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Winds of change

Filed under: — group @ 11 June 2009

Gavin Schmidt and Michael Mann

There was an interesting AP story this week about possible changes in wind speed over the continental US. The study (by Pryor et al (sub.)), put together a lot of observational data, reanalyses (from the weather forecasting models) and regional models, and concluded that there was some evidence for a decrease in wind speeds, particularly in the Eastern US. However, although this trend appeared in the observational data, it isn’t seen in all the reanalyses or regional models, leaving open a possibility that the trend is an artifact of some sort (instrumental changes, urbanization etc.). If the effect is real though, one would want to see whether it could be tied to anything else (such as forcing from greenhouse gas or aerosol increases), and indeed, whether it had any implications for wind-generated electricity, water evaporation etc.

Amusingly, both of us were quoted in the story as having ostensibly conflicting views. Mike was quoted as finding the evidence for a trend reasonably convincing, while Gavin was quoted as being unconvinced of the evidence for an anthropogenic climate change signal (note that the two statements are not in fact mutually inconsistent). As one should expect in any news story, these single lines don’t really do justice to the longlonger interviews both of us gave the reporter Seth Borenstein. So what is the bigger context?

First some background. It’s important to note that ‘windiness’ is not a globally uniform field, and that changes will occur in different regions for very different reasons. Also, note that mean wind speed is not the same as storminess*.

Winds in the mid-latitudes are a function of the jet stream and of the ‘baroclinic instability’ that we see as low-pressure systems. In the tropics, winds locally depend strongly on convective activity and on a larger scale, the Hadley circulation. In monsoonal regions (West Africa, India etc.), winds are a function of the temperature contrasts over land and sea during the warm seasons. Winds can be affected by the ozone hole in the Southern Ocean, a change in the orbit of the Earth in the tropics, or by the presence or absence of an ice sheet. So the concept of winds changing in a general sense is not unusual or unexpected. However, because of the many distinct influences you wouldn’t expect all winds to increase or decrease together.

In the free atmosphere off the equator, wind is essentially ‘geostrophic’ which means that it’s driven by the (predominantly north-south) gradients in air pressure, and follows contours of constant pressure (’isobars’). Near the surface, friction slows the winds, and causes them to cross the isobars from high to low pressure (hence we get ‘convergence’ in the center of surface low pressure regions). Nonetheless, changes in surface winds will follow approximately from the associated change in the surface pressure field.

The business-as-usual projections show a general poleward shift of the current subtropical surface high pressure belt into the mid-latitudes, especially during summer (a poleward shift of the descending branch of the so-called “Hadley Cell”). The high pressure belt is a region of low pressure gradient, and hence low wind. A northward shift displaces the region of maximum westerly surface winds poleward, from the U.S. into, say, southern Canada. A decrease in the mean strength of the surface westerlies over the U.S. would therefore appear to be consistent with projected changes in large-scale circulation. However, it’s not that simple. The average wind speed at these latitudes depends as much on the day-to-day variance (driven primarily by mid-latitude storms) as it does on the mean strength of the climatological westerly surface winds. The gradient in temperature between subtropics and pole tends to decrease with global warming (due to the ‘polar amplification’ of warming) and this, in turn, diminishes the “baroclinicity” of the atmosphere, and thus, the degree of storminess. So both a decrease in baroclinicity and a poleward shift in the extratropical band of westerly surface winds would therefore seem to work in the direction of decreasing wind in mid-latitudes.

But even this reasoning is somewhat questionable, as wind anomalies over a region as small as the U.S. are unlikely to be representative of the trend for the entire latitude band on the whole. Factors such as El Nino, and the “Northern Annular Mode” have an important role on wind patterns over the U.S., and changes in the behavior of these phenomena could easily overwhelm the average trend for the mid-latitude band. So in short, the observations of decreasing wind speeds over the U.S. are in a rough sense consistent with these ideas, but given the uncertainties in factors that are important in determining wind patterns over the scale of the U.S. continent, it’s hard to say precisely what would be expected.


Figure 1. The trends in the station winds and in the N. American reanalysis (from fig.4 in Pryor et al.)

In the specific case of the GISS-ER model, we can easily see what the model suggests. The picture below gives the annual mean wind speed change for a business-as-usual scenario out to 2100 (we picked this just because the changes are large, but a picture for simulated trends over the last 50 years is similar).

The first thing to note is that the expected changes are complex. There is a clear increase in the Southern Oceans (related to changing temperature trends in the lower stratosphere associated with both the ozone hole and greenhouse gas increases). There is also a change near the equator associated with increases in convective activity and a shift in the Hadley Cell. Note also that changes over land are very small, and in particular, over the US no significant changes are seen. The situation might be different in different models (or different seasons, or in the day-to-day variance), and so one wouldn’t want to read to much into this single figure, but it makes clear that a change in US windiness is not a strong ‘a priori’ expectation from global warming. This doesn’t of course shed any light on whether the observed trends are real, but it does speak to the attribution part of the discussion.

Indeed, you would need a careful detection/attribution analysis to see if the observed changes in wind speeds are consistent with the multi-model climate change projections. This has been done for surface temperature, precipitation, and sea level pressure changes, and there is no obvious reason it can’t be done for wind speeds if the data holds up.

Regardless of the cause of the indicated decline, is this likely to have a direct impact on wind power generation? There is a study by Archer and Jacobson that explores the potential for wind power over the US, and the results can be seen in this graph:

Wind speed class 3 (usable for power generation) and above (dark blue, green, yellow, red and black dots) are not that widespread, and are concentrated over the plains and offshore. Comparison to the trend map in the Pryor et al study (figure 1 above) shows only a limited overlap, so even if all these sites were being used, it’s not clear the trends would hamper wind-power generation much. However, this is highly speculative and will need to be looked at much more carefully in future.

Whether the wind of change is truly blowing through this continent remains to be seen…

Note that an apparent quote from David Deming that the possibility of decreased wind speed over the Eastern US is somehow in contradiction with the possibility of increased tropical storm intensity in the tropical Atlantic is embarrassing in the inappropriateness of the comparison.


180 Responses to “Winds of change”

  1. 151
    Mark says:

    “As far as I can figure, whenever observational data is studied, evidence indicates warming moderates severe weather.”

    Cite?

    As far as I can remember, evidence indicates worse weather.

  2. 152
    David B. Benson says:

    Brian Dodge (141) — Their certainly will be such changes.

  3. 153
    G. Karst says:

    Mark: Sources in addition to the above study which found decreasing wind speed.

    Easterling, et al (2000), who said, “the number of intense and landfalling Atlantic hurricanes has declined.” In the Gulf of Mexico there is “no sign of an increase in hurricane frequency or intensity,” according to Bove, et al (1998). For the North Atlantic as a whole, according to the United Nations Environment Programme of the World Meteorological Organization, “Reliable data … since the 1940s indicate that the peak strength of the strongest hurricanes has not changed, and the mean maximum intensity of all hurricanes has decreased.”

    Zhang, et al (2000) Using a variety of indices, including storm surge water levels, the authors found “no significant trend in storm activity during this century along the East Coast.” The real problem along the coastline, they say, is not changing climate but changing land use, as more and more development occurs along the shorelines, creating greater susceptibility to storm damage.

    Gulev, et al (2000) employed NCEP/NCAR reanalysis data since 1958 to study the occurrence of winter storms over the northern hemisphere. They found a statistically significant (at the 95% level) decline of 1.2 cyclones per year for the period, during which temperatures reportedly rose in much of the hemisphere.

    Kunkel et al. (1999) concluded, they saw “no apparent trend in climatic drought frequency” and “no evidence of changes in the frequency of intense heat or cold.” Climate change is not a major factor because “trends in most related weather and climate extremes do not show comparable increases with time.”

    Ryan Maue of Florida State University recently measured the frequency, intensity and duration of all hurricanes and cyclones to compile an Accumulated Cyclone Energy Index.
    His findings? The energy index is at its lowest level for more than 30 years.

    Atlantic hurricanes were much more numerous from 1950 to 1975 than from 1975 to present.

    Tornado(s) seem to decline
    http://www.climate-movie.com/wordpress/wp-content/uploads/2009/06/tornadotrend-500×393.jpg

    I would not say any of the above is definitive but indicative instead.

  4. 154
  5. 155
    Hank Roberts says:

    Brief excerpt from the above Chapman Conference link, for those who don’t click:
    ————–
    …. At the AGU Chapman conference today… The artifacts date from about 2190 B.C., when cities and towns of the Akkadian empire in Mesapotamia were being abandoned en masse as the region suffered crushing drought.

    “This site is the Pompeii of ancient Mesapotamia,” says Weiss. “They walked away.”

    Weiss reviewed evidence that a rapid change in storm tracks in the North Atlantic – yet to be satisfactorily explained – dried out the Tigris and Euphrates valley 4,200 years ago. And that valley wasn’t alone. Around the same time, deflection of the Indian Monsoon hit the Nile with a drought, and Egypt’s Old Kingdom went down. The extreme events are also mirrored in North America from New Jersey to the Yukon. In a separate talk today, glaciologist Lonnie Thompson showed new ice core data from Huascarán in Peru, the highest tropical mountain, with a huge spike in dust deposition around this time. The dust probably blew off an aridifying West Africa ….

  6. 156
    Mark says:

    “G. Karst Says:
    23 June 2009 at 12:53 AM

    Mark: Sources in addition to the above study which found decreasing wind speed.”

    And they say this is due to what physical process?

    Extraordinary claims require extraordinary proof.

    Winds are cause by a pressure difference.

    A pressure difference is powered by differential heating.

    The higher the base temperature, the more the same fractional difference in temperature is.

    Therefore from a basic first-guess you’d expect higher windspeed.

    Proofs against that are needed and you’ve just said “there are papers…”.

  7. 157
    Mark says:

    Looking through your sources, they all seem to be talking about hurricanes.

    This is not the only wind on the planet.

    If you want to expand what you mean when you say “windspeed” maybe we can avoid talking at cross-purposes.

    (PS we still need a causation)

  8. 158

    Mark,

    It’s established from the GCMs that equatorial and tropical temperatures will rise less than summer hemisphere polar regions. That alone implies a decline in the temperature gradient from equator to pole, which gives you your lower temperature differential and reduced winds. There are other claims that overnight low temperatures will increase. How much that will contribute, I haven’t a clue, but a first guess is that it would potentially reduce temperature gradients if the overnight low rises more than the daytime high.

  9. 159
    Mark says:

    re 158, FCH, OK, so when he was talking about “reduced winds” he really WAS only considering hurricanes.

    Not being in hurricane country (as is most of the world, really), these are not what I consider “winds”.

    They are also not the winds that are considered winds for the use of widnfarm energy extraction.

    So quite why they are being brought up in the context of the utility of windfarms remains opaque.

    PS His quotes and papers are all talking about the ***number*** of hurricanes. Not much on the windspeed. Adding quite a few more questions begging from his post.

  10. 160
    G. Karst says:

    Mark:

    Increasing temps causes an increase in H2O vapor. Due to the large heat capacity of water, there is an increase in the transport of heat energy from the equatorial zones to the poles. As the poles warm, delta T(s) decrease and consequently delta P(s) decrease. Therefore, ergo, winds and severe weather should moderate as per observed data.

    This is pretty basic stuff, I am sure you are capable of sourcing it.

  11. 161
    Greg Simpson says:

    While global warming may reduce a global wind driver, there should be an increased temperature gradient between ocean and land, which could increase regional winds. A transient effect, I suppose, but a very long lasting one.

  12. 162
    G. Karst says:

    Greg Simpson:

    Possibly, however that delta T exists during the day now, and results in a sea breeze. It reverses every night. I am not sure if that effect is modeled or not. Doubt it.

  13. 163
    Mark says:

    G, that’s the sort of thing I’m looking for.

    Now the release of that energy when the rains fall. Are we going to see massively heavier downpours and very deep convection cloud from it?

    And is this effect going to effect windfarms (which relates to the discussion)?

    Your links were all hurricane related.

    Not too important at 50N.

    Rain, however…

  14. 164
    G. Karst says:

    Mark:

    Like most things, influences are double edged swords, that cut in two directions. While less severe weather safeguards the capital investments in wind machines (severe weather can wreck them), decreasing prevailing winds could demand relocation or re-engineering to appropriate capacity.

    Beyond that, I cannot speculate further and remain on scientific grounds. I am sure there are others here who wouldn’t mind speculating with you, on future weather patterns and effects.

  15. 165
    Jerry Toman says:

    One technology for the conversion of renewable energy (CAPE from the troposphere) to electricity doesn’t depend on surface wind speed:

    http://vortexengine.ca

    This technology still looking for support from atmospheric scientists as it attempts to lifts itself up into prominence by its own “bootstraps”.

    Are there no Contributors to Realclimate who would consider adding their names to the current Endorsement List, which include, beside myself, Don Cooper, CPEng., of Australia or M. Francios Maugis, a Consulting Engineer from France, or to the list of notable atmospheric scientists such as Nilton Renno (who says there is no reason why it shouldn’t work), or Kerry Emanuel who say that the idea is worth further developmental investment?

    “All that is necessary for the triumph of evil is that good men do nothing.” Edmund Burke

  16. 166
    Mark says:

    I prefer B5′s “Truth is a three-edged sword: Your side, their side and the truth.” And all of them will cut.

  17. 167
    L. David Cooke says:

    Re: 158/159

    Hey All,

    Here are a few thoughts we may need to consider in relation to atmospheric heat transport.

    http://www.geosc.psu.edu/Courses/Geosc320/Lau.pdf

    Tropical slowdown of the Walker circulation is a likely effect.

    http://www.gfdl.noaa.gov/cms-filesystem-action?file=user_files/kd/pdf/gfdlhighlight_vol1n3.pdf

    Along with that is a temperate inversion in the Arctic winter (primarily near the pole) and associated dust capping the severity of tropical events as observed with the Namma Expedition, Which is attributed to contributing to the loss of polar warmth.

    http://www.atmos.ucla.edu/csrl/publications/Hall/boe_et_al_submitted.pdf

    The problem with the value of the thermal inversion value is the empirical data that seemed to be collected in Greenland.

    http://dsc.discovery.com/tv/project-earth/lab-books/greenland/greenland-guide1.html

    (However, the use of the polypropylene fabric versus a conductive foil cover may point to a partially failed experiment, if the intent was to identify the cause and effect. This is where the creation of the experiment by a professional is important. The use of a “canvas” does not remove the possibility that air temperature or CO2 radiant re-radiation could be involved.)

    As to night time temperatures being higher, again it generally depends on the processes for which the documentation are still being researched. The main issue is though it is possible that tropical circulation may slow the polar circulation is demonstrating regional changes in wind speed; but, overall Arctic winds seem balanced and follow the ENSO/PDO/NAO oscillations.

    http://pafc.arh.noaa.gov/papers/brw_winds1.pdf

    Here is a paper derived from remote sensing data and the indications suggest there may have been regional changes; but, the overall wind speeds do not show a significant increase.

    http://marine.rutgers.edu/~francis/pres/Francis_TOVS_winds_Jcli_final.pdf

    Here is an article suggesting that changes in the Ozone in the polar region may be affecting polar wind speeds which appears to be different from the idea of decreasing wind speeds.

    http://dsc.discovery.com/news/2009/06/12/ozone-oceans-climate.html

    In conclusion, it is entirely likely that the Walker Circulation may decrease in the tropical zones which could be contributing to the increasing energy content in the ITCZ region. If we consider that the slower tropical winds it may increase surface temperatures which would then translate to a greater convective volume moving northward rather then westward, in essence reducing apparent westward rate while the winds are redirected northward.

    The change in atmospheric heat content would seem to lead us to the issue of differences in the zonal regions. If as has been analyzed, the ITCZ and lower temperate zone demonstrate reduced wind speeds it should suggest that there may be a greater difference between inter-zonal surface temperatures. The issue in most of these measures is we seem to forget that circulation of heat content in the atmosphere is not a closed system, there are also radiant components. If the radiant rate has been reduced it is entirely likely that would provide additional fuel for temperature differences re-enforcing increased Hadley Cell poleward wind speed movement.

    If the actual measures and derivations are suggesting a reduction in temperate wind speeds there should also be a reduction in the trend of Air Pressure max/min or center proximity. If as we have seen in the last 20 years, that there have been significant increasing air pressure differential measures in the ITCZ zone, it would seem to suggest the average wind speeds should have increased.

    The lack of a reduction in average air pressure would seem to suggest the issue must be the proximity of air pressure cells. Proximity of Air Pressure Cells may point to the root cause of the observations. As of yet, I have not found supporting data in relation to decreasing proximity for air pressure centers, this suggests there may be more work to be done before the new IPCC Climate Model is developed. Then again the work may already be “in the can” and I am simply ignorant of it.

    Cheers!
    Dave Cooke

  18. 168
    L. David Cooke says:

    RE: 167

    Hey All,

    I need to make a small correction:
    “Along with that is a temperate inversion in the Arctic winter (primarily near the pole) and associated dust capping the severity of tropical events as observed with the Namma Expedition, Which is attributed to contributing to the loss of polar warmth.”

    It should have read:
    Along with that is a temperate inversion in the Arctic winter (primarily near the pole) and dust events capping the severity of tropical events as observed during the Namma Expedition. (Where the former contributes to the slowing of the loss of polar warmth and the later likely reduces the inter-zonal heat content transport.)

    Sorry for the delay, I figured someone would question the thought and I would correct it then. Else where there was an “it” that needed to be dropped; however the context would have been correct…

    Cheers!
    Dave Cooke

  19. 169
    L. David Cooke says:

    Re: 169

    Hey Nicolas,

    I guess if it were proven that the data used or the methods applied were in error that questioning the validity or discrediting of scientific studies would be justified. However, if the works have been found to be sound and the principles supported by experimental and or observational fact then the basis of science should be in lock step for the limited scope of the dataset.

    It is when we try to apply theories derived from laws and principles to new or under studied phenomena that there is a room for differences. If we were to focus on the science and attempt to create universal datasets, by combining valid datasets together, that seem to be in conflict, that we begin to see understand the new/under studied phenomena.

    Being a layman forms a tendency for me to assume that peer reviewed long standing theories, principles and laws must likely be accurate, for that time and place. If the underlying physics is correct, then it is likely the data set and philosophies derived from that study must be enduring. Hence, though there may appear to be conflict, in essence, it is simply a blind man documenting “his portion of the elephant”… After all how can one man eat an elephant…?, Answer, One byte at a time…

    Cheers!
    Dave Cooke

  20. 170
    Phillip Huggan says:

    I’d think it should be possible to measure this by looking at past low elevation cloud formations in the satellite data along with sand transport.

  21. 171
    Iscar says:

    You have a great blog here and it is Nice to read some well written posts that have some relevancy…keep up the good work

  22. 172
    tidal says:

    wouldn’t be surprised if this is already noted somewhere around here, but it was new to me! A good interview with Gavin regarding the skill and constraints of climate models (and much more – with extensive video excerpt) was posted yesterday at edge.org.

    or http://www.edge.org/documents/archive/edge292.html#tc

  23. 173
    Abi says:

    Re 172

    I tried to suggest the following earlier, but ran into CAPTCHA.

    Gavin – speaking of interviews, I saw the one you gave to Edge, where you explained how GCMs work. You might want to consider adding it to your Start Here section, for those who might be daunted by the idea of reading a book on the subject.

    http://www.edge.org/3rd_culture/schmidt09/schmidt09_index.html

  24. 174
    RichardC says:

    172 and 173 link to Gavin’s explanation of models and reality

    Sounds like a meta-model is in order. Lots of assumptions, lots of variables. Take the average of each assumption/variable and build a new model. More importantly, what nationality are you Gavin? You sound half bloke and half yank.

    [Response: Mostly bloke. - gavin]

  25. 175

    Jerry @ 165:

    There is absolutely no reason it shouldn’t work. I suspect the real concerns are going to be energy density and environmental impact.

    Build a 40M model and see what happens to its appetite for air and land.

  26. 176
    David B. Benson says:

    tidal (172) — Thank you. New to me as well.

  27. 177
    David B. Benson says:

    “New Type Of El Nino Could Mean More Hurricanes Make Landfall”:
    http://www.sciencedaily.com/releases/2009/07/090702140835.htm

  28. 178
    Doug says:

    WRT climate modeling generally

    As a non-scientist, I came across this statement on another website:

    “you appear to be trying to describe phenomena of energy centers of varying scales of time and space. There is an incipient field trying to describe patterns of relationships among them…commonly described as teleconnections. One example is the phenomena commonly labeled in the public as “El Nino-La Nina”. There are other, stronger relationships or teleconnections we are discovering. Again, we are just scratching the surface of understanding and all are poorly understood w/r/t causality…as *** correctly referenced above.

    Atmospheric variables are limited to the Navier Stokes Primitve Equations. These are not in the hundreds or thousands.

    Since *** brought this up, the interested reader should ask how many of these natural variability phenomena are replicated in the climate models?

    ANSWER: NONE.”

    I really don’t understand the science, but am curious about your take. The author claims to be an atmospheric physicist.

    Thanks,
    Doug

    [Response: It makes very little sense. Is the commenter suggesting that the diurnal cycle, the seasons, variations in jet stream, tropical Pacific variability, multi-decadal changes in the N. Atlantic are not in the models? In that case, he/she is simply wrong. If they are saying that the exact variability - the weather - is not perfectly synced up with the real world, then that is correct (due to chaos), but almost completely irrelevant to climate change. - gavin]

  29. 179
    Jeffrey Davis says:

    re:173 and Gavin’s recorded comments

    The clip was great, but I was struck by the “irreducible complexity” of the problem. Part of the effectiveness of Gavin’s comments came from their thoroughness. If you truncated the information, you’d degrade their impact. I don’t know why but I keep hearing the Eddie Izzard skit where the teenage Izzard is talking to his high school career advisor. No matter what ambition Izzard had the guidance counselor countered with, “Well, just remember you’re English so scale it back a bit.” But once you scale it back a bit in AGW, you come close to the sound-bite level, and you’re stuck playing the game with denialists.

  30. 180
    Greg Simpson says:

    The edge video of Gavin was well done. The video, at least as played by my computer, only covered about the first half of the transcript. It was worth reading the rest.


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