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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614. weatherbro
11:21 PM GMT on May 21, 2008
that low next week won't do anything to Florida except reinforce continental dry air down the peninsula.
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613. pearlandaggie
11:40 PM GMT on May 21, 2008
today at home and at work I saw hail for the third time

you should ask Floodman about hail...we saw it nearly EVERY year at least once a year in Dallas...
Member Since: September 14, 2007 Posts: 3 Comments: 3963
612. FLWeatherFreak91
7:37 PM EDT on May 21, 2008
Will in survive 'til it reaches the coast?
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609. 882MB
11:32 PM GMT on May 21, 2008
Just seen the new GFS 18Z and it continues to show development in the carribean,however I noticed some good news in the new model run for florida,looks like the backdoor coldfront thats going to hit southflorida this weekend will stall over the bahamas and a low pressure trough will move right over the state bringing some beneficial rain.
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608. pearlandaggie
11:31 PM GMT on May 21, 2008
606. i dunno...the gradient is certainly tight over the central GOM
Member Since: September 14, 2007 Posts: 3 Comments: 3963
605. Drakoen
11:27 PM GMT on May 21, 2008
LOL pearl!!!
Member Since: October 28, 2006 Posts: 57 Comments: 29864
604. pearlandaggie
11:27 PM GMT on May 21, 2008
JF...

this Link shows that the shear is increasing near the middle of the GOM
Member Since: September 14, 2007 Posts: 3 Comments: 3963
603. KEEPEROFTHEGATE (Mod)
7:24 PM EDT on May 21, 2008
since about 6 FLWF
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602. pearlandaggie
11:24 PM GMT on May 21, 2008
596. FIFY
Member Since: September 14, 2007 Posts: 3 Comments: 3963
600. KEEPEROFTHEGATE (Mod)
7:21 PM EDT on May 21, 2008
mlc oil will be 150 by july after the canes it will rise to 220 by christmas
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599. FLWeatherFreak91
7:20 PM EDT on May 21, 2008
Whoa, when did that complex of storms develop in the gomex. And I really hope this one actually holds together 'til Tampa b/c we've been missing out on everything lately
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598. StormJunkie
11:18 PM GMT on May 21, 2008
Ok, see y'all in a bit ☺
Member Since: August 17, 2005 Posts: 26 Comments: 15626
597. moonlightcowboy
5:25 PM CDT on May 21, 2008
For the last couple of months I've been measuring and keeping up with the dollar's value.


The US dollar status with world currencies:


1.00 USD = 10.7624 MXN Peso
1.00 USD = 0.650470 EUR Euro
1.00 USD = 0.990710 CAD Canada
1.00 USD = 7.10600 CNY China Yuan
1.00 USD = 103.157 JPY Japan Yen
1.00 USD = 40.4100 INR India rupees
1.00 USD = 23.8484 RUB Russian Rubies
1.00 USD = 1.02995 CHF Switzerland Francs
1.00 USD = 7.78747 ZAR S. African Rand

As of 3/12/08.

1.00 USD = 10.5337 MXN Peso (LOSS)
1.00 USD = 0.633070 EUR Euro (LOSS)
1.00 USD = 1.02150 CAD Canada (GAIN)
1.00 USD = 7.00650 CNY China Yuan (LOSS)
1.00 USD = 100.975 JPY Japan Yen (LOSS)
1.00 USD = 39.9400 INR India rupees (LOSS)
1.00 USD = 23.4755 RUB Russian Rubies (LOSS)
1.00 USD = 1.00115 CHF Switzerland Francs (LOSS)
1.00 USD = 7.80890 ZAR S. African Rand (GAIN)

As of 4/12/08.

1.00 USD = 10.5071 MXN Peso (LOSS)
1.00 USD = 0.643544 EUR Euro (GAIN) slight
1.00 USD = 1.00259 CAD Canada (LOSS)
1.00 USD = 6.98781 CNY China Yuan (LOSS)
1.00 USD = 104.639 JPY Japan Yen (GAIN)
1.00 USD = 40.6277 INR India rupees (GAIN) slight
1.00 USD = 23.7122 RUB Russian Rubies (GAIN) slight
1.00 USD = 1.04952 CHF Switzerland Francs (GAIN) slight
1.00 USD = 7.52090 ZAR S. African Rand (LOSS)

As of 5/05/08.

1.00 USD = 10.3712 MXN Peso (LOSS)
1.00 USD = 0.633510 EUR Euro (LOSS)
1.00 USD = 0.983900 CAD Canada (LOSS)
1.00 USD = 6.96020 CNY China Yuan (LOSS)
1.00 USD = 103.046 JPY Japan Yen (LOSS)
1.00 USD = 42.7300 INR India rupees (GAIN)
1.00 USD = 23.5650 RUB Russian Rubies (LOSS)
1.00 USD = 1.02573 CHF Switzerland Francs (LOSS)
1.00 USD = 7.72818 ZAR S. African Rand (LOSS)

As of 5/21/08.


I've been doing this monthly for comparison, but I'm going to try to update it now every two weeks. Loss or Gain is from Apri 12 figures. The ONLY gain since March 12 was against the Indian rupee. All other comparative currency shows the dollar still declining, but with slight periods of increase against some currencies.

It will be interesting to see how this next week's trading goes with the additional spike in oil prices, the fed rate-cut a few weeks ago, consumption, production down in Mexico and Russia, US supply down, the Myanmar storm, cane season on us, etc. With the dollar continuing to decline, oil prices can only increase.

Todays, record crude barrel price reached $134.06 with new record prices being set, now, almost daily regularity. One year forecast is at $174 bl, but at these rates, that's liable to come much faster.

Photobucket
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595. pearlandaggie
11:15 PM GMT on May 21, 2008
590. we sure could use some rain in Texas...last summer by July 7, we had received an entire year's worth of rainfall. this year...very dry.
Member Since: September 14, 2007 Posts: 3 Comments: 3963
594. Drakoen
11:16 PM GMT on May 21, 2008
If anything this would be a welcome system.
Photobucket
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593. IKE
6:15 PM CDT on May 21, 2008
590. JFLORIDA 6:13 PM CDT on May 21, 2008
Station 42055 - Bay of Campeche

Atmospheric Pressure (PRES): 29.62 in


That's a rather low BP.
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591. StormJunkie
11:12 PM GMT on May 21, 2008
That was the big point of it Drak, and in that regards it is fairly unique I think.

Last night the model was not showing a symmetrical warm core though.
Member Since: August 17, 2005 Posts: 26 Comments: 15626
589. StormJunkie
11:11 PM GMT on May 21, 2008
SC2007, GFS now shows it weak, but warm core symmetrical over the Bahamas, and then headed back towards S Fla. See how quickly things can change with these stalled frontal type systems.
Member Since: August 17, 2005 Posts: 26 Comments: 15626
588. Drakoen
11:11 PM GMT on May 21, 2008
584. StormJunkie 11:11 PM GMT on May 21, 2008
Thank-ya Drak, very kind!

Still, I am not a fan of that time frame though.

Seriously though, you getting any thoughts on the stalled front?


I like your site because everything is just there easy to find.

Not really thinking much on the front other than what I told you yesterday.
Member Since: October 28, 2006 Posts: 57 Comments: 29864
587. IKE
6:11 PM CDT on May 21, 2008
585. TheWeatherMan504 6:11 PM CDT on May 21, 2008
Thats cool IKE.


I corrected it...
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586. HIEXPRESS
7:09 PM EDT on May 21, 2008
Having to "double clutch" modify comments again.
Isn't that gulf rain out over the loop current?
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585. TheWeatherMan504
11:10 PM GMT on May 21, 2008
Thats cool IKE.
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584. StormJunkie
11:07 PM GMT on May 21, 2008
Thank-ya Drak, very kind!

Still, I am not a fan of that time frame though.

Seriously though, you getting any thoughts on the stalled front?
Member Since: August 17, 2005 Posts: 26 Comments: 15626
581. pearlandaggie
11:08 PM GMT on May 21, 2008
574. don't fret. it'll shape up when the season begins in earnest.
Member Since: September 14, 2007 Posts: 3 Comments: 3963
580. Drakoen
11:08 PM GMT on May 21, 2008
576. atmoaggie 11:06 PM GMT on May 21, 2008
Drak, guess you haven't seen some of the spam circulating that has a collection of about 100 random words, all lower case, with no punctuation, etc. I assume it is written that way to get past spam filters looking for certain phrases.


I have seen some. I have been wondering why thats happening.
Member Since: October 28, 2006 Posts: 57 Comments: 29864
579. Stormchaser2007
11:08 PM GMT on May 21, 2008


I think he was referring to the E coast low, which is what most should be doing right now imho...


Oh yeah what ever happened to that??
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578. IKE
6:07 PM CDT on May 21, 2008
569. TheWeatherMan504 6:03 PM CDT on May 21, 2008
it brought me to http://weather.unisys.com/gfsx/loop/gfsx_pres_loop.html


I see what you mean....I'll edit that post...not sure how that happened.
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577. StormJunkie
11:04 PM GMT on May 21, 2008
553.

I think he was referring to the E coast low, which is what most should be doing right now imho...
Member Since: August 17, 2005 Posts: 26 Comments: 15626
576. atmoaggie
11:04 PM GMT on May 21, 2008
Drak, guess you haven't seen some of the spam circulating that has a collection of about 100 random words, all lower case, with no punctuation, etc. I assume it is written that way to get past spam filters looking for certain phrases.
Member Since: August 16, 2007 Posts: 6 Comments: 12461
575. Drakoen
11:04 PM GMT on May 21, 2008
You get a better view of the track here for those of us the are track-challenged.
Link
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574. Stormchaser2007
11:01 PM GMT on May 21, 2008
.
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573. HIEXPRESS
6:51 PM EDT on May 21, 2008
I still have a working XT. "512K ought to be enough for anybody"
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572. IKE
6:04 PM CDT on May 21, 2008
You see that?

I see that now on visible satellite. Blow up of thunderstorms.
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571. StormJunkie
10:52 PM GMT on May 21, 2008
Thanks KOTG, that is what it was! Could not remember the acronym for the life of me!

atmo, it is ok, most would not know a TRS-80 if it jumped up from the dead and bit them in the....

CoCo

Commodore 64

And notice this from the list of computers running the FSU Experimental page...Last line of table

Also take note of the color on that last line and what it means in the legend at bottom :~)
Member Since: August 17, 2005 Posts: 26 Comments: 15626
570. IKE
6:02 PM CDT on May 21, 2008
567. JFLORIDA 6:02 PM CDT on May 21, 2008
Now the rain is actually in the gulf now IKE you should see it Friday or so.


Yeah...a good chance the next 2 days.
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569. TheWeatherMan504
11:02 PM GMT on May 21, 2008
it brought me to http://weather.unisys.com/gfsx/loop/gfsx_pres_loop.html
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566. Drakoen
11:00 PM GMT on May 21, 2008
558. atmoaggie 10:58 PM GMT on May 21, 2008
Great spelling and grammar

Don't diss it...loved his work in the erectile spam.

The immaturity in this blog is beyond even me.
Member Since: October 28, 2006 Posts: 57 Comments: 29864
565. CaneAddict
11:00 PM GMT on May 21, 2008
Im off to the shower, BBL!
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564. IKE
6:00 PM CDT on May 21, 2008
560. TheWeatherMan504 5:58 PM CDT on May 21, 2008
550. IKE 10:53 PM GMT on May 21, 2008
18Z GFSLink

Why dont you go to the NWS to see the models??Link


That's the same page I got it from.
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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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