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The future of wind shear: will it decrease the number of hurricanes?

By: Dr. Jeff Masters , 1:55 PM GMT on May 21, 2008

Could global warming increase wind shear over the Atlantic, potentially leading to a decrease in the frequency of Atlantic hurricanes? Several modeling studies are now predicting this, and it is a reasonable hypothesis. The most recent study, "Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions", was published Sunday in Nature Geosciences. The authors, led by Tom Knutson of NOAA's GFDL laboratory, showed that global warming may reduce the number of Atlantic tropical storms by 27% and hurricanes by 18% by the end of the century. However, their model also found that the strongest hurricanes would get stronger.

An important reason that their model predicted a decrease in the frequency of Atlantic hurricanes was due to a predicted increase in wind shear. As I explain in my wind shear tutorial, a large change of wind speed with height over a hurricane creates a shearing force that tends to tear the storm apart. The amount of wind shear is critical in determining whether a hurricane can form or survive.

The main sources wind shear over the tropical Atlantic:
1) The jet stream is the primary year-round source of high wind shear over the Atlantic. The jet can have two branches--the main northerly polar jet, and a weaker subtropical jet that blows over the Gulf of Mexico or Caribbean. In winter, the jet stream is far to the south, bringing very high levels of wind shear to the tropical Atlantic. The Caribbean Sea is warm enough year-round to support hurricane formation, but high levels of wind shear from the southerly position of the jet stream prevents wintertime hurricanes from forming. In the summer, the jet stream retreats to the north, but can still loop far enough south to create hurricane-hazardous wind shear.

2) The large-scale tropical atmospheric circulation pattern known as the Walker Circulation (Figure 1) can bring high wind shear to the Atlantic. A weak Walker Circulation brings high wind shear, while a strong Walker Circulation--rising air over the tropics near Australia, combined with sinking air of the coast of South America near Peru--brings weak upper-level winds over the Atlantic, resulting in low levels of wind shear.

3) The presence or absence of an El Niño event has a critical impact on wind shear levels. El Niño events weaken the Walker Circulation, bringing strong upper-level winds out of the west to the Atlantic, creating high wind shear.

4) In summer and fall, Tropical Upper Tropospheric Troughs (TUTTs) and upper-level cold-core low pressure systems ("cold lows") that are cut off from the jet stream often wander through the tropics, bringing high wind shear with them.

5) A strong east-to-west flowing jet of air is frequently found at the southern boundary of the Saharan Air Layer (SAL), a hot, dry region of air found off the coast of Africa during hurricane season. This easterly jet often is strong enough to cause significant wind shear over the hurricane development region of the tropical Atlantic.

Figure 1. Schematic drawing of the Pacific Ocean's Walker Circulation. Warm ocean waters over the Western Pacific near Australia heat the air above, causing it to rise. When the rising air reaches the top of the troposphere, it can't rise any further, and is forced to flow eastwards towards the Atlantic. This air then sinks back to the surface near the Pacific coast of South America, then flows back towards Australia as easterly trade winds. Image credit: Wikipedia.

The future of wind shear
In their 2007 paper, "Increased Tropical Atlantic Wind Shear in Model Projections of Global Warming", Gabe Vecchi of NOAA's GFDL laboratory and Brian Soden of the University of Miami looked at 18 of the models used to formulate the "official word" on the science of climate change, the 2007 Intergovernmental Panel on Climate Change (IPCC) climate report. Vecchi and Soden found that in the scenario where CO2 doubles to 720 ppm by year 2100 (the so-called "A1B" scenario), these models predict a 1.5-3.5°C increase in global surface air temperature. However, in the Caribbean and some surrounding regions, at least 13 of the 18 models predict that the amount of wind shear rises by 1-2 mph per degree C of warming (Figure 2). The shear increases largely as a result of a weakening of the Walker Circulation. This weakening brings strong upper-level westerly winds to the Eastern Pacific and Caribbean.

The implications
If true, Vecchi and Soden's results imply that we may see fewer hurricanes in the Atlantic and Eastern Pacific by the end of the century, since wind shear is such an important ingredient in their formation. How reliable are these model predictions? If global warming is expected to cause a slowdown in the Walker Circulation and increased wind shear over the tropical Atlantic, shouldn't we be able to see these effects already? There is some evidence that we are seeing these effects. According an article by the same authors published in 2006 in Nature, the observed 0.5-0.6°C global warming in the past century has caused the Walker Circulation to slow down by 3.5%--in line with what theory predicts. Moreover, Wang and Lee (2008) documented a 3 mph increase in wind shear over the tropical Atlantic between 1949-2006 (despite some rather low shear years recently, such as during the record-breaking Hurricane Season of 2005). These results, plus the fact that 13 of the 18 IPCC models predict a tropical Atlantic wind shear increase in the coming century, make the hypothesis that we may see increased wind shear over the Atlantic in coming decades a reasonable one. However, climate scientists Ray Pierrehumbert and Rasmus Benestad argue in a 2006 post on realclimate.org that we need another ten years of observations of the Walker Circulation to confirm that we really are seeing a slowdown. In addition, we need to see if the model predictions of increased wind shear hold up when improved simulations with better data and higher resolutions are performed. These models are fairly primitive in their abilities to simulate these sort of regional climate shifts, and some models predict a strengthening of the Walker Circulation in coming decades--the opposite of what Vecchi and Soden found.

Figure 2. Top: predicted change by 2100 in wind shear (in meters per second per degree C of warming--multiply by two to get mph) as predicted by summing the predictions of 18 climate models. Bottom: The number of models that predict the effect shown in the top image. The dots show the locations where tropical storms formed between 1981-2005. The box indicates a region of frequent hurricane formation where wind shear is not predicted to change much. Image credit: Geophysical Research Letters, "Increased Tropical Atlantic Wind Shear in Model Projections of Global Warming", by Vecchi and Soden, 2007.

All other things remaining constant, an increase in wind shear will cause fewer hurricanes to form. However, all other things will not remain constant. As the climate warms, Sea Surface Temperatures (SSTs) will warm, which may partially or completely offset the effects of increased wind shear. Vecchi and Soden's research also show a substantial increase in wind shear over most of the Southern Hemisphere's hurricane breeding grounds during their hurricane season, but a significant decrease in wind shear over the Western Pacific and North Indian Oceans. Typhoons and cyclones in these ocean basins may well get more numerous and stronger in the future as a result of the lower wind shear. Much more research remains to be done, and it is far too early to be confident of how wind shear might change in a warming world.

Vecchi, G.A., B.J. Soden, A.T. Wittenberg, I.M. Held, A. Leetmaa, and M.J. Harrison, 2006, "Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing", Nature, 441(7089), 73-76.

Vecchi, G.A., and B.J. Soden, 2007, "Increased Tropical Atlantic Wind Shear in Model Projections of Global Warming", Geophysical Research Letters, 34, L08702, doi:10.1029/2006GL028905, 2007.

Wang, C., and S. Lee, 2008, "Global warming and United States landfalling hurricanes", Geophysical Research Letters 35, L02708, doi:10.1029/2007GL032396, 2008.

realclimate.org has a nice discussion of the Veccu and Soden paper.

Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Reader Comments

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764. FLWeatherFreak91
5:34 AM EDT on May 22, 2008
Well, the even the radar at long-range is completely filled with rain... and the cold tops you were referring to in the central gulf are moving right toward us.... We should get sufficient rain today
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762. FLWeatherFreak91
5:28 AM EDT on May 22, 2008
I'm up JFL and I'm am ecstatic to know it will be raining all day. And how tropical does this thing look! If a low happens to form (like you think it's doing) this may have a chance to develop after it moves into the ATL.... do you think this is the system the models have been picking up on?
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758. moonlightcowboy
11:53 PM CDT on May 21, 2008
Have a good sleep, Baha!
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757. moonlightcowboy
11:50 PM CDT on May 21, 2008
Gotcha, so then, a high positioned like that looks as if it ups the odds for you and the Carolinas and hopefully fish? Sound about right?
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756. BahaHurican
12:50 AM EDT on May 22, 2008
Anyway, I'm headed out. I've an early day tomorrow; I should have hit the sack an hour ago . . . g'night!
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755. BahaHurican
12:43 AM EDT on May 22, 2008
Based on that map on 750? If you had a break in the ridge like that, u might end up with something across eastern Hispaniola, western PR. Most of the lesser Antilles would get something. After that, where it ended up would be dependent on whether that little low was still there.
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754. BahaHurican
12:41 AM EDT on May 22, 2008
Actually we haven't been that bad this spring. I think we had rain for most of April - nothing heavy, but some light to moderate showers, mainly at night, every week. We've been dry for about 2 1/2 weeks or so.
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753. moonlightcowboy
11:41 PM CDT on May 21, 2008
Looking at the sfc map, and say we had some thing out of the ITCZ just east of Pottery, where do you think we'd see a track based on that high position (of course I know steering would be a factor); but, just based on the high position?
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752. moonlightcowboy
11:40 PM CDT on May 21, 2008
I think I read a post today that said Key West was like 41 days, now, without a drop.
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751. BahaHurican
12:38 AM EDT on May 22, 2008
Yeah, that's the one I was thinking about.

Rain would be great.
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750. moonlightcowboy
11:36 PM CDT on May 21, 2008
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749. BahaHurican
12:33 AM EDT on May 22, 2008
Hmmm. . . . looking at that makes me think we might even get a drop or two by Friday . . . however, unless that high pressure ridge backs off seriously, most of the Bahamas as well as SFL will hardly get wet.

I'm assuming, BTW, that the goldish yellow green blot in the Gulf is the low that is supposed to head this way by Friday . . .
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748. moonlightcowboy
11:31 PM CDT on May 21, 2008

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747. BahaHurican
12:31 AM EDT on May 22, 2008
Thanks for the link, MLC. I hadn't seen that particular site before . . .
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746. moonlightcowboy
11:27 PM CDT on May 21, 2008
Yeah, I guess you would, lol. Hey, I've found some new links, one of which is centered on the Bahamas. See if you've seen this one before:

HNWS Here's the one of the Bahamas:

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745. BahaHurican
12:24 AM EDT on May 22, 2008
BTW, this is the Dennis I think about when u say Dennis:

This is the one that hit just before Floyd. Looking at the map now, I can visualize the location of the High, and then how the ridge that pushed Floyd into the NW Bahamas just pushed out enough for us to get the whammo. Dennis was like a foreshadowing, an omen.
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744. BahaHurican
12:21 AM EDT on May 22, 2008
mlc, that low ITCZ is one reason I have been reluctant to endorse the idea of an early start to the season. I don't think we are going to see much action, serious or otherwise, before mid-July or even early August. The SSTs are part of that reasoning.
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743. moonlightcowboy
11:10 PM CDT on May 21, 2008
Baha, Pottery and I were chin-wagging back-n-forth last night about the ITCZ. It stills seems low, especially in the wATL and we primarily attributed that to late warming SST's. It's still running kind of low, now, too. Her's my question: If these storms track t thru the ITCZ like that, I know they have trouble with coriolis; but, as it gets slightly higher, where do you guess we could see any developing storm first make a move north?

Ivan and I have speculated that we might see some cyclogenesis origin like Dennis. Any thoughts?

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742. BahaHurican
12:07 AM EDT on May 22, 2008
If one could be sure we'd get the fishies, one wouldn't mind so much. But sometimes what LOOKS like a fish storms ends up tagging us.
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741. moonlightcowboy
10:59 PM CDT on May 21, 2008
Hope not, Baha! Hopefully, we'll get some good shear, a bunch of sand storms - and things we'll just dissolve! Ugh!
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740. BahaHurican
11:53 PM EDT on May 21, 2008
Yesterday's weather:

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739. BahaHurican
11:51 PM EDT on May 21, 2008
Today's Nassau weather:

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738. BahaHurican
11:44 PM EDT on May 21, 2008
Yeah, MLC, I was just looking at that. What a setup! I dunno what to think about that. It's been hanging about, that low, and variably further east and west, for about 7 - 10 days now. I'm thinking that's just GREAT for steering things around to the NW Bahamas and east US coast. :o(
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11:16 PM EDT on May 21, 2008
695. atmoaggie 9:29 PM
RE: 692
For whom? 767s going west?
The jokes on me, I guess. Are you saying that graphic, and the others linked to it, say nothing about the potential for severe weather?
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736. moonlightcowboy
10:41 PM CDT on May 21, 2008
Hey, Baha! Check out this weak high parked out there well east of Bermuda.

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735. BahaHurican
11:37 PM EDT on May 21, 2008
2005 had some storms that bombed out. Katrina wasn't the only one.
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734. Vanagew
3:28 AM GMT on May 22, 2008
Gidday everyone... looking at the pic of Katrina, sdhe exploded over 24 hours.
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733. BahaHurican
11:18 PM EDT on May 21, 2008
Evening everybody. Just popping in for a couple. Still no rain in Nassau, Bahamas today, though it did do a practice (dry run :o) of the 3 p. m. downpour. In other words, it clouded up nicely, but we didn't get a drop. So it was just another really hot day. I expect we will get a downpour some time in the next 3 - 6 days, since it seems the moisture content in the air has really increased. In a minute I'll try posting a pic of the non-raining clouds . . .
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732. ajcamsmom2
9:41 PM CDT on May 21, 2008
Water Temperature (WTMP): 82.8 °F
Conditions at 42056 as of
0150 GMT on 05/22/2008

We've reached the magic number....
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730. stormhank
2:23 AM GMT on May 22, 2008
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729. stormhank
2:19 AM GMT on May 22, 2008
Im thinking the numbers tomm. from NOAA n Dr M. will prob average near the 13-15 named 7-9 hurricanes and 3-4 major. category. but thats just my guess.
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728. Patrap
9:15 PM CDT on May 21, 2008
Powerful Imagery for sure stormhank.
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727. Drakoen
2:14 AM GMT on May 22, 2008
723. JFV 2:10 AM GMT on May 22, 2008
Me too, anyways, Drak what are your thoughts this evening on the potential carib system for next week my cyber friend?

I don't know really. Really need more models to jump on board.
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726. stormhank
2:13 AM GMT on May 22, 2008
I wonder what NOAA's numbers will be tommorow on their outlook?
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725. stormhank
2:11 AM GMT on May 22, 2008
nice links of Katrina pat
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724. Patrap
9:09 PM CDT on May 21, 2008
In 3 hours Ill be watching Indiana Jones.

Large coke and a Clark Bar too.
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722. Drakoen
2:06 AM GMT on May 22, 2008
I thought David Archuletta was going to win oh well
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721. Patrap
9:02 PM CDT on May 21, 2008
Life cycle of Katrina GOM (~33MB) Link

* Close-up of Eye (~33 MB)Link

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720. FLWeatherFreak91
10:03 PM EDT on May 21, 2008
David Cook won AI fyi
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9:53 PM EDT on May 21, 2008
nice blow up over s texas tex mex border tornadic
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9:51 PM EDT on May 21, 2008
night sj
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9:48 PM EDT on May 21, 2008
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716. StormJunkie
1:47 AM GMT on May 22, 2008
Night CA, see ya tomorrow

Ok, now y'all can start talking about me again :~)

Night everyone
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715. CaneAddict
1:45 AM GMT on May 22, 2008
714. KEEPEROFTHEGATE 1:45 AM GMT on May 22, 2008
poppers over s ga again

Hey man, Have a good night, Sorry about the little agrument today, We both were wrong...I have just had some issues lately and should have no been so quick to fly off the handle over a comment. No hard feelings, Look forward to your humor this season and your contribution. Night all, SJ,atmoaggie, pearl, drak, all4, EVERYONE!
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9:38 PM EDT on May 21, 2008
poppers over s ga again
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Category 6™


Cat 6 lead authors: WU cofounder Dr. Jeff Masters (right), who flew w/NOAA Hurricane Hunters 1986-1990, & WU meteorologist Bob Henson, @bhensonweather

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