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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When we start seeing some consensus among the ensembles.
Then we give more interest,least for me.
The seasonal Tropics (reg.Patent.Pending)...are arriving on schedule.
updates are in the comment section
CURRENT STATUS as at 21st May 2008
Next update expected by 4th June 2008 (two weeks after this update).
Summary: Pacific climate patterns generally neutral
Pacific climate patterns remain generally neutral, although some aspects of the 2007/08 La Niña persist. Lingering La Niña signals include the continued presence of cooler than average water on the equator near the date-line, accompanied by reduced cloud and, in the first few weeks of May, enhanced Trade Winds.
Overall though, the tropical Pacific has been warming gradually during the past few months, with ocean temperatures now near-normal in central and eastern areas. In addition, the Trade Winds have decreased to near-normal levels in the western half of the Pacific and, confirming the return to neutral conditions, the Southern Oscillation Index (SOI) is about zero for the past 30 days.
Computer model predictions show Pacific temperatures continuing to gradually increase over the next two seasons, but remaining near-average. The models indicate a low chance of either a stronger warming to El Niño levels or a re-intensification of La Niña conditions during 2008.
weather456 that is from november.
That was a mistake...Now the one below....look at the time frame of the possible disturbance, that is May 29-June 5
LINK
Wow, if I'm analyzing correctly
You must be thinking Ne..and East
2008
Most conditions are expected to be the same except for reduce wind shear from La-Nina-Neutral in 2008.
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.
NEXRAD Radar
New Orleans Base Reflectivity 0.50 Degree Elevation Range 124 NMI Link
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?!
ASO refers to the months of August, September and October
Source: Noaa's Outlook
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.
Denver Base Reflectivity 0.50 Degree Elevation Range 124 NMI Link
Buy that NOAA radio and your Hurricane Supplies Yet JFV?
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
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 focus on real time events and impacts.
Glad to see you got that radio.
Next Week is Hurricane Preparedness Week Link
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.
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