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

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

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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.

Caveats
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.

References
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

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414. Drakoen
8:47 PM GMT on May 21, 2008
405. Patrap 8:45 PM GMT on May 21, 2008
We arent here to Please.
Its a info sharing multi layered medium for expressing Ideas, related topic material and an Occasional reminder to Florida that its Louisiana's first Barrier Island..so take care of it and post with insight and wisdom one has to offer..


Yep. and hopefully its not BS from somewhere else.
Member Since: October 28, 2006 Posts: 57 Comments: 30823
413. Floodman
8:41 PM GMT on May 21, 2008
392. pearlandaggie 8:39 PM GMT on May 21, 2008
387. hey, hey, HEY! no need for aggie jokes here! :)



Do I detect a soft spot? Hmmm?
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412. hurricane23
4:46 PM EDT on May 21, 2008
Iam good.....

Just waiting for my rainy season to hit but unfortunatly its been put on hold atleast for a few days.

NWS discussion-MIA

MAY 20 IS THE MEDIAN DATE OF THE ONSET OF THE SOUTH FLORIDA RAINY
SEASON. DUE TO DRY AIR MIXING DOWN TO THE SURFACE EACH OF THE
PAST TWO AFTERNOONS LIMITING CONVECTION...IT APPEARS THAT RAINY
SEASON WILL NOT START UNTIL NO EARLIER THAN TOMORROW...MAY 22. THE
COLD FRONT EXPECTED TO MOVE ACROSS THE AREA SUNDAY WILL PUT A
TEMPORARY HALT TO THE WET PATTERN EXPECTED TO COMMENCE LATE THIS
WEEK INTO THE WEEKEND. AS A RESULT...THE ACTUAL START DATE OF THE
RAINY SEASON WILL PROBABLY NOT BE DETERMINED UNTIL NEXT WEEK WHEN
THE EXTENT OF THE DRYING BEHIND THE COLD FRONT WILL BE BETTER
NOTED. /MOLLEDA
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411. pearlandaggie
8:46 PM GMT on May 21, 2008
405. LMAO!

by the way, pat, speaking of duds...

Sunspot cycle more dud than radiation flood
Member Since: September 14, 2007 Posts: 3 Comments: 3963
410. Drakoen
8:46 PM GMT on May 21, 2008
.
Member Since: October 28, 2006 Posts: 57 Comments: 30823
409. atmoaggie
8:45 PM GMT on May 21, 2008
reminder to Florida that its Louisiana's first Barrier Island

Wow, Pat. I've thought that but I have never had the gumption to say it out loud.

Bold...and spicy. LMAO.
Member Since: August 16, 2007 Posts: 6 Comments: 12463
408. Patrap
3:45 PM CDT on May 21, 2008
Thats $9.75 for the Nachos and Pickle
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407. smmcdavid
3:45 PM CDT on May 21, 2008
Pat... that was great! I actually LOLed and the dog looked at me like I'm crazy or something.
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406. smmcdavid
3:44 PM CDT on May 21, 2008
Calm down peeps... save some energy for when you need it.
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405. Patrap
3:43 PM CDT on May 21, 2008
We arent here to Please.
Its a info sharing multi layered medium for expressing Ideas, related topic material and an Occasional reminder to Florida that its Louisiana's first Barrier Island..so take care of it and post with insight and wisdom one has to offer..

Phew.
LOL
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404. atmoaggie
8:44 PM GMT on May 21, 2008
QPF is also synonymous with WTF. (Wednesday Thursday Friday for you kids watching at home.)
Member Since: August 16, 2007 Posts: 6 Comments: 12463
403. hurricane23
4:44 PM EDT on May 21, 2008
400. Weather456 4:42 PM EDT on May 21, 2008
390. hurricane23 4:39 PM AST on May 21, 2008
382. Weather456 4:37 PM EDT on May 21, 2008
378. hurricane23 4:34 PM AST on May 21, 2008

You can please some of the people, some of the time but u can please all persons, all the time.

Hey thats fine with me 456 but just save the rude and negative comments for someone else.

That wasnt a rude comment...its just a saying....even though you try respect persons...it will have the exceptional few.

456 not you buddy! Its just sometimes the fun gets taking out of blog when negativity/rudeness are present.
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402. hahaguy
4:40 PM EDT on May 21, 2008
milk duds where lol
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401. pearlandaggie
8:42 PM GMT on May 21, 2008
ahhhh...makes sense, now, smmc! i figured something like that was up! :)
Member Since: September 14, 2007 Posts: 3 Comments: 3963
400. Weather456
4:40 PM AST on May 21, 2008
390. hurricane23 4:39 PM AST on May 21, 2008
382. Weather456 4:37 PM EDT on May 21, 2008
378. hurricane23 4:34 PM AST on May 21, 2008

You can please some of the people, some of the time but u can please all persons, all the time.

Hey thats fine with me 456 but just save the rude and negative comments for someone else.


That wasnt a rude comment...its just a saying....even though you try respect persons...it will have the exceptional few.
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399. pearlandaggie
8:41 PM GMT on May 21, 2008
thanks, drak. not nearly as cool as my imagination let me believe!
Member Since: September 14, 2007 Posts: 3 Comments: 3963
398. Floodman
8:39 PM GMT on May 21, 2008
383. atmoaggie 8:37 PM GMT on May 21, 2008
Sensitive egos are being tainted by opposing subliminal egotism.

Big words...careful or you'll get accused of obfuscating ;-)



Obfuscation is an acceptable strategy LOL
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397. smmcdavid
3:41 PM CDT on May 21, 2008
Yeah, thanks for clearing that up Drak?! Huh?
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396. Drakoen
8:40 PM GMT on May 21, 2008
383. atmoaggie 8:37 PM GMT on May 21, 2008
Sensitive egos are being tainted by opposing subliminal egotism.

Big words...careful or you'll get accused of obfuscating ;-)


lol I hope not that would be rather detrimental.
Member Since: October 28, 2006 Posts: 57 Comments: 30823
395. smmcdavid
3:40 PM CDT on May 21, 2008
Pat, get me a large pickle and some nachos!

Pearland, I take it you haven't looked at my profile/blog yet...
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394. atmoaggie
8:38 PM GMT on May 21, 2008
QPF = Quantitative Precipitation Forecast

QPF GFS bomb details: here.
Member Since: August 16, 2007 Posts: 6 Comments: 12463
393. Drakoen
8:39 PM GMT on May 21, 2008
QPF bomb is the explosion in convection you see typically on the GFS long range associated with random mid level vorticity maximum.
Member Since: October 28, 2006 Posts: 57 Comments: 30823
392. pearlandaggie
8:39 PM GMT on May 21, 2008
387. hey, hey, HEY! no need for aggie jokes here! :)
Member Since: September 14, 2007 Posts: 3 Comments: 3963
391. Patrap
3:37 PM CDT on May 21, 2008
I think Im going with raisinets,rather than the milk duds.
Anyone need anything from the Lobby?
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390. hurricane23
4:37 PM EDT on May 21, 2008
382. Weather456 4:37 PM EDT on May 21, 2008
378. hurricane23 4:34 PM AST on May 21, 2008

You can please some of the people, some of the time but u can please all persons, all the time.

Hey thats fine with me 456 but just save the rude and negative comments for someone else.
Member Since: Posts: Comments:
389. pearlandaggie
8:38 PM GMT on May 21, 2008
383. or bloviating! LOL
Member Since: September 14, 2007 Posts: 3 Comments: 3963
388. Floodman
8:38 PM GMT on May 21, 2008
380. Drakoen 8:35 PM GMT on May 21, 2008
Everyone think they won't get into it but the slightest comment can set of a QPF bomb in the blog lol.



Precisely...
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387. smmcdavid
3:38 PM CDT on May 21, 2008
It must be an aggie thing pearland!
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386. pearlandaggie
8:36 PM GMT on May 21, 2008
380. alright, i'm the dummy...what's a QPF bomb?
Member Since: September 14, 2007 Posts: 3 Comments: 3963
385. smmcdavid
3:36 PM CDT on May 21, 2008
Ohhh... what's a QPF bomb? Sounds exciting!
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383. atmoaggie
8:35 PM GMT on May 21, 2008
Sensitive egos are being tainted by opposing subliminal egotism.

Big words...careful or you'll get accused of obfuscating ;-)
Member Since: August 16, 2007 Posts: 6 Comments: 12463
382. Weather456
4:34 PM AST on May 21, 2008
378. hurricane23 4:34 PM AST on May 21, 2008

You can please some of the people, some of the time but u can please all persons, all the time.
Member Since: Posts: Comments:
381. smmcdavid
3:35 PM CDT on May 21, 2008
hurricane23, I'm pretty sure everyone is just trying to have a good time today... not really being critical of anyone (even you). Have some fun, it's okay.
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380. Drakoen
8:35 PM GMT on May 21, 2008
Everyone think they won't get into it but the slightest comment can set of a QPF bomb in the blog lol.
Member Since: October 28, 2006 Posts: 57 Comments: 30823
379. Floodman
8:33 PM GMT on May 21, 2008
Well, I will say that I'm a firm believer in the ignore button...works quite well in fact
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378. hurricane23
4:34 PM EDT on May 21, 2008
lol sorry but theres nothing disfunctional ive done in this blog as far as iam concerned.I for one always try to be polite to everyone on this blog and always do the best of my ability to help everyone during difficult times.Iam not here to bash or criticize anyone thats just not who i am.But anyway it does not matter.



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377. Drakoen
8:33 PM GMT on May 21, 2008
374. atmoaggie 8:32 PM GMT on May 21, 2008
If this season turn out to be too active we could have the most drama episodes in blog history on Wunderground.

You are correct of course. For some reason a lot of folks here lately are wearing their feelings on their sleeves and being real touchy about even percieved slights.

Folks this ain't the NBA. A fight over nothing at all will only make you look stupid here.


Sensitive egos are being tainted by opposing subliminal egotism.
Member Since: October 28, 2006 Posts: 57 Comments: 30823
376. Weather456
4:30 PM AST on May 21, 2008
If this season turn out to be too active we could have the most drama episodes in blog history on Wunderground.

so true....that is wen i'll slip into lurk-mode...dont want be apart of that this season.
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375. Drakoen
8:32 PM GMT on May 21, 2008
And StormJunkie, for the record, i'm not guilty of doing anything lol.
Member Since: October 28, 2006 Posts: 57 Comments: 30823
374. atmoaggie
8:29 PM GMT on May 21, 2008
If this season turn out to be too active we could have the most drama episodes in blog history on Wunderground.

You are correct of course. For some reason a lot of folks here lately are wearing their feelings on their sleeves and being real touchy about even percieved slights.

Folks this ain't the NBA. A fight over nothing at all will only make you look stupid here.
Member Since: August 16, 2007 Posts: 6 Comments: 12463
373. Floodman
8:31 PM GMT on May 21, 2008
356. sporteguy03 8:23 PM GMT on May 21, 2008
600 Acre Wildfire in Lake County Florida, Lake County Firefighters are trying to fight it. Fire is out of control though, fatigued firefighters from Volusia, Orange Counties trying to assist.



What part of Lake, sporteguy?
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372. Drakoen
8:31 PM GMT on May 21, 2008
368. Floodman 8:30 PM GMT on May 21, 2008
You're right, if last year was any indication, Drak; I'll let folks pick on me...I am but an egg, but when they start in on the rest, I tend to get a little loud LOL


Yea. I already see the core influence is already underway.
Member Since: October 28, 2006 Posts: 57 Comments: 30823
371. Floodman
8:30 PM GMT on May 21, 2008
353. Drakoen 8:22 PM GMT on May 21, 2008
349. smmcdavid 8:20 PM GMT on May 21, 2008
One big, happy, disfunctional family. How sweet!

SJ and Adrian are the dysfunctional ones. Everyone else is pretty neutral lol.


That! LOL
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370. Floodman
8:30 PM GMT on May 21, 2008
365. smmcdavid

LOL
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369. Drakoen
8:29 PM GMT on May 21, 2008
364. Floodman 8:27 PM GMT on May 21, 2008
OMG, Drak, you didn't!

Didn't what?
Member Since: October 28, 2006 Posts: 57 Comments: 30823
368. Floodman
8:28 PM GMT on May 21, 2008
You're right, if last year was any indication, Drak; I'll let folks pick on me...I am but an egg, but when they start in on the rest, I tend to get a little loud LOL
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367. pearlandaggie
8:28 PM GMT on May 21, 2008
...just keep thinking, "dust and shear, dust and shear, dust and shear".....
Member Since: September 14, 2007 Posts: 3 Comments: 3963
366. Drakoen
8:27 PM GMT on May 21, 2008
362. Floodman 8:26 PM GMT on May 21, 2008
349. smmcdavid 8:20 PM GMT on May 21, 2008
One big, happy, disfunctional family. How sweet!



You're dead on it, smmc...and God help anyone who comes in and starts in on anyone of that family; last year there were some pretty fgood dust ups in here


If this season turn out to be too active we could have the most drama episodes in blog history on Wunderground.
Member Since: October 28, 2006 Posts: 57 Comments: 30823
365. smmcdavid
3:26 PM CDT on May 21, 2008
I'll try to mind my P's and Q's... "try" being the operative word in that statement.
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364. Floodman
8:26 PM GMT on May 21, 2008
OMG, Drak, you didn't!
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