Arguments
Two Expert — and Diverging — Views on Arctic’s Impact on Weather ‘Whiplash’
Posted on 5 July 2013 by greenman3610
This is a re-post from the Yale Forum on Climate Change and the Media
In discussing their diverging views on impacts of rapid shrinking of Arctic sea ice, two leading scientists provide witness to the kinds of evidence-based exchanges of views not uncommon among top researchers in the climate field.
Over the past year, Jennifer Francis, Ph.D., of Rutgers University has produced compelling evidence of links between the rapid reduction in Arctic sea ice and extreme weather events in the Northern Hemisphere.
Her hypothesis — that the reduced temperature gradient between Arctic and temperate zones causes the jet stream to slow — was examined in early June’s Yale Forum “This is Not Cool” video.
The science is compelling, but — as with all scientific evidence — not yet what could be called “settled.”
For this month’s “This is Not Cool” video, independent videographer and regular Yale Forum contributor Peter Sinclair interviewed both Francis and Kevin Trenberth, Ph.D., a highly regarded senior researcher from the National Center on Atmospheric Research, NCAR, in Boulder, Co. Trenberth expresses his reservations about the jetstream/Arctic connection, and he says questions remain about whether the Arctic Ocean is a sufficiently large heat reservoir to affect hemispheric circulation patterns in the jet stream. He asks if the answer to the riddle may lie in much larger energy exchanges in the tropical oceans.
Francis and Trenberth, in their comments in this month’s video, provide insight not only into the specific issue at hand, but also into the kinds of ongoing dialogue among top scientists trying to provide evidence-based answers to some of the most complex issues posed by our changing climate.
I don't think Kevin's & Jennifer's views are diverging by much. Kevin does not contradict the theory of meandering jetstream's signifficance on NH weather, he's just a bit skeptic (in a good sense) and is looking for more evidence. Note how he sais "there might be influence somewhere in Europe, for example, but not over entire NH". To which I may reply: well, the same jetstream flies over Canada US and in Europe, why do you think America is "immune" to it?
Personally, I give more edge to Jennifer's view because she's more knowledgeable expert in this particular topic. And some evidence confirming meandering jetstream's theory at work in America (e.g. tropical cyclone Sandy) do exist to back her.
Fascinating article, thanks for printing it.
And thanks chriskoz for voicing my thought:
"Note how (Trenbirth) says "there might be influence somewhere in Europe, for example, but not over entire NH". To which I may reply: well, the same jetstream flies over Canada US and in Europe, why do you think America is "immune" to it?
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And sorry for doing this, but it is only slightly off topic considering how science skeptics love pointing out the 1920/30s arctic warming (where they fail to mention the warming was rather localized, originating around Spitsbergen) trying to imply that there is nothing unusual about current events.
I've been reading: "The Arctic Warming 1919 to 1939" by: Arnd Bernaerts
http://www.arctic-heats-up.com/pdf/chapter_8.pdf
Which has introduced me to the notion that intense navel activity caused a great deal of ocean mixing, disrupting the thermohaline current, and leading to the freak warming event that started off Spitsbergen and spread outward from there:
" a long barrage between the Orkney Islands and Norway ... (USA and UK mines laid) 73.000 mines __about 5,000 exploded prematurely soon after laying __20,000 mines were disposed of while the work was in progress __from the remaining ca. 50,000 mines __more than 30,000 mines were already ‘gone’ in spring 1919, either drifted away, or exploded during winter storms; __rest 20,000 were swept in 1919." {ch.8 p.94}
In all something like 200,000 mines were laid in various North Atlantic locations.
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As for ships and U-boats:
"The situation became dramatic when U-boats destroyed more ships than Britain could build in early 1917. In April 1917, the same total rate of the previous annual rate of 1916, ca. 850,000 tons, was destroyed by U-boats. In April 1917, Britain together with the Allies lost 10 vessels every day. During the year of 1917, U-boats alone sank 6,200,000 tons, which means more than 3000 ships, and, during the war months of 1918, another 2,500,000 ship tonnage. The total loss of the Allies ship tonnage during WWI is of about 12,000,000 tons, namely 5,200 vessels. The total loss of the Allies together with the Axis naval vessels (battle ships, cruisers, destroyers, sub-marines, and other naval ships) amounted to 650, respectively 1,200,000 tons." {ch.8 p.88}
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It's one of those thoughts that at first blush is way out there, but then on reflection and considering the new found appreciation for the amount of deep ocean churning whales and other sea creatures produce as they dive to deep depths then return to surface (http://www.biogeosciences-discuss.net/9/8387/2012/bgd-9-8387-2012.html) - it doesn't seem so far fetched.
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I'm curious how has the scientific community reacted to Bernaerts' hypothesis?
Not an expert, but I could follow both arguments; I also found Dr. Francis' to be more compelling.
As I understand it: The jet stream(s) are not driving forces; they are a product of the large circulation patterns for polar, intertropical, and Hadley cellss; they exist where the circulation patterns meet. Physics predicts, and we have observed, that warming anamolies increase toward the poles. The waves in the jet stream are largely a result of the different speeds of the larger circulation patterns. When air (or any fluid) flows past other air, turbulence is created. For less deltas, the turbulence has larger, slower waves. I suppose an analogy could be that shorter frequency waves have higher energy than longer frequency waves; and in this case the energy in the wave is a product of the energy differences between the air masses in the different circulation patterns.
If someone has a better understanding, I'd be glad to hear it.
So, if this is by-and-large correct, then we would expect to see changes in the jet stream waves (amplitude, frequency, and location) with any lessoning of temperature gradient between equator and pole. The Arctic ice is particularly interesting if you think of it as an temperature buffer. There is a fair amount of energy delta between several million cubic meters of ice and the same mass of water, at the same temperature. So, I think that there would be a change in the jet stream pattern regardless of if the Arctic were land or sea, but I suspect that the difference is enhanced by the larger differences we are seeing being ice minimum and ice maximum.
Jeff Masters site has had several recent posts on this as well including this: http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2452 from just a couple days ago.
yup, also see:
Weather Extremes Tied to Jet Stream Changes
By: Seth Borenstein
Published: June 25, 2013
http://www.wunderground.com/news/heat-wave-alaska-jet-stream-may-be-blame-20130625
In 2005, I had an intuition about the link between the Arctic ice cap and the behavior of the jet stream. It became the predicate of my novel, A Change in the Weather. I continue to be stunned and amazed--and really, anguished--that my intuition appears to have been accurate.
The story is a thought experiment about the imminence and abruptness of this change, and how it might affect the economy and social organization of America as seen through the lens of a single family. The politics of it would certainly be off-topic here, but I think it's safe to say the first consequence would be the failure of agriculture, which will put tremendous strains on society not only in the US but across the globe.
Look at what happened in Michigan in March 2012. Nighttime temperatures set all-time record highs for the entire month because the jet sream had lifted its skirt so far north. The tropical air that wafted underneath coaxed the apple trees into an early blossom. When the jet stream dropped back to its habitual latitudes, the blossoms couldn't survive the cold. Michigan lost 90% of its apple crop.
I think when the ice cap finally inverts from white heat reflector to black heat absorber, even just for a few weeks in late summer, these erratic excursions will be greatly amplified and interfere with the rainfall patterns that agriculture has depended on for millennia. In my book, it happens in 2018. That's just five years from now. I've checked this with Dr. Francis, Dr. Masters, and Dr. Mark Serreze at the NSIDC, and Dr. Ken Dunton at UT-Austin. They think this timeframe is plausible.
I think a number of the above commentators are right on the money. If I understand the situation correctly, the strength of the jet streams is a function of the speed of rotation of the two Hadley cells that create the jet stream in question. The polar jet stream is located where the Polar Hadley cell and the Ferrel cell meet. The Ferrel cell is an 'idler gear' between the Equatorial Hadley cell and the Polar Hadley cell, but the Polar Hadley cell, is a powered cell, powered by polar air made dense by radiating heat into space and lacking heat from the sun to counter this cooling. With a complete cover of snow and ice, the arctic absorbs very litte heat. Looking at the NSIDC website for October 2012, about half way down we see a report of rising air due to heat given off from open water and winds from the South West. In other words, a reversal of the Polar Hadley cell. As we have more open water for longer periods, this reversal should become more persistant and stronger. The jet stream which is becoming weaker and wobbling should disappear and we will have, essentially, a two cell system in the Northern Hemisphere. Weather patterns which are creeping northward, should lurch northward. The effect should initially be seen in Autumn and should spread further back into summer over the ensuing years. We will have, in essence, an offshore wind as the land cools off in the fall but a lot of heat is available in the open Arctic ocean. This offshore wind, though, will persist day and night rather than being a phenomenon that occurs only each evening as in temperate areas. The effect on agriculture should be pretty much as described in "A change in the Weather" mentioned above. What I find worrying is that if you take a bit of lisence in interpreting the preliminary results from El'gygytgyn, it seems that we already are in a new climate regime but the effects are not yet being felt. In other words, the climate is not yet in equilibrium with the present CO2 levels. We have set the system in motion towards a new equilbrium but now all sorts of feed back mechanisms will work their way through until we are in the new climate. If so, and as we head with gay abandon towards 500ppm, it should be exciting times.
william@7,
You understand the things correctly (but your "Polar Hadley cell" is just called "Polar cell"), it's worth quoting this excellent article by John Mason here based laregely on Francis 2012.
However that's not the primary point of contention between Kevin & Jennifer above. The point is, that Kevin looks to me like climate scientist who averages everything (i.e. colder conditions in Arctic mean slower heat transfer therefore no large T swings expected) denying the variability of weather where he does not seem to specialise. To which Jennifer (both climate & weather specialist) replies that heat transfer is not by convection but by fast blowing wind which is simple and modest and dead-on accurate in rebuting his claim.
The mechanism linking Arctic amplification (AA: increased sensitivity of the Arctic to global warming or cooling) to changes in the jet stream that Steve Vavrus and I proposed and demonstrated in our March 2012 paper involves more than just sea-ice loss. While the ice loss is the largest contributor to AA in fall and early winter, rapid Arctic warming is also driven by the pronounced negative trend in snow cover on high-latitude land in spring and early summer, and even more strongly by the increase in water vapor at high latitudes, which affects AA in all months. That water vapor warms the Arctic in all seasons in 3 ways: it is a greenhouse gas, it releases latent heat into the atmosphere when it condenses into clouds, and those additional clouds also trap heat below them. Most of the modeling studies to date that examine the Arctic’s effects on the large-scale atmospheric circulation have focused only on the influence of sea-ice loss. I believe this is the reason that most of those studies fail to find a robust response, while the real world (which DOES include all the contributions to AA) is responding more strongly.
As commentor #8 points out, most of the modeling studies also average over time and over many ensemble members, which would smear a signal of a more amplified jet stream if those meanders did not occur in the same location. Take March 2012 versus March 2013, for example. In both cases the jet stream was highly amplified, but in 2012 there was a trough over the western U.S. and a large ridge over the east, which led to the breaking of thousands of high temperature records. A year later, the trough-ridge location was opposite but equally anomalous. Averaging over the two years would produce no signal.
It is also important to note that AA has emerged from the noise of natural variability only in the last decade or two (challenging tests of statistical significance), it is strongest in the fall and near the surface (because of sea-ice loss), and it is now becoming evident in all seasons and through a deep atmospheric layer (because of increased water vapor). This must inevitably have a more pronounced effect on the polar jet stream as we continue to warm the planet by burning fossil fuels.
This was a very interesting post and certainly displays that even two experts on the "consensus" side can disagree about the details of the disruptions caused by anthropogenic climate change. The contrast in perspectives between Dr. Trenberth and Dr. Francis is actually quite refreshing. I tend to personally favor Dr. Francis in this particular difference of opinion.
From a larger perspective, the high amplitude jet stream patterns we are see are yet one more global example of a positive feedback mechanism whereby the overall increase in energy in the Earth system is trying to balance between pole and equator. The meridonal advection of energy brought about by this pattern is enhanced such that we get more vigorous equator to pole mixing. We are seeing the exact same thing higher in the stratosphere and even the mesosphere with an enhanced Brewer-Dobson circulation.
william:
Further to chriskoz's comment about the naming of the cells (just the tropical one is the Hadley cell), the boundary between the Polar and Ferrell cells is a bit more complicated than the old three-cell visual.
Winds blow because of a difference in air pressure. We are used to looking at air pressure at the surface, but atmospheric circulation also depends on pressure at different heights in the atmosphere - and differences in pressure. The idea of the Polar/Ferrell/Hadley cell circulation fits the early knowledge of surface pressure and wind patterns, but the high-altitude winds are a bit different.
Even with the same surface pressure in two regions, different air temperatures would mean different pressures at altitude. Cold air is more dense than warm, so starting with the same surface pressure and moving upwards, the colder location will see pressure dropping more rapidly. (Pressure is just the weight of the overlying air.) Consequently, at high altitude there will be a lower pressure in the cold location than in the warm, so you have a high-altitude pressure difference, and thus can have high altitude winds even though the surface pressures are equal. The pressure relationship to temperature differences is call baroclinicity. Wikipedia has an entry on thermal winds, and one on baroclinicity.
There is a strong temperature contrast between the Polar and Ferrel cells. The resulting thermal conditions lead to the jet stream - high altitude baroclinic winds. Just as surface winds can be seen to follow the patterns of surface pressures, the jet stream can be seen to follow the patterns of the high-altitude pressures.
Just to confuse things, meteorologists do not draw maps of the pressure at a constant altitude for upper air analysis. Instead, they pick a constant pressure, and determine what altitude you find that pressure at. The map will have the same pattern of highs and lows, but it's a slightly diffeerent way of thinking of things. Here is the web page of Environment Canada showing some of the maps available for analysis, and here is the map for the 500mb (500 hPa in SI units).
Dr Francis talks about the effects of diminishing heat gradient between the Arctic and Northern tropics resulting in weakening of the jet stream with consequential transfer of heat northwards and cold southwards. It seems to me that her argument that this diminishing gradient is brought about by transfer of water vapour into the Arctic producing temperature amplification and loss of sea ice is both plausible and likely - at least in summer.
However, I find it difficult to believe that this could occur during the winter months when sunlight hours are low, as are Arctic temperatures. So, should we not expect jet stream weakening to occur only in the warmest Arctic months? Is there evidence that it occurs outside these months?
I have a somewhat contrary view than previous commenters about this subject. As near as I can tell the scientific consensus through the early 2000's was for a stronger jet (less meridional variation). An example paper is http://www.esrl.noaa.gov/psd/people/dezheng.sun/lectures/nao/thompsonetal2000.pdf I have seen no comprehensive study that contrasts any new theory to this old one.
The AO as mentioned in the paper is not a one-for-one proxy for the sub-polar jet. But generally the more positive the AO index, the stronger the jet. The trend was positive to around the early 1990's; see this thread for the graph: http://www.skepticalscience.com/jetstream-guide.html The main reason is that the cooling stratosphere, including the Arctic, and the warming troposphere which includes AA, should increase both the vertical and horizontal temperature gradients at the tropopause at the boundary of the Arctic. This would induce a stronger jet, on average.
The recent weakening of the jet, or more negative AO, is likely IMO to be natural.
I agree with Agnostic that various effects should be seasonal. While the recent drop in AO in winter may be natural IMO (following a cycle of rising from the 70's through the earily 90's), there ought to be an anthropogenic rise in summer basically due to higher pressure at the north pole. An annual plot would obscure this trend. It is also contrary to the argument I made above about the gradient. It is also noted by Chris G above, that the jet is driven by the patterns of high and low pressure rather than vice versa.
A related consideration is the strength of the Aleutian and Icelandic lows which would likely deepen in all seasons helping sustain a stronger jet and positive AO. But they could also be unbalanced (they take turns being strong and weak).
Is the Antarctic Jet Stream unchanged? Because of the differences between them (Southern Ocean not land mass, land at the pole etc) I would expect that the Antarctic Jet Stream should be unchanged. If it has also slowed and increased its meandering then I would reconsider the causes of the Arctic Jet Stream.
AAO shows less cyclical variation: than AO:
I have found that the monthly plots above obscure some week-long negative excursions that correlate well to extreme weather. But the plots can be used more generally as an index for the polar jets, red being faster and less meandering, blue being slower and meandering.
[RH] Hot linked URL's that were breaking page formatting.
Thanks for fixing the links above. I will practice doing them correctly with a paper describing the "old" theory, SH oriented: http://journals.ametsoc.org/doi/pdf/10.1175/3221.1 which states: The resulting zonal mean response demonstrates a strengthening of the meridional gradient in extratropical H500 and H200 and thus increased zonal mean geostrophic winds.
The "new" theory, applied to the NH is from this paper: http://marine.rutgers.edu/~francis/pres/Francis_Vavrus_2012GL051000_pub.pdf which states: 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).
Hopefully the links work this time and illustrate the contrast. The SH paper briefly describes the modeled NH changes and shows in figure 1 that the change from AGW is seasonal with an increase in the gradient in NH summer (dashed line showing greater warming at low latitudes than high latitudes) but a large decrease in the gradient in NH winter (solid line). That is consistent with the NH paper's statement about the NH fall.