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National Hurricane Center and the Likelihood of Hurricanes

Posted by Stuart Staniford on March 14, 2006 - 9:36pm
Topic: Environment/Sustainability
Tags: climate change, hurricanes, national hurricane center, peak oil [list all tags]

Consider that in their December 2004 forecast, the NHC reported a 69% chance of a major (Category 3-4-5) hurricane hitting the US, but said of 2005:
We do not, however, expect anything close to the U.S. landfalling hurricane activity of 2004.
Well, they got that wrong...

This begs the question of how likely were the 2004 and 2005 hurricane seasons given the NHC forecasts? I'm going to argue that they were quite unlikely, suggesting the NHC, at least in recent years, is systematically underestimating the seriousness of the problem.

Update [2006-3-16 0:8:34 by Stuart Staniford]: Flabdablet caught some sloppiness in the calculation that follows, which is corrected in this comment. It doesn't change the conclusion.

This is a simple high-school probability argument based on the following data. In December 2003 the NHC predicted a 68% chance of a major hurricane hit on the US (which I'll call a MLH - major landfalling hurricane), while in December 2004, they estimated a 69% chance of a major storm hitting the US coast.

In fact, in 2004 there were three major hits on the US (Charlie, Ivan, and Jeanne), and in 2005 there were four (Dennis, Katrina, Rita, and Wilma).

If we make one additional assumption - that the probability of any subsequent major landfalling storms is independent of the first and each other, then we can compute the probability of the observed outcomes given the forecast.

Specifically, the probability P that they give (the 69% or 68%) is the sum of the probability of getting one MLH, two MLHs, three, etc. We need to know p, the probability of getting exactly one, rather than P, the probability of getting one or more. P = p + p2 + p3 + .... If you remember algebra, and you imagine factoring a p out of the RHS, you should be able to see that P = p(1+P), so p = P/(1+P). To a good enough approximation for this purpose, 68% = 69% = 2/3. Therefore, p = 2/5 in both 2004 and 2005.

So what are the chances of getting 3 or more MLHs in a year? Well, p3+p4+.... That turns out to be about 11%. What are the chances of getting 4 or more MLHs in a year? Well, about 4%. If you want 3 or more in one year and four or more in the other, the chances of that are 4%*11% *2 (the factor of two comes because we would have viewed a reversal of the two years as equally significant). That suggests the overall chances of the 2004 and 2005 seasons, based on the forecasts, are 0.9%. If we had been willing to accept 2 storms one year and 5 another as equally significant (I wouldn't), that would raise it to 1.8%

The usual level of statistical significance is 5%, and the usual level for evidence to be "very significant" is 1%. Thus, the last two hurricane seasons constitute statistically very significant evidence that the forecasts understated the probability of major landfalling hurricanes in the US.

67 comments on National Hurricane Center and the Likelihood of Hurricanes

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strandk on March 14, 2006 - 10:19pm Permalink | Subthread | Comments top
How good were they for hurricane forcast before 2003?  How do they forcast hurricanes? If they forcast the number of hurricanes based on the data about climate patterns and incidence of hurricanes in the last fifty years, this won't work well because of rapid climate changes in recent years. The news said that this winter season we experienced the warmest winter in Canada on record (http://news.yahoo.com/s/ap/20060314/ap_on_sc/canada_warmest_winter_3;_ylt=AhgHyFUFqXxMHBNB57wztw5rAl MA;_ylu=X3oDMTBiMW04NW9mBHNlYwMlJVRPUCUl).  Then we technically entered an uncharted territory of climate in terms of temperature in North America this year. Then how can they forcast weather six months later without having any data relevant with the current climate conditions. The only way to do this is to set up some sorts of models and to extrapolate the past data to the current situation. But I doubt that we have good models for weather forcast.  
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Stuart Staniford on March 15, 2006 - 12:42am Permalink | Subthread | Parent | Comments top
Interestingly, the seasons in the 2000-2003 timeframe where much milder and don't to the casual eye look like much evidence of anomaly. (You can check out the archives). So either 2004-2005 have a strong component of unlucky fluke to them, or we are crossing some kind of non-linear threshold...

One possible factor is the state of the loop current in the GoM.

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strandk on March 15, 2006 - 10:46pm Permalink | Subthread | Parent | Comments top
Thank you for your reply. I think that the climate is the result of combination of chaotic random factors and predictable factors which follow laws of physics. As many data indicate, the temperature has increased globally in the last one hundred years. Although this is merely a trend, it is very unlikely that the temperature next year would drop down to the level of temperature one hundred years ago. This is predictable because the temperature must obey the laws of energy. The size of polar ice is pretty much dependent on the temperature in the polar region. Therefore the size of polar ice is one of good indicators of global warming. On the contrary, the number and strength of hurricanes are dependent on many factors including both chaotic random factors and predictable factors. When the sea temperature is higher, there are more chances for hurricanes to become stronger. This is predictable because hurricanes are getting energy from heat energy retained in the sea. However, it is unpredictable how, when, and where hurricanes are formed because this belongs to the realm determined by chaotic factors. There might be some patterns in the relationship between hurricane formation and other climatologic phenomenon. But I think that these patterns are only temporary at their best.
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Dan Ur on March 16, 2006 - 2:52am Permalink | Subthread | Parent | Comments top
But chaos can be predicted if we have a big enough chaos theory predicting program.  But this is not true chaos, this is a big set of variables that we have not yet wrapped our hands around to see the outcomes of the things that are going on.  Cause and Effect are happening on a vast scale, but they are happening in a Logical manner even in the oceans.  So technically speaking its not Chaos, Just not yet understood by us humans.

Maybe I have just written to much science fiction and Read to many papers on Chaos and how to make the complex simple.
I almost feel as if I know where this is all heading but can't quite put my finger on it.

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Magnus Redin on March 16, 2006 - 6:38am Permalink | Subthread | Parent | Comments top
It is very hard to conclude what the boundaries of outcome are for this chaotic system, and how can we conclude if the chaos inside these boundaries is predicatble or not? It might be like a very simple multiple hinged pendulum problem, we can predict how the swings will peter out on average but not in wich direction it will swing a number of swings into the future since imperfections give changes that gets larger for each swing. The need to know everything about the starting conditions is sooner or later limited by our ability to measure.
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futurebird on March 15, 2006 - 8:24am Permalink | Subthread | Parent | Comments top
I have no doubt they are underestimating -- however I don't think you can look at only two years and call the results in to question. Also: almost all long term weather forecasts are bull
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Nate Hagens on March 14, 2006 - 10:21pm Permalink | Subthread | Comments top
Stuart - do you ever play craps?
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Stuart Staniford on March 14, 2006 - 10:33pm Permalink | Subthread | Parent | Comments top
Never have :-)
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Dave Cohen on March 14, 2006 - 10:38pm Permalink | Subthread | Comments top
There is a major disconnect in communications between the NHC and the climate change people. In fact, not only do they not talk much with each other, they take potshots at each other on the various news sources I've seen or listened to.

Consider who is looking right and who is looking wrong....

Sea surface temperatures (SSTs) are higher. The North Atlantic Oscillation (NAO) is positive and there is a strong historical correlation between increased hurricane activity and the NAO in its current state. Combine that with the SST's and I'd say there's little question significant future chances of major hurricanes making landfall in the Gulf over the next few years IMHO.

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The Interloafer on March 14, 2006 - 11:13pm Permalink | Subthread | Parent | Comments top
And don't forget NOAA's prediction that La Nina will be in effect for the next six months at least. (La Nina typically reduces upper level shear in the Atlantic basin and therefore fosters Hurricane development.)
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Gets IT on March 15, 2006 - 6:08am Permalink | Subthread | Parent | Comments top
Links to oscillations,
http://www.ngdc.noaa.gov/paleo/ctl/resource100.html

North Atlantic Data
http://www.ldeo.columbia.edu/NAO/

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futurebird on March 15, 2006 - 8:25am Permalink | Subthread | Parent | Comments top
This is what I find most disturbing-- the game is changing and they are using an old model.
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Halfin on March 14, 2006 - 10:44pm Permalink | Subthread | Comments top
Two comments. First, here is the corresponding forecast for 2006:

http://hurricane.atmos.colostate.edu/Forecasts/2005/dec2005/

It's up to 81% this year! That's after 68% and 69% the past two years. So I guess they're learning...

Second, it's probably not right to assume that the hurricanes are independent. I don't remember the whole history, but Katrina and Rita had similar tracks (at least at the end) and came in pretty close to each other. It might well be that once conditions arise that are favorable for a hurricane to hit, that we could be vulnerable to more than one.

Here's one odd thing from their forecasts. They are all like this, but this is the one for 2006:

Entire U.S. coastline - 81% (average for last century is 52%)

U.S. East Coast Including Peninsula Florida - 64% (average for last century is 31%)

Gulf Coast from the Florida Panhandle westward to Brownsville - 47% (average for last century is 30%)

The thing is, the probabilities don't add up, even though the subregions are geographically distinct. The probability for the whole coastline is 81%, the probability for the western part is 47% and for the eastern part is 64%. Those two parts add up to 111%.

What does this mean? Presumably, that the odds are not independent. The chances are not zero for hurricanes to hit both west of Florida and in the eastern part. I wonder if this could shed any light on their imputed probability for more than one hit, if we did some statistical mojo with these numbers?

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plucky underdog on March 14, 2006 - 11:33pm Permalink | Subthread | Parent | Comments top
That's easy

Letmebefirstletmebefirstletmebefirst

p(X) = probability of event X
E = Hurricane in East, W = Hurricane in West
~X means NOT event X
& means logical OR
| means logical OR

p(E) = 0.64    so p(~E) = 0.36
p(W) = 0.47    so p(~W) = 0.53

so p(~E & ~W) = 0.36 * 0.53 = 0.19

so p(~(~E & ~W)) = p(E|W) = 1 - 0.19 = 0.81

QED. Cigar?

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plucky underdog on March 14, 2006 - 11:34pm Permalink | Subthread | Parent | Comments top
Errata

& means logical AND

Oops

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Halfin on March 15, 2006 - 1:29am Permalink | Subthread | Parent | Comments top
Thanks, Plucky, that does make sense! So it really does add up properly.
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Stuart Staniford on March 15, 2006 - 12:52am Permalink | Subthread | Parent | Comments top
At an unconditional level, the independence assumption has to be wrong because we know there are things like sea surface temperatures in the tropical atlantic, the ENSO index, etc, that have a lot to say about whether there are lots of bad hurricanes or not.

However, those variables are available for NHC to incorporate into their predictions. So if you want to argue that the hurricanes are highly non-independent even when conditioned on the NHC forecast, that suggests there might be good features that NHC is not incorporating into their forecasts.

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plucky underdog on March 15, 2006 - 1:22am Permalink | Subthread | Parent | Comments top
Correct - but surely the NHC's statisticians aren't that naive? Or maybe we've just learned an important lesson about the quality of Gubmint stats. Or maybe those probabilities are based on a much more sophisticated model, and the numbers just came out that way.

Poisson process, anyone? Or is there a system recovery time between major storms in a small province like Florida? Or did they just run a zillion simulations with random initial conditions and report the frequency of outcomes?

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Dan Ur on March 16, 2006 - 3:10am Permalink | Subthread | Parent | Comments top
Having read the discussion sections for most of the major and some of the minor storms for the past 3 years over at NHC's website.  You get the picture that there are about 5 to 10 computer models that they use almost all the time to predict the path of a storm then they use a few human guesses ( either a mix of the models, or a best guess from the group that have been doing this for years ).  They have in the past, used human guessing to get things right or model's averaged to get things right.  But they admit that they do not know all the factors and repeat a lot of the time that their models are NOT prefect.  

The window of 3 to 6 hours ahead can be shaky at best, and anything longer than that has error factors.  Last season those error factors came to full light, when they could not predict the outcomes of several late season storms and you could tell from reading them there was a massive amount of frustration going on.

The computer models available to them are only as good as the data streams they were designed around.  Things are changing and they know it is happening, but they just can't tell you for sure what and how!!  This makes their job even harder, because so many people now depend on them getting it right, when they the NHC know they can't get it right if the ocean keeps changing and they don't have new modeling programs that predict those changes.

Its like us asking Dave and Stuart to predict the Oil movement in a basin between March 16th at 5 pm and March 18th at 8 pm and tell us where the next blowout will occur before it happens.

Models and programs of a changing world are only so good as the "Past" data used to make them.

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Gets IT on March 15, 2006 - 6:16am Permalink | Subthread | Parent | Comments top
http://www.fema.gov/hazus/dl_rat.shtm

FEMA has developed a companion software tool to HAZUS-MH called the HAZUS-MH Risk Assessment Tool (RAT) to help you produce your risk assessment outputs for earthquakes, floods, and hurricanes. This tool was developed as a third-party model to support HAZUS-MH and is used to display the outputs from the HAZUS-MH risk assessment in an easy-to-use format. The RAT pulls natural hazard data, inventory data, and loss estimate data into pre-formatted summary tables and text. These summaries can support the presentation of data to decision-makers and other stakeholders and in your mitigation plan.

bullet Risk Assessment Tool (zip, 19.3 MB)

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Halfin on March 16, 2006 - 3:36pm Permalink | Subthread | Parent | Comments top
I worked out the corresponding probabilities for 2006 using the Poisson distribution as Stuart described, and came up with the following probabilities for this season based on their 81% estimate:

p(1 or more) = 81%
p(2 or more) = 49%
p(3 or more) = 23%
p(4 or more) =  9%

So I think they would have to say again that the probability of this year being as bad as the previous year would be quite low, even with these larger probabilities.

Just out of curiosity, what would it have to be to get the probability with 4 or more to be at least 50%? This corresponds to a "lambda" of 3.672 which means a probability of 1 or more big storm hitting of 97.5%! I don't know if even the most stringent advocate of a linkage between global warming and hurricanes would say we are in that territory yet. So I suspect that any way you look at it, we had a substantial bit of bad luck these past two years, and are unlikely to be that unlucky again this year.

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Stuart Staniford on March 16, 2006 - 4:14pm Permalink | Subthread | Parent | Comments top
This analysis has made it clear to me that there's a real mystery here in 2004,2005. While I stand by my general sense that the Webster/Emmanuel/Hansen "global warming is driving SSTs higher and storms are going to dissipate ever more power" idea is correct as an explanation of the last few decades, it also seems that these last two years are anomalous (in a highly significant way) relative to the trend in terms of what hit the US.

One possibility, as you suggest, is that they are just exceptionally bad luck, and 2006 and 2007 will mostly return to the trend of gradual worsening that's been going on for decades (in which case the NHC's 81% is a reasonable looking estimate). The other possibility is that we have hit some kind of non-linearity in the way the climate responds to increasing global warming forcing, and we are in a new (and much worse) regime. In that case, our best estimate of lambda should be 3.something and the probability of 1 or more storms should be 90%+. I don't think we have enough information to argue one way or the other very convincingly at this point.

If we are crossing some kind of threshold, there may be some flickering back and forth for a while. On the other hand, if we were to have another season or two like 04/05, I don't think that would leave much room for doubt. It would also help to have some kind of physical explanation for what switch had flipped (I have no idea at present, though changes in the Atlantic circulation seem like an interesting place to start looking given the Bryden et al paper).

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Cherenkov on March 15, 2006 - 12:19am Permalink | Subthread | Comments top
Don't forget that due to the weakening of the Gulf Stream in the north, the loop that circles near the gulf must proportionally increase. And, the extra trapped heat energy due to global warming also adds a little extra oomph to that loop.

The hurricane conveyer belt is that much stronger.

Weeeeee! Let's fire up some more technology!!!

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Stuart Staniford on March 15, 2006 - 12:47am Permalink | Subthread | Parent | Comments top
The Gulf Stream is pretty constant actually. However, Bryden et al's measurements last year suggested that the amount of water from the Gulf stream returning in the wind driven gyre to the tropics (rather than continuing up the North Atlantic towards Europe) was increasing. That would show up as warmer SSTs, however, which are presumably available to the NHC as a prediction variable.
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jeffcc on March 15, 2006 - 1:29am Permalink | Subthread | Comments top
Last weekend's storm was unusually strong for this time of the year. Up here in Minnesota, snow and wind closed U.S. 52 for the first time in at least 35 years. Further south, 110 tornadoes in 24 hours was about double the 30 year average for the entire month of March. (Please see NOAA's monthly tornado statistics, http://www.spc.noaa.gov/climo/torn/monthlytornstats.html). Further west, storm-related high winds led to wild fires that burned 700,000 acres, over 100 square miles.

Does anyone know about the current state of SSTs in the Gulf of Mexico? All week long the Minnesota weather forecasts were talking about great moisture flow/energy transfer coming from the gulf. Are SSTs still uncommonly high? If so, is there any link between high SSTs and land-based destructive storms like those of March 11-13?

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Gets IT on March 15, 2006 - 5:37am Permalink | Subthread | Parent | Comments top
Current weather including sea surface temps can be found at

http://www.nowcoast.noaa.gov/
Set Louisiana (its a GOM state, not a coastal state?)
Set sea surface temp

Then zoom out to see the entire GOM

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Essex Land Rover Man on March 15, 2006 - 6:25am Permalink | Subthread | Parent | Comments top
The national weather service have a map of the tropics with weekly sea surface temperatures colour coded.

Link :-

http://www.cpc.ncep.noaa.gov/products/hurricane/atlsst_lastweek.html

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jkissing on March 15, 2006 - 12:59pm Permalink | Subthread | Parent | Comments top
Great charts. It clearly shows more warm water than normal coming down from the north atlantic, supporting the idea that the gulf stream is being pushed away from europe. And, interesting to see that the gulf is already warmer than normal...
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Stuart Staniford on March 15, 2006 - 1:20pm Permalink | Subthread | Parent | Comments top
Here's the picture:

Sure does look like the gyre is bringing more warm water back as Bryden's paper suggested to me it would (we discussed this last fall. That makes things nice and warm where the hurricanes get started and then we have that hot spot in the Gulf starting to build up to turn them into monsters before they hit.

Does anyone know if similar maps are available for historical conditions in prior years?

One caution is that the depth of the warm water layer is just as important as the surface temperature. Hurricanes self limit by churning up cold water from below, so to get a really powerful hurricane, it helps to have a deep layer of warm water (or else a shallow sea where's it warm all the way to the bottom).

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jkissing on March 15, 2006 - 9:38pm Permalink | Subthread | Parent | Comments top
Looks like an ideal feedback loop - hotter gulf melts more greenland ice pushing warm water south before it has a chance to cool off (and warm europe), then repeat. So the poor overheated gulf gets more and more warm water, and now the only remaining mechanism left to cool is evaporation, usefully enhanced by the lengthening hurricane season.
I wonder if the warmer than usual gulf water flowing north along the eastern seaboard has any effect on the jet stream, which last winter dammed arctic air in upper canada, moderating NA climate, and then released it to flow south into western europe, now bereft of the gulf stream...
Western europe might become used to siberian winters...

Maybe things will get better when greenland has no ice left to melt (or, maybe at that point hurricanes transport sufficient fresh water distilled from the gulf to the northeast to stabilize climate in a new mode.) gw means more rain, but of course not evenly distributed...

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totoneila on March 15, 2006 - 4:32pm Permalink | Subthread | Parent | Comments top
Hello Jeffcc,

Good point about a possible link between SSTs and land-based destructive storms.  Global warming combined with shrinking polar icecaps means violent storm energy transfers are longer lasting and moving northward causing more tornadoes and more destructive land-falling hurricanes [just a supersized tornado].  Stuart's ocean temp anomaly graph shows a hotspot off the California coast-- maybe CA might see a mild hurricane/tropical storm this year, last tropical storm was in 1939 linked here:

http://www.usatoday.com/weather/whhcalif.htm

If this CA hotspot continues to grow, it could be similar to the GoM thermal hotspot providing energy to jackup storm intensity levels.  We will see.  I agree with most other posters that the weather models are not incorporating accurate changes as fast as Nature is changing the weather.

Bob Shaw in Phx,AZ  Are Humans Smarter than Yeast?

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jkissing on March 15, 2006 - 9:47pm Permalink | Subthread | Parent | Comments top
I doubt ca will get a hurricane - the water might be a little warmer than normal, but normal is very cold. We get the japanese current, nicely chilled as it passes alaska. actually, I thought cold sea water was the one bad thing about living in southern ca, now beginning to think its not so bad...
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Don Sailorman on March 15, 2006 - 10:20pm Permalink | Subthread | Parent | Comments top
If memory serves, California was a hurricane coast as recently as the first half of the nineteenth century.

(Ooops, now I've let it slip: I am really old:-)

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jkissing on March 16, 2006 - 2:02am Permalink | Subthread | Parent | Comments top
Can you provid a reference?
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Don Sailorman on March 16, 2006 - 11:04am Permalink | Subthread | Parent | Comments top
Two problems here: Lack of data and lack of consistent usage in terminology. "Hurricanes" may be referred to as tropical storms, Pacific cyclones, and (west of the International Dateline) typhoons. A good starting place is

www.islandnet.com/~see/weather/almanac/arc2003/alm03aug.htm

Sometimes the statement is made that no hurricane has ever hit California. On the face of this, the statement is highly questionable, because how the heck could this be known???? True the cool California current can be expected to dissipate the energy of tropical storms, but there is no good reason to think this current is eternal or even particularly ancient.

Eastern Pacific hurricanes (Let's agree to call them that for now.) are little known because most of the time they are far from land and far from established shipping lanes. What data we do have prior to 1850 is mostly from the logs of whaling ships that were close to the California coast in search of whales. The best compilation of this sketchy data I've seen is at the NOAA office in Honolulu, but so far as I know it is not available online.

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jkissing on March 16, 2006 - 11:55am Permalink | Subthread | Parent | Comments top
Many boomers these days are wondering where to go in retirement. SOme of these would like to be close to the ocean, and are reasonably interested in chances of a devastating hurricane arriving after they do. If there is not convincing data for a hurricane in california for 100 or 150 years, thats good enough for their time frame, particularly in comparison with the gulf and even the atlantic. Times before the current off california flowed south is not, for these, germane.
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Don Sailorman on March 16, 2006 - 3:07pm Permalink | Subthread | Parent | Comments top
I lived in California for twenty years and know many people who grew up there. Every single Californian I know at or near retirement age plans to leave there--overpopulation, pollution, congestion, worsening race problems, water problems, insanely overpriced real estate, rampant property crime combined with more crimes of violence, deteriorating schools . . . . and the litany of woe goes on.

BTW, do you like earthquates?

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jkissing on March 31, 2006 - 1:54am Permalink | Subthread | Parent | Comments top
They're far more benign than cat 4/5 hurricanes, which have beocme fairly common in areas where retirees used to go. and, it is possible to avoid major faults, whereas hurricanes go where they want to.

It is true that many CA boomers are finding they can afford to retire if they sell their CA home and move to, say, Phoenix or las vegas, but cannot retire if they stay in CA. So, CA is rewarding those who otherwise were unable to plan for their retirmenent. THose who left earlier may have had to actually save some money...

There are certainly some areas of CA where the quality of life does not seem to be high enough to compensate for the world's best climate, such a LA, but most areas are pretty nice - we still manage to attract quite a few tourists.

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Essex Land Rover Man on March 17, 2006 - 11:06am Permalink | Subthread | Parent | Comments top
The National Weather Service has an Atlantic- Carribean Sea - Gulf of Mexcio area and an Eastern Pacific (out to 140W) area. I have looked at these for a couple of years and although there are tropical storms and hurricanes, they tend to move away from America and Mexico rather than towards it. 2004 all tropical storms and hurricanes formed north of 10N and moved north west generally.

The link to the main web page:-
http://www.nhc.noaa.gov/

Archive of hurricane seasons east and west :-
http://www.nhc.noaa.gov/pastall.shtml

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Brother Kornhoer on March 15, 2006 - 1:53am Permalink | Subthread | Comments top
Looking at Dr. Gray's December forcast (http://hurricane.atmos.colostate.edu/Forecasts/2005/dec2005/), in Section 7, reveals:

Figure 2 provides a flow diagram showing how these forecasts are made.  Net landfall probability is shown linked to the overall Atlantic basin Net Tropical Cyclone activity (NTC; see Table 8) and to climate trends linked to multi-decadal variations of the Atlantic Ocean thermohaline circulation as inferred from recent past years of North Atlantic SSTA*.

Higher values of SSTA* generally indicate greater Atlantic hurricane activity, especially for intense or major hurricanes.  Atlantic basin NTC can be skillfully hindcast, and the strength of the Atlantic Ocean thermohaline circulation can be inferred from the value of SSTA* which is North Atlantic SST anomalies (in the region 50-60°N, 10-50°W) from current and prior years.  See our previous papers located online at http://hurricane.atmos.colostate.edu/Forecasts for further discussion of SSTA*.

Interesting - so (sez Dr. Gray et al) the thermohaline circulation does affect SSTA*, which in turn correlates with intense or major hurricanes....but it's an increase in Atlantic thermohaline circulation that is linked increased Atlantic hurricane intensity & activity (reading from Section 8 now):  

The Atlantic has seen a very large increase in major hurricanes during the last 11-year period of 1995-2005 (average 4.0 per year) in comparison to the prior 25-year period of 1970-1994 (average 1.5 per year).  This large increase in Atlantic major hurricanes is primarily a result of the multi-decadal increase in strength of the Atlantic Ocean thermohaline circulation (THC) which is not directly related to global temperature increase.   Changes in ocean salinity are believed to be the driving mechanism.

Notice the key word "directly."  Also, the area they're measuring SSTA* in is basically the North Atlantic south of Greenland and Iceland between Ireland and Newfoundland.  So...what does this imply?  If global warming suppresses THC as some suspect, will this dampen hurricanes?  Or am I reading this wrong and drawing the opposite conclusion from what I should?