Weather forecasters can predict Britain’s notoriously variable weather as it changes from day to day or even hour to hour.
But what about year to year? After all, 2013 saw weeks of unbroken sunshine whereas summer this year has barely got going – despite some blazing hot days now and again – even by late August. Some winters are wet and stormy, others mild and dry.
While we can now predict winter seasonal weather with some accuracy, our summer efforts are less advanced. But why shouldn’t we be able to forecast a scorching summer with more accuracy? A new study, published in Climate Dynamics with colleagues at the University of Sheffield, should help.
It all depends on the jet stream, the ribbon of strong westerly winds which blows across the Atlantic 10km or more above the surface. Britain is located near the easterly end of the North Atlantic jet and so the jet stream influences much of the variability in its weather.
In summer, a jet lying to the north of the British Isles steers any rain-bearing, low-pressure systems northward towards Norway and away from Britain. This results in the UK experiencing the sort of warm dry weather seen for weeks on end in 2013.
However, when the jet stream lies further to the south, those same low-pressure systems tend to hit north-western Europe, resulting in wet weather. A difference in rainfall between north-west Scotland and south-east England highlights this difference. In 2012, for example, while most of the British Isles experienced well above average rainfall, north-west Scotland was unusually dry. In drier summers, however, north-west Scotland tends to experience the higher rainfall associated with a northward jet displacement.
In most recent years, this southerly shift in the jet stream has prevailed. This has in part been attributed to warm sea surface temperatures in the Atlantic which lead to lower pressure and increased summer rainfall.
A number of recent summer extremes such as the record-breaking wet summers of 2007 or2012 and the warm dry summer of 2013 have heightened the need for improved understanding of summertime jet stream variability. After all, improvements in seasonal forecasting would lead to great socio-economic benefits – imagine being able to plan your summer months in advance, already knowing whether rain and cloud or endless sunshine is more likely.
Ultimately, our research found that there was no single factor that causes the jet stream to shift around. Changes in the sun’s strength from year to year play a role, as does the extent of sea-ice in the Arctic and sea surface temperatures in both the North Atlantic and, perhaps more surprisingly, the tropics near Indonesia. However there is little evidence that the well-known El Niño phenomenon has any influence in summer.
It takes quite a while for changes to have an impact on the jet stream. For example, it would seem that solar variability three to five years previously is more significant than solar variability at the time of any given summer.
Likewise, our work found an intriguing and as yet unexplained link between sea ice extent in the previous autumn in the Barents and Kara seas, north of Norway and Russia, and the position of the jet stream in the following summer.
Increased sea ice is associated with a northward shift in the jet, which means drier summers in the UK. Given how fast the Arctic is melting, less ice could contribute to a southward shift in the jet and an increased probability of wet British summers. However, we still don’t know exactly how this will play out in future, and it’s possible that continued sea-ice decline will have some unexpected effects.
At present, dynamical forecasting models are showing some skill in making seasonal predictions for the North Atlantic winter, but demonstrate almost no skill for the summer months. Our work found that, while much of the jet variability is attributed to seemingly random atmospheric “noise”, the predictable component of summer jet stream variability may be as high as 35%. This may seem quite small but ensuring such factors are adequately represented in the forecasts will help us predict what’s in store next summer.