Hurricane season 2005--why so active?

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

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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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241. Alec
5:50 PM EDT on August 02, 2005
im leaving since its so slow. will come back on if it warrents....Alec.........OUT!!!
240. weatherboyfsu
9:39 PM GMT on August 02, 2005
Bermuda should have some fun for the next couple of days.....
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239. Alec
5:39 PM EDT on August 02, 2005
hi. my computer has real psychological,physiological, and metaphysical problems. it just loves to boot me off. TD#8 is getting chased by an upper low. It wont strengthen much as long as that upper low stays on its heels.
238. HurricaneKing
9:36 PM GMT on August 02, 2005
My real name is one of those two 147257.
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237. 147257
09:29 PM GMT op 02 Augustus, 2005
Posted By: pirateotobx at 5:00 PM EDT on August 02, 2005.
the storm I'm looking out for is Irene..or maybe Katrina..with those names they just sound like they're going to be tough ones...lol...later folks..back after while

I dont agree with this ;) i think Ophelia and Lee going to be the bad ones
Member Since: August 2, 2005 Posts: 7 Comments: 68
236. 147257
09:27 PM GMT op 02 Augustus, 2005
someone thinks that a hurricane wil hit the ABC islands?

And TP depression is currently moving north
Member Since: August 2, 2005 Posts: 7 Comments: 68
235. fredwx
5:09 PM EDT on August 02, 2005
Looks like TD 8 has formed 350 miles SW of Bermuda and is heading NE.
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234. weatherboyfsu
9:03 PM GMT on August 02, 2005
Theres an upperlevel low right on its tail....and forecasted to stay on its tail.....so it will be something to watch.....the closer it gets obviously the more shear will hurt it.....
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233. HurricaneKing
9:05 PM GMT on August 02, 2005
yes
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232. Denials
4:02 PM CDT on August 02, 2005
Am I correct in thinking that the southern outflow looks restricted? If this is the case, could this hamper development?
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231. pirateotobx
5:00 PM EDT on August 02, 2005
the storm I'm looking out for is Irene..or maybe Katrina..with those names they just sound like they're going to be tough ones...lol...later folks..back after while
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230. HurricaneKing
9:02 PM GMT on August 02, 2005
lalalalala It could do a loop.
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229. pirateotobx
4:58 PM EDT on August 02, 2005
I've been through enough hurricanes...I like to stay on top of, or ahead of the game if possible..;-)
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228. HurricaneKing
8:58 PM GMT on August 02, 2005
we have TD8.Aint that great.
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227. weatherboyfsu
8:54 PM GMT on August 02, 2005
I didnt know you guys had it officially......okkkkk
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226. HurricaneKing
8:43 PM GMT on August 02, 2005
we have td 8.
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225. hurricanecrab
8:44 PM GMT on August 02, 2005
yeah and our good ol' Wunderground had it a while ago.
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224. weatherboyfsu
8:42 PM GMT on August 02, 2005
TD #8 ON THE NHC.....WOW
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223. hurricanecrab
8:43 PM GMT on August 02, 2005
Which reminds me to encourage everyone to spend the five bucks a year to subscribe to WUNDERGROUND.
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222. hurricanecrab
8:41 PM GMT on August 02, 2005
And one needs to make their own decisions during such times. There is nowhere to run on this 12 X 1 mile chunka rock but UP.
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221. hurricanecrab
8:38 PM GMT on August 02, 2005
We live 12 tiny little feet above sea level and 150 feet from the ocean When the big ones come, it's an easy decision -- go to the cave w/cats. When they noodle around and/or are on the cusp of 'big enough' we need to access our own info. when Ivan was headed here, we went to the cave and stayed there even after we could see it was turning toward Grand Cayman and away from us because we couldn't access any reliable info.
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220. hurricanecrab
8:35 PM GMT on August 02, 2005
Pirate thanks for the info. Think I will subscribe too. Most of that info is available through independentwx.com, but I like the way it's organized on Alley. Plus easy and quick access to steering currents. good stuff. thanks again
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219. pirateotobx
4:29 PM EDT on August 02, 2005
I agree...but it just shows how quickly things can change from absolutely nothing going on to a, possibly, new storm in a mattter of hours.
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218. STORMTOP
8:24 PM GMT on August 02, 2005
one affecting the us not for another 2.5 weeks...even if harvey forms so what he will be a problem to the fish only...
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217. pirateotobx
4:16 PM EDT on August 02, 2005
Whirlwind, It may form today...

Hurricanecrab I just subscribed...i'll let you know as soon as I get my log-in info if it's worth it. It was 6.00 for 1 month so I figured I'd try it. Some of the features that are available I think it will be a good resource.
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215. whirlwind
8:10 PM GMT on August 02, 2005
When do you guys think the first storm of aug will form? Now it is really starting to slow down... it was looking good with 25 continuous days of activity.
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214. pirateotobx
4:09 PM EDT on August 02, 2005
no I haven't subscribed yet... I've been leaning towards it more recently....they seem to stay on top of things and I can find out data, like that graphic, sooner than most places announce anything.
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213. pirateotobx
4:06 PM EDT on August 02, 2005
I haven't found where they have "officially" called it as a storm but they are beginning tracking. If this looked like it would be a threat to the US coast we would probably be getting more info.
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212. hurricanecrab
8:06 PM GMT on August 02, 2005
Pirate, seems like it might be worth the subscription price; do you subscribe? The overview looks like all the info is organized into a logical flow for a nonforcaster like me
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210. HurricaneKing
8:04 PM GMT on August 02, 2005
Mine was weather related.
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209. HurricaneKing
8:03 PM GMT on August 02, 2005
so.
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208. Hawkeyewx
2:56 PM CDT on August 02, 2005
You know, guys, if we don't cut way back on these non-weather related, one-word conversation type posts, of which there are several just above this post, this blog thread will have a million damn posts by the time Dr. Masters gets back from vacation.
Member Since: July 5, 2005 Posts: 2 Comments: 1925
207. HurricaneKing
7:58 PM GMT on August 02, 2005
does that mean its a storm?
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206. hurricanecrab
7:58 PM GMT on August 02, 2005
great link Pirate, thanks. Now I have yet another resource to add to my favourites.
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205. hurricanecrab
7:57 PM GMT on August 02, 2005
Carryover from the other blog Alec. yah they are. Almost etherial.
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204. HurricaneKing
7:57 PM GMT on August 02, 2005
wha does anyone else think?
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203. pirateotobx
3:55 PM EDT on August 02, 2005
Link

Link to forecast path and strength of 08L(Harvey)
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202. Alec
3:56 PM EDT on August 02, 2005
integrals are cool. -sorry that was too random. now im gone from jeffs blogg also for a while.........see ya later guys!!...........
201. hurricanecrab
7:56 PM GMT on August 02, 2005
me either. One one hand looking too hard, sometimes seeing things that don't matter. On the other, don't want anything to develop. The whole area here has had ENOUGH
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200. Hawkeyewx
2:47 PM CDT on August 02, 2005
The water vapor loop shows the Bahamas upper low actually moving back northeastward toward the surface low, so that will likely hamper the development of the surface low for a while longer.
Member Since: July 5, 2005 Posts: 2 Comments: 1925
199. HurricaneKing
7:55 PM GMT on August 02, 2005
not sure.
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197. hurricanecrab
7:53 PM GMT on August 02, 2005
you're right. My apologies, however that's no excuse for not leaving any snacks. Now. What about the 32 n 44? Think we're looking at anything?
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196. HurricaneKing
7:51 PM GMT on August 02, 2005
I was talking to you go back and look.
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195. hurricanecrab
7:51 PM GMT on August 02, 2005
32 & 44 looks possible
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193. hurricanecrab
7:50 PM GMT on August 02, 2005
King how rude...... leaving me talking to myself in your blog. Didn't even leave any crackers there or anything
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192. HurricaneKing
7:49 PM GMT on August 02, 2005
told you
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Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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