On Wednesday, August 12, 2015 at 5:46:06 PM UTC-4, Graham P Davis wrote:
On Wed, 12 Aug 2015 13:10:22 -0700 (PDT)
Stephen Davenport wrote:

On Tuesday, August 11, 2015 at 5:35:02 AM UTC-4, xmetman wrote:
After listening to Carole Kirkwood talking about what a wet and
cold summer its been over Scotland this year on this mornings
Breakfast Weather, it does seem to me that as far as the media are
concerned most of today's weather woes in this country today,
including this one can be blamed on the jet stream, or lack of jet
stream, or the position of the jet stream. So if the answer to most
of the world's climate problems is the ubiquitous "jet stream",
what about its part in global warming? I haven't seen any research
that looks into the role jet streams play in global warming, but
I'm sure it won't be long before there is a slew of scientific
papers about it.

If this is true, just what controls the position and strength of
the jet stream, and the answer to that must be the general
atmospheric circulation, but of course that's not half as catchy,
dynamic or sexy as the phrase "jet stream". I just can't imagine
John Hammond for instance spending an hour in a BBC Horizon program
to explain the intricacies of the Hadley cell and the northeast
trade winds, when he can talk over a 3D animation and fly through
of a North Atlantic super jet blowing at 250 knots.

I'm sure that the phrase "jet stream" will be cropping in many
conversations about the weather in the coming years, you can easily
picture one old dear saying to another in a bus queue "I know Enid
the weather has been simply terrible, and you know it's all down to
the orientation of the Jet stream over Iceland you know".

=============

It's not so much what affect a fundamentally changed jet stream would
have on global warming, although feedback mechanisms are no doubt set
in train. The effect of a weaker and meridionally exaggerated jet
stream would be in the distribution of heat and cold rather than net
warming or cooling. Contrarily, it's the warming that might impact
the jet stream. Arctic warming being more rapid than elsewhere, the
argument goes that the thermal gradient north to south would lessen
and thus reduce atmospheric zonality.

As others have pointed out, it is posited that this could result in
greater-amplitude Rossby waves and a greater frequency of blocked
patterns - thus also of extended periods of weather types, or more
severe local or regional conditions. Jennifer Francis at Rutgers has
done a lot of research in this area and there was a 2013 workshop
that explored this: The Committee on Linkages Between Arctic Sea Ice
Loss and Mid-Latitude Weather.

I'm not yet thoroughly convinced, and there are many inter-annual and
inter- and intra-seasonal forcings to consider - no doubt including
some "unknown unknowns".


As I've already pointed out, this theory dates back a long way, perhaps
even to before WWII. It was also found not to work. Not should
it. For one thing, you get larger amplitude waves and more blocking with
stronger jet-stream winds, not weaker ones.

--
Graham P Davis, Bracknell, Berks. [Retd meteorologist/programmer]
http://www.scarlet-jade.com/
I wear the cheese. It does not wear me.
Posted with Claws: http://www.claws-mail.org/

========

Yes, it had been theorized a long time back, and as I said I'm yet to be convinced. But perhaps we are looking at a different climate paradigm now. The argument seems to now be thus, and I quote from the paper to which I am linking:

"Two separate effects on upper-level characteristics are anticipated: weaker poleward thickness gradients cause slower zonal winds, and enhanced high-latitude warming causes 500 hPa heights to rise more than in mid-latitudes, which elongates the peaks of ridges northward and increases wave amplitude.. Both of these effects should slow eastward wave progression."

I don't pretend to have any answer or opinion on the causation/correlation argument nor whether this is largely down to differential rates of long-term warming between the Arctic and the mid-latitudes / tropics (Arctic Amplification).

Anotehr extract:

"The strength of the poleward thickness gradient determines the speed of upper-level zonal winds. As the gradient has decreased with a warming Arctic, the upper-level zonal winds during fall have also weakened since 1979 (Figure 3, right), with a total reduction of about 14% (95% confidence). Winter winds are more variable but exhibit a steady decline since the early 1990s. When zonal wind speed decreases, the large-scale Rossby waves progress more slowly from west to east, and weaker flow is also associated with higher wave amplitudes [Palmén and Newton, 1969]. Slower progression of upper-level waves causes more persistent weather conditions that can increase the likelihood of certain types of extreme weather, such as drought, prolonged precipitation, cold spells, and heat waves. Previous studies support this idea: weaker zonal-mean, upper-level wind is associated with increased atmospheric blocking events in the northern hemisphere [Barriopedro and Garcia-Herrera, 2006] as well as with cold-air outbreaks in the western U.S. and Europe [Thompson and Wallace, 2001; Vavrus et al., 2006].

The second effect - ridge elongation - is also expected in response to larger increases in 500-hPa heights at high latitudes than at mid-latitudes. This effectively stretches the peaks of ridges northward, as illustrated schematically inFigure 2b, and further augments the wave amplitude. Higher amplitude waves also tend to progress more slowly."

From: "Evidence linking Arctic amplification to extreme weather in mid-latitudes", Jennifer A. Francis, Stephen J. Vavrus, 17 March 2012 DOI: 10.1029/2012GL051000

http://onlinelibrary.wiley.com/doi/1...2GL051000/full

Stephen.