Friday, May 23, 2014

A chance of snow

Here’s my latest piece for BBC Future.


So what happened to the severe winter that we in the UK were warned about last November? Instead there was less snow than in recent years, but it rained incessantly. Sure, the winter was unusually bitter – if you live in Buffalo, not Bristol. Why can’t the forecasters get it right?

Well, here’s the thing: the UK’s Met Office didn’t forecast anything like this at all. They said simply that “temperatures are likely to remain near or slightly below average for the time of year, but otherwise fairly normal conditions for early winter are most likely”.

But more importantly, the Met Office has tried repeatedly to explain that “it’s not currently scientifically possible to provide a detailed forecast over these long timescales”. As far as snow is concerned, they said last November, “because there are so many factors involved, generally that can only be discussed in any detail in our five day forecasts.” To which commendable honesty you might be inclined to respond “a fat lot of good.”

Yet hope is at hand. In a new paper published in Geophysical Research Letters, a team of Met Office scientists say that, thanks to advances in modelling the North Atlantic weather system, “key aspects of European and North American winter climate… are highly predictable months ahead”.

So what’s changed? Well, in part it all depends on what you mean by a weather forecast. You, I or Farmer Giles might want to know if it is going to rain or snow next Thursday, so that we can anticipate how to get to work or whether to gather in the herds. But the laws of physics have set a fundamental limit on how far in advance that kind of detail can be predicted. The weather, crucially dependent on the turbulent flows of atmospheric circulation, is a chaotic system, which means that the tiniest differences in the state of the system right now can lead to completely different outcomes several days down the line.

Beyond about ten days in the future, it is therefore mathematically impossible to forecast details such as local rainfall, no matter how much you know about the state of the weather system today. All the same, advances in satellite observations and computer modelling mean that forecasting has got better over the past few decades. No matter how cynically you like to see it, the five-day forecast for Western Europe is now demonstrably more accurate than the three-day forecast was in the late 1980s.

Despite this limited window of predictability, some aspects of weather – such as general average temperature – can be forecast much further in advance, because they may depend on features of the climate system that change more slowly and predictably, such as ocean circulation. That’s what lies behind the Met Office’s current optimism about winter forecasting in the North Atlantic region.

This area has always been difficult, because the state of the atmosphere doesn’t seem to be as closely linked to ocean circulation as it is in the tropics, where medium-range forecasting is already more reliable. But it’s precisely because the consequences of a colder-than-usual winter may be more severe at these higher latitudes – from disruption to transport to risk of hypothermia – that the shortcomings of winter weather forecasts are more keenly felt there.

The most important factor governing the North Atlantic climate on the seasonal timescale is an atmospheric phenomenon called the North Atlantic Oscillation (NAO). This is a difference in air pressure between the low-pressure region over Iceland and the high-pressure region near the Azores, which controls the strength of the westerly jet stream and the tracks of Atlantic storms. The precise size and location of these differences see-saws back and forth every few years, but with no regular periodicity. It’s a little like the better-known El Niño climate oscillation in the tropical Pacific Ocean, but is confined largely to the atmosphere whereas El Niño – which is more regular and fairly well predictable now – involves changes in sea surface temperatures.

If we could predict the fluctuations of the NAO more accurately, this would give a sound basis for forecasting likely winter conditions. For example, when the difference between the high- and low-pressure “poles” of the NAO is large, we can expect high winds and storminess. The Met Office team reports that a computer model of the entire global climate called Global Seasonal Forecast System 5 (GloSea5) can now provide good predictions of the strength of the NAO one to four months in advance. They carried out “hindcasts” for 1993-2012, using the data available several months before each winter to make a forecast of the winter state of the NAO and then comparing that with the actual outcome. The results lead the team to claim that “useful levels of seasonal forecast skill for the surface NAO can be achieved in operational dynamical forecast systems”.

What is it that makes the notoriously erratic NAO well predicted by this model? The researchers can’t fully answer this question yet, but they say that the fluctuations seem to be at least partly governed by the El Niño cycle. The NAO is also linked to the state of the Arctic sea ice and to a roughly two-year cycle in the winds of the tropical lower stratosphere. If the model can get those things right, it will be likely to forecast the NAO pretty well too.

If we can make good predictions of general winter climate, says the Met Office team, then this should help with assessing – months in advance – the risks of dangerously high winter winds and of transport chaos, as well as predicting variations in wind energy and setting fuel-pricing policies. All of which, you must admit, is probably in the end more useful than knowing whether to expect a white Christmas.

Paper: A. A. Scaife et al., Geophysical Research Letters 41, 2514-2519 (2014)

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