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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364. Floodman
8:26 PM GMT on May 21, 2008
OMG, Drak, you didn't!
Member Since: August 2, 2006 Posts: 10 Comments: 9922
363. StormJunkie
8:25 PM GMT on May 21, 2008
I hear ya :~)

Ok, really got to get in the chair before she passes me over!
Member Since: August 17, 2005 Posts: 26 Comments: 16295
362. Floodman
8:23 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
Member Since: August 2, 2006 Posts: 10 Comments: 9922
361. hurricane23
4:25 PM EDT on May 21, 2008
351. StormJunkie 4:21 PM EDT on May 21, 2008
Come on guys, that was funny!

No offense 23, just a good laugh for all :~)

Are you for real?

Its useless trying to be nice to some people now a days.For me personally ive given 100 full complete respect to everyone on this blog since day 1.
Member Since: May 14, 2006 Posts: 8 Comments: 13707
360. Drakoen
8:24 PM GMT on May 21, 2008
357. StormJunkie 8:24 PM GMT on May 21, 2008
Well if that ain't the pot calling the kettle black Drak!


:~)
Member Since: October 28, 2006 Posts: 57 Comments: 30157
359. hurricane23
4:22 PM EDT on May 21, 2008
.
Member Since: May 14, 2006 Posts: 8 Comments: 13707
358. pearlandaggie
8:23 PM GMT on May 21, 2008
352. even an approved image has to be reapproved if you make it the default (at least that's what i read)....
Member Since: September 14, 2007 Posts: 3 Comments: 3963
357. StormJunkie
8:23 PM GMT on May 21, 2008
Well if that ain't the pot calling the kettle black Drak!
Member Since: August 17, 2005 Posts: 26 Comments: 16295
356. sporteguy03
8:20 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.
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355. smmcdavid
3:22 PM CDT on May 21, 2008
Good to know Drak. Thanks.
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354. StormJunkie
8:21 PM GMT on May 21, 2008
Ok, time to get in the chair, see y'all in a little while
Member Since: August 17, 2005 Posts: 26 Comments: 16295
352. 69Viking
3:17 PM CDT on May 21, 2008
333. smmcdavid

Thanks, will do. Seems strange though because the photo I uploaded already shows approved.
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351. StormJunkie
8:19 PM GMT on May 21, 2008
Come on guys, that was funny!

No offense 23, just a good laugh for all :~)
Member Since: August 17, 2005 Posts: 26 Comments: 16295
350. Patrap
3:20 PM CDT on May 21, 2008
4 panel GOM

Photobucket
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349. smmcdavid
3:19 PM CDT on May 21, 2008
One big, happy, disfunctional family. How sweet!
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348. pearlandaggie
8:18 PM GMT on May 21, 2008
341. oh, oh, oh! it was just a joke...c'mon man! :)
Member Since: September 14, 2007 Posts: 3 Comments: 3963
346. StormJunkie
8:15 PM GMT on May 21, 2008
69, if you hit modify comment and then copy everything in your post that was supposed to have the image, paste it in a WUmail to me and I may be able to help. Got to hurry though I am sitting in the barber shop now waiting.

MichFan, I like my stalled fronts every year :~) They are always interesting to watch.

That latest phase shows the system going warm core as well so, imvho it will be the best thing in the ATL to watch over the next few days. That said, it is likely to not amount to much except fun blob watching
Member Since: August 17, 2005 Posts: 26 Comments: 16295
345. atmoaggie
8:16 PM GMT on May 21, 2008
Foreign car, cramped engine compartment, but other then that it went great today pat

I actually had a foreign car that had a stud to hang the starter on and a bolt hole on the opposite side. Thought that was the coolest/simplest thing I ever saw. Why not have a large bolt to hang the 40 pounds on while you line up the other pair of holes?
Member Since: August 16, 2007 Posts: 6 Comments: 12461
344. Michfan
3:17 PM CDT on May 21, 2008


Not hurricane related but impressive to say the least. Something nice to look at at.
Member Since: September 7, 2007 Posts: 0 Comments: 1679
343. hurricane23
4:16 PM EDT on May 21, 2008
332. StormJunkie 4:14 PM EDT on May 21, 2008
I see 23 is holding top secret data again...Kinda like ST never being able to tell us how he got his info :~)

SJ you are a trip brother for real....It seems you always got something smart for me lol.Its all good man.

Member Since: May 14, 2006 Posts: 8 Comments: 13707
342. Weather456
4:14 PM AST on May 21, 2008
332. StormJunkie 4:14 PM AST on May 21, 2008

Stormtop's idea of top secret data was his washing machine in his basement forecasting center that when turned on, the spin gave similar resemblance of tropical cyclones seen on computer models.
Member Since: July 24, 2005 Posts: 407 Comments: 19076
341. Floodman
8:15 PM GMT on May 21, 2008
325. pearlandaggie 8:11 PM GMT on May 21, 2008
322. I'm hoping for lots of DUST and SHEAR! :)

(that's not my prediction, just HOPING for a CHANGE)



That's not funny, Pearland...you sound a lot like somneone I just ignored LOL
Member Since: August 2, 2006 Posts: 10 Comments: 9922
340. Drakoen
8:16 PM GMT on May 21, 2008
337. atmoaggie 8:16 PM GMT on May 21, 2008
I see 23 is holding top secret data again...Kinda like ST never being able to tell us how he got his info

Now that is hitting below the belt...not nice.


lol its called a "low blow". We already know how that season went... :~)
Member Since: October 28, 2006 Posts: 57 Comments: 30157
339. Drakoen
8:16 PM GMT on May 21, 2008
trade wind anomalies show that the trades have died down the normal levels.
Member Since: October 28, 2006 Posts: 57 Comments: 30157
338. hurricane23
4:12 PM EDT on May 21, 2008
Looks like the reason season for south florida will be on hold for a few days but it maybe around the corner.

Piece of the NWS updated 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

Almost forgot its 95 degrees outside the house!Man thats crazy hot especially if your out during the day like myself.
Member Since: May 14, 2006 Posts: 8 Comments: 13707
337. atmoaggie
8:15 PM GMT on May 21, 2008
I see 23 is holding top secret data again...Kinda like ST never being able to tell us how he got his info

Now that is hitting below the belt...not nice.
Member Since: August 16, 2007 Posts: 6 Comments: 12461
336. StormJunkie
8:14 PM GMT on May 21, 2008
Foreign car, cramped engine compartment, but other then that it went great today pat. Didn't get rained on and I already had it most of the way out yesterday, so today was pretty easy. Thanks for asking!
Member Since: August 17, 2005 Posts: 26 Comments: 16295
335. Michfan
3:12 PM CDT on May 21, 2008
SJ is loving his stalled front!
Member Since: September 7, 2007 Posts: 0 Comments: 1679
334. pearlandaggie
8:13 PM GMT on May 21, 2008
329. you have to make it the default image (check box near bottom) and it has to be approved by admin...then it will show up.
Member Since: September 14, 2007 Posts: 3 Comments: 3963
333. smmcdavid
3:13 PM CDT on May 21, 2008
69viking... I think it might have to be approved before it shows up. Give it 24 hours before trying again.
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332. StormJunkie
8:11 PM GMT on May 21, 2008
I see 23 is holding top secret data again...Kinda like ST never being able to tell us how he got his info :~)
Member Since: August 17, 2005 Posts: 26 Comments: 16295
331. IKE
3:12 PM CDT on May 21, 2008
326. Patrap 3:11 PM CDT on May 21, 2008
vary Interesting....


That moisture is headed NE.
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330. Patrap
3:13 PM CDT on May 21, 2008
How dat starter go SJ?
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329. 69Viking
3:05 PM CDT on May 21, 2008
Will anyone help? How do I put an image in to make the "Click to add an Image" box go away? I clicked to add an image and added one but it's not showing up.
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328. Patrap
3:13 PM CDT on May 21, 2008
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327. StormJunkie
8:08 PM GMT on May 21, 2008
Afternoon all :~)

I see the GFS is still toying with the stalled front and what may or may not form along it. Weak system, but interesting positioning. Phase Evbolution 12z GFS

You can find the phase evolution page as well as other model, sat imagery, marine data, and wind data pages from here.
Member Since: August 17, 2005 Posts: 26 Comments: 16295
326. Patrap
3:08 PM CDT on May 21, 2008
vary Interesting....


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325. pearlandaggie
8:09 PM GMT on May 21, 2008
322. I'm hoping for lots of DUST and SHEAR! :)

(that's not my prediction, just HOPING for a CHANGE)
Member Since: September 14, 2007 Posts: 3 Comments: 3963
323. hurricane23
4:08 PM EDT on May 21, 2008
Pat whats the joke for?

I just answered your question above.Boy iam telling you.
Member Since: May 14, 2006 Posts: 8 Comments: 13707
322. Weather456
4:07 PM AST on May 21, 2008
313. hurricane23 3:57 PM AST on May 21, 2008
Thats correct 456!There outlook should be out around 11:15am tommorow morning.Look for Jeff Masters to also blog on his new thoughts for this season.


Looking forward to thier thoughts about 08.
Member Since: July 24, 2005 Posts: 407 Comments: 19076
321. Patrap
3:06 PM CDT on May 21, 2008
Learn More about RAMSDIS here Link

RAMM Advanced Meteorological Satellite Demonstration and Interpretation System
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320. pearlandaggie
8:06 PM GMT on May 21, 2008
LOL Patrap and Atmoaggie! :)
Member Since: September 14, 2007 Posts: 3 Comments: 3963
319. IKE
3:05 PM CDT on May 21, 2008
Water temps along the northern GOM coast are approaching 80 degrees.........

Panama City, FL 79.0 °F
Apalachicola, FL 79.0 °F
Grand Isle, LA 78.1 °F
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318. hurricane23
4:00 PM EDT on May 21, 2008
PAT they obtain there data under a special agreement from the European satellite agency (EUMETSAT).
Member Since: May 14, 2006 Posts: 8 Comments: 13707
317. Drakoen
8:04 PM GMT on May 21, 2008
Bahamas and Caribbean shaping up real nicely and the 26C isotherm has moved to the north and east in the Central Atlantic.


Member Since: October 28, 2006 Posts: 57 Comments: 30157
316. Patrap
3:01 PM CDT on May 21, 2008
That Movie dosent come out to June 20th I thought?



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315. atmoaggie
8:00 PM GMT on May 21, 2008
There is a reason for that which i liked to
elaborate on but cant.There are alot of other views out there SEE HERE.


Cant? LOL

NSA protocol 23?


He could tell us but then he would have to ask Dr. Masters for all of our IP addresses to hunt us down and...
Member Since: August 16, 2007 Posts: 6 Comments: 12461
314. Patrap
2:59 PM CDT on May 21, 2008
306. Weather456 3:52 PM EDT on May 21, 2008
300. Drakoen 3:48 PM AST on May 21, 2008
RAMSDIS shut of their west Africa view to the public again.

There is a reason for that which i liked to
elaborate on but cant.There are alot of other views out there SEE HERE.



Cant? LOL

NSA protocol 23?
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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.