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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964. mississippiwx23
6:36 PM GMT on May 22, 2008
1802 8 W GREELEY WELD CO 4042 10490:

DIRECT HIT ON STATE FARM BUILDING. CARS TOSSED OFF OF ROAD. TREES DOWN. SWIFT COMPANY ROOF TORN OFF. TORNADO 3/4 TO 1 MILE WIDE (BOU)
Member Since: August 20, 2007 Posts: 3 Comments: 683
963. AWeatherLover
6:36 PM GMT on May 22, 2008
Hey all, I just spoke with my buddy at the Ruskin NWS and although he has been out of town and hasn't seen the model he did say, "the GFS loves to spin up storms." I trust him so my confidence of this happening has dropped a bit but time will tell I suppose.
Member Since: November 2, 2007 Posts: 0 Comments: 431
962. IKE
6:32 PM GMT on May 22, 2008
NEW BLOG!
Member Since: June 9, 2005 Posts: 23 Comments: 37858
961. IKE
6:31 PM GMT on May 22, 2008
Extended discussion from Miam,FL...

"For the extended period including the rest of the Memorial Day
Holiday weekend...Sunday will be the transition day with continuing
above climatology chances for showers and thunderstorms mainly Alligator
alley south, but drying out north of Alligator alley. Back to a
Spring air mass with lower humidities and northeasterly winds Monday
and Tuesday moderating with easterly flow returning late Tuesday and
Wednesday. The Atlantic water temperature is close to 80, so expect
onshore moving marine layer showers anytime beginning Memorial Day.


Even farther out, the GFS is hinting at a much wetter period of
several days beginning Thursday but much can happen between now and
then."
Member Since: June 9, 2005 Posts: 23 Comments: 37858
960. smmcdavid
6:28 PM GMT on May 22, 2008
Thanks
Member Since: September 20, 2005 Posts: 31 Comments: 2309
959. mississippiwx23
6:28 PM GMT on May 22, 2008
New tornadic storm just east of Boulder, CO moving northwest.
Member Since: August 20, 2007 Posts: 3 Comments: 683
958. IKE
6:27 PM GMT on May 22, 2008
LOL...or as Patrap says..."ghost"LOL.
Member Since: June 9, 2005 Posts: 23 Comments: 37858
957. Cavin Rawlins
6:27 PM GMT on May 22, 2008
941. cchsweatherman 2:11 PM AST on May 22, 2008
Question for 456,
Looking at the GFS model, why doesn't the possible system strengthen much at all during the run? It would seem to be in a very favorable environment. Is it interaction with Eastern Pacific activity? Thanks.


I was also wondering that. Zonal shear is expected to be weak to modest. In fact, the upper level high that is forecast to station over the Leeward Islands is expected to provide poleward outflow but at the same time increase upper level winds by a bit. There are two other factors, interaction with land, Central America and, as you said, the nearby storm system in the EPAC. Only when the system begins to enter the open enviroment of the NW Caribbean it begins breathing and expands.
Member Since: July 24, 2005 Posts: 407 Comments: 19076
956. nash28
6:26 PM GMT on May 22, 2008
LOL Ike!

Pre blob.
Member Since: July 11, 2005 Posts: 190 Comments: 16972
955. IKE
6:25 PM GMT on May 22, 2008
The GFDL won't run on a pre-blob.
Member Since: June 9, 2005 Posts: 23 Comments: 37858
954. cchsweatherman
6:24 PM GMT on May 22, 2008
946. IKE 2:21 PM EDT on May 22, 2008
The GFS serves a purpose, It was right on, last year with Dean...a few days in advance. It's been consistent with the western Caribbean system/ghost. It's almost within the 144 hour time frame that several of these other models go out to. It'll be interesting to see if they jump on board. From almost every run the GFS has had, it takes it well east and south of me, so I'm not wishcasting from myself to be involved.


If you take a close look at the model run, you see development starting right around the 120 hour, which is five days out. It should be interesting to see the computer models this weekend once it gets into good range.

Member Since: April 14, 2007 Posts: 8 Comments: 5169
953. IKE
6:24 PM GMT on May 22, 2008
949. smmcdavid 1:22 PM CDT on May 22, 2008
Where is this "system/ghost" expected to go if it does form?


A general NNE direction........
Member Since: June 9, 2005 Posts: 23 Comments: 37858
951. nash28
6:23 PM GMT on May 22, 2008
The GFDL only initializes systems that are formed. It does not run for non existing.
Member Since: July 11, 2005 Posts: 190 Comments: 16972
950. weatherboyfsu
6:22 PM GMT on May 22, 2008
LIVE coverage of Denver tornado!!!!!


Link
Member Since: July 17, 2005 Posts: 0 Comments: 1025
949. smmcdavid
6:22 PM GMT on May 22, 2008
Where is this "system/ghost" expected to go if it does form?
Member Since: September 20, 2005 Posts: 31 Comments: 2309
948. IKE
6:22 PM GMT on May 22, 2008
I don't think the GFDL runs until there's an invest #.
Member Since: June 9, 2005 Posts: 23 Comments: 37858
947. cchsweatherman
6:21 PM GMT on May 22, 2008
939. AWeatherLover 2:10 PM EDT on May 22, 2008
What kind of confidence do you all have in the GFS model? I know last season the CMC seemed to develop many things that never materialized so I guess I'm just wondering what kind of track record the GFS has, and if you all think this really might develop. And if you do think it will develop where does the track look to be headed and how strong might it be? Sorry so many question but I can't view the model on my phone. Thanks.


Based upon the 12z GFS, the model forecasts the potential system to begin developing right around Panama (can't tell if its in the Pacific side or Caribbean side) in about four to five days. Due to a strong surface ridge over the Central Atlantic, it keeps the system at bay near Nicaragua for about 48 to 60 hours with no strengthening (I would think there would be some strengthening). At about one week out, a deep trough will develop and swing across the northern US moving the surface ridge east and finally allowing the system to move northward. The system continues northward with minor strengthening and crosses Cuba moving into Southern Florida by around June 1. It persists around South Florida for about two days bringing some much needed tropical rainfall, then speeds off paralelling the Eastern Seaboard.

This has been the forecast for the past six runs now, so the model is gaining some consistency necessary to warrant at least some consideration. This possible scenario looks quite reasonable and I see no reason why it won't happen. Just need to see a few more models get on board and need to wait until the weekend to truly gauge the situation more carefully.


Member Since: April 14, 2007 Posts: 8 Comments: 5169
946. IKE
6:21 PM GMT on May 22, 2008
The GFS serves a purpose, It was right on, last year with Dean...a few days in advance. It's been consistent with the western Caribbean system/ghost. It's almost within the 144 hour time frame that several of these other models go out to. It'll be interesting to see if they jump on board. From almost every run the GFS has had, it takes it well east and south of me, so I'm not wishcasting for myself to be involved.
Member Since: June 9, 2005 Posts: 23 Comments: 37858
944. nash28
6:17 PM GMT on May 22, 2008
I am paying more attention to the GFS since it has been on this for several days now. Regarding strength, don't even put stock into the model to depict strength. It is not a dynamical model such as the GFDL.
Member Since: July 11, 2005 Posts: 190 Comments: 16972
942. Patrap
6:14 PM GMT on May 22, 2008
I dont track Ghost JFV.
I focus on real time events and impacts.
Glad to see you got that radio.

Next Week is Hurricane Preparedness Week Link
Member Since: July 3, 2005 Posts: 426 Comments: 128661
941. cchsweatherman
6:11 PM GMT on May 22, 2008
Question for 456,
Looking at the GFS model, why doesn't the possible system strengthen much at all during the run? It would seem to be in a very favorable environment. Is it interaction with Eastern Pacific activity? Thanks.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
939. AWeatherLover
6:10 PM GMT on May 22, 2008
What kind of confidence do you all have in the GFS model? I know last season the CMC seemed to develop many things that never materialized so I guess I'm just wondering what kind of track record the GFS has, and if you all think this really might develop. And if you do think it will develop where does the track look to be headed and how strong might it be? Sorry so many question but I can't view the model on my phone. Thanks.
Member Since: November 2, 2007 Posts: 0 Comments: 431
938. cchsweatherman
6:09 PM GMT on May 22, 2008
Good afternoon Nash. Could you give us your opinion on the GFS model forecast right now? It has been remarkably consistent over the past five or six runs in terms of track and intensity. Thanks.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
937. Patrap
6:08 PM GMT on May 22, 2008
Upslope storms to the NW are rare there.
Member Since: July 3, 2005 Posts: 426 Comments: 128661
936. GBlet
6:07 PM GMT on May 22, 2008
Thanks Pat, our cape values are over 3000! A very odd direction for that colorado storm.
Member Since: September 13, 2007 Posts: 0 Comments: 435
935. NEwxguy
6:05 PM GMT on May 22, 2008
925. Patrap 5:56 PM GMT on May 22, 2008
Florida best be careful..your my First Barrier Island to Se Louisiana...from the east


You better watch out for that end around run around Cuba
Member Since: September 6, 2007 Posts: 881 Comments: 15840
934. nash28
6:05 PM GMT on May 22, 2008
Afternoon everyone.
Member Since: July 11, 2005 Posts: 190 Comments: 16972
933. cchsweatherman
6:03 PM GMT on May 22, 2008
Looks like Lake Okeechobee will be getting a real good soaking for the next few hours with this action.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
932. mississippiwx23
6:02 PM GMT on May 22, 2008
I hope that tornado missed the most populated areas. Wow is all I can say about that storm.
Member Since: August 20, 2007 Posts: 3 Comments: 683
931. Patrap
6:01 PM GMT on May 22, 2008
Be prepared come June First.
Buy that NOAA radio and your Hurricane Supplies Yet JFV?
Member Since: July 3, 2005 Posts: 426 Comments: 128661
929. Patrap
6:00 PM GMT on May 22, 2008
NEXRAD Radar TVS signature
Denver Base Reflectivity 0.50 Degree Elevation Range 124 NMI Link
Member Since: July 3, 2005 Posts: 426 Comments: 128661
928. Patrap
5:58 PM GMT on May 22, 2008
Denver Warning TOR Link
Member Since: July 3, 2005 Posts: 426 Comments: 128661
927. Patrap
5:57 PM GMT on May 22, 2008
Just looking out fer ya FLWeatherFreak91 ..I never implied you were. Not my style.
Member Since: July 3, 2005 Posts: 426 Comments: 128661
926. mississippiwx23
5:57 PM GMT on May 22, 2008
If the storm that just formed near Denver and put down a 'large tornado' is a hint at what is to come today, it could be a busy day.
Member Since: August 20, 2007 Posts: 3 Comments: 683
925. Patrap
5:56 PM GMT on May 22, 2008
Florida best be careful..your my First Barrier Island to Se Louisiana...from the east

Member Since: July 3, 2005 Posts: 426 Comments: 128661
924. FLWeatherFreak91
5:56 PM GMT on May 22, 2008
Sorry about that error PATRAP... I'm actually not that stupid :)
Member Since: December 1, 2006 Posts: 2 Comments: 3625
923. Patrap
5:55 PM GMT on May 22, 2008


It should be remembered that CAPE represents potential energy, and will only be used should a parcel be lifted to the level of free convection. When values are above 3500 j/kg and storms do develop, they may build rapidly and quickly become severe. Often these storms are referred to as "explosive storms" by chasers and professionals. In a high CAPE environment storms that develop can usually be seen by the human eye as rising rapidly. Chasers discussing these kind of storms have been overheard saying these storms resemble a nuclear explosion as in this photo, because they rise up so quickly and powerfully.
Member Since: July 3, 2005 Posts: 426 Comments: 128661
921. mississippiwx23
5:54 PM GMT on May 22, 2008
Nasty storm just north of Denver, CO. Impressive hook echo, moving to the northwest.
Member Since: August 20, 2007 Posts: 3 Comments: 683
920. Cavin Rawlins
5:54 PM GMT on May 22, 2008
Historically, we have almost never (only once in 100 years) seen a La Nina transition from its current present strength to an El Nino by ASO. Therefore, there is a likelihood that the current La Nina patterns of tropical convection and winds will persist and affect the hurricane season, even if La Nina dissipates.

ASO refers to the months of August, September and October

Source: Noaa's Outlook
Member Since: July 24, 2005 Posts: 407 Comments: 19076
919. Patrap
5:53 PM GMT on May 22, 2008
Cape Values Link
Member Since: July 3, 2005 Posts: 426 Comments: 128661
918. FLWeatherFreak91
5:53 PM GMT on May 22, 2008
911. weatherdead 5:45 PM GMT on May 22, 2008
It seems "global warming" is used to explain a lot of things. More hurricanes, less hurricanes, even global cooling. Is it just possible that no one really knows what is going on?



That's exactly right! Finally! No one knows what is going on bc the Weather and the systems of the Earth operate on a much larger scale than humans can ever fully understand. Global warming is our current answer to the "abnormalities" we find in the systems. The earth is warming slightly, yes, that's a fact, but the truth is we don't know the cause of this warming for sure- so our THEORY is global warming caused by humans. Now we use this term to justify ANY fluctuation which we can't explain. More hurricanes, less hurricanes, hotter temps, cooler temps... Who knows?!
Member Since: December 1, 2006 Posts: 2 Comments: 3625
917. Patrap
5:51 PM GMT on May 22, 2008
Boomers

NEXRAD Radar
New Orleans Base Reflectivity 0.50 Degree Elevation Range 124 NMI Link
Member Since: July 3, 2005 Posts: 426 Comments: 128661
916. GBlet
5:50 PM GMT on May 22, 2008
Pat, could you explain cape values to me please?
Member Since: September 13, 2007 Posts: 0 Comments: 435
915. Patrap
5:49 PM GMT on May 22, 2008
Gulf of Mexico - AVN Color Infrared Loop Link
Member Since: July 3, 2005 Posts: 426 Comments: 128661
914. cchsweatherman
5:49 PM GMT on May 22, 2008
...TROPICAL WAVES...

TROPICAL WAVE IS ALONG 23W S OF 12N MOVING W 10 KT. THIS WAVE
REMAINS FAIRLY WELL DEFINED WITH VISIBLE IMAGES SHOWING
LOW-LEVEL CYCLONIC TURNING ALONG THE AXIS AND A CLEAR
PERTURBATION IN THE ITCZ. HOWEVER...MUCH OF THIS TURNING IS LOW
LATITUDE CENTERED NEAR 6N. SCATTERED SHOWERS AND TSTMS ARE
WITHIN 150 NM EITHER SIDE OF THE AXIS S OF 9N.
Member Since: April 14, 2007 Posts: 8 Comments: 5169

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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.