Jet stream: Is climate change causing more ‘blocking’ weather events?

[knocker]

The past few months have seen some remarkable weather, from the UK’s sunniest spring on record to Siberia’s dramatic heatwave and “zombie wildfires”.

Key to this unseasonable weather are persistent high-pressure “blocking” weather systems, which bring clear, dry conditions on the ground below for many days or weeks.

Blocking events bat away oncoming low-pressure systems that would bring the prospect of clouds and rain. They are particularly synonymous with heatwaves and drought in summer and bitterly cold conditions in winter.

But what are the prospects for blocking events in a warming climate? And could a rapidly warming Arctic also have a role to play?

In this Q&A, Carbon Brief takes a closer look at the causes of blocking events and the potential changes in the future.

https://www.carbonbrief.org/jet-stream-is-climate-change-causing-more-blocking-weather-events

[knocker]

A further note

For the jet stream as well as the storm tracks, the crucial ingredient in the climate change problem seems to  be this underlying need for poleward energy transport. This, after all, is the single most fundamental reason why the jet and the storms are there to start with: to balance the energy budget, the atmosphere needs to move energy from the tropics towards the poles.

We have just said that in the tropics, up at peak jet level, the warming will be stronger than elsewhere. Surely this means that the all-important contrast between tropical and polar temperatures will increase: does this not imply stronger jets and storms in the future, after all?

We now arrive at one of the central causes of uncertainty over how the jet will change. In the upper troposphere, the warming is indeed strongest in the tropics, as we have just heard. Down at the surface, however, the situation is precisely the opposite.

Another clear and very robust feature or the climate model predictions is that surface warming will be strongly amplified in the Arctic, not the tropics. This arises because of local feedbacks, for example the loss of sea ice which uncovers the relatively warm ocean beneath. Winter sea ice coverage is expected to decline by around 30%  by the end of the century, and in summer this number is closer to 90%. Some completely ice free summers are expected by this point in the strong warming scenarios, a very dramatic change indeed since the days of William Scoresby.  The result, then, is a veritable battle for the future of the jet stream (see Fig. 15.3). While the upper level changes driven by the tropics act to strengthen the temperature contrast, the lower changes driven by the Arctic act instead to weaken it. The future changes to the jet stream predicted by climate models are actually quite small, but this is at least partly due to a stalemate in the tug-of-war for the jet between tropics and pole. If our predictions of either of these two regions prove wrong, the changes in the jet may be much stronger than expected.  It is not, however, a complete stalemate. One of the two effects does seem to be stronger than the other. And the winner, to the best of our current knowledge, is the tropics. Many of the predicted changes in the atmospheric circulation can be traced back to the amplified warming high up in the air over the equator

Source - 'Jet Stream' by Tim Woollings

[iapennell]

@knocker

As the upper warming over the tropics with modest global warming would appear to be the more dominant influence this does tie in with IPCC predictions that higher latitudes will be warmer and wetter in winter with more intense storms as a result of global warming. The reduction in the frequency of prolonged high-latitude blocking bringing bitterly cold northerlies and easterlies for long periods in the UK in winter in recent years does bear this out. The earlier global warming of the 1920's and 1930's was accompanied with few severe winters (1928-29 excepted), and more frequent strong westerlies and heavy rains; the 0.5C global cooling from 1940 to 1970 brought about more frequent blocking and some noteworthy severe winters to the UK, of which 1946-47 and 1962-63 were the most extreme manifestations.

For more recent decades, with the exception of December 2010, there has not been a CET with a mean temperature below 0C since January 1987. In the 1980's both February 1986 and January 1987 had a mean CET below 0C- and December 1981 was within three-tenths of a degree of being another month with a mean CET below 0C, as was January 1985. Severe night frosts in April and October were not unknown in the 1980's, but have been virtualy unknown in lowland England in the last eighteen years. All of which suggests that, at least during the winter half-year, high-latitude blocking has been decreasing as the North has got warmer.

Other factors help explain why the Circumpolar Vortex and storms should become stronger in winter as the Northern Hemisphere warms- in spite of the Arctic warming faster. One is that a warmer North Atlantic and warmer North Pacific can furnish more moisture (and more latent heat energy to power stronger storms) in a warmer world. The other is to do with the mean latitude of the Westerlies shifting polewards (following the retreating edge of Arctic ice-cover and attendant zones of strong baroclinicity), as the Westerlies shift polewards they have to blow harder to act as a sink for Westerly Atmospheric Angular Momentum put into the atmosphere in the tropics by surface tropical easterlies, that is because the Westerlies blow closer to the axis of the Earth's rotation.

A warmer World also means hotter air containing more moisture in the zone of hot rising air near the Equator, which would fuel stronger thunder-storm type convection there, this is something that readily becomes apparent during strong El Niños. More vigorously rising air near the Intertropical Convergence Zone supports stronger NE and SE Trade Winds, and stronger Westerlies aloft in an invigorated Hadley Circulation and greater transfer of Westerly AAM to higher latitudes.  Stronger convection near the Equator would lead to greater latent heat transfer aloft and a consequent latent-heat induced warming of the upper air over the tropics, consistent with the article referred to above. This is all consistent with greater Westerly AAM transport to higher latitudes, stronger Westerlies reaching the UK and deeper depressions passing to the north: Strong Westerlies at the surface and aloft tend to preclude high-latitude blocking patterns.

If, however, the Arctic basin became very much warmer in winter, perhaps as a result of further climatic warming leading to a complete thaw of the pack-ice and then warm southerly winds from the Atlantic associated with frequent depressions helping to prevent the re-freezing of the Arctic Ocean well into the winter blocking patterns could increase rapidly: The the low atmosphere over the Arctic could be over 20C warmer, which would be enough to greatly reduce baroclinic temperature gradients that fuel the North Atlantic (and North Pacific) depressions- leading to a weakening of the Westerlies. The sinks for Westerly AAM would be pushed into lower latitudes (associated with troughs in a weaker Circumpolar Vortex), as well as the Arctic interior becoming a sink for Westerly AAM associated with depressions. Were that to happen, intense blocking highs over northern Europe could become very frequent- with major implications for winter weather in the UK.    

Edited November 1, 2021 by iapennell

[Roger J Smith]

Let's say the theory is correct that warming in the arctic is causing various mid-latitude blocking and displacement events.

The problem I have with this is that the proposed solution to climate change would restore the climate to some cooler baseline and in all past climate periods for which we have observations, these cold events, blocking patterns and variability in general seemed at least equal to the recent past if not greater in frequency.

So what are we concerned about? The solution to climate change problems might fix other issues but it won't fix this one, any climate we reset towards will produce (by natural variability) all sorts of freakish anomalies. I have followed the weather for about six decades now and cannot see any evidence of increasing frequency of cold weather events in recent years. In fact the 1970s probably had a peak in blocking, it was quite regular for several years in a row (1975 to 1979 probably the peak for this) for large omega block patterns to set up and bring very anomalous weather conditions of both warm/dry and cold/wet types. 

As an example, in Ontario and surrounding parts of the U.S., April 1975 was freakishly cold, started out with a midwinter blizzard that left two feet of snow on the ground, followed by two weeks of clear, cold weather. That blocking ridge moved a bit further east and by May, 1975, it was record warm and stayed like that for much of the summer to follow. By Jan 1976 we were back into record cold temperatures. And by Easter (mid-April) 1976 it was record warm again, followed by a snowstorm that came after the leaves had all come out (three weeks earlier than average) on deciduous trees. 

Whatever has happened since then has never been much more "block crazy" than that. 

If we did somehow manage to reverse the AGW signal and if the climate slowly cooled (and it will be slow, this excess of greenhouse gas is not going to disappear the moment we stop adding to it), there would continue to be high variability and displaced polar vortex situations. Anyone who imagines that all this is being caused by the AGW signal would have to explain why it used to happen before there was an AGW signal. 

It was widely reported with the Texas freeze of Feb 2021 that it was the most intense cold and snow event in Texas since 1895. Okay, so what happened in 1895? (the records showed that it was somewhat more widespread then so it was clearly the extension of a large scale pattern in the upper atmosphere, Feb 1895 was much below normal in Toronto and NYC as well, whereas this past Feb the anomalies there stayed positive). 

I think the rising sea levels are enough of an incentive to deal with AGW if we can, without introducing these other aspects which seem like arguments meant to appeal to people who don't live near sea level and aren't as concerned. And personally I don't think we can change the climate by very much, nor is AGW the only reason for warming, after all we had about half a dozen strong El Nino events from 1980 to 2012 and then the massive warm bulge of 2015-16 that seemed to be correlated with the Pacific hurricane Patricia's track into central North America (dragging a lot of subtropical air north, and it stayed around for 2-3 months). My guess is that whatever political and economic changes are made now to 2030, the atmosphere will continue a slow warming trend until the Sun gets more active again, then we will see another accelerated warming like 1990-2006. Whatever we do, there will be a significant increase in temperatures. 

But having said that, at some point if the global climate warms by a large enough amount, this idea of displaced polar vortex and mid-latitude freak cold spells will be washed away in the shift of all frequencies of events to a higher bell curve and we would see fewer rather than more of these events. It won't be the case that global mean temperature would rise and variability would increase. Very warm climates probably will have smaller variability. Shorter term warming events in the past have come with more persistent or consistent warmth. 

[Bradley in Kent]

On 24/10/2021 at 13:53, knocker said:

A further note

For the jet stream as well as the storm tracks, the crucial ingredient in the climate change problem seems to  be this underlying need for poleward energy transport. This, after all, is the single most fundamental reason why the jet and the storms are there to start with: to balance the energy budget, the atmosphere needs to move energy from the tropics towards the poles.

We have just said that in the tropics, up at peak jet level, the warming will be stronger than elsewhere. Surely this means that the all-important contrast between tropical and polar temperatures will increase: does this not imply stronger jets and storms in the future, after all?

We now arrive at one of the central causes of uncertainty over how the jet will change. In the upper troposphere, the warming is indeed strongest in the tropics, as we have just heard. Down at the surface, however, the situation is precisely the opposite.

Another clear and very robust feature or the climate model predictions is that surface warming will be strongly amplified in the Arctic, not the tropics. This arises because of local feedbacks, for example the loss of sea ice which uncovers the relatively warm ocean beneath. Winter sea ice coverage is expected to decline by around 30%  by the end of the century, and in summer this number is closer to 90%. Some completely ice free summers are expected by this point in the strong warming scenarios, a very dramatic change indeed since the days of William Scoresby.  The result, then, is a veritable battle for the future of the jet stream (see Fig. 15.3). While the upper level changes driven by the tropics act to strengthen the temperature contrast, the lower changes driven by the Arctic act instead to weaken it. The future changes to the jet stream predicted by climate models are actually quite small, but this is at least partly due to a stalemate in the tug-of-war for the jet between tropics and pole. If our predictions of either of these two regions prove wrong, the changes in the jet may be much stronger than expected.  It is not, however, a complete stalemate. One of the two effects does seem to be stronger than the other. And the winner, to the best of our current knowledge, is the tropics. Many of the predicted changes in the atmospheric circulation can be traced back to the amplified warming high up in the air over the equator

Source - 'Jet Stream' by Tim Woollings

It does seem we have an increase in Westerly winds from tropical warming as our weather is more driven by the upper atmosphere than lower atmosphere. When looking at archived charts from years ago, you look at how blocked those scenarios are for weeks at a time and think 'that would hardly ever happen now'. As the world warms, things become energised and more mobile where even if a high latitude block does form (like now at the time of typing this) the extra energy creates more shortwaves which break any block down soon after. We can still get cold, but it doesn't last anywhere near as long as it used to, simply because the Atlantic comes back much sooner. 

Perhaps other parts of the world may experience slower moving systems, but for the UK our winters are becoming a zonal fest!

Edited November 16, 2021 by Bradley in Kent

[Paul]

@BornFromTheVoid posted an interesting blog on a very similar subject last year:

Variable British Weather: Is Something Up?

WWW.NETWEATHER.TV

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In recent years there's been a lot of discussion about weather patterns getting stuck more frequently. Is the weather actually changing, or is it media hype? And, if it really is changing, do we have any clues...

 

[Downburst]

Change in Atlantic jet stream making for more powerful storms, finds research 

https://www.irishtimes.com/news/environment/change-in-atlantic-jet-stream-making-for-more-powerful-storms-finds-research-1.4809003

The average winter northern hemisphere jet stream position over the North Atlantic has moved northwards by up to 330 kilometres, and the mean winter jet speed has increased by 8 per cent to 212km/h over a 141-year period, confirmed Dr Samantha Hallam of the ICARUS Climate Research Centre at Maynooth University and the National Oceanography Centre (NOC) in the UK.