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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464. CaneAddict
9:48 PM GMT on May 21, 2008
BLOG HAS FROZEN AGAIN!!!!!
Member Since: October 5, 2007 Posts: 3 Comments: 2151
463. IKE
4:48 PM CDT on May 21, 2008
I didn't know you got out of the FEMA trailer...congrats!
Member Since: June 9, 2005 Posts: 23 Comments: 37858
462. StormJunkie
9:47 PM GMT on May 21, 2008
lol pat, you know damn well you are much more then that, at least to us!

Speaking of that, I have been meaning to tell you/ask you...If you ever have things that you think are not up to par in the preparedness area, I am all ears!
Member Since: August 17, 2005 Posts: 26 Comments: 15633
461. CaneAddict
9:45 PM GMT on May 21, 2008
<
Member Since: October 5, 2007 Posts: 3 Comments: 2151
460. Patrap
4:46 PM CDT on May 21, 2008
Im just an Blogger, not a met type by any means.
SJ has the info in a concise easy to read and navigate site.

Im only a Hack blogger 4 months removed from a FEMA trailer.

LOL
Member Since: July 3, 2005 Posts: 420 Comments: 127512
459. HurricaneSammy
9:46 PM GMT on May 21, 2008
Oh Ok i see it now
458. HurricaneSammy
9:45 PM GMT on May 21, 2008
My Posts aren't Showing up !
457. HurricaneSammy
9:45 PM GMT on May 21, 2008
.......
456. smmcdavid
4:43 PM CDT on May 21, 2008
... continued from last post

To both SJ and Pat
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455. StormJunkie
9:43 PM GMT on May 21, 2008
What a kwinky dink!

Thank-you kind sir!
Member Since: August 17, 2005 Posts: 26 Comments: 15633
454. CaneAddict
9:40 PM GMT on May 21, 2008
445. smmcdavid 9:40 PM GMT on May 21, 2008
I thought the models were indicating a possible storm at the end of next week...

Pat, what can you tell me about models? I don't know how to use them at all. Any advice?


The GFS has been consistently hinting at a disturbance forming in the SW Caribbean, However track and intensity of this disturbance has been everywhere. I would'nt count on it though, Unless more models began to predict the same scenario.

Also some models have been forecasting the development of something cold-core (sub-tropical) in nature off the S.E. coast of the U.S.....This also remains to be seen.
Member Since: October 5, 2007 Posts: 3 Comments: 2151
453. 69Viking
4:40 PM CDT on May 21, 2008
Off to mow the lawn before the storms decide to pop. Nothing on the radar yet but under a Severe Thunderstorm Watch in FL Panhandle. Keep up the good conversations, I'll check back this evening.
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452. HurricaneSammy
9:40 PM GMT on May 21, 2008
..................
451. smmcdavid
4:43 PM CDT on May 21, 2008
Will do. Thanks.
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450. StormJunkie
9:40 PM GMT on May 21, 2008
smmc, I know I am not pat, so I hope he does not get offended I am answering his question !~)

Check these out, it will take about 20 minutes to watch them, but I think it will help you get a good understanding of the basics of the models, and of how to use some of the various model pages. Then you will have to learn more from the experts! But at least it is a good start.
Member Since: August 17, 2005 Posts: 26 Comments: 15633
449. Patrap
4:40 PM CDT on May 21, 2008
I suggest you vist StormJunkies site for a good tutorial on the Tropical Puter models.
Member Since: July 3, 2005 Posts: 420 Comments: 127512
448. smmcdavid
4:40 PM CDT on May 21, 2008
Thanks 69Viking... it is a full-time job and worth every second. I do believe that some moms are not given the credit they are due. Me not included: my family really appreciates me and they let me know daily.
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447. TEXASYANKEE43
9:38 PM GMT on May 21, 2008
443. Patrap
..vary soon maybe.


Reminds me of a song by Jeff Beck, Definitely Maybe

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446. CaneAddict
9:37 PM GMT on May 21, 2008
50, 100, 200 Comments Sort: Newest First - Order Posted Filter: Show All Show Bad Show Below Average Show Average Show Good Show Best

442. floridastorm 9:36 PM GMT on May 21, 2008
is there going to be a 90L this week


Probably not likely this week but next week (If the GFS is correct on either system) it will be possible if one of the following scenarios play out.

1. A disturbance forms in the SW Caribbean. (As the GFS says)
2. A subtropical disturbance forms along a frontal boundary on the SE coast of the U.S.

Member Since: October 5, 2007 Posts: 3 Comments: 2151
445. smmcdavid
4:38 PM CDT on May 21, 2008
I thought the models were indicating a possible storm at the end of next week...

Pat, what can you tell me about models? I don't know how to use them at all. Any advice?
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444. StormJunkie
9:33 PM GMT on May 21, 2008
391. lmao :~)

375.

Well at the very least you are guilty of not using proper punctuation...396. :☺

QPF Bomb - I thought that was something you got in New York when a regular F-Bomb wasn't good enough.

lmao some more!
Member Since: August 17, 2005 Posts: 26 Comments: 15633
443. Patrap
4:37 PM CDT on May 21, 2008
Never feel the wundergound dosent reach the World.


My Map-Loco today.Link


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442. floridastorm
9:36 PM GMT on May 21, 2008
is there going to be a 90L this week
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441. TEXASYANKEE43
9:33 PM GMT on May 21, 2008
440

That brings back mememories!Thanks Pat.....
Man you sure changed that quick...WOW!
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440. Patrap
4:32 PM CDT on May 21, 2008
For the GOM and Atlantic Views in 3 Channels Link
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439. MasterForecaster
9:18 PM GMT on May 21, 2008
HEY EVERYBODY!

Don't usually post much aside from the usual "WILL IT HIT SOUTH FLORIDA" question every now and again. I have realized however, my love for tracking and studying tropical systems so I will be here to post my opinion and ask all of you yours. I love this blog even though there tends to be some drama (lol).

I wanna start by asking if the expected rain tomorrow in south Florida will help the drought/fire issues enough to make a difference?

Looking forward to a great season tracking with everyone!
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438. HIEXPRESS
5:13 PM EDT on May 21, 2008
331. IKE 4:14 PM EDT on May 21, 2008
326. Patrap 3:11 PM CDT on May 21, 2008
vary Interesting....

That moisture is headed NE.


So is this Brush Fire - now 800 plus acres -

Always learning here. Today Walker circulation (probably basic to you real mets), & yesterday QPF Bomb - I thought that was something you got in New York when a regular F-Bomb wasn't good enough. CBL
Member Since: October 13, 2005 Posts: 4 Comments: 2156
437. 69Viking
4:13 PM CDT on May 21, 2008
Kudos to you smmc for being able to blog and "Play Mom" because playing mom is a full time job and I mean it.
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436. IKE
4:13 PM CDT on May 21, 2008
7th day HPC........

Link
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435. Floodman
9:08 PM GMT on May 21, 2008
Okay folks, out for a while...
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434. Floodman
9:05 PM GMT on May 21, 2008
LOL, Pearland
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433. pearlandaggie
9:05 PM GMT on May 21, 2008
pat, buddy, I tHInk you'RE havING SOme capitALIzation issuES! :)
Member Since: September 14, 2007 Posts: 3 Comments: 3963
432. Patrap
4:02 PM CDT on May 21, 2008
LSU ESL Image of the Week.Link
Click Image to ENlarge
Member Since: July 3, 2005 Posts: 420 Comments: 127512
431. pearlandaggie
9:00 PM GMT on May 21, 2008
425. yikes! lotsa Texans on dis heer blog!

427. let it lie, big 'un......let it lie.
Member Since: September 14, 2007 Posts: 3 Comments: 3963
430. Drakoen
9:01 PM GMT on May 21, 2008
428. Floodman 9:00 PM GMT on May 21, 2008
423. Drakoen

We begin to see some consensus in the models?


Who knows lol...
Member Since: October 28, 2006 Posts: 57 Comments: 29883
429. Patrap
4:01 PM CDT on May 21, 2008
This season has begun in the sense that were seeing things beginning to awaken.
Member Since: July 3, 2005 Posts: 420 Comments: 127512
428. Floodman
8:58 PM GMT on May 21, 2008
423. Drakoen

We begin to see some consensus in the models?
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427. Floodman
8:56 PM GMT on May 21, 2008
418. pearlandaggie 8:51 PM GMT on May 21, 2008
413. nope....strong like bull! :)



Despite huge temptation, I will leave that one alone...LOL
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426. Patrap
3:56 PM CDT on May 21, 2008
Hold um Tigers..Link

LSU ESL Tropical Site Link
Member Since: July 3, 2005 Posts: 420 Comments: 127512
425. smmcdavid
3:56 PM CDT on May 21, 2008
Well, I have to go play Mom for a bit... be back soon.

BTW pearland, I'm originally from Austin so I'm a Longhorn at heart (plus my dad and husband both went there)... don't tell anyone!
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424. atmoaggie
8:51 PM GMT on May 21, 2008
Do I detect a soft spot? Hmmm?

We/they sometimes are touchy about that...for reasons I cannot fathom.

Howdy from a 5th generation New Orleans native Aggie former student(not a drop out, that's what we call alumni)

...I shout Whoop while bleeding purple and gold. Geaux Tigahs. (I have been called freak before...so go ahead.)
Member Since: August 16, 2007 Posts: 6 Comments: 12461
423. Drakoen
8:51 PM GMT on May 21, 2008
I got caught up in all of this I forget to mention the UKMET is showing a similar migratory area of 850mb positive vorticity maximum as the surface flow becomes westerly. That being said, if the GFS pulls in on the time frame we could actually see the UKMET pick up on the system.
Member Since: October 28, 2006 Posts: 57 Comments: 29883
422. Patrap
3:52 PM CDT on May 21, 2008
You should be able to answer the following questions before a hurricane threatens:

*
What are the Hurricane Hazards?
*
What does it mean to you?
*
What actions should you take to be prepared?
Member Since: July 3, 2005 Posts: 420 Comments: 127512
421. smmcdavid
3:51 PM CDT on May 21, 2008
Everyone (even aggies) enjoy a good aggie joke every now and then! It just kinda comes with the school choice.
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420. pearlandaggie
8:51 PM GMT on May 21, 2008
417. reminds me of that old joke about florida and cuba....!
Member Since: September 14, 2007 Posts: 3 Comments: 3963
419. Patrap
3:50 PM CDT on May 21, 2008
In all seriousness though.

Next Week is Hurricane Preparedness week..Link
Member Since: July 3, 2005 Posts: 420 Comments: 127512
418. pearlandaggie
8:50 PM GMT on May 21, 2008
413. nope....strong like bull! :)
Member Since: September 14, 2007 Posts: 3 Comments: 3963
417. hahaguy
4:48 PM EDT on May 21, 2008
reminder to Florida that its Louisiana's first Barrier Island


i agree we are just hanging there
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416. Floodman
8:48 PM GMT on May 21, 2008
408. Patrap 8:46 PM GMT on May 21, 2008
Thats $9.75 for the Nachos and Pickle



Spoken like a true New Orleanian...LOL
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415. Patrap
3:48 PM CDT on May 21, 2008
90L..? Link
Member Since: July 3, 2005 Posts: 420 Comments: 127512
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: 29883

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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.