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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564. IKE
6:00 PM CDT on May 21, 2008
560. TheWeatherMan504 5:58 PM CDT on May 21, 2008
550. IKE 10:53 PM GMT on May 21, 2008
18Z GFSLink

Why dont you go to the NWS to see the models??Link


That's the same page I got it from.
Member Since: June 9, 2005 Posts: 23 Comments: 37860
563. cchsweatherman
6:58 PM EDT on May 21, 2008
559. IKE 6:58 PM EDT on May 21, 2008
553. cchsweatherman 5:55 PM CDT on May 21, 2008
545. TheWeatherMan504 6:49 PM EDT on May 21, 2008
It makes it go in the gulf this time with a strong High to the East.

Where do you see the system going into the Gulf? It still maintains a track across Central Cuba and just barely past South Florida, not in the Gulf.

At the end of the run.........


Sorry there TWM504. I jumped the gun before saying that. I made a mistake.
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562. pearlandaggie
10:59 PM GMT on May 21, 2008
atmo....i'm afraid to even ask.......
Member Since: September 14, 2007 Posts: 3 Comments: 3963
561. pearlandaggie
10:57 PM GMT on May 21, 2008
557. that darn shear means lots of wave in the GOM....that sux!
Member Since: September 14, 2007 Posts: 3 Comments: 3963
560. TheWeatherMan504
10:56 PM GMT on May 21, 2008
550. IKE 10:53 PM GMT on May 21, 2008
18Z GFSLink


Why dont you go to the NWS to see the models??Link
Member Since: Posts: Comments:
559. IKE
5:57 PM CDT on May 21, 2008
553. cchsweatherman 5:55 PM CDT on May 21, 2008
545. TheWeatherMan504 6:49 PM EDT on May 21, 2008
It makes it go in the gulf this time with a strong High to the East.

Where do you see the system going into the Gulf? It still maintains a track across Central Cuba and just barely past South Florida, not in the Gulf.


At the end of the run.........
Member Since: June 9, 2005 Posts: 23 Comments: 37860
558. atmoaggie
10:56 PM GMT on May 21, 2008
Great spelling and grammar

OK, you guys didn't get it. Ummm, I agree.

If you got some of the junk I do in my inbox...
Member Since: August 16, 2007 Posts: 6 Comments: 12463
556. Drakoen
10:56 PM GMT on May 21, 2008
553. cchsweatherman 10:55 PM GMT on May 21, 2008
545. TheWeatherMan504 6:49 PM EDT on May 21, 2008
It makes it go in the gulf this time with a strong High to the East.

Where do you see the system going into the Gulf? It still maintains a track across Central Cuba and just barely past South Florida, not in the Gulf.


18z run it shows the system passing South Florida and into the Gulf. Why even worry about track. The only thing to watch is development.
Member Since: October 28, 2006 Posts: 57 Comments: 30833
555. CaneAddict
10:54 PM GMT on May 21, 2008

528. KEEPEROFTHEGATE 10:39 PM GMT on May 21, 2008
cane addict we will get what ever dev or not devs in the gom area in weather nothing can be ruled out and a non tropical sys is very possible they like high shear
yes sj imo backdoor is going to occur interesting to say the least
Action: | Ignore User


Great spelling and grammar.

Member Since: October 5, 2007 Posts: 3 Comments: 2151
554. atmoaggie
10:53 PM GMT on May 21, 2008
Green or Amber were the only graphics flavors for your ASCII creations.

Can you believe I still have about 30 machines like that? No mouse, no graphics...all intentional. 4-3.0GHz processor cores, each, for raw number crunching only. Difference between then and now is the sheer power.
Member Since: August 16, 2007 Posts: 6 Comments: 12463
553. cchsweatherman
6:52 PM EDT on May 21, 2008
545. TheWeatherMan504 6:49 PM EDT on May 21, 2008
It makes it go in the gulf this time with a strong High to the East.


Where do you see the system going into the Gulf? It still maintains a track across Central Cuba and just barely past South Florida, not in the Gulf.

Member Since: Posts: Comments:
552. Drakoen
10:54 PM GMT on May 21, 2008
SJ, i'm all out of jokes now lol. Maybe some will come back to me later :~)
Member Since: October 28, 2006 Posts: 57 Comments: 30833
551. pearlandaggie
10:52 PM GMT on May 21, 2008
i never even owned a PC or computer of any kind (abacus excepted :) ) until 1998...and I STILL have that one and it still works!
Member Since: September 14, 2007 Posts: 3 Comments: 3963
550. IKE
5:52 PM CDT on May 21, 2008
18Z GFSLink
Member Since: June 9, 2005 Posts: 23 Comments: 37860
549. KEEPEROFTHEGATE (Mod)
6:50 PM EDT on May 21, 2008
i remember that sj com 64 my first intro to comp age that and dos
and bbs was right (bulletin board service)
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548. StormJunkie
10:50 PM GMT on May 21, 2008
Not me pearl, I had a slight pc hiatus in the mid 90s, and i was never really one of the "cool" guys anyway ☺
Member Since: August 17, 2005 Posts: 26 Comments: 16874
547. atmoaggie
10:48 PM GMT on May 21, 2008
TRS-80

Whoa! Hey, no cussin in here, this is a family-oriented blog.
Member Since: August 16, 2007 Posts: 6 Comments: 12463
546. pearlandaggie
10:49 PM GMT on May 21, 2008
the COOL guys in '96 had to partition their HUGE 1GB hard drives.....were you that guy? :)
Member Since: September 14, 2007 Posts: 3 Comments: 3963
545. TheWeatherMan504
10:46 PM GMT on May 21, 2008
It makes it go in the gulf this time with a strong High to the East.
Member Since: Posts: Comments:
544. pearlandaggie
10:47 PM GMT on May 21, 2008
hey, the state-of-the-art PCs when I entered college were Pentium 90s! they told us we needed to setup our email account, and coming from a small town I though, "But I already have a bank account. Why do I need another account?"

LOL
Member Since: September 14, 2007 Posts: 3 Comments: 3963
543. StormJunkie
10:46 PM GMT on May 21, 2008
No way peraland, and as funny as it sounds, I was even blogging to an extent back then. Had the old put your phone on the modem deal and you could log into BBS's, At least I think that is what they were called.

1 meg of memory in the early 80's was like 5 to 10k Can you imagine!
Member Since: August 17, 2005 Posts: 26 Comments: 16874
541. KEEPEROFTHEGATE (Mod)
6:44 PM EDT on May 21, 2008
what message drak
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540. pearlandaggie
10:45 PM GMT on May 21, 2008
i thought I WAS the only one that remembered the TRASH-80! :)
Member Since: September 14, 2007 Posts: 3 Comments: 3963
539. pearlandaggie
10:44 PM GMT on May 21, 2008
534....atmo-the GOD-aggie...NOW you have my full ATTENTION! :)
Member Since: September 14, 2007 Posts: 3 Comments: 3963
538. StormJunkie
10:44 PM GMT on May 21, 2008
JF, I was thinking more of the CoCo or TRS-80 :~)
Member Since: August 17, 2005 Posts: 26 Comments: 16874
537. Drakoen
10:44 PM GMT on May 21, 2008
18z is still bent on the Caribbean system developing brings it close to Florida with a big high in the east.
Member Since: October 28, 2006 Posts: 57 Comments: 30833
536. StormJunkie
10:42 PM GMT on May 21, 2008
Watch out Drak, pearl is walking away with your comedy show!

Thank you KOTG. At least we are getting some more eyes on it now :~))
Member Since: August 17, 2005 Posts: 26 Comments: 16874
534. atmoaggie
10:41 PM GMT on May 21, 2008
chlorophyll??? more like BOROPHYLL! :)

How would like to have a clear map of where to find all of the Mackerel, Marlin, Sails, and dolphin you could stand?
Member Since: August 16, 2007 Posts: 6 Comments: 12463
532. StormJunkie
10:40 PM GMT on May 21, 2008
Just watched a discover show on the pc coming of age era.

Hard to believe we have come so far from the days of labor intensive DOS programing.

RUN
Member Since: August 17, 2005 Posts: 26 Comments: 16874
531. Drakoen
10:41 PM GMT on May 21, 2008
528. KEEPEROFTHEGATE 10:39 PM GMT on May 21, 2008
cane addick we will get what ever dev or not devs in the gom area in weather nothing can be ruled out and a non tropical sys is very possible they like high shear
yes sj imo backdoor is going to occur interesting to say the least


Your subliminal message isn't that well hidden...
Member Since: October 28, 2006 Posts: 57 Comments: 30833
530. pearlandaggie
10:40 PM GMT on May 21, 2008
Running a home-brew oceanic chlorophyll front detection code many times over with minor tweaks to get the best possible result.

chlorophyll??? more like BOROPHYLL! :)

/Billy Madison
Member Since: September 14, 2007 Posts: 3 Comments: 3963
529. pearlandaggie
10:39 PM GMT on May 21, 2008
526. LOL

10 GOTO 20
20 GOTO 10

:)
Member Since: September 14, 2007 Posts: 3 Comments: 3963
528. KEEPEROFTHEGATE (Mod)
6:31 PM EDT on May 21, 2008
cane addict we will get what ever dev or not devs in the gom area in weather nothing can be ruled out and a non tropical sys is very possible they like high shear
yes sj imo backdoor is going to occur interesting to say the least
Member Since: Posts: Comments:
527. atmoaggie
10:36 PM GMT on May 21, 2008
pat...atmo..you guys are STILL here? and i thought I had no life! :)

Still working. Running a home-brew oceanic chlorophyll front detection code many times over with minor tweaks to get the best possible result.

Front detection in image processing of geophysical data is a pain...

Every time I change a variable and run it, I have about 180 seconds to do something useful or look here. Nothing useful happens that quickly.
Member Since: August 16, 2007 Posts: 6 Comments: 12463
526. StormJunkie
10:37 PM GMT on May 21, 2008
525.

I refer you to 518.
Member Since: August 17, 2005 Posts: 26 Comments: 16874
525. Drakoen
10:35 PM GMT on May 21, 2008
Still nothing on the GFS 18z...
Member Since: October 28, 2006 Posts: 57 Comments: 30833
523. pearlandaggie
10:34 PM GMT on May 21, 2008
pat...atmo..you guys are STILL here? and i thought I had no life! :)
Member Since: September 14, 2007 Posts: 3 Comments: 3963
522. atmoaggie
10:30 PM GMT on May 21, 2008
Yeah, JFlorida, but the wind is far from interesting. Normal, and even weak, for there...


Open this text file for last month's historical records, look at column 7 and multiply by 2.2369 (to get mph). 15 to 20 mph is perfectly normal at this buoy.

Pressure keeps falling, but apparently the pressure gradient hasn't changed much (or maybe the anemometer is busted...hmmm)
Member Since: August 16, 2007 Posts: 6 Comments: 12463
521. HIEXPRESS
6:22 PM EDT on May 21, 2008
511. JFLORIDA 6:21 PM EDT
Some showers are starting to build Mid Gulf.

But for the shear, it might look like this>
http://www.wunderground.com/hurricane/at197601.asp
Member Since: October 13, 2005 Posts: 4 Comments: 2156
520. NorthxCakalaky
10:23 PM GMT on May 21, 2008
So in the future there could be only 1storm per season in the Atlantic Ocean and perhaps it being stronger than a cat5.

Could this later mean super-storms like in other inhabited planets?

STOP GLOBAL WARMING
Member Since: Posts: Comments:
519. CaneAddict
10:24 PM GMT on May 21, 2008
500. KEEPEROFTHEGATE 10:09 PM GMT on May 21, 2008
you keep beleiving that cane addick


Whats there to believe? It's true shear is to high for tropical development....Why the disagreement? Look below, Shear is 40Knots and higher....I'm not saying anything towards you i am just stating in general shear is to high for development, However shear is decreasing.

Image Hosted by ImageShack.us

Member Since: October 5, 2007 Posts: 3 Comments: 2151
518. StormJunkie
10:23 PM GMT on May 21, 2008
This should raise excitement in here...

Latest GFS is showing the E Coast low heading back towards S Fla. Seems to have a little larger wind field with it too, when it is over the Bahamas.

Folks, the most interesting game in the next 7 days is this front that is going to pull off shore in the next few days, at least imvho!
Member Since: August 17, 2005 Posts: 26 Comments: 16874
516. Drakoen
10:21 PM GMT on May 21, 2008
511. JFLORIDA 10:21 PM GMT on May 21, 2008
Station 42055 - Bay of Campeche

Wind Direction (WDIR): ESE ( 110 deg true )
Wind Speed Wind Speed (WSPD): 9.7 kts
Wind Gust (GST): 11.7 kts
Wave Height (WVHT): 2.0 ft
Dominant Wave Period (DPD): 5 sec
Average Period (APD): 3.6 sec
Atmospheric Pressure (PRES): 29.64 in
Pressure Tendency (PTDY): -0.06 in ( Falling )
Air Temperature (ATMP): 82.6 °F
Water Temperature (WTMP): 81.7 °F


Some showers are starting to build Mid Gulf.


The pressure reading is suspicious 1003mb. There is a low over the Yucatan Peninsula but it was around 1006mb according the NHC.
Member Since: October 28, 2006 Posts: 57 Comments: 30833
515. Patrap
5:22 PM CDT on May 21, 2008
Surrounded by water..and still a drought.
Member Since: July 3, 2005 Posts: 428 Comments: 129902
514. hurricane23
6:21 PM EDT on May 21, 2008
41 days today without a drop of rain in keywest.
Member Since: May 14, 2006 Posts: 8 Comments: 13841

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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.

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