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The 2020–21 European windstorm season kicked off with Storm Alex in early October as a microcosm of this century so far, promising great winds then delivering a lot of water. The previous winter saw many similar weak windstorms, with just one notable event in Ciara/Sabine causing roughly €1.5 billion in losses in February. Will this season extend the run of relatively benign winter winds, or will we see a return to the stormier 1980s and 1990s?

In the first part of this blog, we will look at the indications for the storminess across Europe this winter. The forecast is completed by a discussion of its uncertainties, which leads to the second part: What is the reliable view of risk for European windstorm? The basis for the long-range outlook is described in an RMS® review of the latest scientific literature on European windstorm activity, made available to clients earlier this year.

Distinguishing cause from effect can be difficult in the climate system, and as a result, many different drivers of winter climate have been proposed. We will focus on four main European winter drivers, which together account for most of the predictability: 

  • El Niño-Southern Oscillation (ENSO): a two- to seven-year cycle of changing east-west heat gradients in the tropical ocean and atmosphere, which modifies extratropical winds
  • Quasi-Biennial Oscillation (QBO): a 28-month cycle of wind changes in the tropical stratosphere, perturbing the polar vortex, hence European storminess
  • Solar flux: changes in ultraviolet radiation following solar cycles (chiefly 11 years, and longer term) modify stratospheric heat and winds, affecting polar vortex, hence midlatitude storminess
  • Arctic sea ice: more ice means a colder polar air mass, which in turn is one of two basic ingredients of storms (the other is warmer southern air mass); sea ice in the Kara and Barents Seas may be particularly relevant to European windstorm by regulating the strength of the Siberian high

The expected impacts of these four drivers on European weather this winter are portrayed in Figure 1 (below), which shows the predicted change in mean winter surface pressure from its long-term climatology (“pressure anomaly”). Black arrows show the mean surface wind anomaly corresponding to such pressure anomalies; when pointing from west to east they indicate a strengthening of the mean westerlies, hence more storminess, and vice versa.

Both the solar and sea-ice drivers are expected to produce an easterly wind anomaly through the windstorm domain, implying less storminess, while the QBO and ENSO favor westerly anomalies and increased storm activity. How will all these drivers combine into the single realization this winter?

Expected change in mean winter surface pressure
Figure 1: Expected change in mean winter surface pressure (contours, in hectoPascals) produced by each of the four major drivers this winter, based on historical experience. The anomalous surface wind is depicted by black arrows.

At first glance, the easterly anomalies from solar and sea-ice forcing look like they may be larger than the westerly (stormy) anomalies from QBO and ENSO. This simple view tacitly assumes the impacts from various drivers combine linearly. However, some published work undermines this assumption. Labitzke et al. (2006) found a remarkably nonlinear interaction between solar flux and QBO in the Arctic winter (see their Figure 3).

The coupling of the polar vortex with European winter weather indicates we need more insight into the interactions between these drivers before confident predictions can be made. Perhaps the most that can be said is that the upcoming winter may be less stormy than the long-term average, and that the state of research means the true outcome is highly uncertain.

What Is a Dependable Reference View?

The above analysis leads to a couple of questions: Will the RMS reference view continue to use all available storm information since the early 1970s to represent the longer-term storm climate? Or should the current climate variability view (CVV), innately more weighted to the recent lull, be considered as a reference view?

The short answer to the second question is no. A candidate for a reference view must offer a robust and reliable perspective on potential storm activity over the next few years. The most recent RMS update on multidecadal variability found growing understanding of the decline from the stormy end of the twentieth century to the twenty-first century lull. However, predictions of future storminess at decadal scales contain several sources of uncertainty, including:

  • Incomplete understanding of known multidecadal drivers.
  • High risk that new, unknown drivers could dominate in the future, a concern heightened by a changing climate, and a limited study size of a single multidecadal cycle.
  • An unpredictable major eruption of a tropical volcano could dramatically raise windstorm risk in the two or three following winters.
  • The past multidecadal cycle has important regional-scale variations that are not replicated by the current generation of climate models.
  • Noise in the climate system could overwhelm deterministic forcings.

Using the CVV as a reference view is essentially assuming that future storm rates will be more like the recent lull, and that the stormy 1980s and 1990s are less likely to occur than long-term experience suggests. Such an expectation for future storminess amounts to a forecast, and the multiple sources of uncertainty listed above explain why such a prediction is inherently risky.

In addition to meteorological uncertainty, the losses are highly sensitive to storm location – whether a storm tracks over an area with a high concentration of exposure (read this RMS blog on different scenarios for Storm Lothar in 1999). In short, the reference view based on storm events since the 1970s is a more robust and reliable view of the next few years of windstorm risk. We will continue to review this situation for changes and improvements to our understanding of decadal-scale variability.

RMS is currently redeveloping our Europe Windstorm Model using the most modern science and data. However, the model will not be available prior to 2022. In this update, rather than using less storm data, the current plan is to boost the length of the historical record used in the reference view and calibrate it to a wind climate that includes all storms since the current model was released.

RMS recently released to clients an updated catalog of significant historical windstorms up to 2018 that can be analyzed in RiskLink® as footprint files. The impact of adding the most recent years to the wind loss climate can be approximated by comparing their losses with storms from the 1980s and 1990s.

In summary, the main seasonal drivers of European windstorm activity indicate a quieter year – but with significant uncertainty. Also, going forward, RMS strategy will be to provide a CVV view of risk for sensitivity studies in addition to a more reliable reference view calibrated to the long-term storm climate. Eventually, different views of risk can be used to inform business decisions, but this should always be made in full understanding of the underlying assumptions and uncertainties.

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Stephen Cusack
Stephen Cusack and Laurent Marescot
Stephen Cusack

Senior Director, Model Development

Stephen is a Senior Director in the Hazard Climate team. After joining RMS in 2009, most of Stephen’s focus has been on researching and developing the Europe Windstorm (EUWS) model, with particular focus on the hazard. Stephen also spent 18 months leading the recalibration of the U.S. and Canada Severe Convective Storm model, released in January 2014. Before RMS, Stephen worked in a wide variety of research and development posts during 13 years at the U.K. Meteorological Office.

 

Laurent Marescot

 

Dr. Laurent Marescot 

Senior Director, Market and Product Specialists at RMS

Laurent is a catastrophe risk management expert at RMS, advising some of the largest companies in the (re)insurance industry how to best manage their nat cat, agriculture, cyber and terrorism risks. He also interacts as an expert for governmental and regulatory authorities. Laurent initially joined RMS in 2008 as part of the account management team, servicing the European (re)insurance and ILS market. He then headed the model product management group for all EMEA and APAC climatic/weather risk perils, such as windstorm, typhoon, severe convective storm and flood, as well as RMS global agricultural risk.

Prior to RMS, Laurent worked 3 years at the Swiss Federal Institute of Technology Zurich (ETHZ) as a Research Associate and Lecturer, managing multidisciplinary research projects. Laurent still lectures regularly on catastrophe modeling and insurance risk quantification at universities and gives seminars and invited talks in international industry conferences. Laurent co-authored numerous industry publications, peer-reviewed scientific articles and proceeding papers. He holds an MSc in Geology from the University of Lausanne and a PhD in Geophysics from the University of Lausanne and the University of Nantes.

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