Why ... is the jet stream so far south?

[Trevor Harley]

I realise there is no simple answer to the question of why is the jet stream so far south at the moment, and hence why is our weather so variable at the moment, but perhaps someone can cast some light on it.

Much as I enjoy the 9.55 BBC News Channel forecast (apart from their inclination to call the jet stream "that fast moving ribbon of air high in the atmosphere" every night), I wish they would show the wider world picture more often. I would like some sense of how the European and Atlantic weather is being affected by events elsewhere - and what is affecting them. Perhaps this information is already out there somewhere, but I don't know it? Is there a BBC World forecast somewhen, for example?

Trevor

[Smartie]

SSTs are usually the first  suspect. NOAA 1/4 deg SST mean and anomaly visualised with IDV.

NB the North Pacific gradient but the key thing is likely the western Atlantic anomaly helping to drive  the strong Atlantic jet. Away from the surface there is ridging in western North America and central Europe with temperatures above normal there. Greenland area blocking seems to have played a role in locating the trough in western Europe the past few weeks.

[Smartie]

Probably worth posting the current 300 hPa jet pattern as well

[Scott W]

On Wednesday, August 2, 2017 at 8:08:21 AM UTC+1, Trevor Harley wrote:

I realise there is no simple answer to the question of why is the jet stream so far south at the moment, and hence why is our weather so variable at the moment, but perhaps someone can cast some light on it.

Much as I enjoy the 9.55 BBC News Channel forecast (apart from their inclination to call the jet stream "that fast moving ribbon of air high in the atmosphere" every night), I wish they would show the wider world picture more often. I would like some sense of how the European and Atlantic weather is being affected by events elsewhere - and what is affecting them. Perhaps this information is already out there somewhere, but I don't know it? Is there a BBC World forecast somewhen, for example?

Something to do with current situation of Arctic ice? Not all the cause but must have some bearing. What about the very low occurance of sun spots? If it is this far south in the summer I might start getting excited for the winter...

[Martin Rowley [West Moors/East Dorset]]

... this article, based on research at the University of Sheffield published last year, confirms that as yet we don't really have a decent idea of the precise causes of these pattern shifts:-

http://theconversation.com/britains-recent-wet-summers-can-be-blamed-on-the-atlantic-jet-stream-says-new-study-64337

... from which I've extracted this section:

" 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."

It's going to take much more work to sort all this out IMV. And the old friend 'cause and effect' (or 'chicken and egg' if you like) will figure heavily. Sunspots have been mentioned as an 'external' factor and we are certainly experiencing low sunspot numbers, both at any particular point in time and also when integrated over the entire cycle (compared with the previous three cycles). Whether that's a factor .... Only Mr Corbyn knows (perhaps: the other Corbyn of course!)

Martin.

[Trevor Harley]

Thanks for the replies so far.

Of course "causality" is a complex notion anyway, particularly so in the case of a complex system like the weather (not to mention the brain/mind). We can always push back from one cause and demand another. So why is the sea particularly warm in the NW Atlantic (if I understand the SST maps correctly)?

Doubtless as well we sometimes observe apparent circularity in our putative casual chain - something like our jet stream being further south affects the weather over Asia affects the Pacific circulation affects the North American pressure system affects the Atlantic sea temperature affects the jet stream (say; I've just made that up).

Solar variability five years ago determining our weather now? Wow. No wonder long-range weather forecasting is a mug's game.

(About the interconnectedness of things: I think I remember that 20 years ago we used to call anyone who said that we get the weather of the USA five days later "idiots". They were perhaps less idiotic than we (I) thought.

[Alastair B. McDonald]

I find the idea that the atmosphere is like a water bed rather appealing. If the atmospheric pressure in the Pacific rises then it must fall somewhere else. There is a fixed mass for the atmosphere, and pressure depends on the mass of air over the surface. So, if there is more air over the Pacific then there must be less air above somewhere else.

This means that if the Jet Stream is pushed north in the Pacific it will move south in the Atlantic or Indian Oceans. The starting point for the circum global Jet Stream is always its end point, perhaps!

[Stephen Davenport]

On Wednesday, August 2, 2017 at 1:50:19 PM UTC-4, Alastair B. McDonald wrote:

I find the idea that the atmosphere is like a water bed rather appealing. If the atmospheric pressure in the Pacific rises then it must fall somewhere else. There is a fixed mass for the atmosphere, and pressure depends on the mass of air over the surface. So, if there is more air over the Pacific then there must be less air above somewhere else.

This means that if the Jet Stream is pushed north in the Pacific it will move south in the Atlantic or Indian Oceans. The starting point for the circum global Jet Stream is always its end point, perhaps!

========

I like that analogy. It's similar to one that I have always used: if you have one of those long half-deflated sausage-shaped balloons, squeezing and removing air from one end means that it bulges at the other.

As you point out and others suggest, if the jet stream is looping unusually far south over the east Atlantic / Europe then it must reach farther north elsewhere. So there is ridging in the W & NW Atlantic, troughing over the east interior of North America and a particularly strong ridge near the West Coast, which has been producing and will continue to produce very hot and dry conditions for the northwestern States and B.C. - not good for regions that have already been ravaged by wildfires.

Anomalous heights extend across Greenland and the NAO is slightly negative, hence the southerly jet and low pressure track near the UK. The question, as always, is what is dictating that pattern? You can end up down a rabbit hole chasing that answer but as somebody else noted Atlantic SSTs (among  other things) are likely influential. The AMO (Atlantic Meridional Oscillation) is strongly positive at the moment and probably helps to dictate the broad pattern we are seeing (analogs certainly suggest so), with intra-seasonal variations driven by other teleconnections. 

Stephen 

Indianapolis IN

[Scott W]

It has been mentioned previously that cold winters (and hence southerly tracking jets) occur when there is a marked cold pool south of the Grand Banks. The cold pool was very negative at the beginning of June but began to disappear rapidly mid July.

 

[Len W]

We are never going to sort it.
The best we can do is formulate the equations of motion.
Unfortunately these are second order , non linear , partial differential equations which have no exact solutions.

Sorry about this negativity.
Just back from the pub.

Len
Wembury

[Trevor Harley]

Is the Atlantic Meridional Oscillation the same as the Atlantic Multi-decadal Oscillation?

Ah, but Len, it has been fun speculating. In fact I'm a bit clearer now about the interconnectedness of everything, and what are the big factors having an effect on our weather. It would be easier though if the earth weren't a damned sphere (or spheroid).

Talking of second order nonlinear partial differential equations, to soak up some of the spare time in my "retirement", when I'm not writing about psychology and futurology, and doing very minor TV and radio work, I'm doing an OU maths/physics degree. Some of it is familiar from Cambridge natural sciences in the mid 70s before I switched to psychology. I struggled with partial differentiation, and we didn't get as far as PDEs. In fact I found div, grad, and curl remarkably difficult - I blame the teaching of course. A lot has gone wrong in universities over the last forty years, but the quality of the teaching has improved immensely.

I'm hoping it will all help with my understanding of the weather and climate.

[Jack Frost]

I did my undergraduate physics degree with the OU, and everyone I have spoken to since has said how good the course books are. I completed it 7 years ago, so courses may have changed since, but I really enjoyed them!

Liam

[Stephen Davenport]

On Thursday, August 3, 2017 at 5:51:05 AM UTC-4, Trevor Harley wrote:

Is the Atlantic Meridional Oscillation the same as the Atlantic Multi-decadal Oscillation?

========

Sorry, Trevor, I mis-typed. I of course meant the Atlantic Multi-Decadal Oscillation. Not the same as the Atlantic Meridional Overturning Circulation at all. A brain burp on my part, evidently.

Stephen. 

[Trevor Harley]

On Thursday, August 3, 2017 at 4:22:11 PM UTC+1, Jack Frost wrote:

I did my undergraduate physics degree with the OU, and everyone I have spoken to since has said how good the course books are. I completed it 7 years ago, so courses may have changed since, but I really enjoyed them!

Liam

 

I have the S207 Physical World and MST209 Mathematical methods and models course books, but they have just changed and updated the course, so this year it's different. I've already done M208 (Pure mathematics - I liked group theory and linear algebra but found analysis hard going).

I assume the most important third level course for meteorology is Mathematical methods and fluid mechanics (MST326), although there is also a course called Deterministic and stochastic dynamics (MS327). However there are also interesting looking courses in Relativity and Astrophysics, which should take me through to my 100th birthday.

[Jack Frost]

Ah yes. I think it was S206 when I did it. I thought it was too waffly, and didn't really enjoy it that much. I'm sure it's been re-written for the updated version. for maths I did MST209 and then MS324 (waves, diffusion and variational principles). I think that last one might have been discontinued now. It was certainly hard! Interesting though! I also did the electromagnetism, quantum mechanics and astrophysics 3rd level courses, and they too were really interesting.

Since completing my OU degree I went on to do a PhD (at the OU) and now I'm postdoc-ing in planetary science. I've read numerous books about atmospheric dynamics, and to be honest most don't need anything more difficult than what is covered in MST209. If you can do the maths in MST209 I recommend 'An introduction to dynamic meteorology' by James Holton. It's a great book!

Liam

[Trevor Harley]

On Friday, August 4, 2017 at 5:58:00 PM UTC+1, Jack Frost wrote:

 I recommend 'An introduction to dynamic meteorology' by James Holton. It's a great book!

Thanks, I've looked that up. I love the word "vorticity".

 

[Len W]

Yes, vorticity is a 'shear' delight.

 🤣

Len
Wembury

[Freddie]

I love the spin James Holton puts on the subject :-)

[Alastair B. McDonald]

Trevor,

I came across this video today: https://www.youtube.com/watch?v=_nzwJg4Ebzo Does it help?

Cheers, Alastair.

[Alastair B. McDonald]

I think that video is from 2010, but the one following it is from a conferenc in Jan 2016 so it should be more up to date.

[Alastair B. McDonald]

Trevor,

I have now watched that second video by Professor Jennifer Francis https://youtu.be/wE53Or56eNM At about 1 hour 10 minutes in, she answers your question! But I found the whole video entertaining as she links weather to climate change through the Jet Stream.

Cheers, Alastair