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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864. Patrap
11:10 AM CDT on May 22, 2008
The Climate Prediction Center outlook calls for considerable activity with a 65 percent probability of an above normal season and a 25 percent probability of a near normal season. This means there is a 90 percent chance of a near or above normal season.

The climate patterns expected during this years hurricane season have in past seasons produced a wide range of activity and have been associated with both near-normal and above-normal seasons. For 2008, the outlook indicates a 60 to 70 percent chance of 12 to 16 named storms, including 6 to 9 hurricanes and 2 to 5 major hurricanes (Category 3, 4 or 5 on the Saffir-Simpson Scale).
Member Since: July 3, 2005 Posts: 426 Comments: 128661
863. cchsweatherman
12:07 PM EDT on May 22, 2008
861. MasterForecaster 12:04 PM EDT on May 22, 2008
cchs are you still in school or what? Im just wondering because I see you posting 24/7 and am wondering how you do it lol...

...also can you post a link to the models so I can see this week system you are talking about? I'd welcome a strong tropical or even minimal cat 1, we need that rain!!!


FYI I just completed my first year at BCC about two weeks ago, so yes I'm out of school. Besides that, I'm not scheduled to work all week until Saturday, so that is why it may seem that I'm on 24/7, but I'm really not.

But, as per the second question, I'm waiting for the 12z GFS to finish running. Thus far it has only gotten up to 102 hours out.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
862. Patrap
11:06 AM CDT on May 22, 2008
NOAA Predicts Near Normal or Above Normal Atlantic Hurricane Season
As With Any Season, Preparation is Essential

May 22, 2008
Link

NOAA's Climate Prediction Center today announced that projected climate conditions point to a near normal or above normal hurricane season in the Atlantic Basin this year. The prediction was issued at a news conference called to urge residents in vulnerable areas to be fully prepared for the onset of hurricane season, which begins June 1.

Living in a coastal state means having a plan for each and every hurricane season. Review or complete emergency plans now - before a storm threatens, said retired Navy Vice Adm. Conrad C. Lautenbacher, Ph.D., undersecretary of commerce for oceans and atmosphere and NOAA administrator.Planning and preparation is the key to storm survival and recovery.




Member Since: July 3, 2005 Posts: 426 Comments: 128661
861. MasterForecaster
3:59 PM GMT on May 22, 2008
cchs are you still in school or what? Im just wondering because I see you posting 24/7 and am wondering how you do it lol...

...also can you post a link to the models so I can see this week system you are talking about? I'd welcome a strong tropical or even minimal cat 1, we need that rain!!!



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860. cchsweatherman
11:53 AM EDT on May 22, 2008
Just to update you on the latest with the new 12z GFS coming out right now, through 60 hours it has come back on board with a weak East Coast system.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
859. AWeatherLover
3:41 PM GMT on May 22, 2008
Thanks cchs!
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858. GBlet
10:36 AM CDT on May 22, 2008
Speaking of the farmers, we sure don't need this today. Right now we are looking at upwards at 54 to 60 bushels an acre for the wheat crop that should be about ready to come out of the fields in a few weeks. I would sure like to see the farmers win over the weather this year.
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857. cchsweatherman
11:33 AM EDT on May 22, 2008
Here you go AWeatherLover.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
856. GBlet
10:32 AM CDT on May 22, 2008
Don't feel that way Pat, nothing here over 12yr. Farmers don't go for that fancy crap.
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855. TEXASYANKEE43
3:31 PM GMT on May 22, 2008
851. GBlet


I'd trade places wid ya if I could git there.
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854. medic2luv
11:30 AM EDT on May 22, 2008
Good morning everyone. Going camping in N FL tomorrow just north of tampa. When does it look like the rain will clear out in that area?? Thanx!!
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853. AWeatherLover
3:20 PM GMT on May 22, 2008
Could someone please post a pic of Carib or GOM for me? Thanks!
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852. Patrap
10:29 AM CDT on May 22, 2008
Think of the 30 year Malt Surrounding you right now. Im Jealous.
Member Since: July 3, 2005 Posts: 426 Comments: 128661
851. GBlet
10:24 AM CDT on May 22, 2008
Thanks TY, but am stuck in of all places, a liquor store all day today. I will be bugging out if need be, cuz you can't pay me enough to lock myself in a box of glass and ride out a storm!
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850. Patrap
10:25 AM CDT on May 22, 2008
Tyvm GBlet..them FEMA Folks were tough,But we made it thru without to many nights in the Box..well almost.

But Carl Liked Gumbo,,and I made a Lot of Gumbo.Gulfscotsman had to eat a Bunch of them Eggs one day though. Link
Member Since: July 3, 2005 Posts: 426 Comments: 128661
849. CajunSubbie
3:23 PM GMT on May 22, 2008
so global warming leads to increased wind shear.. which leads to less frequent hurricanes.. so does global warming lead to decreased tropical SSTs because of the increased wind shear? lol sounds kinda ironic.. warming leads to cooling... man this global warming theory can be fit for any theory.
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848. cchsweatherman
11:22 AM EDT on May 22, 2008
Don't get me wrong, I respect the NHC very much - in fact, I'll be visiting the NHC tomorrow afternoon to meet with one of the meteorologists there - but I find these predictions to be utterly absurd. Just look at the ranges. Based upon those ranges, it can be anywhere between an average season to well-above average season. That doesn't help much. Anybody could get those numbers. Even more telling is the difference between the two predicted ACE values; 110%. Wow!
Member Since: April 14, 2007 Posts: 8 Comments: 5169
847. TEXASYANKEE43
3:19 PM GMT on May 22, 2008
845. GBlet


Be safe today and always GBlet. Turn on your weather radio!
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846. TEXASYANKEE43
3:17 PM GMT on May 22, 2008
I like the chicken Bones



Only you Pat. lol
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845. GBlet
10:10 AM CDT on May 22, 2008
I will be a kind Texan today and leave that one alone. Vort, thanks for the info. Good morning cch and patrap. I was very glad to hear you are no longer a prisoner of Fema Pat! Rest assured TY,I will be back on blog tomorrow.
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844. Patrap
10:14 AM CDT on May 22, 2008
Numbers for nothing.
Seasonal forecasts cant say when or where.

I like the chicken Bones and a roll of the dice method on a Left sided sidewalk on Cloudy day for mine.

But I dont publish it.

It scares people.



Member Since: July 3, 2005 Posts: 426 Comments: 128661
843. cchsweatherman
11:06 AM EDT on May 22, 2008
Good morning JFV. Right now, I'm anxiously awaiting the NOAA Hurricane Season Preview as well as Dr. Master's own preview on the upcoming season.

Based upon the past four or five GFS model runs, it appears that IF this potential Western Caribbean storm does form, it would take a track that would target South Florida by the beginning of June. It seems to offer great hope for South and Central Floridians as it keeps the system weak (of course the models are never certain and can never tell what the true intensity of a system will be, especially this far in advance) and forecasts a prolonged soaking rainfall event. Just based upon the GFS model, I would think that whatever system would form would come as a strong tropical storm or weak Category 1 hurricane. But that of course is IF this system develops. But, the GFS may have begun gathering some support as the European model develops a East Pacific system that then crosses Central America into the Western Caribbean and the latest CMC 850 vorticity model displays a clearly tropical system moving up into Panama before the model ends.
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842. nrtiwlnvragn
11:12 AM EDT on May 22, 2008
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841. TEXASYANKEE43
3:05 PM GMT on May 22, 2008
I live in Great Bend, Ks. Dead center of the state. There is definately a strange feeling in the air here today.


No offence but I would have used a better word in this situation.....
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840. nrtiwlnvragn
11:05 AM EDT on May 22, 2008
2008 NHC Outlook
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837. GBlet
9:58 AM CDT on May 22, 2008
I feel like Chicken Little today, my husband thinks I'm some sort of weather fanatic. I have warned all of my friends to stay alert today. I just hope they pay attention.
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834. GBlet
9:53 AM CDT on May 22, 2008
Good morning everyone! Does anybody have some input for me today? I live in Great Bend, Ks. Dead center of the state. There is definately a strange feeling in the air here today.
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833. cchsweatherman
10:53 AM EDT on May 22, 2008
MTJax,
That is very heavy rainfall like I had stated. There could be some localized flooding where drainage is poor and where the heaviest rainfall occurs.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
832. smmcdavid
9:50 AM CDT on May 22, 2008
What's going on with the blog...? Anyone else having problems?
Member Since: September 20, 2005 Posts: 31 Comments: 2309
831. weatherboyfsu
2:45 PM GMT on May 22, 2008
Good morning,

Rain, rain, rain,,,,,,,,,,,,,,,,awwwwwwwwwww!
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830. MTJax
2:28 PM GMT on May 22, 2008
CCH are you expecing any flooding areas in FL over the next 72 hrs? The QPF shows less than 3 inches in most places but you stated you expect more than that.
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829. smmcdavid
9:23 AM CDT on May 22, 2008
Good morning all, what's new?
Member Since: September 20, 2005 Posts: 31 Comments: 2309
828. mississippiwx23
2:21 PM GMT on May 22, 2008
Florida looks like your run of the mill Florida summer thunderstorms today.


However, the conditions are getting very interesting in Kansas...
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827. moonlightcowboy
9:08 AM CDT on May 22, 2008
China says death toll in quake more than 51,000 - full story.

BEICHUAN, China - China said the death toll from last week's powerful earthquake jumped to more than 51,000, as it appealed Thursday for millions of tents to shelter homeless survivors. The confirmed number of dead rose to 51,151 - a jump of almost 10,000 from the day before - Cabinet spokesman Guo Weimin told a news conference. Another 29,328 people remained missing and nearly 300,000 were hurt in the May 12 quake centered in Sichuan province, he said.

The disaster also left 5 million people homeless and leveled more than 80 percent of the buildings in some remote towns and villages areas near the epicenter. In bigger cities whole apartment blocks collapsed or are now too dangerous to live in because of damage and worries about aftershocks. "We need more than 3.3 million tents," Foreign Ministry spokesman Qin Gang told reporters, adding that 400,000 tents have already been delivered. It was the second call for tents from China in recent days.

"We hope and welcome international assistance in this regard. We hope the international community can give priority in providing tents," he said.


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826. surfmom
1:49 PM GMT on May 22, 2008
Based on weather choice of workout will be running - love it in the light rain here in sarasota, hope the waves are good tomorrow and the wind lays down. I am "battered" from yesterday's attempt, but it was sweet to get wet w/out a wetsuit.

I am so happy to have a cloudy rainy day -- horses and I will just relax, get groomed, and watch the grass grow. Many workers were out at the polo club yesterday working on their pastures, priming them for this rain. Natural Water holes are beginning to fill and aren't bone dry.
Member Since: July 18, 2007 Posts: 30 Comments: 26536
825. IKE
8:50 AM CDT on May 22, 2008
From CNN...

"Traders say that even though you're already paying for the hurricane season, the price could spike to $6 a gallon if catastrophe strikes."
Member Since: June 9, 2005 Posts: 23 Comments: 37858
824. sporteguy03
1:41 PM GMT on May 22, 2008
Surf,
In Brevard County it is 0-1 flat not much to ride on by Cocoa.
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823. Patrap
8:40 AM CDT on May 22, 2008
Welcome to the Weather Underground's new Climate Change page!
We're just getting started, so keep checking back over the next few months for new additions.
Link
Member Since: July 3, 2005 Posts: 426 Comments: 128661
822. surfmom
1:30 PM GMT on May 22, 2008
Good AM - bit too rainy here for me to surf b/4 work but for those GOMEX surfers - there is 1-2ft ridable swell at south facing beaches(although IMHO yesterday was barely ridable & more like being in a washing machine - but here on the GOMEX you take whatever you get) Should also be good Friday due to enough of a South Fetch by Cuba. Winds go light tomorrow (and after Thursdays wind & slight rip-tide) that's going to be my choice
Member Since: July 18, 2007 Posts: 30 Comments: 26536
821. hurricane10
1:34 PM GMT on May 22, 2008
WOW LINK to gfs 288 hrs
look at that low off florida!
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820. hydrus
1:18 PM GMT on May 22, 2008
PATRAP-Those are some cool links you posted,I wonder if it was raining bats in texas?
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819. Patrap
8:21 AM CDT on May 22, 2008
Nothing beats a LucasFilm in the Theater.
Yes Id go again.
Member Since: July 3, 2005 Posts: 426 Comments: 128661
818. kingy
1:09 PM GMT on May 22, 2008
Interesting views from Masters on the possibility of fewer quantity of canes but more stronger ones. This will not be good for the GOM drill rigs nor the insurance companies. Remember that the cat 1 storms are 'relatively' manageable. The financial damage by the cat 4 and cat 5 hurricanes is disproportionately higher. A strong cat 4/5 coming ashore near a major city/port brings with it catastrophic personal as well as financial cost. So in some ways we would be forgiven from drawing little comfort from this interesting and well written blog.
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817. sporteguy03
1:19 PM GMT on May 22, 2008
Patrap,
Do you think Indiana Jones will be better in theater or DVD? Would you see it again?
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816. Patrap
8:09 AM CDT on May 22, 2008
I give Indiana Jones an "8" from last Night.
Member Since: July 3, 2005 Posts: 426 Comments: 128661
814. Patrap
8:08 AM CDT on May 22, 2008
no Gale,no sale..
Member Since: July 3, 2005 Posts: 426 Comments: 128661

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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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