Global warming and hurricanes part 1: The natural cycle
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
Reader Comments
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Katrina movies, click on IR next to "Hurricane Katrina Movies", it should open in Windows Media Player.
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.
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.
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.
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.
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
You should post your e-mail to the nhc as well. I would like to read what you have to say.
- Nick
"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.
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?
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