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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92. oriondarkwood
2:17 PM GMT on August 02, 2005

Where at in NC, I grew up near Ft. Bragg?
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91. Alec
10:15 AM EDT on August 02, 2005
sittingwetinflorida, was your thermometer facing the sun because that sometimes gives an inaccurate real outside temp? Not saying you did it but want to know where its placed.
90. HurricaneKing
2:15 PM GMT on August 02, 2005
no rain yet today.
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89. HurricaneKing
2:13 PM GMT on August 02, 2005
I'm thinking about changing my name to sittingwetinnorthcarolina. No joke.
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88. Alec
10:14 AM EDT on August 02, 2005
Tallahasee area is cooler today because of the clouds and rain. there wont be temps in the mid/upper 90's here at all.
2:08 PM GMT on August 02, 2005
hurr king if and i emphasize if it develops it will be harvey but its only going to do one thing out to sea with the may clip nova sctia like the last one did...
86. HurricaneKing
2:12 PM GMT on August 02, 2005
What's going on?
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85. sittingwetinflorida
10:13 AM EDT on August 02, 2005
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84. sittingwetinflorida
10:11 AM EDT on August 02, 2005
i know down here were i live it got to about least that was the temp. on my back porch yesterday when i got home.
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83. Alec
10:08 AM EDT on August 02, 2005
hurricane king welcome!
82. HurricaneKing
2:07 PM GMT on August 02, 2005
Its the twilight zone.
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81. Alec
10:06 AM EDT on August 02, 2005
that wont happen stormtop. i guarantee you that because we're getting lots of cloud cover and rain and it didnt even get close to 90 yesterday.
2:06 PM GMT on August 02, 2005
anything that forms now will go out to sea the high will surpress all activity hitting th us for the next 4 days...
79. HurricaneKing
2:05 PM GMT on August 02, 2005
Both stormtop and alec. What a shock.
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2:05 PM GMT on August 02, 2005
where you are at and me ....for the next 3 days very hot..
77. Alec
10:03 AM EDT on August 02, 2005
where will the temps reach the upper 90's?
76. Alec
10:00 AM EDT on August 02, 2005
1:56 PM GMT on August 02, 2005
the dust may stop these storms all together from the african coast developing...i still think we are going to get to the t im dropping my prediction to 20.....3 more major hurricanes and only 4 hitting the predicting 7 more hurricanes...this will not be a record season when it goes out...i am predicting a mild august and a busy september and october....
1:34 PM GMT on August 02, 2005
this whole week will be boaring like alec and toyota lol j/k...conditions have changed a big high is expected to build in tomorrow sending temps in the upper 90s...squashing all tropical activities..august may start out as a dull month for the first 2 weeks..thats my prediction...
73. Alec
9:37 AM EDT on August 02, 2005
Now the problem is if a storm like Charley got into the Gulf when its a hot tub. If the conditions are right there can be significant development. The lull will give the Caribbean and Gulf time to consume vast amounts of heat. The NW Caribbean is notorious for producing some of the strongest hurricanes because of it's high heat content.
72. punkasshans
8:34 AM EST on August 02, 2005
Look out to Africa for the next REAL storm. The circulation next to the Bahamas I guess could still become something. It still has a circulation and lots of convection, so if the sheer dies down, we could get something. But even then, i dont expect it to be a news breaker. Africa is where the next storm is coming from (or at least a wave that is within 1000 miles from it, hurricanes dont really come OFF the continent!)
71. Alec
9:34 AM EDT on August 02, 2005
i agree with you punk. Nothing going on down there for the forseeable future.
70. punkasshans
8:32 AM EST on August 02, 2005
The gulf is dead. Actually, very little to speak of over the entire ocean. Boring day today.
69. Alec
9:27 AM EDT on August 02, 2005
Now with global warming scientist are still trying to find whether global warming has an impact on hurricane intensities. While global warming can shift climate in the long run we must look deeper into the other factors that make hurricanes so intense. Sea surface temps help but the lack of wind shear, a moister environment, the situation of the upper highs that help ventelate the the hurricanes, and the contents of our atmosphere will determine more precisely what the reasons are. Still there is no definite answers with respect to global warming but i think it'd help to see the whole picure.
68. Alec
9:17 AM EDT on August 02, 2005
Well hawkeye, thanks yesterday again for correcting me in one of my explanations. i got it confused with something else. I like it when we learn like that because the atmosphere is still a place of discovery. Now as the heat of the hurricane season ramps up so does the environment over near the cape verdes. It becomes moist, and the dust becomes less of a factor in those tropical waves. I think the dust will taper off as the climatological peak starts to happen the next few weeks. Just like last yr. at the beginning dust hampered the moist atmosphere over the cape verdes but soon got replaced by a moist environment where Charley, Frances, Jeanne, and Ivan turned the yr into a record setter!
67. Denials
8:19 AM CDT on August 02, 2005
Pcola...while I'm not disagreeing with what you are saying, it could also be that we've been *overmeasuring* past ocean temperatures, not *undermeasuring* them, as your argument seems to assume. While I would have to assume that we have indeed gotten better at measuring sea temps, do you know which way the previous bias was? If the bias were towards overmeasuing in the past, there might actually be *more* of an increase than it looks like.

I have no way of knowing which way it is, just throwing out a thought.
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66. Hawkeyewx
7:53 AM CDT on August 02, 2005
Alec, I wonder if all the dust coming from Africa could dampen the Cape Verde season, assuming we keep getting dust for the next couple months.
Member Since: July 5, 2005 Posts: 2 Comments: 1928
65. Alec
7:48 AM EDT on August 02, 2005
Oh yeah i forgot to say: GOOD MORNING TO ALL!!!
64. Alec
7:31 AM EDT on August 02, 2005
The anaysis for today in the tropics is pretty much quiet. That wave ne of the bahamas has a surface low and convection filled in on the eastern side of it because of shear from an adjacent upper level low to its west/nw. If this surface low can get into a better environment it has a chance to develop in the next couple of days. The Gulf remains quiet with nothing imminent. The Caribbean has little convection for now. The hurricane season will start heating up the next few weeks as we reach the climatalogical peak somewhere between Aug20-September.
62. DauphinIslandDunes
8:58 AM GMT on August 02, 2005
Thunder on the Dauphin Island got me up just now. PCola makes a good point about alarmism with regard to global warming studies - that bent does muddy waters on the science. Let's look at numbers, I made a quick count on number of storms in 1995-2004, the past ten year period, it's 138. How many other ten year periods have this number of storms? Also I looked at last five years, 2000-2004, the number of storms for this 5 year period is 72. How many other 5 year periods have 72 or more storms? I'm definitely curious. The answer would help us to get some beginning sense concerning the normalcy or unusualness of the current period.
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61. Alec
12:14 AM EDT on August 02, 2005
NOTHING WILL DEVELOP IN THE GULF ANYTIME SOON. nada. do you understand!!!!!!!!!!!
60. Alec
12:08 AM EDT on August 02, 2005
How do you know that toyota's unexperienced? You guaranteed a depression for the last couple of days and it hasn't panned out. you're not the jugdge on who is smarter on here. accurate predictions speak for themselves.
59. Alec
12:05 AM EDT on August 02, 2005
im watching stormtop. what was that comment a second ago using my name supposed to mean?
3:58 AM GMT on August 02, 2005
my prediction by the way is 22 storms for this year i revised it up...also 6 major hurricanes...i think 7 will hit the us 5 on the gulfcoast 2 east coast...
57. pcolaFL
3:51 AM GMT on August 02, 2005
It's late here, so I will say goodnight. I'll check back tomorrow to see if anyone responded. BTW--happy trails, Dr. Masters! Enjoy your well deserved rest, and thanks for the excellent info you always include in your blog!
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3:44 AM GMT on August 02, 2005
i think the next TD8 will come froim the gulf of is a beginner and needs to learn a lot more to really study how a storm acts and alec well he isnt playing with a full deck at the moment...this blog should be strictly for weather alec and nothing else...DO YOU UNDERSTAND!!!!!!!!!!!!with that said keep your eyes on the gulf and good night...
55. pcolaFL
3:34 AM GMT on August 02, 2005
I question the science behind assertions that average sea temperatures have been increasing and are now higher than ever in the past. Isn't it true that we simply have gotten better at measuring sea surface temperatures? Technology has advanced since we began recording these figures, and now we can be even more accurate than ever. Sea temps may have been just as high in different historical eras, but we did not know how warm they were because measurements weren't as accurate. I think linking hurricane intensity to global warming is a huge assumption. Two claims being made by MIT, if I understand them correctly, are based on average SST, monetary damage assessments (which I think most people understand *have* to be higher now with more multimillion dollar homes etc on the coastlines than in the past). The report seems, well, alarmist.
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54. Alec
11:29 PM EDT on August 01, 2005
nite dunes, dont blow away.
53. DauphinIslandDunes
3:11 AM GMT on August 02, 2005
I think global warming can create both dampening effects (e.g., dust storms and perhaps shear) and enhancing effects (e.g., warm seas and higher moisture in atmosphere). I would guess, I not an expert on any of this, that overall warmer seasons will be more intense storm-wise as we seem to be experiencing right now. Thanks for the discussion - got to get some sleep now. I'll read comments tomorrow.
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52. HurricaneKing
11:07 PM EDT on August 01, 2005
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51. Alec
11:02 PM EDT on August 01, 2005
i think the earth pretty much tries to get into a state of equilibrium. with pressure differences over the entire globe, systems can change their cycles if extreme differences such as heat play into the mix.
50. outrocket
9:55 PM CDT on August 01, 2005 are you suggesting that maybe since land heats faster..that gloabal warming could be causing the african dust stormsto be more intense and actually dampen the season??..
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49. DauphinIslandDunes
2:11 AM GMT on August 02, 2005
The potential effects of global warming or pre-existing hurricane decadal cycles will be debated for quite awhile I imagine. However, assuming there is global warming, which most climate scientists seem convinced of, then the immediate question which arises is not whether we can prove that global warming is the cause of more intense hurricane seasons, but whether global warming is a plausible explanation for more intense hurricane seasons. I would like to know if there is any ten year period with as many storms as the period from 1995 to 2004. If so, then we could easily be dealing with cycles. If not, while we could be dealing with very long cycles, this is not much to hold onto until we at least know how long of a cycle we are dealing with. On the other hand, the explanation offered by global warming is far more straight forward and has predictable results. Global warming will warm the seas and this will tend to produce storm seasons that are longer and of greater intensity. Of course warming could produce other effects which might tend to dampen the season. I suspect however that we will generally continue to see intense seasons and that warming effects will become better predictors of upcoming seasons than theories about hurricane season cycles.
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48. HurricaneKing
10:40 PM EDT on August 01, 2005
come to my blog everyone is there.
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47. moocrew
2:29 AM GMT on August 02, 2005
Thank you outrocket for your explanation.....i find your view point interesting.
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46. Alec
9:35 PM EDT on August 01, 2005
everyone we're going to hurricaneking's blog. see ya there......
45. HurricaneKing
9:33 PM EDT on August 01, 2005
thankyou but like I said I'm going to my blog you should come to.
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44. outrocket
8:29 PM CDT on August 01, 2005
really that was remanants from 92L? did get kinda sheared off course,high was weak and the shear became it's steering current.It does look as it may be trying to make a comeback.that front is a wash,so the HIGH could ridge back in...and if may be right about NNW
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43. HurricaneKing
9:30 PM EDT on August 01, 2005
going to my blog you should follow.
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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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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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