Here in the northern mid-latitudes (much of Canada and the US, Europe, and the northern half of Asia) our weather is governed by the jet stream. This high-altitude wind current, flowing rapidly from west to east, separates cold Arctic air (to the north) from warmer temperate air (to the south). So on a given day, if you’re north of the jet stream, the weather will probably be cold; if you’re to the south, it will probably be warm; and if the jet stream is passing over you, you’re likely to get rain or snow.
The jet stream isn’t straight, though; it’s rather wavy in the north-south direction, with peaks and troughs. So it’s entirely possible for Calgary to experience a cold spell (sitting in a trough of the jet stream) while Winnipeg, almost directly to the east, has a heat wave (sitting in a peak). The farther north and south these peaks and troughs extend, the more extreme these temperature anomalies tend to be.
Sometimes a large peak or trough will hang around for weeks on end, held in place by certain air pressure patterns. This phenomenon is known as “blocking”, and is often associated with extreme weather. For example, the 2010 heat wave in Russia coincided with a large, stationary, long-lived peak in the polar jet stream. Wildfires, heat stroke, and crop failure ensued. Not a pretty picture.
As climate change adds more energy to the atmosphere, it would be naive to expect all the wind currents to stay exactly the same. Predicting the changes is a complicated business, but a recent study by Jennifer Francis and Stephen Vavrus made headway on the polar jet stream. Using North American and North Atlantic atmospheric reanalyses (models forced with observations rather than a spin-up) from 1979-2010, they found that Arctic amplification – the faster rate at which the Arctic warms, compared to the rest of the world – makes the jet stream slower and wavier. As a result, blocking events become more likely.
Arctic amplification occurs because of the ice-albedo effect: there is more snow and ice available in the Arctic to melt and decrease the albedo of the region. (Faster-than-average warming is not seen in much of Antarctica, because a great deal of thermal inertia is provided to the continent in the form of strong circumpolar wind and ocean currents.) This amplification is particularly strong in autumn and winter.
Now, remembering that atmospheric pressure is directly related to temperature, and pressure decreases with height, warming a region will increase the height at which pressure falls to 500 hPa. (That is, it will raise the 500 hPa “ceiling”.) Below that, the 1000 hPa ceiling doesn’t rise very much, because surface pressure doesn’t usually go much above 1000 hPa anyway. So in total, the vertical portion of the atmosphere that falls between 1000 and 500 hPa becomes thicker as a result of warming.
Since the Arctic is warming faster than the midlatitudes to the south, the temperature difference between these two regions is smaller. Therefore, the difference in 1000-500 hPa thickness is also smaller. Running through a lot of complicated physics equations, this has two main effects:
- Winds in the east-west direction (including the jet stream) travel more slowly.
- Peaks of the jet stream are pulled farther north, making the current wavier.
Also, both of these effects reinforce each other: slow jet streams tend to be wavier, and wavy jet streams tend to travel more slowly. The correlation between relative 1000-500 hPa thickness and these two effects is not statistically significant in spring, but it is in the other three seasons. Also, melting sea ice and declining snow cover on land are well correlated to relative 1000-500 hPa thickness, which makes sense because these changes are the drivers of Arctic amplification.
Consequently, there is now data to back up the hypothesis that climate change is causing more extreme fall and winter weather in the mid-latitudes, and in both directions: unusual cold as well as unusual heat. Saying that global warming can cause regional cold spells is not a nefarious move by climate scientists in an attempt to make every possible outcome support their theory, as some paranoid pundits have claimed. Rather, it is another step in our understanding of a complex, non-linear system with high regional variability.
Many recent events, such as record snowfalls in the US during the winters of 2009-10 and 2010-11, are consistent with this mechanism – it’s easy to see that they were caused by blocking in the jet stream when Arctic amplification was particularly high. They may or may not have happened anyway, if climate change wasn’t in the picture. However, if this hypothesis endures, we can expect more extreme weather from all sides – hotter, colder, wetter, drier – as climate change continues. Don’t throw away your snow shovels just yet.
Thank you so much for this excellent summary. I’ve been struggling for a couple of months about how to make this concept more accessible to the general public, and here it is, you’ve done it!
Great job!
Great explanation. Thank you.
Reblogged this on Standard Climate.
With projections made from PIOMAS giving total loss of summer time sea ice within only a few years, it seems reasonable to suppose that we can expect the effects on the jet stream (and accordingly the weather) to rapidly become much worse (with attendant threat to agriculture and infrastructure).
The loss of the Arctic albedo (snow and ice) represents a very fundamental transition occurring in the earth system – all we’re seeing so far is little changes at the beginning. A little thought about albedo feedback, insolation and the physics of ice leads towards that conclusion.
You make so uncomplicated. Thank you! So many fake skeptics are fond of pointing to things like our northern midwest and Canada, and Siberia, as “disproving” global warming. The simpler and more direct the explanation, the more likely that some magic thinking will be dispelled.
Also, your comment policy is so rational and straightforward, though I’m sure it is work for you to keep the discussion sane and reality-based. Thanks again.
Good stuff! I live in South East Australia (Beechworth) and have lived here for 25 years. The last 12 months have been the windiest I have ever experienced here. We have been having some extreme wind events in recent years. This year not so much extreme but just constant wind which is really unusual for this part of Victoria. The logical connection between overall climate warming generating more energy and thereby increasing wind – speed, amount, whatever (I’m no scientist) has been ringing bells with me and your explanations of what’s happening with the jet stream and northern hemisphere makes sense for down here – the blocking too – how parts of Australia get very prolonged heat waves. Anyway, very interesting and thankyou. I think wind is the climate change factor that is yet to make its impact felt.
Nice explanation. Thanks.
I have a basic question about jet streams.
Is a trough in a jet stream a southward bend or is it a dip in elevation ? I ask this because when shown on a two dimensional map even a vertical bend ( dip in altitude ) would appear as a latitudinal bend.
so have I read this right us humans are making the weather worse if thats the case what are all the government doing about it nothing because all they are worried about is money there will be no planet if they keep this up and im fed up nobody seems to speak up about this anymore just been pushed aside I love this planet but the way its going is very bad i got to say this is the worst winter I have know no sign of spring just more cold weather coming because its got stuck again and that’s all to do with jet stream which we are messing up people need to start acting now not later
I had the idea to look for is the jetstream becoming wavier from chaos theory. When energy is added to a system flow goes from linear to oscillating but what next at some point the system goes from oscillating to chaos.
Curiouser and curiouser, many thanks for a good explanation, there also seems to be a further effect. I have noticed that recently (start of April ’13) that there seems to be a correlation between the North Atlantic Current and the jet stream, (it makes sense as it is a large heat source). This year the Gulf Stream is very southerly and the North Atlantic Current is, or rather appears to be stuttering, that is due, one assumes, to the increased volume of fresh water making its way south and my using insufficient data. I also expect that this is a simplistic interpretation of an extremely complex scenario, however it is very scary as bears a stark resemblance to the start of the Younger Dryas period.
Peter Golding, there are now several excellent expositions (I won’t call them explanations, exactly,, as the subject appears to be complex) about the recent Sudden Stratospheric Warming events; also we have lost a lot of multi-year ice in the Arctic and the ice is breaking up it seems a mite prematurely.
http://gmao.gsfc.nasa.gov/researchhighlights/SSW/
I live in Florida, and have seen in just the last two years things I thought I’d not see here. It is April and I still wear a sweat shirt in the morning. Some days the clouds have the same low gray structure I’ve seen far north of here. The heat in the summer is physically unbearable now. I’m in my 60′s and can remember big news in the 50′s to have a day that hit 90. NEWS with pictures of an egg frying on the sidewalk.
How can people not notice? The sciencecan be complex, but really just basic Earth Science from High School. But not to notice?
I fear it is too late and the artic conveyer turnover will come to a stop in the next two to ten years and my children will have hell to pay and whoever survives, I hope they leand to trust in what they know and see.