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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714. KEEPEROFTHEGATE (Mod)
9:38 PM EDT on May 21, 2008
poppers over s ga again
Member Since: July 15, 2006 Posts: 178 Comments: 56059
713. StormJunkie
1:44 AM GMT on May 22, 2008
Nothings the LC press!
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712. CaneAddict
1:43 AM GMT on May 22, 2008
Night folks!
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711. presslord
9:43 PM EDT on May 21, 2008
it ain't the Lowcountry SJ...
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710. StormJunkie
1:42 AM GMT on May 22, 2008
Almost press

So how is Hotlanta?

Mail for you
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709. CaneAddict
1:41 AM GMT on May 22, 2008
Before i hit the bed, I want to post a very amazing video of how quick Hurricane Katrina EXPLODED over the loop eddy current! Also how fast it grew in size...in a matter of days.

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708. presslord
9:40 PM EDT on May 21, 2008
I'm in Atlanta tonight..no hurricane worries here....
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707. Floodman
1:38 AM GMT on May 22, 2008
I'll tell ya what, the weather can do whatever it wants in the midwest tomorrow, but early Friday I'm driving to St Louis, so I'm putting in an order for blue skies
Member Since: August 2, 2006 Posts: 10 Comments: 9922
706. pearlandaggie
1:39 AM GMT on May 22, 2008
i'm out too, folks...have a good evening
Member Since: September 14, 2007 Posts: 3 Comments: 3963
705. presslord
9:37 PM EDT on May 21, 2008
Sj...ARe you gone?
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704. pearlandaggie
1:35 AM GMT on May 22, 2008
later Atmo...i'll be looking for my copy of that BOROphyll map!
Member Since: September 14, 2007 Posts: 3 Comments: 3963
703. StormJunkie
1:34 AM GMT on May 22, 2008
BOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO

hehe...

Night atmo

Night pearland

:~)
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702. atmoaggie
1:34 AM GMT on May 22, 2008
I'm out. 3 year-old and 18-month-old a-calling.

Later, SJ...PlandAg, everyone.
Member Since: August 16, 2007 Posts: 6 Comments: 12463
701. pearlandaggie
1:32 AM GMT on May 22, 2008
alright, everyone! Stormjunkie's gone...now you can resume talking about him!

LOL

:)

'night SJ
Member Since: September 14, 2007 Posts: 3 Comments: 3963
700. atmoaggie
1:31 AM GMT on May 22, 2008
I hate a Headwind at 34K.

My arms Ice up sometimes too.


Knew you couldn't resist that. Just what I thought you might say.
Member Since: August 16, 2007 Posts: 6 Comments: 12463
699. Ivansrvivr
1:24 AM GMT on May 22, 2008
CC, the things you have mentioned all point towards the onset of the rainy season here. Among other things, It seems that the start of the FL (peninsula) rainy season often coincides with strong storms moving southward through AL/GA towards the Gulf Coast. That happened yesterday.
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698. StormJunkie
1:28 AM GMT on May 22, 2008
674...Holy Mackerel, another one jumps on board with the fabled E Coast low :~)

cchs, imvho that stalled front will be the most interesting feature over the next 5 days or so.

Ok, I am off for the night. See y'all tomorrow!

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697. Patrap
8:30 PM CDT on May 21, 2008
GOES-12 Atmospheric Imagery,Atlantic Basin Link
Member Since: July 3, 2005 Posts: 428 Comments: 129841
696. Patrap
8:29 PM CDT on May 21, 2008
I hate a Headwind at 34K.

My arms Ice up sometimes too.
Member Since: July 3, 2005 Posts: 428 Comments: 129841
695. atmoaggie
1:28 AM GMT on May 22, 2008
Fixn 2 get ugly

For whom? 767s going west?
Member Since: August 16, 2007 Posts: 6 Comments: 12463
694. pearlandaggie
1:26 AM GMT on May 22, 2008
688. just nit-picking (not trying to be an arse), but i'm not sure probabilities <1% constitute a "significant" increase. look at the area east of Maryland...not exactly in the MDR.
Member Since: September 14, 2007 Posts: 3 Comments: 3963
693. Patrap
8:26 PM CDT on May 21, 2008
If one just links the image. All is well.

I dropped a pen anyone seen it?
Its a Bic.
Blue,...Clear plastic.
Member Since: July 3, 2005 Posts: 428 Comments: 129841
692. HIEXPRESS
9:26 PM EDT on May 21, 2008
Fixn 2 get ugly
Member Since: October 13, 2005 Posts: 4 Comments: 2156
691. atmoaggie
1:26 AM GMT on May 22, 2008
Weird, cchs...this version hasn't. Wonder whom to believe now.

Dang HGW beat me to it.
Member Since: August 16, 2007 Posts: 6 Comments: 12463
690. HadesGodWyvern (Mod)
1:24 AM GMT on May 22, 2008


why is this map so different... i find this more reliable though that pinpoint the area with best tc potential
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689. pearlandaggie
1:21 AM GMT on May 22, 2008
Dang, the size of this pic really ruined the important data to be garnered from it. I forget, what is the html for pic size?

people can just right-click and click on "View Image" to see the pic better, then hit the back button.

i don't find it to be much of an issue if i do that.

(these instructions are for FireFox users)
Member Since: September 14, 2007 Posts: 3 Comments: 3963
688. cchsweatherman
9:23 PM EDT on May 21, 2008
Just wanted to post this. There has been a significant increase in potential for tropical development throughout the basin.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
687. cchsweatherman
9:16 PM EDT on May 21, 2008
Maybe JFlorida and Kepper were onto something with the disturbed weather in the Gulf of Mexico as convection has continued to increase across the region. If this maintains itself, South and Central Florida may get some late thunderstorms that could spark off.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
686. extreme236
1:17 AM GMT on May 22, 2008
West Caribbean shear

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685. atmoaggie
1:18 AM GMT on May 22, 2008
The shear in SW Kansas is setup just right for them things...you know what I mean



Dang, the size of this pic really ruined the important data to be garnered from it. I forget, what is the html for pic size?
Member Since: August 16, 2007 Posts: 6 Comments: 12463
684. KEEPEROFTHEGATE (Mod)
9:14 PM EDT on May 21, 2008
you may want to change your yellow to red taz
Member Since: July 15, 2006 Posts: 178 Comments: 56059
683. CaneAddict
1:14 AM GMT on May 22, 2008
Folks, I off to bed, Some reason i am very exhausted! Tomorrow looks to be very interesting with Jeff's thoughts on the season and NOAA's! Night all!

atmoaggie, you will have to slide by my house and pick that cash up ;) LOL!

Night!
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682. Weather456
8:58 PM AST on May 21, 2008
Just an observation or two...never totally discount features below 10N. I have been watching a tropical wave all day just off the coast of africa, that has been associated with cyclonic turning along its axis just around 7N. Showers and thunderstorms have diminsh tonight and development is not expected, but it was rather interesting how we think that storms cannot aquire turning below 10N. Not normal but at the same time not impossible.

Member Since: July 24, 2005 Posts: 407 Comments: 19076
681. KEEPEROFTHEGATE (Mod)
9:04 PM EDT on May 21, 2008
very good taz
lift inflow rtn flow combine with strong dynamics promotes good chances of severe weather
a wide spread high impacting event for the next couple of days taz
Member Since: July 15, 2006 Posts: 178 Comments: 56059
680. Tazmanian
1:07 AM GMT on May 22, 2008
i may want to book mark this site

Link
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679. all4hurricanes
1:06 AM GMT on May 22, 2008
For once there is no rain in the forecast here.
There is a very large low pressure system by the Philippians, it probably won't develop. 10 days till hurricane season. Each season and each storm has it's own personality. A new season is like meeting a new person.
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678. atmoaggie
1:09 AM GMT on May 22, 2008
...I'm in. Will this help?

Heck, yeah it will. Can you email it to me? ;-)
Member Since: August 16, 2007 Posts: 6 Comments: 12463
677. atmoaggie
1:06 AM GMT on May 22, 2008
Experimental thunderstorm probability almost certain...

Note that this only covers through 12 Z tomorrow (7am CDT)
Link
Member Since: August 16, 2007 Posts: 6 Comments: 12463
675. GBlet
8:04 PM CDT on May 21, 2008
Atmo, I'm ready!!! I'm from TX. 1979 will forever be with me. Greensberg last year scared the crap out of me.
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674. cchsweatherman
8:58 PM EDT on May 21, 2008
See the 18UTC GFs shows a very interesting scenario occuring in Florida as it seems a weak warm-core surface low will form in the Bahamas off the backdoor cold front and move into south-central Florida late Sunday into Monday then following with a tropical storm (maybe even weak Cat. 1 hurricane) moving over the Bahamas and then coming back over South Florida by the following week. Would be terrific news for the entire state.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
673. Ivansrvivr
12:59 AM GMT on May 22, 2008
There is occasional tropical or sub-tropical development in the W-Caribbean that occurs with the arrival of the Florida "rainy season". It takes a soaking to start the cycle of daily afternoon rains here and it usually comes from the south. Hopefully that will put an end to all the "muckfire" smoke here.
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672. KEEPEROFTHEGATE (Mod)
9:01 PM EDT on May 21, 2008
it is done
Member Since: July 15, 2006 Posts: 178 Comments: 56059
671. GBlet
8:02 PM CDT on May 21, 2008
SSSHHH, I'm busy painting the bullseye!!!
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670. atmoaggie
1:02 AM GMT on May 22, 2008
Taz, what a mouthful! I live in Great Bend,Ks. I can also tell that it's coming too

I would leave the WX radio on if I were you.
Member Since: August 16, 2007 Posts: 6 Comments: 12463
668. GBlet
7:54 PM CDT on May 21, 2008
Taz, what a mouthful! I live in Great Bend,Ks. I can also tell that it's coming too.
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667. atmoaggie
12:59 AM GMT on May 22, 2008
This is why...strong flow bringing Gulf moisture-a-plenty to the dry line...

Member Since: August 16, 2007 Posts: 6 Comments: 12463
666. bappit
12:57 AM GMT on May 22, 2008
Akron CO 33mph sustained and gusts to 56

no thunderstorm nearby, that's just the wind
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665. CaneAddict
12:58 AM GMT on May 22, 2008
662. HurricaneJosh20 12:57 AM GMT on May 22, 2008
maybe it'll bring rain to us parched ppl in Fl.....can wish can't we? lol
Action: | Ignore User


I am also located in Florida, We can wish all day, Lets hope it comes true! My name is also Josh by the way.
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664. atmoaggie
12:56 AM GMT on May 22, 2008
Probability outlook pretty heady, too

(Probablility of severe wx...wind/hail/nados within 25 miles of a point)

Member Since: August 16, 2007 Posts: 6 Comments: 12463

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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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