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

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

Share this Blog
2
+

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

Reader Comments

Comments will take a few seconds to appear.

Post Your Comments

Please sign in to post comments.

or Join

Not only will you be able to leave comments on this blog, but you'll also have the ability to upload and share your photos in our Wunder Photos section.

Display: 0, 50, 100, 200 Sort: Newest First - Order Posted

Viewing: 314 - 264

Page: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20Blog Index

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?
Member Since: July 3, 2005 Posts: 427 Comments: 129439
313. hurricane23
3:55 PM EDT 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.
Member Since: Posts: Comments:
312. hurricane23
3:55 PM EDT 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.
Member Since: Posts: Comments:
311. Patrap
2:55 PM CDT on May 21, 2008
GOES-12 GOM View in 3 Channels Link
Member Since: July 3, 2005 Posts: 427 Comments: 129439
310. Weather456
3:52 PM AST on May 21, 2008
Anything thoughts on what NOAA will put out tommorow?

Did I miss something? NOAA is issuing their hurricane season outlook 2mr?
Member Since: July 24, 2005 Posts: 407 Comments: 19076
309. Patrap
2:52 PM CDT on May 21, 2008
Nothing is Shut off from the Public.
Get a Grip.

RAMSDIS isnt the Sat owner..or operator.
Member Since: July 3, 2005 Posts: 427 Comments: 129439
308. atmoaggie
7:51 PM GMT on May 21, 2008
I can understand, there has been some inconsistency in the model track...basically jumping all over the place....but the GFS has been showing something down there for days now. Confidence still remains low since its only one model showing it.

Also, NAM and GFS both put a weak high there, at times, with counter-clockwise flow around it...doesn't lend itself much confidence that way.
Member Since: August 16, 2007 Posts: 6 Comments: 12463
307. Patrap
2:52 PM CDT on May 21, 2008
Could vary well be
Member Since: July 3, 2005 Posts: 427 Comments: 129439
306. Weather456
3:49 PM AST 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.


Yep but the NRL Navy and Emusat, makes up for that.
Member Since: July 24, 2005 Posts: 407 Comments: 19076
305. TEXASYANKEE43
7:48 PM GMT on May 21, 2008
295. Patrap 7:45 PM GMT on May 21, 2008
I feel so Negatively tilted.



Is that a double negative?
Member Since: Posts: Comments:
304. hurricane23
3:50 PM EDT on May 21, 2008
Anything thoughts on what NOAA will put out tommorow?

Iam thinking they might go with a slightly above average year.No matter what the numbers say its all about the steering pattern which i might add has been on our side the past 2 seasons.Could we go for a 3rd year?There's a good chance the storms will be returning to the Southeast in 08.That's not to say it's going to be another 2005 though....hopefully we'll never see a season as destructive as 2005 in our lifetime. But it will be fun for tropical weather enthusiasts.
Member Since: Posts: Comments:
303. Floodman
7:49 PM GMT on May 21, 2008
300. Drakoen

Someone had noted that earlier...is this a regular occurence for them?
Member Since: Posts: Comments:
302. pearlandaggie
7:49 PM GMT on May 21, 2008
300. Why do they do that? I noticed it, too, but I don't think I've ever heard the reason why...
Member Since: September 14, 2007 Posts: 3 Comments: 3963
301. IKE
2:48 PM CDT on May 21, 2008
300. Drakoen 2:48 PM CDT on May 21, 2008
RAMSDIS shut of their west Africa view to the public again.


Yeah...that was posted earlier.
Member Since: Posts: Comments:
300. Drakoen
7:47 PM GMT on May 21, 2008
RAMSDIS shut of their west Africa view to the public again.
Member Since: Posts: Comments:
299. atmoaggie
7:47 PM GMT on May 21, 2008
One might call this turning with a L somewhere near Oaxaca...but is it significant?

Member Since: August 16, 2007 Posts: 6 Comments: 12463
297. Drakoen
7:44 PM GMT on May 21, 2008
292. Floodman 7:44 PM GMT on May 21, 2008
271. Drakoen

How are you Drak?

They've been building something there at around a week for the last two days...


I'm good! You?
Member Since: Posts: Comments:
296. HIEXPRESS
3:38 PM EDT on May 21, 2008
Central Fl Fire Weather Warning. We need this moisture now. Back to work.
Member Since: October 13, 2005 Posts: 4 Comments: 2156
295. Patrap
2:44 PM CDT on May 21, 2008
I feel so Negatively tilted.
Member Since: July 3, 2005 Posts: 427 Comments: 129439
294. Patrap
2:44 PM CDT on May 21, 2008
Hurricanes

Latest Storm Images and Data From NASA Link

Meet Scott Braun: Hurricane Computer Modeler and Research Meteorologist Link

Scott studied the effects of vertical wind shear (a rapid change in wind speed or direction with height) on hurricanes using the MM5 Forecast model to simulate 1998's Hurricane Bonnie and 2001's Hurricane Erin. Scott also noticed that the vortex (the swirling wind area) of the hurricane tilted when there was wind shear.
Member Since: July 3, 2005 Posts: 427 Comments: 129439
293. Drakoen
7:43 PM GMT on May 21, 2008
291. Weather456 7:43 PM GMT on May 21, 2008
I can understand, there has been some inconsistency in the model track...basically jumping all over the place....but the GFS has been showing something down there for days now. Confidence still remains low since its only one model showing it.


Yea, I agree.
Member Since: Posts: Comments:
292. Floodman
7:33 PM GMT on May 21, 2008
271. Drakoen

How are you Drak?

They've been building something there at around a week for the last two days...
Member Since: Posts: Comments:
291. Weather456
3:39 PM AST on May 21, 2008
I can understand, there has been some inconsistency in the model track...basically jumping all over the place....but the GFS has been showing something down there for days now. Confidence still remains low since its only one model showing it.
Member Since: July 24, 2005 Posts: 407 Comments: 19076
290. cchsweatherman
3:41 PM EDT on May 21, 2008
284. Weather456 3:39 PM EDT on May 21, 2008
271. Drakoen 3:32 PM AST on May 21, 2008
I don't understand Jeff Masters saying that the GFS has been inconsistent with development in the Caribbean. That being said the GFS 12z continues to show development in the Southern Western Caribbean in a week.

Ur not the only one.


I must join your group and agree with you Drak. It has become very interesting now that it has begun to appear that the ECMWF is beginning to join the GFS.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
289. KEEPEROFTHEGATE (Mod)
3:39 PM EDT on May 21, 2008
hey doc can you find some help for help4u i can seem to find any lol
good to see ya post hope your winter was relaxing
Member Since: July 15, 2006 Posts: 176 Comments: 55649
288. tornadofan
7:40 PM GMT on May 21, 2008
Post 277 - thanks!
Member Since: Posts: Comments:
287. JimNtexas
7:39 PM GMT on May 21, 2008
Whatever direction the change is, global warming is the cause.
Member Since: Posts: Comments:
286. atmoaggie
7:39 PM GMT on May 21, 2008
Bay of Camp buoy plot for the last 5 days shows lowering pressure, but winds are stuggling to pick up. 9 knots is pretty average for there...

Member Since: August 16, 2007 Posts: 6 Comments: 12463
285. Drakoen
7:39 PM GMT on May 21, 2008
283. IKE 7:38 PM GMT on May 21, 2008
271. Drakoen 2:32 PM CDT on May 21, 2008
I don't understand Jeff Masters saying that the GFS has been inconsistent with development in the Caribbean. That being said the GFS 12z continues to show development in the Southern Western Caribbean in a week.

Did you see the 12Z ECMWF?

Link


Yes, I saw the ECMWF 12z run.
Member Since: Posts: Comments:
284. Weather456
3:37 PM AST on May 21, 2008
271. Drakoen 3:32 PM AST on May 21, 2008
I don't understand Jeff Masters saying that the GFS has been inconsistent with development in the Caribbean. That being said the GFS 12z continues to show development in the Southern Western Caribbean in a week.


Ur not the only one.
Member Since: July 24, 2005 Posts: 407 Comments: 19076
283. IKE
2:36 PM CDT on May 21, 2008
271. Drakoen 2:32 PM CDT on May 21, 2008
I don't understand Jeff Masters saying that the GFS has been inconsistent with development in the Caribbean. That being said the GFS 12z continues to show development in the Southern Western Caribbean in a week.


Did you see the 12Z ECMWF?

Link
Member Since: Posts: Comments:
282. smmcdavid
2:36 PM CDT on May 21, 2008
I'm just now getting back in the habit... my semester is over and staying home all summer with my son. So, I'll have some free time to annoy everyone!
Member Since: Posts: Comments:
281. Drakoen
7:35 PM GMT on May 21, 2008
273. smmcdavid 7:33 PM GMT on May 21, 2008
Hi Drak... nice of you to join us today.


Hello! I'm always here everyday around this time.
Member Since: Posts: Comments:
280. Weather456
3:20 PM AST on May 21, 2008
224. Floodman 2:54 PM AST on May 21, 2008
220. Weather456

I believe it's 6, 456...does anyone know what the average is for this yime of year (African waves, that is)


On average, there are about 60 tropical waves annually between May and December.

60/8 months = 7.5 per month or 7-8 tropical waves per month

This is superficial, however. It does not account for the varying frequency between months. For example, May and December accounts for the least number of tropical waves, while August and September accounts for the highest. On a typical surface map in May, there maybe 2-3 tropical waves at a time but in August, there maybe more than 5 at a time.
Member Since: July 24, 2005 Posts: 407 Comments: 19076
279. pearlandaggie
7:34 PM GMT on May 21, 2008
272. smooth....just smooth! LOL
Member Since: September 14, 2007 Posts: 3 Comments: 3963
278. Buhdog
7:34 PM GMT on May 21, 2008
Do we have any bouy readings going down downn there?
Member Since: Posts: Comments:
277. JeffMasters (Admin)
3:34 PM EDT on May 21, 2008
4. tornadofan wrote:

Nice blog about shear. My question - will increased shear lead to greater and more frequent tornado outbreaks across the US?


Global warming is expected to reduce wind shear across Tornado Alley in the U.S., which would tend to reduce tornado activity. However, instability is expected to increase, which should more than offset the shear decrease. I've got a blog entry I posted in February that talks about the latest research on this issue.

Jeff Masters

276. pearlandaggie
7:31 PM GMT on May 21, 2008
270. LOL!

well, my last summer there we had 43 consecutive days over 100F!
Member Since: September 14, 2007 Posts: 3 Comments: 3963
275. 786
7:31 PM GMT on May 21, 2008
I did, lol I guess its been an on-going downhill ride. Well I guess ignore is in order for that one then.
Member Since: Posts: Comments:
274. 69Viking
2:31 PM CDT on May 21, 2008
Ok, I'm a computer guy but can't figure out how to assign an icon to my posts, can anyone help, I feel pretty stupid.
Member Since: Posts: Comments:
273. smmcdavid
2:32 PM CDT on May 21, 2008
Hi Drak... nice of you to join us today.
Member Since: Posts: Comments:
272. KEEPEROFTHEGATE (Mod)
3:31 PM EDT on May 21, 2008
theres no help4u
Member Since: July 15, 2006 Posts: 176 Comments: 55649
271. Drakoen
7:30 PM GMT on May 21, 2008
I don't understand Jeff Masters saying that the GFS has been inconsistent with development in the Caribbean. That being said the GFS 12z continues to show development in the Southern Western Caribbean in a week.
Member Since: Posts: Comments:
270. atmoaggie
7:24 PM GMT on May 21, 2008
233. we lived in DFW for almost 5 years. 97F at 10PM is INSANE! the first summer there, the temperature hit 117F!

When I was at A&M I spent 2 summers tending to research cotton in the fields. Being research cotton, no machinery allowed. Chop the cotton with a hoe, pick by hand. We saw plenty of 110+ days and out in it from 7 to 5.

Most days, I could probably cure a ham with the dried salt on my forehead.
Member Since: August 16, 2007 Posts: 6 Comments: 12463
269. TEXASYANKEE43
7:30 PM GMT on May 21, 2008
267.

You must have missed the last week or so....
Member Since: Posts: Comments:
268. Patrap
2:29 PM CDT on May 21, 2008
Im a Cajun Cher..I have Nutria and muskrats fer dat chore,LOL
Member Since: July 3, 2005 Posts: 427 Comments: 129439
267. 786
7:28 PM GMT on May 21, 2008
lol, well did see some rational in the very first post but its gone downhill from there
Member Since: Posts: Comments:
266. pearlandaggie
7:26 PM GMT on May 21, 2008
260. Amen, brotha! Preachin' to the choir!
Member Since: September 14, 2007 Posts: 3 Comments: 3963
265. smmcdavid
2:27 PM CDT on May 21, 2008
264. love it!
Member Since: Posts: Comments:
264. 786
7:25 PM GMT on May 21, 2008
ok help now you are going overboard...one man ain't gonna do shizznits
Member Since: Posts: Comments:

Viewing: 314 - 264

Page: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20Blog Index

Top of Page

About

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

Local Weather

Overcast
32 °F
Overcast