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on nhess-2022-268', Anonymous Referee #1, 28 Jan 2023
  • CC1: 'Comment on nhess-2022-268', Matthias Klawa, 11 Feb 2023
  • RC2: 'Comment on nhess-2022-268', Anonymous Referee #2, 13 Feb 2023

Stephen Cusack

Stephen Cusack


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Short summary
Decadal climate research circumvents the lack of a complete European windstorm loss history by using climate indices. In this study, a full loss timeseries from 1950 to 2022 was developed, and the key finding is that climate proxies diverge from losses in recent times. Including low-level baroclinicity in the climate indices, because damaging gusts derive their strength from winds in the lower troposphere, will narrow the gap between climate indices and losses.