Supported by leading meteorological organisations, the view is that this winter has the potential to be wetter and milder than normal, with a higher likelihood of stormy conditions.
Looking beyond the daily weather forecast requires different techniques and produces predictions, which require careful interpretation. Seasonal predictions presented as probabilities are often delivered as an increased or decreased likelihood of above average, average or below average conditions. So, for example, while warmer/wetter conditions may be more likely in a forecast, that doesn’t rule out the possibility of colder/drier conditions occurring, albeit at a lower probability. This is important to recognise when thinking about how to apply seasonal forecast information.
Major climate drivers influencing seasonal variability for the current season are assessed to provide a rationale for considering models that produce seasonal predictions run by the world’s leading meteorological organisations. The models (General Circulation Models) are physics-based and use supercomputers to run the millions of calculations necessary to resolve the equations required to make predictions of climate conditions months in advance.
| Climate driver | Status | Impact |
|---|---|---|
| North Atlantic Oscillation (NAO)1 | Slightly positive | Encourages more unsettled weather patterns for Europe. Positive NAO means increased potential for stormy conditions. |
| Indian Ocean Dipole (IOD)2 | Slightly positive | Disrupts rainfall patterns in tropics, which influences European weather. Positive IOD supports wetter and milder winter. |
| Stratospheric Polar Vortex(SPV)3 | Strong for first half of winter, but weakening later in the winter | Enhances global circulation around the North Pole. When SPV is strong, it encourages wetter and milder conditions in Europe. |
| El Nino-Southern Oscillation (ENSO)4 | Neutral | Limited influence on Europe. Neutral conditions anchor the probabilities towards average. |
Good agreement between many of the major long-range prediction centres around the world gives confidence in the seasonal forecast. UK Met Office5 , ECMWF6 , Meteo-France7 , NCEP8 and JMA9 are in general agreement on the major drivers described previously and a westerly flow (weather coming from the west across the Atlantic) dominating for Europe, leading to milder and wetter conditions than normal.
Seasonal forecasts predict temperature, precipitation and various other atmospheric characteristics at a broad scale, but while these models do not predict individual storms, the strong suggestion of wetter and milder conditions for the rest of winter are indicative of an enhanced probability of stormy and unsettled conditions. More information on the skill of seasonal prediction systems is available on request or from the relevant organisation's website.
The positive phase of the NAO is associated with increased storminess for Europe. European windstorms exhibit a high degree of clustering (Vitolo et al. 200910 ) due to the effects of the NAO on storm tracks and therefore storm frequency for Europe, leading to increased aggregation of industry loss events. This clustering characteristic of European Windstorms can be incorporated in an annual view of risk in industry catastrophe models through adjustments to event sets. Our Willis Re Catastrophe Analytics11 teams can provide a View of Risk based on this increased clustering of storms to be ready for enhanced storminess this season should it occur.
Risk managers may find it beneficial to be aware of the seasonal variations, and the increasing skill available from seasonal forecasting capabilities. Incorporating these enhanced probabilities of stormier conditions for the next couple of months, may prove useful in managing claims, capital allocation, and can also help with reviewing placement requirements in post-renewal discussions. However, seasonal prediction is based on probabilistic modelling, and so benefits are more likely to be achieved by adopting a process that incorporates seasonal forecasts over many years rather than just one attempt. Testing on past data in an unbiased study is advised before making financial decisions using seasonal forecasts.
While the seasonal predictions indicate a positive NAO through this winter, influencing storm tracks for Europe as described above, it is much more difficult to quantify the effect of a warming climate on winter storms. The NAO signal is largely a representation of the internal variability in the climate system, describing the major atmospheric patterns driving European Weather extremes, and therefore external forces from greenhouse gas increases related to climate change are difficult to assess, other than the ability of the NAO to provide a clue to the future range of the variability (Deser et al. 201712 ). Furthermore, a strong consensus has not yet been reached in the scientific community on the effects of climate change on storminess in the past, however a large proportion of studies suggest an increase in winter storm intensity in a future warmer climate for the North Atlantic and Western Europe, albeit with total storm numbers remaining uncertain (Feser et al. 201513 ).
On the seasonal scale however, the underlying temperature increases observed as a result of climate change (IPCC Stocker et al. 201314 ) will contribute to the increased likelihood of a warmer than average current winter season, when compared to recent decades. As mentioned before, this is a probabilistic forecast, so below average temperatures remain possible, and of course on a sub-seasonal timescale (day-to-day weather) there will most likely be short colder spells during the winter, but given the current climate conditions, over the remainder of the winter, colder-than-average temperatures are less likely.
There are valid technical reasons, supported by leading meteorological organisations, to take a view that this winter has the potential to be wetter and milder than normal, with a higher likelihood of stormy conditions. While the predictions are broadly consistent across models, and confidence is relatively high, it is difficult to make a quantitative assessment of the direct impact of the predictions on storminess, beyond a suggested directional change towards above normal. However, seasonal forecasts are probabilities, so this only ‘loads the dice’ in favour of the predicted conditions, and opposite outcomes of colder and drier than normal conditions can still occur.
| January | February | March | |
|---|---|---|---|
| Storminess potential | Increased | Increased | Increased |
| Precipitation | Wetter-than-average | Wetter-than-average | Wetter-than-average |
| Temperature | Milder-than-average | Milder-than-average | Milder-than-average |
1
https://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/JFM_season_nao_index.shtml: accessed January 2020.
2
http://www.bom.gov.au/climate/enso/#tabs=Indian-Ocean: accessed January 2020
3
https://www.metoffice.gov.uk/weather/learn-about/weather/atmosphere/polar-vortex
4
https://iri.columbia.edu/our-expertise/climate/forecasts/enso/current/
5
https://www.metoffice.gov.uk/
6
https://www.ecmwf.int/
7
http://www.meteofrance.fr/
8
https://www.cpc.ncep.noaa.gov/
9
https://www.jma.go.jp/jma/indexe.html
10 Vitolo, R., Stephenson, D.B., Cook, I.M. and Mitchell-Wallace, K., 2009. Serial clustering of intense European storms. Meteorologische Zeitschrift, 18(4), pp.411-424.
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Catastrophe Analytics
12 Deser, C., Hurrell, J.W. and Phillips, A.S., 2017. The role of the North Atlantic Oscillation in European climate projections. Climate dynamics, 49(9-10), pp.3141-3157.
13 Feser, F., Barcikowska, M., Krueger, O., Schenk, F., Weisse, R. and Xia, L., 2015. Storminess over the North Atlantic and northwestern Europe—A review. Quarterly Journal of the Royal Meteorological Society, 141(687), pp.350-382.
14 Stocker, T. ed., 2014. Climate change 2013: the physical science basis: Working Group I contribution to the Fifth assessment report of the Intergovernmental Panel on Climate Change. Cambridge University Press.
Scientists from Columbia University and the National Center for Atmospheric Research were invited to testify to the Committee on Science, Space and Technology in September, in a hearing entitled “Understanding, forecasting and communicating extreme weather in a changing climate”.
