Jet stream, America's storm maker, moving slowly northward??

[Tim Johnson]

Being no scientist myself, I'd be interested in hearing from someone who is knowledgeable enough to say whether or not this article holds much truth.

http://timesunion.com/AspStories/story.asp?storyID=681664&BCCode=BNNATION&newsdate=4/17/2008


Also spoken of here... (Carnegie Institution of Washington's Department of Global Ecology

http://globalecology.stanford.edu/DGE/CIWDGE/CIWDGE.HTML


So...is the Jet Stream Nudging Northward?
And if so, why?


(I searched ST for this and didn't turn anything up...my apologies if this has already been covered somewhere on ST).

 

[Donald Giuliano]

Being no scientist myself, I'd be interested in hearing from someone who is knowledgeable enough to say whether or not this article holds much truth.

http://timesunion.com/AspStories/story.asp?storyID=681664&BCCode=BNNATION&newsdate=4/17/2008


Also spoken of here... (Carnegie Institution of Washington's Department of Global Ecology

http://globalecology.stanford.edu/DGE/CIWDGE/CIWDGE.HTML


So...is the Jet Stream Nudging Northward?
And if so, why?


(I searched ST for this and didn't turn anything up...my apologies if this has already been covered somewhere on ST).

I don't know anything about this research, but IIRC years in the late 1970s were characterized by some very cold winters in the U.S. with relatively low temperatures worldwide. I don't think there's any doubt global warming is occurring and see no reason why their statement about the jet stream moving north would be false, but I always take global warming statements that begin with "From 1977/8/9 to..." with a grain of salt as there's the possibility of cherry-picking dates to make the most impressive sound-bite. Beyond that, I can't say much, as climatology is not my field of expertise. Perhaps someone with a bit more knowledge of climatology could weigh in here...

 

[Terry Tyler]

So...is the Jet Stream Nudging Northward?
And if so, why?

man, i aint no scientist either...but i will tell you this...

i was taught that the jet-stream AUTOMATICALLY moves northward during the summer months, and then it moves back south again in the winter...

as for WHY...like i said, i aint no scientist...but, i was told that its more of a seasonal thing, and it has to do with i guess the way the earth spins, and the corolios effect that determines where the jet-streams are, and where they move...there are 6 different jet streams in the world, and they all move different places in different seasons...im sure some of these experts can elaborate, there are some incredibly gifted minds on this forum...

personally, i wouldent even trip on that article...nothing in there but science fiction and men crazier then me posting their ideas...

the jet stream moves north in the summer and south in the winter...sometimes it kinks, or might come down hard, but generally it migrates to the north in the summer, and then south in the winter...

 

[rdale]

It's referring to a trend with time, not the seasonal variation.

In any case, a 1.25 mile average annual change just seems to be like 1) of little consequence and 2) something WAY too small to be measurable given our current set of observation points.

And it "may" be connected to GW.

Or may not.

 

[Kenny Drake]

The jet stream does not "automatically" move northward. It's a function of the thermal wind at a global scale. The thermal wind is really hard to explain physically as it is not a force but it is not a wind. The relation basically says there will be a wind parallel to an interface between two air masses of differing temperatures; obeying the right hand rule (its a cross product.) In the northern hemisphere (edit: just realized this is the same in s. hemi. too) there exists cold air poleward, and relatively warm air towards the equator due to uneven heating. It is where these 2 air masses meet that a strong thermal wind exists, pointed to the east (in the northern hemisphere).

So in summary, the jet stream location is a function of the cold polar air and warm tropical air. It is far more complicated as I'm neglecting a huge factor - the global circulation patterns. These too are a function of the uneven surface heating. In the end it is fair to say the jet stream is thermodynamically driven.

Observations have shown that the arctic is warming at a higher rate than anywhere else in the world (including the tropics.) This in effect will shift the jet stream northward. Also, since the arctic is warming faster than the tropics the thermal gradient is being decreased. Since the thermal wind is proportional to the magnitude of the thermal gradient, the thermal wind is decreasing. This in effect decreases the strength of the jet stream in the NH.

I think the change in the spatial distribution of summertime severe wx across the US over the last few decades also reflects this northward shift in the jet stream rather well.

This is why climate models are telling us northern latitudes will become more wet while areas within 30 degrees of the poles will become drier. This article actually makes me happy because its more proof that climate models have been verifying. I'd be happy chasing in the northern Canadian provinces if there were roads.

 

[Eric Sipes]

I think Kenny pretty much has this one down but, according to several studies, the climate's latest warming trend has basically peaked. As a result, the northward trend of the jet stream will most likely come to an end in the next few years.

This is assuming that man is not the cause of global warming. Climate change is a natural process and, I have not seen any credible evidence of it being a product of man. (Just my opinion though.)

-Eric

 

[Aaron Kennedy]

The thermal wind is really hard to explain physically as it is not a force but it is not a wind.

Not really... thermal wind is simply the difference between the geostrophic wind speed vectors at two different heights. In layman terms, It says that if a temperature gradient exists, there must exist wind shear (jet streak aloft). The jet streak itself can be explained due to the imbalance of heating Kenny pointed out and the fact that at a given pressure level, there is a mass imbalance (gradient of geopotential height).

Ironically, I was just writing a paper on the impacts of global warming on the general circulation for a class. If the poles are truly warming faster than the equator, then our mean temperature gradient will decrease. If that decreases, then I'd also expect our polar jet to weaken *SLIGHTLY*. Keep in mind we're talking changes of a a degree or two. Basically, radiative forcing due to greenhouse gases and their associated feedbacks at the poles (melting ice -> lower albedo -> more radiation absorbed) helps dampen the differential heating. If there is a smaller surplus at EQ and defecit at the poles, then there is a less need for baroclinic eddies to transfer heat/moisture poleward.

Of course as one might expect, there are more caveats than words in this post. HUGE sticking points include the impacts/influences of the oceanic circulation, aerosol and cloud feedbacks, etc. One possibility is if the mid-lat/n lats are warmer, then there would be more evaporation, and a potential for more latent heat release. This is a crucial component for bomb cyclogenesis along the NE seaboard.

according to several studies, the climate's latest warming trend has basically peaked.

Mind citing them? How do we know what the future holds? I'm not trying to start the great global warming debate again, but using a year or two of data to justify global warming OR cooling is suspect.

 

[Mikey Gribble]

Kenny said....
"I think the change in the spatial distribution of summertime severe wx across the US over the last few decades also reflects this northward shift in the jet stream rather well."

This doesn't hold any water IMO. We are talking about long term trends with GW. IMO you can't use short-term seasonal variations for proof of GW. I don't know what the distribution of severe wx events over the last two decades would look like overlayed on a map, but assuming the southern plains has been below average, it's short term in the scheme of things. What maps show this drop off in severe weather? That is far from an adequate sample for determining long term trends. It is basically the same as some people blaming individual storms on global warming.
We were way above average on snowfall this year in Kansas. Does that mean the jet stream is migrating farther South? No. Not IMO.

 

[Kenny Drake]

Kenny said....
"I think the change in the spatial distribution of summertime severe wx across the US over the last few decades also reflects this northward shift in the jet stream rather well."

This doesn't hold any water IMO. We are talking about long term trends with GW. IMO you can't use short-term seasonal variations for proof of GW. I don't know what the distribution of severe wx events over the last two decades would look like overlayed on a map, but assuming the southern plains has been below average, it's short term in the scheme of things. What maps show this drop off in severe weather? That is far from an adequate sample for determining long term trends. It is basically the same as some people blaming individual storms on global warming.
We were way above average on snowfall this year in Kansas. Does that mean the jet stream is migrating farther South? No. Not IMO.

The jet stream location and the location of the greatest severe weather are usually about the same. The study posted at the opening of the thread has looked at the change in the jet stream over the last ~30 years. Severe weather spatial distribution over the US, over the last 30 years, reflects this movement in the jet stream. I'm talking about synoptically-induced severe weather, btw (not those associated with hurricanes, etc.) Finally, I'm not looking at long-term trends. I simply stated that the severe weather distribution over the last 30 years reflects the jet stream location over the last 30 years really well. Of course you have to look at long-term trends when it comes to climate change. However, most scientists agree that a 30 year time-frame is about the minimum time-scale for determining a region's climate (and hence its change.) A good rule of thumb is this: Don't make predictions about a change encompassing a time-scale larger than the time-scale of data you have. By definition climate is a 30 year average. We have sufficiently good data from at least the last 30 years to project 30 years into the future. Of course, this is at a global scale which is where the relationship between space and time comes in with regard to forecasting. No one in their right mind at this time could say precisely what the weather forecast for day __ in 25 years will be, because it requires a much higher spatial resolution (smaller space). In Meteorology, predictions get a lot easier when you increase the spatial scale for which you are predicting. At larger spatial scales and time scales the equations can be simplified quite a bit because the magnitudes of various forcing terms are generally time and space dependent. IE: t^2 will grow much faster than t (t=time). Plug in t=1 second and you end up with both terms being the same. Plug in t=100 and the "t" term can be safely neglected (100^2 vs 100, t term has a 1% effect), whereas t=1 yields both terms contributing equally to the net effect. At small time scales the opposite can happen. The coriolis effect is a great example. Anyways, the point is climate predictions from a fundamental standpoint can be easier. However, try to include human nature and you are left nowhere. IE: How does a model predict if/when humans decide to cut greenhouse gas emissions?

 

[Dann Cianca]

We have sufficiently good data from at least the last 30 years to project 30 years into the future.

... I'd like to see some verification of that.

 

[Aaron Kennedy]

From 1979 to 2001, the Northern Hemisphere's jet stream moved northward on average at a rate of about 1.25 miles a year

Here's a conference form of the paper:
http://ams.confex.com/ams/pdfpapers/134671.pdf

A principle dataset for their analysis is the NCEP reanalysis dataset, which I have worked with quite a bit recently. The resolution for this dataset is 2.5 degrees in lat/lon.
From the paper:

and to 0.17-0.19 degrees/decade in the NH

So for their analysis, 1979-2001: that's ~.42 degrees total change or 1/5 the resolution of the dataset they are using. Hmm.

Another nugget of love:

This equation suggests that the thermal structure of the
troposphere can, to a first approximation, affect trends in
jet stream wind speed...


Figure 5 shows slopes from the regression analysis
of zonal wind shear from Eq. (4). In the NH jet, trends of
thermal wind are correlated with trends in jet stream
wind speed (r=0.49 in NCEP and 0.38 in ERA-40),
which suggests that the meridional temperature
gradients have been affecting the jets. In the southern
hemisphere, conversely, trends of jet stream wind speed
appear to be non-correlated (or even anti-correlated)
with trends of thermal wind, which suggests that the
thermal wind equation is too simplified an approach for
the southern hemisphere jet streams.

Uh what??? This is what happens when global ecologists write papers with meteorology

 

[Bill Tabor]

Just skimmed this, so maybe missed this angle... 1.25 miles / year x 22 years = 27.5 miles. This is hardly anything to us chasers. I might be a bit worried if it was 125 miles / year.

Many of us chasers have noted how the jet stream is so far north lately and are concerned about it killing our southern storms and chase areas. I always thought in recent years it's been more like 500 to 1000 miles north of normal. If this is the only change then I am hardly concerned. I suspect last 10 years might have been more per year, but doubt it is significant, and as many of you pointed out for purposes of climate change 22 years is not a valid usable data set to determine any longer range trends. Try more like 220 or 2200 years - maybe more.