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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814. Patrap
8:08 AM CDT on May 22, 2008
no Gale,no sale..
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813. Patrap
8:06 AM CDT on May 22, 2008
Buoy 42040.
I found a sweet spot GSM..LOL
Conditions at 42040 as of
(6:50 am CDT)
1150 GMT on 05/22/2008:
Unit of Measure: Time Zone:

Click on the graph icon in the table below to see a time series plot of the last five days of that observation.
5-day plot - Wind Direction Wind Direction (WDIR): S ( 170 deg true )
5-day plot - Wind Speed Wind Speed (WSPD): 21.4 kts
5-day plot - Wind Gust Wind Gust (GST): 23.3 kts

5-day plot - Wave Height Wave Height (WVHT): 2.3 ft
5-day plot - Dominant Wave Period Dominant Wave Period (DPD): 4 sec
5-day plot - Average Period Average Period (APD): 3.2 sec
5-day plot - Atmospheric Pressure Atmospheric Pressure (PRES): 29.76 in
5-day plot - Pressure Tendency Pressure Tendency (PTDY): 0.02 in ( Rising )
5-day plot - Air Temperature Air Temperature (ATMP): 78.8 F
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812. Patrap
8:05 AM CDT on May 22, 2008
Call the Cruise Line...or

Check your local Forecast by inserting your Zip in the Box at the top left of this and every wu-page.

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811. hurricane10
1:05 PM GMT on May 22, 2008
the GFS model
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809. hurricane10
1:00 PM GMT on May 22, 2008
what do you guys think about the "thing" near south florida june 1st and 2nd cause im going on a cruise on that date around their so give me your input
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808. vortfix
1:02 PM GMT on May 22, 2008
The SPC is not highliting any severe risk areas in FL or the SE regarding this shortwave at the present time.
This is being handled through the local HWO's.

Maybe at the 12:30 update they will. But not now.
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807. vortfix
1:00 PM GMT on May 22, 2008
Surface Based CAPE:

Photobucket



Most Unstable CAPE:

Photobucket
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806. weathermanwannabe
8:51 AM EDT on May 22, 2008
801. cchsweatherman 8:46 AM EDT on May 22, 2008 Just took a look at the latest sounding data from Tallahassee. There is an insanely high CAPE (Convective Available Potential Energy) value nearing 3500 at the surface. This is very favorable for severe weather.

I'm here and will let you know how "bad" it gets; however, spotter activation has not been requested at this time by Tally NWS so I suspect that it will be a "fluid" situation going into the afternoon/eveing hours in terms of potential instability...Since it is already overcast right now, daytime heating may not be as big of an issue.....Guess it boils down to the timing of low, but, it appears to be moving pretty quickly right now to the NE..
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805. Patrap
7:51 AM CDT on May 22, 2008
NEXRAD Radar
Mobile Storm Relative Mean Radial Velocity 1.45 Degree Elevation Range 124 NMI Link

NEXRAD Radar
Mobile Echo Tops Range 124 NMI

Tops to 56K Link

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804. Patrap
7:49 AM CDT on May 22, 2008
A lil surface rotation as the Low winds up and slowly moves North.

NEXRAD Radar
Mobile Base Reflectivity 0.50 Degree Elevation Range 248 NMI Link
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803. Patrap
7:47 AM CDT on May 22, 2008
A Hook echo composite image with Bats on radar too Link
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802. Patrap
7:44 AM CDT on May 22, 2008
SPC convective outlook thru the next 8 days Link
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801. cchsweatherman
8:38 AM EDT on May 22, 2008
Just took a look at the latest sounding data from Tallahassee. There is an insanely high CAPE (Convective Available Potential Energy) value nearing 3500 at the surface. This is very favorable for severe weather.
Member Since: April 14, 2007 Posts: 8 Comments: 5163
800. Patrap
7:43 AM CDT on May 22, 2008
Yeppars,rain and T-storms are welcome for many today and tonight and tomorrow.
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799. weathermanwannabe
8:40 AM EDT on May 22, 2008
798. Patrap 8:39 AM EDT on May 22, 2008

Right Pat, here's the local NWS take:

NATIONAL WEATHER SERVICE TALLAHASSEE FL
305 AM EDT THU MAY 22 2008

AT THE SURFACE...THE STALLED FRONT OVER SOUTHERN ALABAMA AND GEORGIA WILL BEGIN TO LIFT NORTHWARD AS A WARM FRONT TODAY IN RESPONSE TO AN APPROACHING SHORT WAVE AND SURFACE LOW PRESSURE FROM THE SOUTHWEST.
DEEP LAYER MOISTURE OVER THE NORTHEAST GULF WILL SURGE NORTHWARD AND OVERSPREAD THE TRI-STATE REGION FROM SOUTHWEST TO NORTHEAST TODAY
THROUGH FRIDAY. THIS IN CONJUNCTION WITH DECENT UPPER LEVEL ENERGY WILL BRING WIDESPREAD SHOWERS AND THUNDERSTORMS TO THE AREA.
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798. Patrap
7:38 AM CDT on May 22, 2008
The Big Pic frontal Map Link
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797. weathermanwannabe
8:31 AM EDT on May 22, 2008
794. cchsweatherman 8:31 AM EDT on May 22, 2008

Thanks....I'll post some local NWS info on it in a little but, it is another impressive "blob" to look at today; if it were mid-June and the shear levels were lower in the Gulf this blob "coulda been somebody".......
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795. majkia
12:16 PM GMT on May 22, 2008
I'm betting the blob goes east of us FWB/Destin. But we could use the rain.
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794. cchsweatherman
8:22 AM EDT on May 22, 2008
790. HouseofGryffindor 8:19 AM EDT on May 22, 2008
What is that in the gulf? It looks pretty impressive. Lots of rain today it looks like for the area.


Just finished looking at satellite loops, consulting QuikSCAT, and completing some simple analysis for this disturbance. If there is any circulation with this disturbance, it does not exist at the surface, but rather in the mid-levels. Convection has become quite impressive, but this is due to excellent upper-level divergence and lower-level convergence. Shear remains very high over the feature, which will not allow for tropical development, but once this does come ashore in the Florida Panhandle, there could be some significant severe weather. By severe weather, I don't mean tornadoes since you don't have multi-directional shear occuring, but large hail, strong wind gusts, and very heavy rainfall are all hazards for the Florida Panhandle and Southern Georgia as this all comes overland.
Member Since: April 14, 2007 Posts: 8 Comments: 5163
793. Patrap
7:24 AM CDT on May 22, 2008
Northern Gulf of Mexico (Updated every ~10-15 mins.) GOES-12, 3 channels Link

Low Cloud Product,Click to ENlarge Link
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792. Patrap
7:22 AM CDT on May 22, 2008
NEXRAD Radar
New Orleans Storm Relative Mean Radial Velocity 0.50 Degree Elevation Range 124 NMI Link
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791. weathermanwannabe
8:17 AM EDT on May 22, 2008
Good Morning Folks.......I was tied up with work all day yesterday, took a look at the Sat Loops this AM, and, looks like I'm going to get some rain up here in the Florida Panhandle.......What's the deal with this ominous looking low, and, is this the low the models were hinting at a few days ago? (actually I'm thrilled cause I need the rain for my lawn)...
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789. cchsweatherman
8:16 AM EDT on May 22, 2008
For anyone who lives in South Florida or has ever watched Channel 10 News at night, you may know who Dwight Lauderdale is. For the past 32 years, he has become a legendary journalist and news anchor and has been an outstanding citizen. Known for his passion for reporting, he has become a very respected journalist by his colleagues. Well, last night, he signed off the air for the last time as his retirement begins today.
Member Since: April 14, 2007 Posts: 8 Comments: 5163
788. floridastorm
12:12 PM GMT on May 22, 2008
will any land area be affected by the GOM storm or carribean or off the coast of africa?
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785. cchsweatherman
7:57 AM EDT on May 22, 2008
Good morning all! See that the 06UTC GFS continues with tropical development in the Caribbean for next week while it has backed off the East Coast low (for now). I will be waiting for the NOAA and Dr. Masters to post their predictions on the upcoming hurricane season before I update my Tropical Update today.
Member Since: April 14, 2007 Posts: 8 Comments: 5163
784. Thundercloud01221991
11:31 AM GMT on May 22, 2008
Crude oil finished at 134 dollars a barrel today
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783. Weather456
10:16 PM AST on May 21, 2008
The tropics today

Member Since: July 24, 2005 Posts: 407 Comments: 19076
782. Patrap
6:28 AM CDT on May 22, 2008
NEXRAD Radar
New Orleans Base Reflectivity 0.50 Degree Elevation Range 248 NMI Link
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779. IKE
6:09 AM CDT on May 22, 2008
06Z GFS is still on board with a Caribbean system........

Link
Member Since: June 9, 2005 Posts: 23 Comments: 37858
778. IKE
6:07 AM CDT on May 22, 2008
0Z ECMWF...still has an east PAC tropical system crossing over into the Caribbean......

Link
Member Since: June 9, 2005 Posts: 23 Comments: 37858
776. TerraNova
10:51 AM GMT on May 22, 2008
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775. TerraNova
10:48 AM GMT on May 22, 2008
Looking at the NAM out 84 hours it appears something may attempt to develop out of this once it emerges off the east coast. The GFS also seems to be hinting at something.
The GFS cyclone phase diagram would indicate so; and environmental conditions are not very favorable for antthing but. Nevertheless this may need to be watched.
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774. TerraNova
10:43 AM GMT on May 22, 2008
An upgrade to a categorial high risk is possible for this afternoon. The moderate risk area currently includes most of West Kansas. If the SPC were to raise the probability of hail as they mention they may in the latest outlook, a high risk will be issued. It's interesting that they mention the extent of storm coverage as a problem.

STRONG MERIDIONAL COMPONENT OF MEAN FLOW AND POTENTIAL FOR
SCATTERED-NUMEROUS TSTM COVERAGE PROVIDES SOME STORM-SCALE
UNCERTAINTIES PRECLUDING GREATER SIGNIFICANT HAIL/TORNADO
PROBABILITIES ATTM.


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773. Thundercloud01221991
10:33 AM GMT on May 22, 2008
Good morning and what is going on in the gulf
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770. MasterForecaster
10:15 AM GMT on May 22, 2008
thanks jflorida
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767. MasterForecaster
9:54 AM GMT on May 22, 2008
morning everyone...

that GOM system looks impressive...is it moving due east or what?

What part of Florida is gonna get the rain?
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766. FLWeatherFreak91
5:55 AM EDT on May 22, 2008
Wow it feels awful outside! It's 80 degrees with really light rain and absolutely no breeze. At least it's finally raining- I can't complain about that.
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765. seminolesfan
9:29 AM GMT on May 22, 2008
I'm on the blog JF.

I looked at the sat ya posted. I do see the small vort you referred to.
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764. FLWeatherFreak91
5:34 AM EDT on May 22, 2008
Well, the even the radar at long-range is completely filled with rain... and the cold tops you were referring to in the central gulf are moving right toward us.... We should get sufficient rain today
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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.