Clark
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In response to some ongoing debate in the hurricane research community, I decided to perform a very simplistic crunching of numbers. Some recent studies (e.g. by Webster, Curry, Holland, and others) claim that the number of very intense hurricanes -- category 4 and 5s -- has increased two-fold over the past 30-50 years, whereas others (e.g. Landsea, Klotzbach) claim that no such trend exists and that the available storm databases are of too poor quality to draw any definitive conclusions. Each of these studies has looked purely at the numbers, with no break down by other modulating atmospheric factors.
So, based upon conversations with a colleague, I decided to split out hurricane seasons based upon phase. A list of years of phase may be found here. I composited the seasons for each phase and looked at the following statistics: number of storms (NS), number of hurricanes (H), number of intense hurricanes (IH), number of category 4 & 5 hurricanes (N45), and season-long duration of the cat. 4 & 5 hurricanes (hrs C45). I looked only at the Atlantic, given that it is where much of the focus in this debate lies and that it has the best quality storm database, and only at the years from 1970-2005 (e.g. in the satellite and reconnaisance era). The years studied largely match those of Klotzbach's recent work.
Note: by no means is this a scientific-quality study. I've taken efforts to ensure the validity of the results, but the sample sizes are relatively low for each composite of seasons and a complete study would also include the East and Northwest Pacific basins. Statistical tests on whether each of the averages are different from each other would help alleviate the relatively small number of cases.
With that said, here are some results. All seasonal averages exclude the minimum and maximum values of each statistic so as to not have the statistics swayed by extreme seasons in either direction.
NEUTRAL YEARS: 11.35 NS, 6.29 H, 2.47 IH, 1.29 N45, 54.35hr C45
minimum values -- 6 NS (1981 and 1985), 2 H (1981), 0 IH (1985 and 1993), 0 N45, 0hr C45 (both 5 seasons)
maximum values -- 28 NS, 15 H, 7 IH, 5 N45 (all 2005), 308hr C45 (2004)
LA NINA YEARS: 11.00 NH, 6.17 H, 2.17 IH, 1.17 N45, 37.00hr C45
minimum values -- 7 NS, 3 H, 0 IH (all 1972), 0 N45, 0hr C45 (both 1972 and 1975)
maximum values -- 15 NS (2000), 8 H (1999 and 2000), 5 IH, 5 N45, 210hr C45 (all 1999)
EL NINO YEARS: 9.14 NS, 4.71 H, 1.71 IH, 0.57 N45, 44.57hr C45
minimum values -- 4 NS (1983), 3 H (1983 and 1987), 1 IH (5 seasons), 0 N45, 0hr C45 (both 4 seasons)
maximum values -- 16 NS (2003), 10 H (1998), 3 IH (3 seasons), 3 N45 (1988), 228hr C45 (2003)
So, what does this all tell us? Neutral years seem to see the most activity, followed by La Nina years and El Nino years. The differences between neutral and El Nino years are 2.21 NS, 1.58 H, 0.76 IH, 0.72 N45, and 9.78hr C45. On first glance, these would appear to be significant differences, but small sample size issues exist. A quick t-test on the intense hurricane data suggest the difference is significant to the 72% confidence level -- not all that high. The difference in named storms is significant to the 81% confidence level -- a bit better, but still not that high. More years and/or adding in other basins would help establish greater confidence.
Since 1994, all but 2 seasons -- 1997 and 2002 -- have been classified as either La Nina or neutral seasons. Given that these seasons, as a whole, see greater activity than do the El Nino seasons, this could explain part of what we have seen over the past 10 years in this basin. All of the recent seasons in all three groups have seen greater activity than their means for the years between 1970-1995, so performing these analyses with an independent set of data (e.g. not including 1995-2005) would likely not change the results significantly. The question thus becomes one of what has caused the reduced frequency of El Nino years relative to all other years. The multidecadal signal in the basin is purportedly one that reduces the frequency of El Nino events, though the reasons behind this are not well understood. In fact, the reasons why El Nino events would be suppressed in the first place independent of the multidecadal signal are not well understood at all.
Could it be a global climate change signal? Possibly, but the skeptic in me wants to see more data before jumping on that bandwagon. These preliminary numbers suggest that it is likely not an anthropogenic climate change signal, but the questions noted above remain. I just have a hard time believing that we would suddenly see a large shift in activity -- specifically that of intense hurricanes -- solely based upon human-induced global climate change. The authors of those studies state that when you look at all major hurricanes, not just the cat. 4 & 5 storms, the trend does not exist. Could this be due to improved detection of intense storms across the globe? Perhaps.
The overall point: there's still a lot we don't know. But, the more work we do, the better we can hope to understand what exactly is going on with hurricanes and climate change.
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LoisCane
Veteran Storm Chaser
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You're amazing Clark.
Sending this to myself to read tomorrow.
It felt like 100 at least if not higher today in Miami and I can't process it I don't know how you wrote it.
You are amazing.
thanks..Lois
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Nateball
Weather Watcher
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Wow Clark you put a ton of work into that, thanks for the great info
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Clark
Meteorologist
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Let me add another caveat here: while this covers the years from 1970-2005, it does not fully explain what we have seen over the past 10 years. Even within the three phase composites, the numbers do increase for years over the past ten years. Thus, for a complete analysis, the trend and/or the averages need to be removed from the data before even compositing the seasons. This is to ensure that the statistics are valid between each of the composites and to hopefully ensure that the results have some physical merit.
So, ultimately, what this "study" shows you is perhaps a part of why we've seen so much activity -- though most of it would be expected anyway from what we know about cycles. How much of that is tied into the uptick in the past ten years remains up for debate, however. Truthfully, were it not for 1995, 2004, and 2005 -- three years out of 10 -- not a whole lot would seem out of place. Just my two cents, though.
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HanKFranK
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the numbers you've got there are just for the atlantic. they don't take into account the pacific and indian ocean basins. the climatological base period you're using has an inherent upward bias over all time periods prior to it (global satellite coverage has only been around since the 60s/70s period). comparing the windspeeds assigned to hurricanes in the 1970s versus the early 2000s is like comparing apples and oranges, as the flight-level to surface conversion ratios used are different.
clark just demonstrated, to a lesser degree, how you can take this data and squeeze an entirely different result out of it, by being selective or not taking into account glaring systematic inconsistencies in the data. the data quality issue alone eats up any argument that has been made about human influence on hurricane activity. a little familiarity with the existing database of hurricane activity makes you aware that it isn't really accurate enough for long enough to make any meaningful trend assumptions out of... even the AMO cyclical nature is hard to resolve cleanly. right now the HRD folks are slowly working their way up through time to the modern record... so within a few years the existing record used for the research will be tweaked anyway, and likely have storms added or adjusted upward in the early part of the analysis period used. the first time i saw the emmanuel paper and webster i immediately saw the flaws, but knew it would be all over the media (perfect timing, in sync with ) and expected to see it on all the banners of people convinced the environmental apocalypse is at hand. al gore even added predicting hurricane to his list of lofty achievements. take that, internet.
point to take home is, yes, people have created an upward trend in hurricane activity... on paper.
HF 1750z15june
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Clark
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I entirely agree HF, and this was meant just to highlight some of what you can do with the data. It looks pretty and all, but I've included so many caveats in there that it's not even funny. A full, comprehensive study isn't going to be possible for probably 50 years -- and by then we'll know what whether or not what we are seeing now is as substantial as some would claim.
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Kimberley Clark
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Yes, Clark, I think you did a beautiful job. And, as my daddy always says there are three things for sure:
1. Time will tell;
2. S--- will smell; and,
3. Water will seek its level.
These three things we know. 
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Kimberley Clark
Mobile, Alabama
Weather Watcher
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longtime-lurker
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Like the pop-up map of . Just stumbled on it by accident, though. Would be helpful if the text could be coloured to indicate that it's there.
BTW, enjoyed the discussion immensely. Keep it up...
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Margie
Senior Storm Chaser
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I became suspicious when trying to track down the scientific evidence showing the data for the AMO, after reading the 2006 NOAA hurricane forecast, which references the "multi-decadal signal" so many times that it caught my attention (more on this in a minute). If it's so obvious, the data must be somewhere, and it's not as if it would be something that would be impossible to measure. When you try to find the actual measurements of the ocean circulation and similar types of references to hard data among the Landsea-Gray-Goldenberg set, you just come up emptyhanded. Not only that, if you look at the sources for all their papers, they just source themselves. Huge, huge red flag.
An example of the non-science is the AOML webpage answering questions about the AMO; it says nothing convincing.
All this didn't make me very happy. I came to the conclusion that NOAA was pushing a political agenda for the Bush administration to prevent any mention of global warming, as part of a plan to convince us it doesn't exist and isn't affecting our planet.
Once something raises my suspicions, you know how I am more or less of an internet bloodhound, and I have been known to spend hours and sometimes days searching through "billions and billions" of web pages to get information on a topic that has piqued my curiosity.
Well guess what folks. There are measurements taken of the ATL thermohaline current, albeit infrequently, and the current has been decreasing, not increasing, for around a decade. In other words, the AMO (or multi-decadal oscillation, or thermohaline circulation, whatever label you give it) can't account for the increase in SSTs that has been occuring.
This is what I figure.
--All the effects that are increasing the probability of an active season, as stated in the NOAA forecast, are true. That an increase in SSTs is one of the culprits, seems true as well.
--As to whether or the increase in SSTs in the North ATL is due to a cyclic redistribution of heat in the oceans -- no hard evidence exists to support this, in spite of NOAA's rather noisy repeated insistence that this be accepted at face value (I am frequently guilty of overanalyzing things, but hardly ever guilty of taking something at face value).
--Even with no hard evidence, the idea of a thermohaline circulation makes sense.
--Recent scientific studies do show a model that correctly predicts the ocean warming due to another cause (below), that has been incorrectly completely attributed to an 'AMO.'
My conclusion, is that a combination of factors are causing the warming, but that with the evidence at hand, a cyclic circulation could not be causing the bulk of the warming.
Hot off the press, an article on just this topic.
Ok so -- I have a brother in law enforcement. They are constantly being trained in different areas and one of those has to do with the psychology of criminals. One tip-off if someone is lying is if they put additional emphasis on the statement. An example of this was when OJ Simpson was asked if he wanted to plead guilty or not guilty, he said, "absolutely 100 percent not guilty." That was the immediate tipoff that he was guilty, from a psychological perspective. So in the NOAA forecast, they don't just mention the "multi-decadal signal" once or twice, and drop it -- they bring it up again and again. So, what...if I see it enough, it's gospel? And what is really odd is that they never mention global warming. And it did work, initially, because I was doing a lot of reading and didn't have time to go back and check anything. So when I first started looking at this I did assume the AMO existed and the same tired chart of the cycle that NOAA displays at every opportunity was valid. But when I dug into it, I blasted myself for not having more of a scientist's skeptical "show me" mindset.
As far as trending SSTs, check out this link that I found.
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Katrina's Surge: http://www.wunderground.com/hurricane/Katrinas_surge_contents.asp
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LoisCane
Veteran Storm Chaser
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I understand what you say about criminal behavor and after working for the government myself for a while I know what you mean about a "party line" but... I seriously doubt that they are prohibited from researching and being honest about something because Bush doesn't like Global Warming.
First off, the people who he mostly worked with at HRD are not exactly hard right wing types and they are very dedicated to research. They sort of live and breathe science vs politics.
And, you can ONLY find an article by Mann and Emmanuel? Talk about people with agendas.
I respect your work greatly here online over the years. I just think there is no conspiracy on behalf of the Bush Administration to ignore any ideas on Global Warming. And, trust me.. there is a lot I would believe on that topic.
The people, some of whom I know, who don't believe this is 100% Global Warming and who are waiting to see if there is or isn't permanent Climate Change vs temporary Ice Ages or temporary spikes in Global Temps are scientists not politicians and they have no ulterior motives. They have thier own beliefs.
Science is an art of observation over TIME when it comes to Climate and you can't jump to conclusions based on to what us seems like a lifetime (20 or 30 years) because that's not climate..that's weather.
Otherwise people in the times of the Mini Ice Age that lasted a few hundred years would have been talking doomsday like about the end of the world and a frozen period when everyone would die from temps too cold for mankind to survive.
And, then it changed.
So true what you said though about behavior. .. especially criminal behavior.
So so true.
Just my opinon Margie.
Bobbi
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Clark
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The same is true for those on the other side of the debate, however -- they all only cite the same papers, over and over again. The SST link you show has nothing on the Pacific Ocean, which produces the most tropical cyclones globally. No one can explain what is going on there, which leaves a lot of open room for contention.
A reduced heat redistribution (e.g. weakened circulation) in the ocean would tend to pool warmer water in regions where it is already found -- so I don't think all of that is inconsistent with what you found. The reason there is a lot of uncertainty about both the AMO and what the others are saying is because we truly don't know what is going on. Historically, even with the faults in the dataset, there has been evidence on a long-term multi-decadal signal. If it's the AMO, if it's tied to the thermohaline circulation, great -- we don't know enough to really state that it is that. We also don't know enough to state that it is something else, either.
There's a recent article by two researchers at Purdue that shows a 34-40% variance explained in the trend in cyclone intensity as correlated with tropical SSTs. While they attempt to make use of the most objective data set available, the European Centre reanalysis, and acknowledge that this only holds for 1978-onward, there are two logic flaws. One, they show a substantial peak globally around 1985 instead of 1995 -- why is this the case? It's not addressed in their work or in Emanuel's previous work either. Two, the dataset only has a 1.125 degree horizontal resolution. Small storms are going to get lost or represented very poorly, even with improved data assimilation and additional satellite data, just because there are only a few grid points over them. Case in point -- Hurricane Andrew in 1992: at its peak intensity over the Bahamas, the reanalysis data set used had Andrew a little further south than it was in reality and with an intensity only of 1013mb/20kt. The metric they used relied on the wind speed cubed; given that smaller storms also tend to be a bit more intense at their peaks, there is a huge underestimate of these storms' intensity and integrated power using their method.
While I am willing to accept that more storms may equal more smaller storms from sheer numbers along, I don't believe there is a substantial trend in storm size in the dataset; it is more likely to be random noise than anything else, particularly when you consider that the global number of tropical cyclones is fairly constant from year to year (only minor variation noted). This is just one example; there are others. Even assuming their findings and general trend are correct, at best it accounts for 35-40% of the variance; something else is accounting for the other 60-65%.
I don't think anyone disagrees that climate change is occurring and that SSTs, on the whole, are warming. Where the disagreement comes is the acceleration of that by man -- anthropogenic climate change -- and I don't think there is any attempt to gang together and say that it is all due to multi-decadal signals. For one, more money is being funneled into climate science these days -- and inherently into the climate change issue. If politics were playing a role, so would money, and it is working in the opposite manner to what you suggest. Just my two cents.
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Margie
Senior Storm Chaser
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Well as far as "If politics were playing a role, so would money, and it is working in the opposite manner to what you suggest. " I'd have to speculate whether keeping your job and to being able to retire would fall under the category of a money incentive. If Hansen hadn't had the clout that he does, he might have been history from NASA.
I'd also have to disagree with this: "I don't think there is any attempt to gang together and say that it is all due to multi-decadal signals."
What about the NOAA statement that had to be retracted later (see the editor's note at the bottom), discussed in this article. Remember, the one that claimed that there was a concensus among NOAA scientists that there was no connection between global warming and hurricanes:
There is consensus among NOAA hurricane researchers and forecasters that recent increases in hurricane activity are primarily the result of natural fluctuations in the tropical climate system known as the tropical multi-decadal signal.
Oh, look, there's that same phrase, the multi-decadal signal. Why does NOAA's 2006 hurricane forecast spend almost as much time trying to sell this concept as it does on the forecast? It's an excellent forecast on its own, and stands alone without the AMO hard sell. It's an unprofessional thing to do. The hurricane forecast should simply be a forecast, and doesn't need to advertise an unproven point of view that happens to coincide with the leanings of the political party currently in office.
It's not very objective and it's not good science.
As far as the "other side" -- look who the coauthor of the June 13 paper is -- Mann. That would be this same M. E. Mann: "we believe more firmly than before that this is real," says Mann of the AMO. And if you look at the references to their papers, which are peer-reviewed, they do reference work from both sides.
It's sad that the skimpy resources available at NOAA are being scrambled to put together a series of quick-and-dirty refutations of the growing body of work done by respected climate scientists, and projects like the hurricane reanalysis, instead of being utilized for hurricane research that will improve forecasting.
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LoisCane
Veteran Storm Chaser
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Agreed. It is a shame NOAA doesn't have the money it needs to do the important research that is neccesary to better predict and even in some way modify hurricanes.
I don't think Gray's report spent that much time on the debate. It was a small amount and I give them cerdit for mentioning it.
When there is an explosion globally in all basins in the same year of hurricanes I will look into global answers; not just the Atlantic.
And, lastly the information from the hurricane reanalysis is essential as if you use bad data in equations you end up with garbage in and garbage out. You only learn from the past if you have the right history to learn from....
Thanks. Good debate. I learned a lot tonight.
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Margie
Senior Storm Chaser
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What was even more interesting to me than the NAS study result announced on June 23rd was the result of NCAR's Trenberth and Shea, which was overshadowed (and rightly so) by the NAS report. An excerpt from the NCAR press release is below (emphasis mine):
"By analyzing worldwide data on sea-surface temperatures (SSTs) since the early 20th century, Trenberth and Shea were able to calculate the causes of the increased temperatures in the tropical North Atlantic. Their calculations show that global warming explained about 0.8 degrees F of this rise. Aftereffects from the 2004-05 El Nino accounted for about 0.4 degrees F. The Atlantic multidecadal oscillation (AMO), a 60-to-80-year natural cycle in SSTs, explained less than 0.2 degrees F of the rise, according to Trenberth. The remainder is due to year-to-year variability in temperatures.
Previous studies have attributed the warming and cooling patterns of North Atlantic ocean temperatures in the 20th century—and associated hurricane activity—to the AMO. But Trenberth, suspecting that global warming was also playing a role, looked beyond the Atlantic to temperature patterns throughout Earth's tropical and midlatitude waters. He subtracted the global trend from the irregular Atlantic temperatures —in effect, separating global warming from the Atlantic natural cycle. The results show that the AMO is actually much weaker now than it was in the 1950s, when Atlantic hurricanes were also quite active. However, the AMO did contribute to the lull in hurricane activity from about 1970 to 1990 in the Atlantic."
This assessment of the AMO tallies with the most recent measurements of the thermohaline circulation, evaluated in the Bryden paper.
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Katrina's Surge: http://www.wunderground.com/hurricane/Katrinas_surge_contents.asp
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