1. The weather’s been terrible on both sides of the Atlantic for months. But why?
Britain’s Met Office recently released a preliminary report trying to explain just why the winter has been so crappy. It’s 24 pages of rather complicated meteorology. But it gives us a pretty good explanation of why everything is so dreadful.
Basically, it’s all the fault of the jet stream. And Indonesia.
2. Normally the jet stream works something like this:
(That’s the simplified version.)
The jet stream is a basically a long tube of fast-moving air, flowing west to east around the globe, several miles up in the atmosphere. There’s actually two parts of the jet stream that matter here - the Asia-Pacific jet stream (where it leaves Asia over Japan) and the North Atlantic jet stream, which runs up from the Americas in the general direction of Europe. Both have a huge influence over our weather - but when it’s operating normally, it generally means we get fairly standard winters.
3. But 2013 saw substantially more rainfall in Indonesia, due to the recent warming of the West Pacific.
A warmer ocean means more water evaporating into the atmosphere - so more rain. The Western Pacific has been abnormally warm for most of the decade; the area (particularly Indonesia and the eastern Indian Ocean) had much higher rainfall than normal throughout December and January - and it already rains an awful lot there.
And that increased rainfall has a knock-on effect on the path of the jet stream.
4. And that unusual rainfall in the West Pacific pushed the jet stream much further north than usual.
And that meant the jet stream was suddenly pulling cold arctic air down across Canada and the USA - which is why it’s been so cold there. And this in turn helped to fuel a more powerful North Atlantic jet stream - one that was up to 30% stronger than normal.
The North Atlantic jet stream basically steers storms right towards the UK and Ireland. And there’s a self-reinforcing element to it - it creates the conditions for the formation of storms, but it also gains momentum from those storms. Which is why, sometimes, you can get stuck in a rut - storm after storm after storm.
5. Of course, it’s all a bit more complicated than that. (Because the global weather system is insanely complex.)
The main reason the North Atlantic jet stream was so strong was the extra-cold air hitting the warm, moist air over the Atlantic. That temperature difference is what creates storms in the first place - the bigger the difference, the more fuel there is for storms.
The sub-tropical Atlantic is also significantly warmer than average right now, meaning an even larger temperature difference - plus lots of extra moisture in the air, which got picked up and dumped on the UK and Ireland.
Plus, there were additional disturbances in the Eastern Pacific too, which also helped add energy to the jet stream.
6. And all that’s before you take into account the polar night jet, which was also unusually strong this year.
A strong polar night jet - a jet stream which only forms in the polar winter, at very high altitudes - also tends to precede a strong Atlantic jet stream, and this year’s polar night jet was sometimes more than twice its normal strength. The polar vortex (the low pressure area that lurks over the pole, inside the polar night stream) also stretched further south than normal this year, covering part of Canada, which may have contributed as well.
7. So: there were a lot of contributory factors. But the report concludes that “the Pacific Ocean has been a major driver of this winter’s severe weather”.
Damn you, Pacific Ocean.
8. But the big question, of course, is this - was climate change ultimately to blame?
There’s no doubt it was a record-breakingly bad winter in the UK - the Met Office report lays out plenty of stats to show that.
But they’re more cautious about saying whether it can be blamed on climate change. Britain’s weather is notoriously changeable. And this makes it very hard to say for sure whether any single weather event, even one as extreme as this winter, was a one-off or part of a larger pattern.
But there are facts that suggest climate change may be partly to blame.
9. Sea levels are rising, and will keep on rising.
We’re certain to see further sea level rises over the coming decades (compared to the levels in 1990.) And of that, “at least two-thirds will be due to the effects of climate change.”
10. Britain may well be likely to suffer more frequent and more severe downpours in the future.
There needs to be more research to confirm this, but - notably - it also fits exactly with what basic physics would suggest a warming world will produce.
11. And while we’re not getting more storms, the storms we do get are becoming worse.
It’s too early to say for sure if climate change is to blame for these storms - or any specific weather event. But increasingly, the evidence seems fits with what we’d expect from climate change. And the Met Office’s chief scientist, Julia Slingo, has said that “all the evidence suggests there is a link to climate change.”