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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164. KEEPEROFTHEGATE (Mod)
1:25 PM EDT on May 21, 2008
cchs over time it will be a paid per veiw setup
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163. Floodman
5:27 PM GMT on May 21, 2008
Feed da babee! We'll talk later, smmc
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162. IKE
12:26 PM CDT on May 21, 2008
OIL
3.12
+2.42%
$132.10

Bangs fist on computer desk.
Member Since: June 9, 2005 Posts: 23 Comments: 37858
161. Floodman
5:25 PM GMT on May 21, 2008
Wow...the errors look for all the world like DOS attack
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160. smmcdavid
12:26 PM CDT on May 21, 2008
Got it... will respond later, but John just woke up from his nap and wants lunch. Be back soon.
Member Since: September 20, 2005 Posts: 31 Comments: 2309
159. surfmom
5:23 PM GMT on May 21, 2008
Good thinking help4U

cchsweatherman --why would they restrict use of that information??????
Member Since: July 18, 2007 Posts: 30 Comments: 26536
158. Floodman
5:23 PM GMT on May 21, 2008
smmcdavid, did you get an response email from me? I got a "Server overloaded" error when I tried to send
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157. cchsweatherman
1:17 PM EDT on May 21, 2008
NOTICE: Unless you are authorized CIRA personnel, access to the satellite imagery for Africa and Asia is no longer allowed on the Tropical RAMSDIS site. You need a user name and password to access those satellites now. This really sucks.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
156. help4u
5:16 PM GMT on May 21, 2008
$60.OO more on a barrel of oil and the war on global warming will be won.If we drive less and keep our air off,we can win this war.Keep the price going up.Change is coming.
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155. NEwxguy
5:16 PM GMT on May 21, 2008
creates a political talk ignore button
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154. OSUWXGUY
5:18 PM GMT on May 21, 2008
StormW- As I'm sure you know, changes in the Environment Lapse Rate is CRUCIAL in understanding what a warming climate may do to the intensity of hurricanes. Unfortunately, this issue appears highly unresolved at the moment.

In regards to the stratosphere, cooling has been seen in this layer, particularly over the last 20-30 years in the polar regions. As the UV absorbing/reflecting ozone in this layer was depleted by CFCs, energy from the sun was able to pass through the stratosphere and did not heat the layer as much as it previously had. CFC levels are slowly dropping and over time this cooling should slowly diminish.

Changes within the troposphere are less understood...as Mississippiwx's second link suggests.

Higher boundary layer water vapor content associated with climate warming preferentially heats near the surface more than the higher atmosphere - which would lead to higher lapse rates and more instability. Observations seem to bear this out more than the model results showing more warming in the upper atmosphere and less near the surface.

Talking with a couple people in Orlando, the depth of the troposphere is will expand in a warmer climate. This would lead to taller convection which can release heat over a larger column - and a slight increase in intensity.

More research is obviously needed and more consistent global sounding data needs to acquired before this will be resolved...which means the verdict is definitely still out on future intensity changes.


20. mississippiwx23 2:27 PM GMT on May 21, 2008
I have seen a study which found temperatures to actually be dropping rather an increasing in the upper atmosphere. It is really only the surface, or near surface, that is increasing.

16. StormW 2:21 PM GMT on May 21, 2008
I've often wondered though, about a warming atmosphere. How far up in the atmosphere are we speaking? It would seem to me if the atmosphere warmed far enough in altitude, then the ELR would be reduced, and lapse rates wouldn't be as steep for a tropical system to be really strong. Maybe I'm looking to deep into it.


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152. cchsweatherman
1:10 PM EDT on May 21, 2008
141. hurricane23 1:06 PM EDT on May 21, 2008
Whats in the GULF is not anything like barry as this feature does not have a circulation at the surface.The mostly outcome for me atleast is for the first name storm to be named in the pacific in a few days.

You can clearly see overall convection is becoming better organized with time.Could be the first TS in the eatern pacific.Overall upper level conditions are forcasted to become favorable for some development.


That activity in the Eastern Pacific you have referred to does not have any circulation associated with it all either. It only looks impressive since there is insanely high upper-level divergence and lower-level convergence in the same area. Once it loses those factors, the impressive convection will rapidly decrease into showers and isolated thunderstorms, just like the SW Caribbean feature has done over the past few days.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
151. surfmom
5:10 PM GMT on May 21, 2008
CHEERS & LOUD HOOTING stormhype --so true , until we/gov clean up the red, instead of creative government accounting and moving things around on the ledger we will continue to head for more trouble. The red ink must be addressed - people as well as government --taking responsibility for our actions!!
Member Since: July 18, 2007 Posts: 30 Comments: 26536
150. LakeShadow
5:08 PM GMT on May 21, 2008
Hurricaine23 I noticed that blob in the PAC earlier...do you think it could enter the gulf? looks like it could be steered in that direction.
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149. Floodman
5:08 PM GMT on May 21, 2008
LOL surfmom!
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148. smmcdavid
12:08 PM CDT on May 21, 2008
Lets not get started talking politics again... it just leads to trouble.
Member Since: September 20, 2005 Posts: 31 Comments: 2309
147. IKE
12:08 PM CDT on May 21, 2008
136. Floodman 12:04 PM CDT on May 21, 2008
Ike, NEwx, life is too short to mess around...much LOL


I hear ya!
Member Since: June 9, 2005 Posts: 23 Comments: 37858
145. IKE
12:05 PM CDT on May 21, 2008
140. surfmom 12:05 PM CDT on May 21, 2008
Yes, MLC we may hear the D word soon, people will remain quiet like cows in a pasture till they get the hungry belly syndrom --otherwise we graze.

The thing is that the top 10 percent (who are in charge) are TOTALLY unaffected by what's going on out there. They are clueless and have no experience seeing one a days salary going to fill up their gas tank -


A one day salary? If my car sits on "E", it would take about a third of what my wife makes a week, to fill it up.
Member Since: June 9, 2005 Posts: 23 Comments: 37858
144. Floodman
5:05 PM GMT on May 21, 2008
Texas Gulf, I'm not sure...I think that maybe 4 2s would be able to disspate more heat, but then it's all relative...2 small 4s vs 4 larger 2s? I think in theory you're likely right, but the reality is dependent on a number of factors
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143. smmcdavid
12:07 PM CDT on May 21, 2008
Floodman... it's your turn to have mail.
Member Since: September 20, 2005 Posts: 31 Comments: 2309
142. surfmom
5:05 PM GMT on May 21, 2008
Floodman - I'm jealous LOL
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141. hurricane23
1:05 PM EDT on May 21, 2008
Whats in the GULF is not anything like barry as this feature does not have a circulation at the surface.The mostly outcome for me atleast is for the first name storm to be named in the pacific in a few days.

You can clearly see overall convection is becoming better organized with time.Could be the first TS in the eastern pacific.Overall upper level conditions are forcasted to become favorable for some development.

SAT view of the area
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140. surfmom
5:01 PM GMT on May 21, 2008
Yes, MLC we may hear the D word soon, people will remain quiet like cows in a pasture till they get the hungry belly syndrome --otherwise we graze.

The thing is that the top 10 percent (who are in charge) are TOTALLY unaffected by what's going on out there. They are clueless and have no experience seeing one days salary going to fill up their gas tank -
Member Since: July 18, 2007 Posts: 30 Comments: 26536
139. cchsweatherman
1:04 PM EDT on May 21, 2008
134. KEEPEROFTHEGATE 1:04 PM EDT on May 21, 2008
cchs boc build on the heat lets see if it can break though blow up real quick


I don't understand what you are saying.

Member Since: April 14, 2007 Posts: 8 Comments: 5169
138. Floodman
5:04 PM GMT on May 21, 2008
129. LakeShadow

**blush**
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137. Weather456
1:02 PM AST on May 21, 2008
There appears to be some inconsistency in the GFS model run and no model consensus. I wouldnt put a whole lot of $$ on this feature as of yet.
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136. Floodman
4:59 PM GMT on May 21, 2008
Ike, NEwx, life is too short to mess around...much LOL
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135. TexasGulf
4:58 PM GMT on May 21, 2008
Floodman, you asked a good question earlier, which needs a chicken & egg answer.

Which is preferrable; Two Cat-4 hurricanes or Four Cat-2 hurricanes?

Everyone living along the coast or enjoying gas prices less than $6.00/gallon would agree Four Cat-2's is our preference.

However, from the EARTH's prospective (and being honest isn't this really earth's perogative... I mean hello, that's what hurricanes are all about) wouldn't Two Cat-4's dissipate heat from the tropics much more efficiently? Two Cat-4's might provide more beneficial effects for the earth than multiple smaller hurricanes. :>)
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134. KEEPEROFTHEGATE (Mod)
12:49 PM EDT on May 21, 2008
cchs boc build on the heat lets see if it can break though blow up real quick
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133. LakeShadow
5:01 PM GMT on May 21, 2008
somebody should clue Mr. Whitey Mc MoneyPig that there's a goldmine to be made in the manufacturing of hemp products...

speaking of which Oil Tycoons are on capital hill right now in congressional committee explaining away their billions of dollars in profits...
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132. surfmom
5:00 PM GMT on May 21, 2008
Let bring on the rain to SWFL....it would make my day!!!
Member Since: July 18, 2007 Posts: 30 Comments: 26536
131. Weather456
12:58 PM AST on May 21, 2008
12Z Model Runs

Red - GFS

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130. surfmom
4:56 PM GMT on May 21, 2008
In total agreement Lakeshadow - the good thing about hemp crops is that it does NOT depleat the soil of nutrients! No fertilizer (petro based product) needed and in fact makes the soil better for the next crop!

Not growing hemp (and I believe this variety does not provide a buzz) is dark ages thinking. Which we may be on the road to.
Member Since: July 18, 2007 Posts: 30 Comments: 26536
129. LakeShadow
4:58 PM GMT on May 21, 2008
121. Floodman 4:57 PM GMT on May 21, 2008
I have always loved you, Lakeshadow! Hemp is indeed the answer, if only for the obvious reason LOL


Back at you Flood!
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128. Floodman
4:57 PM GMT on May 21, 2008
smmcdavid, you have mail
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126. IKE
11:58 AM CDT on May 21, 2008
121. Floodman 11:57 AM CDT on May 21, 2008
I have always loved you, Lakeshadow! Hemp is indeed the answer, if only for the obvious reason LOL


LOL!
Member Since: June 9, 2005 Posts: 23 Comments: 37858
125. LakeShadow
4:55 PM GMT on May 21, 2008
The thing is that hemp crops do not form buds...infact they are a different species of plant altogether. They are a cannabis plant, but you cant get stoned from them. The reason why they wont allow us to grow and produce manufactured goods from them is because you cant tell real marijuana plants apart from hemp plants from the air. (thats how they find 'em)
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124. NEwxguy
4:57 PM GMT on May 21, 2008
flood,
you don't mess around,lol.
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122. cchsweatherman
12:56 PM EDT on May 21, 2008
We will have to see if the GFS continues this forecast before I give it any credence as it has been very erratic with track and strength. If we see the GFS continue with this track and strength, we may have to give it more consideration, although it still remains a little over a week and a half out in time.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
121. Floodman
4:57 PM GMT on May 21, 2008
I have always loved you, Lakeshadow! Hemp is indeed the answer, if only for the obvious reason LOL
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120. NEwxguy
4:49 PM GMT on May 21, 2008
cchs,keep in mind that trough which has been messing up our spring here in the northeast is moving out this weekend and the midwest ridge is moving eastward,so if that blob wants to do anything for Florida it better do something soon.
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119. Floodman
4:55 PM GMT on May 21, 2008
102. Patrap 4:31 PM GMT on May 21, 2008
Cure for those who cant-stop-thinking.

Zoloft and

Sleep



Maybe Chloral Hydrate?
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118. surfmom
4:47 PM GMT on May 21, 2008
Re post 86 - I do know of one amateur weather watcher, Micah Weaver of Aurasurf (a weekly surf site) who predicted Charely would pummel Port charlotte - in fact, there were several fisherman and surfers who were calling that out --inspite of the general knowledge the news was putting out (this was before I knew about WU) these guys called the landing based on years of experience on the GOMEX WATERS --MORE LIKE INSTINCT (which makes weather forecasting in my mind a science but also an art.
Member Since: July 18, 2007 Posts: 30 Comments: 26536
117. cchsweatherman
12:52 PM EDT on May 21, 2008
288 HRS
Member Since: April 14, 2007 Posts: 8 Comments: 5169
116. IKE
11:54 AM CDT on May 21, 2008
12Z GFS running through 384 hours...

Link
Member Since: June 9, 2005 Posts: 23 Comments: 37858
115. captainhunter
4:51 PM GMT on May 21, 2008
I'm glad I bought 40 gallons of unleaded for last year's hurricane season at last year's prices. I just hope the fuel stabilizer has done its job when it's time to crank the generator.
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114. tornadofan
4:50 PM GMT on May 21, 2008
OSUWXGUY - thanks for your comments regarding tornadoes. It makes sense.
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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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