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

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

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.

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.

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

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Reader Comments

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214. IKE
1:39 PM CDT on May 21, 2008
Statement as of 2:30 PM EDT on May 21, 2008

The NWS Storm Prediction Center has issued a
Severe Thunderstorm Watch for portions of

far southeastern Alabama
northern Florida
southeastern Georgia
coastal waters

Effective this Wednesday afternoon and evening from 230 PM until
1000 PM EDT.

Hail to 2 inches in diameter... thunderstorm wind gusts to 70
mph... and dangerous lightning are possible in these areas.

The Severe Thunderstorm Watch area is approximately along and 55
statute miles north and south of a line from 40 miles southwest
of Crestview Florida to 25 miles east northeast of Brunswick
Georgia. For a complete depiction of the watch see the
associated watch outline update (wous64 kwns wou2).

Remember... a Severe Thunderstorm Watch means conditions are
favorable for severe thunderstorms in and close to the watch
area. Persons in these areas should be on the lookout for
threatening weather conditions and listen for later statements
and possible warnings. Severe thunderstorms can and occasionally
do produce tornadoes.

Discussion... convection has recently developed near quasi-stationary
front as temperatures warmed into the upper 80s. MLCAPES around 2000
j/kg and deep layer shear near 40 kt are favorable for severe
storms. Hail should be the main threat... given the amount of
instability and steep mid level lapse rates. However...
unidirectional wly winds increasing with height may result in short
line segments with wind damage.

Aviation... a few severe thunderstorms with hail surface and aloft
to 2 inches. Extreme turbulence and surface wind gusts to 60
knots. A few cumulonimbi with maximum tops to 500. Mean storm
motion vector 29025.
Member Since: December 31, 1969 Posts: Comments:
213. Floodman
6:32 PM GMT on May 21, 2008
As for the proof of existence of climate change, it's all around you. Denying it's existence is closing your eyes before the crash...

I've said it before and I'll say it again here: I don't believe the 30 meter sea level rise in the next century crowd, any more than I believe the "it's not happeneing, I don't believe it" crowd...the earth is a closed system, and as such can only deal with so much rampant dumping of materials not normally in the system before it starts to show the effects. The change will not be in a straight line, but rather in fits and starts, but over the course time the effects will average, and we will (and can) see it...

I won't be downtown wearing a "The end is near" sandwich board, but I'm mightily concerned, as anyone with an open mind should be
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212. pearlandaggie
6:38 PM GMT on May 21, 2008
smmc...actually, i'm an engineer for a very large chemical company, and we're hurt severely when energy prices are high as a lot of the raw material feedstocks for chemical processes are the same hydrocarbons that drive the energy sector.
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211. pearlandaggie
6:34 PM GMT on May 21, 2008
210. the answer's "No, I'm not in the oil business!" LOL
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210. smmcdavid
1:34 PM CDT on May 21, 2008
Hey aggie... what do you do? Just curious.
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209. pearlandaggie
6:31 PM GMT on May 21, 2008
207. i totally agree. we should put our energy and attention to the future while doing what we have to in the short term to ensure stable energy supplies. vilifying those that produce energy does nothing productive for the future.
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208. pearlandaggie
6:30 PM GMT on May 21, 2008
205. send some of that rain this way! we could sure use it!
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207. Floodman
6:30 PM GMT on May 21, 2008

Okay, disregard that last response, here you go:

The current price is starting to have an effect, but the way we live, dampening the use of oil is problematic. We all have to go to work, we all have to eat and wear clothing, buy stuff, etc. Despite what the oil companies will tell you, there is a finite amount of oil to go around. If you figure for about 3 trillion barrels of oil in the ground at the start of the oil economy (say, 1900 or so)we used the first third in 100 years; at present rate of consumption (which will be unsustainable; we use more every day) the next trillion barrels will be gone in 30 years or so. Not a sustainable economy by any stretch of the imagination.

As for ethanol, it costs more oil to raise the crops that we use for the ethanol than the ethanol can replace; if you spend a dollar to save 25 cents, you haven't saved anything...there are other possibilities for ethanol production: switchgrass is supposed to be a great candidate, but like all of the other alternatives, switchgrass is a weed and could cause food crop yields to suffer (switchgrass can be planted at the edges of fields and in between the rows and will grow with little or no assistance; the yeild in ethanol will be less, but we can augment the production by planting it around food crop fields. This would help us stop using food crops for ethanol, killing two birds with one stone)

We can't stop, as Pearland so eloquently put it, but we'd damned well better start looking at other things we can do to keep this game going; our vast increases in population have been artificailly propped by the use of a material that's finite and in 30, or 50 or 100 years will no longer be available...

**once again steps off the soapbox**
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206. presslord
2:26 PM EDT on May 21, 2008
BTW...for those of you around last night...my offshore sailor buddy made it in alive and well...if a bit shaken...and was grateful for the scotch....
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204. presslord
2:24 PM EDT on May 21, 2008
flood...this would be a good point for you to remind me to shut up....
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203. pearlandaggie
6:22 PM GMT on May 21, 2008
the difference is that there is definitive proof that cancer exists!
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202. presslord
2:18 PM EDT on May 21, 2008
I don't have to know the ins and outs of molecular biology to know I don't want cancer....
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201. 69Viking
6:12 PM GMT on May 21, 2008
This blog is for conversation, not to show a long list of rich people who have no clue or degree, who here cares.
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200. pearlandaggie
6:16 PM GMT on May 21, 2008
198. nope...just that there are a LOT of highly educated folks that don't believe in AGW...but they don't get a whole lot of press. however, seems like a lot of folks without a whole lot of expertise seem to get the attention. that's all....
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199. presslord
2:16 PM EDT on May 21, 2008
actually flood...that's a brilliant idea...wish I'd thought of it
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198. jholmestyle
6:06 PM GMT on May 21, 2008
aggy, what's the point of your post. Only stupid dropouts believe in GW?
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197. Floodman
6:10 PM GMT on May 21, 2008
188. presslord 6:08 PM GMT on May 21, 2008
flood...I'm all over the map on the oil issue...but somewhere someone HAS to have studied and answered this question: At what price per gal will Americans start seriously curtailing consumption? And...At what price per gallon does ethanol become cost effective?

Any ideas?

Press, I'll email you my response...tryiong to keep the oil/GW/AGW talk to a minimum in here...
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196. pearlandaggie
6:13 PM GMT on May 21, 2008
194. totally agree. Anthropogenic effects like deforestation, albedo changes, and the UHIE DO and WILL influence local climate in the future. we SHOULD consider those effects and try to prevent or even reverse them in the future.
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195. pearlandaggie
6:12 PM GMT on May 21, 2008
191. it might not be a popular opinion, but I'd LOVE to see high shear all season long...then again, I'd like to see the Astros win the World Series, too! :)
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194. smmcdavid
1:11 PM CDT on May 21, 2008
People can't actually cause global warming or "climate change" because it's a naturally occuring event. However, we can contribute to the problem of rapidly increasing temperatures... anthropogentic factors do indeed have an effect.
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193. Weather456
2:08 PM AST on May 21, 2008
The NRL Navy has the best high resolution animation of west africa....

NEMOC...updates every 6 hrs....sorry...i cant tolerate that kind of refresh rate for this hurricane season.

http://www.nrlmry.navy.mil/sat-bin/display10.cgi?SIZE=thumb&PHOT=yes&AREA=mediterranean/sahara&PROD =vis&NAV=tropics&DISPLAY=Latest&CGI=tropics.cgi&CURRENT=20070826.1100.msg2.ir.x.sahara_dust.x.jpg&MO SAIC_SCALE=15
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192. pearlandaggie
6:09 PM GMT on May 21, 2008
187. i'm not really sure that's necessary. the post is certainly relevant given the original blog post by the good Dr.
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191. crimson75
5:52 PM GMT on May 21, 2008
I don't mean to be a wet blanket but I don't see shear relaxing anytime soon. I just don't think our models have enough data dealing with a strong negative PDO. One of the byproducts of the negative PDO is the persistence of a low plunging jet stream longer into the spring/early summer. As DrM pointed out in his blog this creates higher shear conditions. I still think we'll have an active CV season but I don't think we'll see any named Atl storms until mid-late July. I think we'll see something along the lines of 2004 where the first storm was July 31.
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190. mississippiwx23
6:06 PM GMT on May 21, 2008
I have yet to meet a scientist (in person) that believes in man made global warming. Yet the media is all about it.

Its global climate change, it happens. We just have to deal with whatever happens and go with it. The real issue isn't global warming, it is how we treat our environment in general and the fact that oil will eventually run out. We need to clean up the environment and find alternative fuels that are not corn-ethanol or oil. Switchgrass (sp) maybe?
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189. pearlandaggie
6:04 PM GMT on May 21, 2008
I am so tempted to invest in it...but then I feel bad because I would be contributing to the problem.

which problem is that? oil is what makes our economy go TODAY, whether one likes it or not. the energy we consume drives productivity and wealth, which in turn allows us to be able to afford to send BILLIONS of dollars in aid to foreign countries (whether they accept it or not is not OUR fault). can oil be sustained forever...probably not. we will have to develop new fuel technologies for the future, but if we kill ourselves by shorting the country of energy NOW, where will we find the money to develop those new technologies? oil drives the economy...the economy drives wealth...and wealth leads to value creation, philanthropy, and ingenuity. it's not really that difficult.......
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188. presslord
2:04 PM EDT on May 21, 2008
flood...I'm all over the map on the oil issue...but somewhere someone HAS to have studied and answered this question: At what price per gal will Americans start seriously curtailing consumption? And...At what price per gallon does ethanol become cost effective?

Any ideas?
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187. Floodman
6:05 PM GMT on May 21, 2008
Pearland, please find another venue for this...
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186. Floodman
6:03 PM GMT on May 21, 2008

Yep, a very active west Africa for the end of May, huh?
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184. mississippiwx23
6:03 PM GMT on May 21, 2008
Too soon to be looking at waves coming off of Africa.

Moderate Risk of severe weather tomorrow. Looks like a good chase day for people in the plains.
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183. Floodman
6:01 PM GMT on May 21, 2008
180. mississippiwx23

Not greed, O I L

Everything we do is oil; you can;t eat, drink, wear or watch anything that doesn't have a compnent of oil in it's past, contributing to the cost...hell, invest in oil! Get in now before it's all gone!

Ooops, now I've done it!
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182. mississippiwx23
6:01 PM GMT on May 21, 2008

Thanks for your post. It was very informative.
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181. moonlightcowboy
1:01 PM CDT on May 21, 2008
Member Since: December 31, 1969 Posts: Comments:
180. mississippiwx23
5:56 PM GMT on May 21, 2008
Considering oil seems to have no upper bound on price, I am so tempted to invest in it...but then I feel bad because I would be contributing to the problem. Since our nation's economy is based on greed, I can see why oil keeps going up.
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179. Floodman
5:49 PM GMT on May 21, 2008
178. moonlightcowboy

It can be significant, but likely not (given it's a site maintained by someone's military). My company's mail site has issues like that and in our case it's just that for encryption reasons we farm out our email and the mail cert doesn't match our web cert...had to change entities for mail and the ID doesn't match. You won't see the error on our web site, but use a company email account and you do; I can't imagine why they'd let it go on for so long though. I've been seeing this error since the fall
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178. moonlightcowboy
12:45 PM CDT on May 21, 2008
Flood, I didn't see that kind of trouble. Is it something I should look for? Implications?
Member Since: December 31, 1969 Posts: Comments:
177. Floodman
5:45 PM GMT on May 21, 2008
176. StSimonsIslandGAGuy

Very good point, StSimon: fluctuations do occur, and given far higher SSTs, a momentary lapse in the shear could be very baaad
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175. Floodman
5:41 PM GMT on May 21, 2008
171. moonlightcowboy

It's very strange that the Navy sites all have security certificate issues...
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174. JRRP
5:41 PM GMT on Mayo 21, 2008

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173. Floodman
5:33 PM GMT on May 21, 2008
167. TexasGulf

TExasGulf, Florida has the most stringent new construction building codes in the country. The Federal government doesn't get overly involved because this is a states issue...before you say the Feds get involved when a storm happens, yes, I agree that they do, but the states aren't capable of handling the vast problems, financial and logisitc involved in trying to recover from something like this. As for more intense building codes, people are people: they don't address issues proactively, it's in their nature...they address issues as they become real time.

As for having the engineering capabilities, yes and no; in '05 I saw a number of "Hurricane proof" roofs peeled like sardine cans...these roofs were rated to 200 mph; what happened there? Engineering failure...bottom line: all new construction in hurricane prone areas should follow the Florida building code for storm safety...will they? Likely not; it's not cost effective. Let's not even talk about retro-fitting to the current code...that's why older structures are grandfathered in
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171. moonlightcowboy
12:35 PM CDT on May 21, 2008
Navy's still got a good site for Africa.

NEMOC Rota Satellite Imagery Portal
Member Since: December 31, 1969 Posts: Comments:
1:32 PM EDT on May 21, 2008
456 nemoc good too
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168. Weather456
1:32 PM AST on May 21, 2008
oh CCH u notice too
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167. TexasGulf
5:23 PM GMT on May 21, 2008
There are a LOT of unanswered questions about tropical weather in the future. More Hurricanes? Stronger? More wind shear? Shifting patterns?

I guess the best response is just to be responsible. If the FED.Gov. keeps allowing people to build homes below sea level on slab foundations in a coastal environment (N.O.La), allows vynil siding, 24" stud spacing, OCB roof decking, non-wind resistant roofing panels, etc... then we aren't being responsible. In these cases, we get what we deserve for ignoring the problem.

We have the experience and engineering know-how to improve house and building construction to be more hurricane resistant. We know how to build properly... it just costs more.

How some coastal counties in Florida become virtual trailer parks is beyond me. Most trailer homes wouldn't stand up beyond a Cat-2 even with the best hurricane anchors. Why doesn't the FED.GOV. enforce strong coastal building laws and resolve most of the property and loss-of-life issues. We know how to do it.
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1:30 PM EDT on May 21, 2008
there appears to be some problems on the site keeps getting server overload message
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165. Weather456
1:10 PM AST on May 21, 2008
Well we had a brief free ride on the Meteosat 4km VIS/IR2 Floater on the Tropical RAMSDIS Online website. Images of West Africa and the eastern atl are now restricted again to CIRA Personnel Only. The images became accessible from about December 2007. Emusat will have to be used from here on end.
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1:25 PM EDT on May 21, 2008
cchs over time it will be a paid per veiw setup
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