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 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. has a nice discussion of the Veccu and Soden paper.

Jeff Masters

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64. cchsweatherman
11:35 AM EDT on May 21, 2008
But, this Bay of Campeche feature could have a great impact on Florida as it appears the entire feature is heading towards Florida. If this does happen, it will interact with the sea breeze and the approaching frontal boundary tomorrow and Friday to create some much-needed widespread rainfall across the state. FYI Buhdog, in the situation we are currently in, I find no fault in rooting for a small system to hit Florida right now. We need it.
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63. IKE
10:37 AM CDT on May 21, 2008
That moisture in the GOM is suppose to get drawn northeast or NNE....
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62. StormJunkie
3:26 PM GMT on May 21, 2008
Morning again y'all

Good to see everyone!

Dr. M Great blog!!! Thanks.

It will be interesting to see how it all plays out. Oh, wait, I likely won't be around to see how things are in 2100 :~(
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61. Buhdog
3:31 PM GMT on May 21, 2008
yeah doc....How much PH increaser would we need for the gulf? And if we think that Global Warming hurts the Gulf....wait till Bio-fuel related pollution destroys it and makes it a dead sea.
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60. cchsweatherman
11:24 AM EDT on May 21, 2008
Just watched various satellite loops and done some quick analysis on the Bay of Campeche feature. I doubt that this feature would get classified as an invest. In my opinion, this is just a non-tropical low (not even closed at the moment) moving into an area with elevated upper divergence and lower convergence creating the appearance that convection may have begun building with this. Wind shear remains way too high for anything to form and there is no convection whatsoever with this feature.
Something interesting to watch? Yes.
Potential to get classified as an invest? Slight chance.
Potential for development? None.
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59. DocBen
3:31 PM GMT on May 21, 2008
BOC doesn't look like much - no cold cloud tops. There is, however, a small but interesting feature in the SW Caribbean. Any potential there?
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58. smmcdavid
10:31 AM CDT on May 21, 2008
Very true Doc... I would post another section of my report but I don't want to bore you!

Love the pic of the family by the way. I have my own reason for caring.
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57. afcjags03
3:28 PM GMT on May 21, 2008
I am leading my self to beleive that we truly have no idea what the effects of any so called global warming may have on the weather or how much or little will change its all shots in the dark

You couldn't be more right.
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56. DocBen
3:27 PM GMT on May 21, 2008
Another factor with coral reefs and other marine life is the decreasing pH of the oceans with increased CO2.
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55. smmcdavid
10:25 AM CDT on May 21, 2008
Buhdog, sorry to hear about the runoff issue in FL... although it doesn't surprise me. Hopefully people will get their act together. There are somethings we can attempt to control.

What's with the blob in the GOM? Didn't think it was favorable time/place for development...
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53. Buhdog
3:17 PM GMT on May 21, 2008

I guess no need for me to add to your report. I can say however that the run off of high nutrients in the water in South Florida have had an impact down here on the alge and reefs...

Bay of camp is looking pretty juicy Pat...

Are we allowed to openly root for a small system to hit us in South Florida? We need the water so bad...I know "careful what you wish for i.e. Charley" but I am almost to that point...a small one would do us good.
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52. cchsweatherman
11:23 AM EDT on May 21, 2008
51. captainhunter 11:22 AM EDT on May 21, 2008
I have kept a curious eye on that BOC feature as well Patrap. Anyone else hae a take on this blob?

Give me a minute to take a look at that feature and I will get back to you with my opinion.
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51. captainhunter
3:17 PM GMT on May 21, 2008
I have kept a curious eye on that BOC feature as well Patrap. Anyone else hae a take on this blob?
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50. smmcdavid
10:21 AM CDT on May 21, 2008
Good morning NE... I think we are all a little confused most of the time.
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49. NEwxguy
3:15 PM GMT on May 21, 2008
!!!I'm confused!!!
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11:07 AM EDT on May 21, 2008
iam leading my self to beleive that we truly have no idea what the effects of any so called global warming may have on the weather or how much or little will change its all shots in the dark
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46. smmcdavid
10:14 AM CDT on May 21, 2008
This is from a recent report I wrote:

According to Hoegh-Guldberg et al. (2007), coral reefs have been deteriorating because of human influences. Atmospheric carbon dioxide concentrations are higher now than they have been in the last 740,000 years and continue to rise. Organisms may not be able to adapt to these changes as quickly as they are occurring. Coral reefs do not seem to be able to adjust to the changing conditions fast enough, partly because they are very sensitive to rising ocean temperatures. “Recent changes to the frequency and scale of disturbances such as mass coral bleaching, disease outbreaks, and destructive fishing, coupled with a decreased ability of coral to grow and compete, are pushing reef ecosystems from coral- to algal-dominated states” (Hoegh-Guldberg et al, 2007). Corals have already been pushed close to their thermal limits. The diversity and density of corals and their associated fish and invertebrate species will continue to decline. Hoegh-Guldberg et al. (2007) and Wilson et al. (2006) have come to the same basic conclusions concerning coral reef systems and global climate change. Coral reefs are in danger of significantly decreasing in number and complexity.
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45. smmcdavid
10:12 AM CDT on May 21, 2008
I feel horrible for the people affected by recent events in Myanmar and also China, however... if they (or their governments) don't want our help, then there's not a lot we can do. We offered, they said no. End of story.
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44. Buhdog
3:11 PM GMT on May 21, 2008
There are multiple reasons as to the reefs dying out including red tide and tissue diseases...or hurricanes, however the "bleaching" of the reefs is strictly from having to warm of water...
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42. smmcdavid
10:06 AM CDT on May 21, 2008
I agree with you Buhdog, but I'm trying to be optimistic... it's sad that most people don't really understand what's at stake here.
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41. mgreen91
10:06 AM CDT on May 21, 2008
The Central Pacific Hurricane Center is predicting slightly fewer than normal tropical cyclones to develop in the Central Pacific basin during the upcoming season.

Forecasters at National Oceanic Atmospheric Administration's hurricane center expect three to four tropical cyclones, while during a typical year four or five form or cross into the area, with two reaching hurricane intensity.

Jim Weyman, the center's director, said La Nina conditions have weakened since February and may become neutral by summer's end.

"We typically see less tropical cyclone activity in the central Pacific when La Nina is active or neutral and more activity during an El Nino cycle," he said.

Hurricane season in Hawaii runs from June 1 through Nov. 30.

The center is introducing new products and technology this season, including an experimental graphical tropical weather outlook will provide a visual representation of the current text-based product.

Also, new video teleconferencing equipment provided by the Federal Emergency Management Agency will facilitate collaboration between local, state and federal officials.

There were two tropical cyclones in the Central Pacific last year
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40. Buhdog
3:04 PM GMT on May 21, 2008

I think it could be even faster than that as far as the reefs go......

Why do I wanna make a tunnel joke right now?
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39. smmcdavid
9:55 AM CDT on May 21, 2008
Love your post Doc...

I asked a very similar early and didn't get any responses. Coral reef systems are in real danger of going extinct within the next 100 years if things don't drastically change.

Even with changes, they will be fighting for survival... which will greatly impact thousands of marine species that depend on these ecosystems.
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38. DocBen
2:49 PM GMT on May 21, 2008
A question/speculation: If the NUMBER of hurricanes decrease does that then leave more heat energy in the tropical waters; thus increasing their temperature further? What does that do to the intensity of the hurricanes that DO form? Are we trading several Cat 1 and 2's for a smaller number of Cat 5's? Or 6's?

Also, what will higher water temperatures do to marine life? Especially coral that is already dying off?
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36. snotly
2:49 PM GMT on May 21, 2008
does more wind shear cause positive or negative feedback on ocean warming?
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35. MrTyphoon
2:35 PM GMT on May 21, 2008
I am happy to read this report from the Atlantic Weather Watchers...Thankyou JM.

It confirms all/most of the theories being discussed within the Asian sectors.

Indeed Global Warming will effect the weather of this century dramatically, and indeed the storms we are seeing now are an indication of what is to come.

In China we have slightly deterred from comparing these latter year's typhoons to those of the past, preferring to note these storms as a new era in storm watching due to the present conditions of Global Warming.

Thankyou again JM for a well-worth-waiting-for report.
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34. smmcdavid
9:50 AM CDT on May 21, 2008
I agree
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32. smmcdavid
9:46 AM CDT on May 21, 2008
Good morning Pat... are you sharing your thoughts and opinions again? What have I told you about that!
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30. LakeShadow
2:33 PM GMT on May 21, 2008
thanks for the update, Dr. M.
That may very well explain last year's wind sheer in the Atlantic eventhough the strong La Nina should have squashed it... Everything seemed to go poof...except the few that made landfall and they were very intense.

and then there was!
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28. Weather456
10:25 AM AST on May 21, 2008
my atlantic tropical cyclone journal:

1. Formation Sources
a. Persistent area of thunderstorms or convection (synoptic in size)
b. The monsoon trough, NECZ, or ITCZ
c. A tropical wave
d. A dissipating frontal boundary or frontal low
e. An extratropical cyclone/a subtropical cyclone
f. An upper level low
g. A surface trough of low pressure (other than NETs)
h. The interaction between remnant disturbances, tropical waves, upper–mid tropospheric troughs
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27. mississippiwx23
2:32 PM GMT on May 21, 2008

A quick google search lead to these:



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25. mississippiwx23
2:30 PM GMT on May 21, 2008

I don't because it was shown to me in a class on a slide. But let me look for a second.
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24. sullivanweather
10:30 AM EDT on May 21, 2008
The 'shear' discussed in these models is speed shear, not directional shear.

One shouldn't confuse the two or the effect it may or may not have on tornados.
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23. cchsweatherman
10:28 AM EDT on May 21, 2008
Based upon current conditions and the latest computer models, it looks like there could be some good thunderstorms over SE Florida tomorrow afternoon as it appears a sea breeze will develop and interact with an approaching frontal boundary to spark off some moderate to strong convection. May have to watch for some strong gusts and dangerous lightning in the storms tomorrow here.
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20. mississippiwx23
2:23 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.

And for all,

The Canadian model is also showing a region of reduced pressure over the eastern east Pacific, close to the same location as the European.
Member Since: August 20, 2007 Posts: 3 Comments: 683
18. mississippiwx23
2:21 PM GMT on May 21, 2008
I have to agree with Dr. M on this one. The European is a superior model, and if it has been consistent with a storm in the Pacific, I would tend to lean that way rather than a storm forming just off Panama. Storms RARELY form off of Panama like that and move north.
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15. mississippiwx23
2:20 PM GMT on May 21, 2008

Just wait until global warming stops over the next 10-20 years...then you'll be really confused.
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14. cchsweatherman
10:15 AM EDT on May 21, 2008
By the way, great update Dr. Masters and one that will allow for much discussion, hopefully civil and objective in nature. Just wanted to refer to Dr. Master's comment below for a second.

The tropical waves that spawn tropical storms are typically too far south to spawn something in the Caribbean in May.

Do you need a tropical wave in order to spawn a tropical storm? To my knowledge, tropical storms can form from tropical lows or from a remnant frontal boundary over the right locations. It seems like something will get going soon in the Caribbean as the environmental conditions should continue to improve for tropical cyclogenesis in the region. So, I would not totally discount the GFS predictions, but I remain somewhat skeptical at this time. But, as the by now worn-out cliche goes, "Only time will tell."
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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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