Seasonal Forecast of European Windstorm Activity: Will a Stormier Atlantic Deliver Increased Losses?
Stephen CusackOctober 31, 2017
Known indicators point to stormier conditions in the North Atlantic this winter. However, what this means for Europe windstorm losses is much less certain.
Our ability to understand and forecast variability of North Atlantic winter storminess continues to improve year-on-year. Research highlights in 2017 include:
A new, and skillful, empirical forecast model for winter climate in the North Atlantic revealed that sea ice concentrations in the Kara and Barents Seas are the main source of predictable winter climate variations over the past three decades. Interestingly, a separate 2017 study supports earlier forecasts of either a slowing or reversal of the sea ice reductions in the Barents and Kara Seas between now and 2020, implying an uptick in storminess over the next few years.
Twelve months ago, the forecasting indicators for the windstorm season broadly pointed to a 2016/17 season characterized by below average storminess — a forecast borne out by subsequent observations. We have already had a fairly active start to the 2017/18 season, with Windstorms Xavier, Herwart, and ex-Hurricane Ophelia causing local damage, but what is the outlook for the rest of the season?
The rapid cooling in the eastern tropical Pacific over the past three months is pointing to a La Niña phase of the El Niño Southern Oscillation (ENSO) this winter. The quasi-biennial oscillation (QBO) is in a westerly phase which could persist through the winter, and the solar phase is near the minimum of its 11-year cycle. This configuration of drivers is typically associated with a deeper polar vortex in late winter, producing a more westerly circulation over the Atlantic and favoring increased storminess in the North Atlantic.
We must also consider the state of sea ice in the Arctic. In September 2017, the extent of sea ice in the Kara and Barents Seas was much lower than the 1981-2010 average. However, the 2017 extent was greater than observed during the previous two Septembers, and far above the record low seen in September 2012. This evidence suggests a slight increase in storminess in the North Atlantic compared to recent years. Furthermore, we speculate that the relatively less severe sea ice declines above Siberia this autumn, compared to near-record declines above northern Canada and Alaska, could be an additional indicator of raised North Atlantic storm activity, via hemisphere-scale changes in upper-level air flow.
Together, these signs suggest a more active North Atlantic basin in the upcoming season relative to the recent quiet period, particularly later in the winter. This is especially the case if the westerly phase of the QBO persists and La Niña conditions continue to develop over the next few months.
But what does this mean for insured losses?
RMS recently reviewed the growing evidence of predictability of winter weather in the North Atlantic, with the aim of identifying opportunities for the insurance industry. Our findings are published in a detailed research paper available to RMS clients.
We find that most research to date has focused on predicting the North Atlantic Oscillation (NAO) index for the upcoming winter period, defined as “December-January-February” (DJF). However, this forecast parameter has both a spatial and temporal mismatch with annual windstorm losses:
The winter NAO describes time-mean weather over the whole North Atlantic, whereas severe windstorm losses arise mainly from winds hitting major European cities which represent small areas of dense exposure much further to the east. (In this regard, we note with interest recent research suggesting European windstorms are better explained using the Scandinavian Pattern, which has centers of action directly over Europe.)
Major windstorm losses can occur outside DJF, including two of the top six historical windstorm losses of the past five decades (Lower Saxony in November 1972 and 87J in October 1987). More recently, the major loss in the 2013-14 season occurred in October (Storm Christian), and the major loss in 2014-15 hit on the last day of March (Storm Niklas).
These dislocations mean that although the winter-mean NAO has some correlation with windstorm losses in the same DJF period, especially in northwestern Europe, the correlation with damage over annual periods is poor. This suggests that even a perfectly skillful forecast of the winter-mean NAO index would have limited utility for insurance purposes.
Thus, we conclude that although there are clear signs indicating the potential for a more active North Atlantic basin during the forthcoming winter, the signs for European windstorm losses over the whole storm season are much more uncertain.
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Examining the European Windstorm Outlook This Winter
Update on Multidecadal Variability of European Windstorms
At this moment, you might expect a blog about European windstorms to be about recent Storms Ciara-Sabine, Dennis and Jorge causing wind and flood losses of a couple of billion euros in Europe. However, the losses this winter are modest in a longer-term context. Instead, I think the recent insights into longer-term variations in wind losses could have much more impact on how we price windstorm risk.
We first noticed multidecadal variability of European windstorm activity ten years ago, with 50 percent lower frequencies of damaging storms in the new millennium than in the eighties and nineties. This variability is important: a company’s length of loss experience is unlikely to match the model calibration period, which impacts model validation. It also held the promise of improved risk management, if the storminess changes could be anticipated. We needed to know more about it.
Hundred-year records of wind data at several stations from the Dutch weather service, KNMI, showed pronounced multidecadal variability of storminess throughout the period. An RMS review of published work found:
A wide variety of observational evidence of multidecadal variability of storminess in Europe
An incomplete understanding of multidecadal drivers, including mixed results from earlier climate models
What has the past decade of wind data and research taught us about these slow variations in storm activity? The next few sections will show we have learned a lot over the past ten years.
Observed Storminess: What are the Changes?
The Dutch study was recently updated with nine additional years of wind observations from KNMI, to span 1910-2019. The wind data was homogenized using the same procedures to ensure changes in storminess reflected real meteorological causes rather than observational changes. Figure 1 below shows the main result: the twenty-first century lull has extended throughout the 2010s in the Netherlands.
Figure 1: Time series of annual storm loss in the Netherlands, with 10-year running meansStorminess on the continental-scale was measured using a preliminary dataset of European windstorm footprints spanning 1972 to 2018. Each footprint consists of winds at 25 kilometer cells, and a Europe-wide loss index was computed for each historical event. The aggregate loss was computed for each year, then transformed into a standard normal distribution. The Europe-wide annual losses are shown in Figure 2 below, together with annual rates of occurrence of damaging windstorms, also standardized for comparison.
Figure 2: Time series of standardized annual aggregate loss index and storm occurrence rates, and their five-year running means, for the whole of EuropeStorm Climate Drivers
What could be the drivers of the multidecadal storm variation? Over the past ten years, researchers have made huge advancements in understanding European winter climate variability through analysis of observations and experiments with better climate models. Specifically, they have identified heat anomalies in the North Atlantic Ocean and the Arctic as two main drivers of our winter wind climate at decadal and longer timescales.
North Atlantic Ocean Forcing
Gulev et al. (2013) examined observations over the 1880-2007 period and identified an ocean region in the central northern North Atlantic which forces the atmosphere at decadal and longer timescales, as shown in Figure 3 below. The resonance in this ocean area is caused by the co-located storm track.
Figure 3: The observed correlation, at decadal timescales, of surface heat fluxes and sea surface temperatures in the period 1880-2007; Figure 1b of Gulev et al. (2013). Positive correlations indicate ocean temperature anomalies produce surface flux changes of same sign, to drive the atmosphere.Researchers had been reporting confusing, mixed results from climate models. The situation has since been clarified by Scaife et al. (2012), who explained the need for a high model top and vertical resolution to simulate mid-latitude winter signals. There are few tests with appropriate models. Omrani et al. (2014) found a significant ocean forcing of atmosphere winds, about one half of the corresponding observed anomalies over Europe, from a high-top model. Peings and Magnusdottir (2014) used a more modern climate model and found roughly the same result, and indicated that the same area as Figure 3 was the key to ocean forcing of atmosphere on long timescales. A cooling in this key area raises storminess, and vice versa.
There have been some remarkable changes in northern hemisphere winters over the past 20 years or so: the Arctic winter warmed at a rapid rate, while mid-latitude winters have warmed slower than the global trend, and even cooled in some areas.
Observations reveal a strong link between declining sea ice and a stronger Siberian High. Mechanisms to explain this were reviewed in Cohen et al. (2020), and they viewed the process depicted in Figure 4 as most robust.
Figure 4: A schematic of the process linking Barents-Kara sea ice to European winter climateThere has been a long debate on the size of the Siberian High anomaly forced by Arctic warming, fanned by mixed climate model results. This is getting resolved by recent research indicating climate models have special requirements to simulate mid-latitude winter signals, such as high model top and fine vertical resolution and fully coupled ocean-atmosphere models. Six modern climate models meeting most requirements had a consistent signal of sea ice loss causing a stronger Siberian High, and a weakening of the westerlies carrying storms into Europe. Their modeled signal is about half of the observed circulation change over the past few decades.
How are the two main drivers likely to evolve over the next several years, and what does this entail for European windstorm activity?
North Atlantic Ocean Outlook
Figure 5 shows the mean temperature anomalies (lower plot) for the key ocean area (upper plot). Will the recent cooling continue, or reverse? There are no published forecasts of ocean heat in the specific key region for the next decade. Instead, we estimate changes based on the known drivers of heat in this area, and conclude the likeliest outcome is for persistence of these cooler anomalies in the next few years, implying the stormier North Atlantic of recent years will continue. Will they raise windstorm loss? That depends more on the second driver, and what it does to the Siberian High.
Figure 5: Time series of mean temperature anomaly in the top 400 meters of the ocean in November to April (lower plot), for the region off Newfoundland indicated by red box (upper plot). Ocean temperatures from EN4 were de-trended to remove global warming signal.Arctic Outlook
Anthropogenic forcing is the main cause of Arctic warming and sea ice decline, and the IPCC indicate this is very likely to continue in the future. On shorter timescales, Årthun et al. (2017) describe how North Atlantic Ocean heat anomalies are carried north to modulate sea ice in the Barents and Kara Seas. The northern Atlantic has been cool since 2015, and Figure 6 below shows winter sea ice area in this region has risen since its nadir in 2016. The northern North Atlantic waters are expected to remain cooler over the next few years, and this could stabilize sea ice, or perhaps even reverse its longer-term decline. Raised sea ice in this region would lift windstorm losses.
Figure 6: The sum of sea ice area in Barents and Kara Seas for December and January. Source: NSIDCUncertainties in the Outlook
There are significant uncertainties in the multiannual forecast:
Could the known drivers evolve unpredictably, in this time of changing climate?
Could an unexpected climate process, such as a mode of tropical variability, or ongoing anthropogenic forcing of the stratosphere, or simply random variability, emerge to dominate?
Will an explosive volcanic eruption occur, to raise windstorm risk for the following few years?
The continental-scale windstorm lull in the first decade of this century continued through the past ten years, with notable regional variability.
There have been several advances in understanding mid-latitude storm climate variability:
Heat anomalies in the North Atlantic Ocean, underneath the storm-track, affect winter storminess in the Atlantic sector
Heat anomalies in the northern North Atlantic modulate Arctic sea ice, notably in the Barents and Kara Seas
Barents and Kara sea ice modulate the Siberian High to affect European storminess
Both observation-based analyses and fit for purpose climate models support these processes
In brief, heat anomalies in the North Atlantic Ocean and Arctic regions explain at least half of the recent multidecadal change in storminess.
The best estimate for the next few years is a slight upward trend in windstorm activity, with significant uncertainty.
The new challenge for insurance is how to benefit from climate forecasting skill, while maintaining safe management of European windstorm risk. We explore this in an upcoming RMS white paper. Debate is positively encouraged, please contact our Product Manager Michele.Lai@rms.com in the first instance.…
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.