The future of wind shear: will it decrease the number of hurricanes?
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
Reader Comments
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Whats in the GULF is not anything like barry as this feature does not have a circulation at the surface.The mostly outcome for me atleast is for the first name storm to be named in the pacific in a few days.
You can clearly see overall convection is becoming better organized with time.Could be the first TS in the eatern pacific.Overall upper level conditions are forcasted to become favorable for some development.
That activity in the Eastern Pacific you have referred to does not have any circulation associated with it all either. It only looks impressive since there is insanely high upper-level divergence and lower-level convergence in the same area. Once it loses those factors, the impressive convection will rapidly decrease into showers and isolated thunderstorms, just like the SW Caribbean feature has done over the past few days.
In regards to the stratosphere, cooling has been seen in this layer, particularly over the last 20-30 years in the polar regions. As the UV absorbing/reflecting ozone in this layer was depleted by CFCs, energy from the sun was able to pass through the stratosphere and did not heat the layer as much as it previously had. CFC levels are slowly dropping and over time this cooling should slowly diminish.
Changes within the troposphere are less understood...as Mississippiwx's second link suggests.
Higher boundary layer water vapor content associated with climate warming preferentially heats near the surface more than the higher atmosphere - which would lead to higher lapse rates and more instability. Observations seem to bear this out more than the model results showing more warming in the upper atmosphere and less near the surface.
Talking with a couple people in Orlando, the depth of the troposphere is will expand in a warmer climate. This would lead to taller convection which can release heat over a larger column - and a slight increase in intensity.
More research is obviously needed and more consistent global sounding data needs to acquired before this will be resolved...which means the verdict is definitely still out on future intensity changes.
20. mississippiwx23 2:27 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.
16. StormW 2:21 PM GMT on May 21, 2008
I've often wondered though, about a warming atmosphere. How far up in the atmosphere are we speaking? It would seem to me if the atmosphere warmed far enough in altitude, then the ELR would be reduced, and lapse rates wouldn't be as steep for a tropical system to be really strong. Maybe I'm looking to deep into it.
cchsweatherman --why would they restrict use of that information??????
3.12
+2.42%
$132.10
Bangs fist on computer desk.
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.
NEMOC Rota Satellite Imagery Portal
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
It's very strange that the Navy sites all have security certificate issues...
Very good point, StSimon: fluctuations do occur, and given far higher SSTs, a momentary lapse in the shear could be very baaad
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
Thanks for your post. It was very informative.
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!
Moderate Risk of severe weather tomorrow. Looks like a good chase day for people in the plains.
Yep, a very active west Africa for the end of May, huh?
Any ideas?
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.......
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?
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
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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