Global warming and hurricanes part 1: The natural cycle

By: Dr. Jeff Masters , 7:39 PM GMT on December 19, 2005

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Before we can discuss the possible influence of global warming on hurricanes, we need to set the stage by talking about this natural cycle of hurricane activity we hear so much about. The Atlantic Multidecadal Oscillation (AMO) is a cyclic variation in the large-scale atmospheric flow and ocean currents in the North Atlantic Ocean that combine to alternately increase and decrease Atlantic sea surface temperatures (SSTs). As its name indicates, the AMO is "multidecadal"--meaning it operates on a time scale of multiple decades. The cool and warm phases last for 25-45 years at a time, with a difference of about 1�F (0.6�C) between extremes. These changes are natural. Analysis of tree rings, fossil coral, and sediments has shown that the AMO has been around at least 300 years, and probably much longer.

What has the AMO done in recent years?
As seen in Figure 1, the AMO has been though about two complete cycles since detailed measurements of the Atlantic began in the mid-1800s. A cool phase lasted 25 years from 1901-1925, a 44-year long warm phase from 1926-1969, and a 25-year long cool phase from 1970-1994. A new warm phase began in 1995, and the AMO index values since 2001 have been the highest on record. This has resulted in sea surface temperatures over the prime hurricane breeding grounds of the tropical North Atlantic being the highest on record, as well. The AMO index in 2004 was about the same as in 2003, but 2005 has seen about a 10% drop from 2004's level.


Figure 1. The AMO index, 1871 to 2003. The index was computed by averaging sea surface temperatures north of the Equator over the Atlantic Ocean, between 75�W and 7.5�W and south of 60�N. The red regions show warmer than average SSTs over the North Atlantic, and the blue regions, cooler than average. The "trend" has been removed, so the mean and long-term increase in SSTs are not visible. Image credit: Rowan, T.S., and Daniel Hodson, "Atlantic Ocean Forcing of North American and European Summer Climate", Science 309, 115-118, 2005. Reprinted with permission from SCIENCE, � 2005 AAAS. Permission from AAAS is required for all other uses.

How do hurricanes vary during warm and cool AMO cycles?
The AMO is thought to strongly influence the incidence of intense hurricanes, which more than double during the years when the warm phase of the AMO. For example, an average of 1.5 intense (category 3, 4, and 5) hurricanes per year formed in the Atlantic during the last cool phase of the AMO, 1970-1994, while 4.1 intense hurricanes/year formed during the current warm phase (1995 - 2005). The number of tropical storms and weak hurricanes do not change much between cool and warm AMO periods.

What causes the AMO, and can we predict it?
The causes of the AMO are not well-understood, but some "coupled" computer models that simulate the behavior of both the atmosphere and the ocean are beginning to shed light on this. One of the leading theories is that changes in the ocean's salt content causes a speed up or slow down of the Gulf Stream, due to the fact that density differences between lighter fresh water and heavier salty water drive weaker and stronger ocean currents, respectively. This circulation (called the "thermohaline circulation") is thought to cause the warm phase of the AMO and warmer Atlantic SSTs when it speeds up, and cooler SSTs and a cool AMO phase when it slows down. It is also possible, but less likely, that changes in atmospheric circulation cause the AMO. We are a long way from being able to predict when a particular phase of the AMO will begin or end. The last warm phase lasted for 44 years. The current cycle began in 1995, so it may not be until 2040 that the current active period of hurricane activity dies down.

What influence does global warming have on the AMO?
Since the instrument record extends back for only two cycles of the AMO, it is difficult to say if the record warmth in the current AMO cycle is unusual. It has been observed that the tropical North Atlantic SSTs have warmed approximately 0.3�C over the past 100 years due to global warming. However, the current thinking of climate experts is that global warming should act to dump increased fresh water into the North Atlantic and reduce the speed of the Atlantic thermohaline circulation. This would lead to a decrease in Atlantic SSTs, and a reduction of intense hurricanes. Either global warming hasn't acted to dump enough fresh water into the North Atlantic to affect the thermohaline circulation, or else the theory is wrong!

Jeff Masters

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146. brianpm
6:19 PM GMT on December 21, 2005
A couple questions about this post.. not directly hurricane related:

1. could you post your references, please. I've been trying to find decent explanations/theories for the atlantic multidecadal mode for some time, and have been unimpressed.

2. I am a little confused by your explanation. The gulf stream is a result of western intensification (aka, western boundary current). It is almost entirely wind driven; basically the trade winds blow the water south and west over the tropical Atlantic and the Coriolis force, conservation of momentum, and the existence of the continent (stopping the water from going further west) all combine to make a very strong, narrow current, which we call the gulf stream. There are similar currents on all the major western boundaries. The thermohaline circulation, as its name tries to imply, is driven by density differences of sea-water that arise from differences in temperature and salinity. Thus, the two currents are fundamentally different phenomena (dynamics versus thermodynamics). The blog entry seems to equate them and their variations, but that doesn't seem right. My guess is that "gulf stream" should not have been mentioned at all, as the rest of the sentence seems to implicate the THC. Is this the right interpretation, or am I missing something?
145. TampaSteve
4:15 PM GMT on December 21, 2005
atmosweather wrote:

"The damage was certainly category 5. Definitely. Now about its actual intensity at landfall. I would normally say it wasn't a category 5, but it really depends what you mean by intensity. If you mean meteorologically then it wasn't a category 5, i would say it was a category 4 145-150 at first landfall and category 4 135 at second landfall. However, if you mean by "pure strength" then it was a category 5 no question. No hurricane that did that much damage should be classified at anything below a category 5"

I'm in agreement with you on the landfall strength...around 145 at first landfall in LA, and 135 at second landfall in MS. Katrina made landfall as a large and very powerful Cat 4 storm, carrying a record-breaking Cat 5 surge (28-30 feet) into the Gulf Coast. The NHCs downgrading of such an historic storm would be unprecedented, AFAIK. We have cases of storms being bumped up in strength after further analysis (Andrew, Charley), but not the other way around, at least not to the extent that Katrina's landfall strength is being "modified" by the NHC.

Katrina was a STRONG Category 4 at landfall...that is what the evidence shows, and that is how the NHC should write the final report. Anything less would ignore reality.
144. tornadoty
3:47 PM GMT on December 21, 2005
And Trouper, look at my blog.
143. tornadoty
3:30 PM GMT on December 21, 2005
Folks, one small correction, the world record is 42 feet, by some Australian cyclone about 100 years ago.
142. Inyo
8:18 AM GMT on December 21, 2005
Theboldman, I live in pasadena but i work in the Angeles National Forest (San Gabriel Mountians) which is why i am also occasionally talking about things like snow and summer thunderstorms LA doesn't usually get.
Member Since: September 3, 2002 Posts: 42 Comments: 873
141. Trouper415
6:28 AM GMT on December 21, 2005
With the types of storms we have had this year, I like the idea of a grading system that calculates pressure, windspeed and storm surge. All three components of the system can back each other up. For instance, katrina made landfall with supposed 125 mph winds and a pressure of 920. However, the storm surge was record breaking, being equally if not more destructive than the winds themselves. Thus, adding in a new catagory and taking up the 'slack of error' from the old classification system. Now, the storm surge catagory, might be been in essence 'not worthy' of making this calculation system in some peoples regards. Say if the storm would have struck a desolate place with nary a home or building, there would not be much structural damage and damage costs wouldnt be in the billions, However, storm surge is equally a factor for judging a storms sheer intensity and might, weather or not the storm strikes a densely populated area or not.
Member Since: September 22, 2005 Posts: 5 Comments: 637
140. phelp
6:07 AM GMT on December 21, 2005
889
You should post your e-mail to the nhc as well. I would like to read what you have to say.
- Nick
139. theboldman
5:41 AM GMT on December 21, 2005
hey inyo i heard your from CA as well were about in ca are you ?
Member Since: September 8, 2005 Posts: 25 Comments: 2
138. StormJunkie
5:32 AM GMT on December 21, 2005
E-
Cat and damage don't always relate the way you think they would. Katrina was a cat 3 imo. She tore up pretty could just prior to landfall.

Surge would be my geuss as to why it was the most costly and geography. She carried a 4 to five surge with her when she made landfall and that coupled with the topography of that area led to tons of damage. And then the fact that we built a city in a fish bowl which was a disaster just waiting to happen. A slow moving cat one could realisticly cause more monetary damage then a fast moving five. or a very very large cat 1 could cause more monetary damage then a very small cat 5. So Category does not determine damage in any real way.

Great discussion though. Wish I had more time to read and comment.

Please check out my blog and help me if you can.

Thanks
SJ
Member Since: August 17, 2005 Posts: 26 Comments: 16519
137. atmosweather
5:26 AM GMT on December 21, 2005
E

The bulk of the damage was caused by Katrina's world record 24-28 feet storm surge in LA and MS. Despite their being a huge amount of wind damage, the catastrophic economic losses and loss of life is attributed to the storm surge that destroyed entire towns and breached the New Orleans' levees. Now, the NHC actually try to provide a reason for that kind of surge with a mid-range Category 3 hurricane that they are trying to make us believe that Katrina was. They say that because Katrina was a massive Category 5 hurricane 12 hours before landfall it made huge ocean swells leading to 30-40 ft wave heights, and those cannot be decreased because they are already high and stick around for 24-36 hours after they generate. However, they forget that storm surge is the height of water above sea level, not the peak wave heights. That is why they measure the marks off buildings, not see how far a wave got up a building. Their reason that I just mentioned cannot be attriubted for storm surge, so they really have little explanation for Katrina's world record storm surge in a Category 3 storm. That is just one reason why the NHC are way off here and also why Katrina was the destructive force that it was.
Member Since: September 24, 2005 Posts: 33 Comments: 9265
136. E
5:18 AM GMT on December 21, 2005
If Katrina was truly only a Cat. 3 at landfall...but yet caused phenomenal damage, what was the reason?

I know what I would say...but that's just because I'm a liberal...although were really not here to talk about politics. So, atmos, to what do you accredit the damage...because even if Katrina was a cat 4 at landfall, the most costly hurricane in U.S. history had to happen for a reason.
135. atmosweather
5:01 AM GMT on December 21, 2005
Yea rmh very strange. Katrina was a freak of nature. It completed an eyewall replacement cycle lasting 24 hrs without losing a single mph in wind lol!!!
Member Since: September 24, 2005 Posts: 33 Comments: 9265
134. rmh9903
4:49 AM GMT on December 21, 2005
I heard a few strange things during all of this that makes no sense... During its Cat 5 period seas were recorded at between 20-30 feet to the north of the eye to 50-60 feet south of the eye. How is that possible? Then seas were 20-40 feet 6 hours before landfall then it weakened more and 1 hour before they were 40-45 feet... did I miss read something or was this storm just a freak of mother nature?
133. Inyo
4:36 AM GMT on December 21, 2005
it seems to me that doppler radar might be prone to error in cases of extreme wind like that of Katrina. It uses the speed of the particles (rain, etc) in wind to determine the speed, right? Well, does anyone really know how rain particles act in 140 mph gusty winds? with all the eddies, variation, turbulence, and the inertia of the raindrops themselves it seems that if anything the doppler will underestimate the winds.

also i was not trying to downplay the tragedy of Katrina at all. In my opinion, it was a human disaster as much as a natural disaster. In todays society no one should have to die in a storm you can see coming 24 hours in advance. The death toll 'could' have been much higher, in a worse case scenario, definitely. However, it could have been much lower with Katrina as well, with better systems of evacuation, more well-maintained levees, and maybe a push to get the poor (and if possible everyone else) out of these below sea level areas... not to mention better protection of the wetlands which naturally buffer storm surge. My friend who works for the park service in the area tells me that the only houses left standing in teh area he was working was the few that happened to be behind the swampland preserve.

Even if the climate never warms another degree, other hurricanes of this force WILL hit new orleans and other major cities along our coast. In my opinion, we (the US) need to do better.
Member Since: September 3, 2002 Posts: 42 Comments: 873
132. KatrinaRitaWilmaZeta
4:18 AM GMT on December 21, 2005
atmosweather when the storm was a cat 5 at one time did they update the winds 180 or 185 or did they keep it at 175 mph
131. atmosweather
4:16 AM GMT on December 21, 2005
Have a great night torn
Member Since: September 24, 2005 Posts: 33 Comments: 9265
130. tornadoty
4:14 AM GMT on December 21, 2005
Good night guys. See some of you in the morning.
129. atmosweather
4:14 AM GMT on December 21, 2005
Have a great night David
Member Since: September 24, 2005 Posts: 33 Comments: 9265
128. KatrinaRitaWilmaZeta
4:07 AM GMT on December 21, 2005
well good night you too i would like to give the nhc a lol for makeing that a cat 3 for land fall my god what will they do next? well night all
127. atmosweather
3:59 AM GMT on December 21, 2005
LOL thanks torn. I agree that my estimates are very robust but I do believe I can back up everything I say, just like everyone else can back up what they say. Your estimates are pretty accruate though.
Member Since: September 24, 2005 Posts: 33 Comments: 9265
126. tornadoty
3:56 AM GMT on December 21, 2005
That makes a ton of sense Atmos (no sarcasm).
125. KatrinaRitaWilmaZeta
3:48 AM GMT on December 21, 2005
atmosweather whats do it call them up
124. atmosweather
3:44 AM GMT on December 21, 2005
LOL David, maybe we should call them and get everyone in one place and put the phone on speaker. Then they can hear all of us complain LOL!!!!!
Member Since: September 24, 2005 Posts: 33 Comments: 9265
123. KatrinaRitaWilmaZeta
3:40 AM GMT on December 21, 2005
what all go tell on the nhc or call them up
122. atmosweather
3:37 AM GMT on December 21, 2005
The expansion of the pressure field also contributes to a much better "mixing down" process. Larger hurricanes can mix down flight level winds much better than small hurricanes. And, judging by the amount of deep convection that wrapped back around the western eyewall near BOTH landfalls, I think that Katrina attained category 4 status near landfall in LA. Since it didn't lose any of its deep convection and only weakened a little before the second landfall, I believe it was still at category 4 intensity.

By the way, I have just noticed a typo which may change your views. I meant to say 140-145, not 145-150 in my assessment that I posted.
Member Since: September 24, 2005 Posts: 33 Comments: 9265
121. tornadoty
3:32 AM GMT on December 21, 2005
I saw it the day it made landfall and saw the burst of convection at landfall. It looked like it was strenghthening at landfall to me. I know where you are coming from. However, I think that the expansion of the pressure field dulled that effect.
120. atmosweather
3:26 AM GMT on December 21, 2005
Yeah I know you're not arguing torn, and neither am I. I am just stating my belief and so are you. If you havent already you should watch the infrared movie of Katrina throughout its lifetime and pay attention to its structure and presentation near landfall on the Gulf coast. If you are not convinced after looking at that then I will give more explanations.

Katrina movies, click on IR next to "Hurricane Katrina Movies", it should open in Windows Media Player.
Member Since: September 24, 2005 Posts: 33 Comments: 9265
119. tornadoty
3:21 AM GMT on December 21, 2005
Atmos, I am not arguing with you. But please explain your reasoning so I can understand.
118. atmosweather
3:14 AM GMT on December 21, 2005
I can assure all of you even though that I respect each and every opinion here that Katrina was a category 4 at both landfalls. If you want me to explain I will. I am just stating my belief.
Member Since: September 24, 2005 Posts: 33 Comments: 9265
117. atmosweather
3:12 AM GMT on December 21, 2005
It is an injustice if the costliest natural disaster in United States' history, the deadliest storm since 1928, and a storm that has displaced over 2 million people is rated as a category 3 storm
Member Since: September 24, 2005 Posts: 33 Comments: 9265
116. tornadoty
3:12 AM GMT on December 21, 2005
I am a bit more conservative than you Atmos. It was probably 135 MPH at LA landfall and 130 MPH at MS landfall. Small, but important differences fromt the NHC report.
115. KatrinaRitaWilmaZeta
3:09 AM GMT on December 21, 2005
i want that hurricane update to a cat 5 or cat 4
114. atmosweather
3:07 AM GMT on December 21, 2005
The damage was certainly category 5. Definitely. Now about its actual intensity at landfall. I would normally say it wasn't a category 5, but it really depends what you mean by intensity. If you mean meteorologically then it wasn't a category 5, i would say it was a category 4 145-150 at first landfall and category 4 135 at second landfall. However, if you mean by "pure strength" then it was a category 5 no question. No hurricane that did that much damage should be classified at anything below a category 5
Member Since: September 24, 2005 Posts: 33 Comments: 9265
113. KatrinaRitaWilmaZeta
3:06 AM GMT on December 21, 2005
atmosweather you give the nhc a lol i never want to her a hurricane being downgrade this look at all the damage that stom did and they call it a cat 3 like come on now
112. atmosweather
3:02 AM GMT on December 21, 2005
A lot of my email comes from my earlier post, and of course I added a lot to those points while also adding a lot on storm surge, wind-pressure relationships amongst many other things
Member Since: September 24, 2005 Posts: 33 Comments: 9265
111. KatrinaRitaWilmaZeta
3:01 AM GMT on December 21, 2005
atmosweather do you think that was cat 5 damage. and a cat 5 that made land fall
110. KatrinaRitaWilmaZeta
2:59 AM GMT on December 21, 2005
atmosweather what did you tell them
109. atmosweather
2:58 AM GMT on December 21, 2005
Well that's another point in itself, David. I didn't even mention that in my email because you really can't prove that a Category 3-4 Hurricane can't do that much damage.
Member Since: September 24, 2005 Posts: 33 Comments: 9265
108. KatrinaRitaWilmaZeta
2:57 AM GMT on December 21, 2005
tornadoty do it do it lol
107. atmosweather
2:57 AM GMT on December 21, 2005
Yeah torn, everyone who can back up their views should definitely write to them. Something has to be done, if not for Katrina then for the next controversial storm. Not sure if they will accept anymore emails after reading my 1400 word opinion LOL!! I think I may have given all possible reasons for not downgrading Katrina!!!!
Member Since: September 24, 2005 Posts: 33 Comments: 9265
106. KatrinaRitaWilmaZeta
2:55 AM GMT on December 21, 2005
can a cat 3 or 4 do that march when it made land fall i am thinking it was a cat 5
105. tornadoty
2:54 AM GMT on December 21, 2005
That's a good idea KRWZ. I just might.
104. atmosweather
2:53 AM GMT on December 21, 2005
I just sent my email so I havent got a reply yet. But the last time I sent them an email they replied within a few hours
Member Since: September 24, 2005 Posts: 33 Comments: 9265
103. KatrinaRitaWilmaZeta
2:52 AM GMT on December 21, 2005
hey tornadoty give what you have to the nhc and have them take a look at it
102. atmosweather
2:51 AM GMT on December 21, 2005
Sorry torn, I forgot to save it and don't know if there is any way to get it back. Most of what I said was in my lengthy post about 30 mins ago, with much more added of course. Generally though, I just expanded on those points and added a lot about wind-pressure realtionships and storm surge.
Member Since: September 24, 2005 Posts: 33 Comments: 9265
101. KatrinaRitaWilmaZeta
2:50 AM GMT on December 21, 2005
atmosweather did they ever get back to you last night when you e mail them i will give them 72hours to get back to me if not i will try one more time
100. tornadoty
2:47 AM GMT on December 21, 2005
Hey Atmos, could you post your letter here.

Also, I plugged all the new data into my scale. Katrina is still a 5 on it. No downgrade.
99. atmosweather
2:47 AM GMT on December 21, 2005
Well they got back to me very quickly (4-6 hours) when I last emailed them. I don't know why they would not reply to you. Anyway, those are the only email addresses they have and I don't know individual forecasters' email addresses, so I can't help you further. Try writing to them again.
Member Since: September 24, 2005 Posts: 33 Comments: 9265
98. KatrinaRitaWilmaZeta
2:44 AM GMT on December 21, 2005
atmosweather i try that and they have not got back to me an i do not think they will
97. atmosweather
2:42 AM GMT on December 21, 2005
For technical assistance, comments, or feedback on the website
contact our webmaster at nhcwebmaster@noaa.gov

For general questions, comments, or feedback on tropical cyclones and the NHC/TPC contact our Public Relations Officer at nhc.public.affairs@noaa.gov
Member Since: September 24, 2005 Posts: 33 Comments: 9265
96. atmosweather
2:41 AM GMT on December 21, 2005
Yeah torn I also emailed Dr. Masters and have just written a 1400 word email to the National Hurricane Center!! I strongly suggest everyone that disagrees with the downgrade of Katrina and have evidence to back up their views to write to them. You're right; if we all send them our views, they might take notice.
Member Since: September 24, 2005 Posts: 33 Comments: 9265

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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.