Hurricane season 2005--why so active?

By: Dr. Jeff Masters , 7:31 PM GMT on August 01, 2005

This will be my last blog entry until August 12; I'm vacationing far from the tropics (Yellowstone!) to appreciate some mountain weather.

Today's monthly summary of hurricane activity for July issued by the National Hurricane Center (NHC) states:

"The month of July saw unprecedented tropical cyclone activity in the Atlantic Basin...with the development of five named storms...Tropical Storm Cindy...Hurricane Dennis...Hurricane Emily... Tropical Storm Franklin...and Tropical Storm Gert. The previous record for named storms that formed in July was four. The two major hurricanes that developed during the month tied a record set in 1916. The July activity follows an unusually active month of June ...And the seven named storms that have formed thus far in 2005 represent a record level of activity for the first two months of the season."

Why has this hurricane season been so active? Part of the reason lies in a decades-long natural cycle in hurricane activity that in 1995 switched to a high-hurricane activity mode. Hurricane activity has been above normal since 1995, and will likely continue to be for the rest of this decade and the next.

Additionally, there are six key ingredients are necessary for tropical cyclone formation (you can read about these in full detail in the Tropical Cyclone FAQ. We'll focus on three of them in particular that have been highly conducive to tropical cyclone formation during this remarkable hurricane season of 2005.

Vertical Wind Shear
Hurricanes need low values of vertical wind shear between the surface and the upper atmosphere (the jet stream level, typically 35,000 - 40,000 feet high in the tropics). Vertical wind shear is the magnitude of wind change with height. High vertical wind shear can disrupt a tropical cyclone trying to form by literally tearing it apart. High wind shear also can weaken or destroy a healthy tropical cyclone by interfering with the organization of deep convection around the cyclone center. Typically, 20 knots (23 mph or 10 m/s) or less difference in wind speed between the surface and upper atmosphere is considered favorable for hurricanes. In June and July of 2005, wind shear values were 20 - 40% below normal for the Gulf of Mexico and Caribbean Sea, the primary genesis locations for the seven tropical cyclones that formed. Wind shear values this low are highly favorable for tropical cyclone formation (see plots below).

Figure 1. Average amount of vertical wind shear (in black) and observed wind shear (in blue) for 2005 for the western Caribbean. Credit: Colorado State University (NOAA/CIRA)

Figure 2. Average amount of vertical wind shear (in black) and observed wind shear (in blue) for 2005 for the eastern Caribbean. Credit: Colorado State University (NOAA/CIRA)

Sea Surface Temperatures
Hurricanes need ocean waters of at least 26.5C (80 F) through a depth of about 50 meters to form or maintain their strength. The warmer the water, the better, since a hurricane is a huge heat engine. Sea surface temperatures (SSTs) are at the highest levels ever observed in the Atlantic, for the 50 years we have records. As of July 31, typical tropical Atlantic SSTs were about 2F (1.1C) above normal.

Figure 3. The Sea Surface Temperature departure from normal (in degrees C) for July 31, 2005. A large area of above normal SSTs (yellows and light greens) covers virtually the entire North Atlantic Ocean. The cold wake of Hurricane Emily is still apparent between the Yucatan Peninsula and southern Texas. Credit: U.S. Navy.

Moist Air
Hurricanes need moist air in the mid-troposphere (5 km or 3 mi altitude). Dry air interferes with the development of the large thunderstorm complexes needed to get a tropical storm going. Until the last week of July, the air over the tropical Atlantic and Caribbean Sea has been very moist. Since then, several large dust storms have moved off of the coast of Africa, accompanied by copious amounts of dry air that has interfered with tropical storm formation. TOMS aerosol data shows a large area of dust covering the entire tropical eastern Atlantic today.

Is Global Warming to Blame?
How much, if any, of this year's activity is due to global warming? That's a difficult question to answer. The research published so far shows that global warming cannot be linked to an increase in the number of hurricanes. So, this season's exceptional number of storms is probably unrelated to global warming. However, there is considerable debate whether or not sea surface temperatures and hurricane intensity have been affected by global warming. It is possible that the remarkable intensity of the hurricanes seen so far this season can be partially blamed on global warming. However, much more research needs to be done on this subject before we can link global warming with hurricane intensity. I plan to write a detailed article on the subject later this season, after I've had time to read the new research linking hurricane intensity to global warming, due to be published in Nature magazine on Sunday, August 7.

Dr. Jeff Masters

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42. HurricaneKing
9:28 PM EDT on August 01, 2005
As my niece would say "yep I do."
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41. Alec
9:27 PM EDT on August 01, 2005
hi. Enough studying for one nite...........just a sec.........will be back in 5-10min.....
40. HurricaneKing
9:28 PM EDT on August 01, 2005
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39. outrocket
8:26 PM CDT on August 01, 2005
does that mean you belive it will come a knocking on your door King...??
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38. HurricaneKing
9:26 PM EDT on August 01, 2005
get stuck and then go NNW.
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37. HurricaneKing
9:25 PM EDT on August 01, 2005
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36. outrocket
8:24 PM CDT on August 01, 2005
it may,would not rule it out,thing is which way will it go..NNE.NE...or get stuck?
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35. HurricaneKing
9:24 PM EDT on August 01, 2005
It has a 1013mb low.
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34. HurricaneKing
9:22 PM EDT on August 01, 2005
The bahamas storm will develop.
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33. outrocket
8:21 PM CDT on August 01, 2005
now..back to canes...LOL..what's next?
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32. HurricaneKing
9:18 PM EDT on August 01, 2005
Go outrocket
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31. weatherwonderer
8:14 PM CDT on August 01, 2005
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30. HurricaneKing
9:11 PM EDT on August 01, 2005
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29. outrocket
8:06 PM CDT on August 01, 2005 can alter your life cycle because you have that choice,I dont think man will out do mother nature.If we alter a cycle,then it may defeat what we do ,destroy us and continue its cycle..but after our demise it's cycle will continue untill the SUN goes Nova..which may be part of the cycle of the universe..even it may have cycles..why should it not?
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28. moocrew
12:56 AM GMT on August 02, 2005
Outrocked do you think that random or not so random things can change a cycle or even alter it all together.
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27. moocrew
12:46 AM GMT on August 02, 2005
Have a good Vac. Dr. Masters. I also want to thank you for giving us your perspective on the matter of the activity. It shall be interesting to see how things shape this month as well. I'm looking forward to your perspective of the issue of storm intensity and global warming and your incite into the data surounding the study. I know you will give us a balanced view point that will be lacking else where.
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26. jeans
12:38 AM GMT on August 02, 2005
I too live on the east coast of Florida and appreciate your comments on the tropics very much. Have a good vacation.
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25. Alec
6:47 PM EDT on August 01, 2005
im gone bye..............
24. outrocket
5:39 PM CDT on August 01, 2005
very possible,everything in life is cyclic,we are.We live a cycle and die.So since the weather has been cyclic(or we would never be able to predict)it stands to reason that even the whole weather cycle has a cycle..SO yes I have to agree with you on gloabal warming....BY the way good point! on Pavement...thats great!
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23. HurricaneKing
10:38 PM GMT on August 01, 2005
going to my blog be back in awhile.
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22. HurricaneKing
10:37 PM GMT on August 01, 2005
Dont talk about rain.
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10:34 PM GMT on August 01, 2005
First...yes I have 23 years of FORECASTING experience but I am not an expert on GLOBAL CLIMATE CHANGE

However, I do understand the SCIENCE and in my humble opinion I don't see a direct connection between so-called GLOBAL WARMING and storms.

Also I think we have seen more WARMING from paving more ground than we do from so-called greenhouse gasses anyway.

Until a researcher can take weather data from the 1960s-1990s and predict what WOULD happen in 2000-2005 I will be a skeptic about climate change.

Isn't it possible that we just haven't been around long enough to SEE a complete cycle??
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20. Alec
6:34 PM EDT on August 01, 2005
did you witness the very heavy rains at fsu weatherboy? it was flooding a bit on campus.
19. outrocket
5:33 PM CDT on August 01, 2005
thats close,I say 4...this weeks looks slow,we deal with one the following week(due to dust inhibitor)3rd week we deal with 1,4t week dust clears and we get 2 as a precurser for Sept:)..oh and 1 will be major..
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18. HurricaneKing
10:32 PM GMT on August 01, 2005
hello anyone?
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17. HurricaneKing
10:29 PM GMT on August 01, 2005
6 storms 4 hurricanes and 2 major hurricanes
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16. weatherboyfsu
10:27 PM GMT on August 01, 2005
Ill say 3 major hurricane....lets go with that......i got to go too.....later....
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15. HurricaneKing
10:25 PM GMT on August 01, 2005
I'm back. Local meteorologist says that the Bahamas low could get pulled in to the old front and push it on shore.
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14. weatherboyfsu
10:23 PM GMT on August 01, 2005
Who knows.....I guess it depends on how much shear sticks around.....we know the sea temps arent going to be a problem....It wouldnt surprise me if we go about a week.....and something pops up, maybe two or three,,,,you know it seems like there is someone that opens the door and lets them out to play with us...opens the flood gates....... man ol man is it kicking butt here....lightning left and right....
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13. Alec
6:22 PM EDT on August 01, 2005
and im going now...........will be back later........Alec out!!!
12. Alec
6:18 PM EDT on August 01, 2005
weatherboy i answered your last question on the other blog. take a look if you havent seen it yet.
11. outrocket
5:16 PM CDT on August 01, 2005
throw a wreath in the sea??? OK..its AUGUST,so how many develop this month?
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10. weatherboyfsu
10:13 PM GMT on August 01, 2005
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9. HurricaneKing
10:11 PM GMT on August 01, 2005
Enjoy your Vacation. Thanks for all the information.
HK out.
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8. outrocket
4:59 PM CDT on August 01, 2005
Have a good trip and hide them pic-nic baskets from yogi..Sure will miss reading your blog everyday.Thanks for today's,you left some good material to keep us in the books.

Enjoy your Vacation,
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7. orlandocanewatcher
6:00 PM EDT on August 01, 2005
Dr. Masters:
Enjoy your vacation and be safe...looking forward to your insightful view of the tropics when you return...thanks for all the useful information thus far...
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6. HurricaneKing
9:45 PM GMT on August 01, 2005
ABNT20 KNHC 012131
530 PM EDT MON AUG 01 2005






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5. Hawkeyewx
3:59 PM CDT on August 01, 2005
Jedkins, that SST chart is "departure from normal" water temp, not the temp itself.
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4. Governor
8:54 PM GMT on August 01, 2005
Dr. Masters, have a great vacation and don't even think about the tropics! I deeply appreciate the information you share and will miss your insights when you're in Yellowstone. have fun!
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3. Jedkins
8:42 PM GMT on August 01, 2005
That SST figure is way off it is trying to say that the water off the coast new england is warmer than it is off the west coast of florida which is way off,the water up there is in the 60's and in the new jersey area mid 70's to maybe 80 now the water off the west coast of florida is upper 80's to low 90's,that is why that is the most inacurate SST graphic I have EVER seen.
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2. Dunhill3
8:32 PM GMT on August 01, 2005
Just found this blog last week. I live on the east coast of Fla. and I think it's a great service, Dr. Masters. Enjoy your vacation! As for the comments from other bloggers, I have to tell you this stuff really matters to us where I live. I don't think I'm being too dramatic when I say any insight you guys share can end up saving lives, so please stop the personal attacks and stick to the weather. we'll ALL bet better off.
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1. iyou
8:24 PM GMT on August 01, 2005
Dr. Masters - Have a wonderful and safe holiday; I hope you will share some of your photos with us with you return! Your direct explanations are appreciated also! :-)
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Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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