Are Category 4 and 5 hurricane increasing globally? (Part II)

By: Dr. Jeff Masters , 2:04 PM GMT on March 31, 2006

In my blog for Monday, I opened the discussion of whether Category 4 and 5 hurricanes were increasing in number globally. Here's Part II of that discussion:

Northwest Pacific typhoon intensities questioned
The question of the integrity of the typhoon intensity data in the Northwest Pacific is critical, since this ocean basin accounts for fully 46% of the global Category 4 and 5 hurricanes since 1970. Dr. Gray and Dr. Knaff both question typhoon intensities measured by reconnaissance aircraft in the Northwest Pacific during the 1973-1986 period. The technique used to determine typhoon intensities during this period (the "Atkinson-Holliday" or AH technique), is thought to have significantly underestimated the maximum winds. Looking at a plot of all Category 4 and 5 activity since 1945 in the Northwest Pacific (Figure 6), one can that intense typhoons were about as common in the 1950s and 1960s as they were during 1990-2004, but took a major dip in the 1970s and 1980s during the period the AH technique was used. I asked Dr. Webster and Dr. Holland about the intense typhoon activity back in the 1950s and 1960s, and they argued that this activity was the result of high SSTs in the Northwest Pacific during that period. On his website, Kerry Emanuel argues that typhoon intensities were overestimated in the 1950s and 1960s. However, Knaff and Zehr (2006) make some convincing arguments that typhoon intensities during the 1973-1986 period were too low due to measurement error, and the number of Category 4 and 5 storms in the region have been roughly constant for the past 50 years. This paper has been accepted for publication in Weather and Forecasting, and will likely be published late this year. Dr. Knaff and Charles Sampson of the Naval Research Laboratory have performed a preliminary re-analysis of maximum typhoon intensities for the period 1966-1987 based on the Knaff and Zehr (2006) results, and this re-analysis will be presented at the upcoming 27th Conference on Hurricanes and Tropical Meteorology (April 24-28, 2006). They show that after correcting for the AH technique errors, the number of Category 4 and 5 typhoons during the 1966-1987 period increased by 1.5 per year, leaving only a slight upward trend in Category 4 and 5 typhoons during the period 1970 - 2004. The 16% increase in Category 4 and 5 typhoons found by Webster et al. during the past 15-year period is reduced to just 3%. Based on this new research, the results of Webster et al. may have to be modified. In particular, their global increase in storms from 1990-2004 compared to 1975-1989, as presented in that paper will be reduced from 57% to 42% if Dr. Knaff's typhoon re-analysis is accepted.

Figure 6. Number of Category 4 and 5 hurricanes in the Northwest Pacific Ocean since reliable records began in 1945. Data taken from the Joint Typhoon Warning Center "best track" database. Typhoon intensities from the period 1973 - 1986 were estimated using the "Atkinson-Holliday" (AH) technique, which may have underestimated typhoon intensity.

Northeast Pacific
Dr. Gray formulates the reasonable hypothesis that if one compares global major hurricane activity for the most recent ten years (1995-2004) with the previous ten years (1985-1994), one should see a significant difference, since global surface temperatures increased about 0.4´┐Ż C between the two periods. He shows that the number of Category 3-4-5 hurricanes stayed exactly the same between these two periods--218 for each time period--if one excludes the Atlantic. I tabulated the results for just Category 4 and 5 hurricanes, and the results were very similar--135 storms storms globally (excluding the Atlantic) from 1985-1994, and 142 for 1995-2004. As most of you are aware, the Atlantic has seen a big increase in the number of intense hurricane the past ten years. Dr. Gray attributes to the Atlantic Multidecadal Oscillation (AMO), a natural cycle I discussed in an earlier blog. Dr. Gray offers another comparison, but just for Category 4 and 5 storms. The most reliable comparison one can make is using data from the Northeast and Northwest Pacific from the past 20 years. This excludes the issues of dealing with the natural AMO cycles in the Atlantic, and the poor data quality in the other ocean basins. Again, the data show essentially no difference between time periods. Indeed, when looking at the plot of Category 4 and 5 hurricane for the Northeast Pacific--the ocean area off the west coast of Mexico (Figure 7), and responsible for 19% of the world's Category 4 and 5 hurricanes--one sees no increasing trend in recent years.

Figure 7. Number of Category 4 and 5 hurricanes in the Northeast Pacific Ocean since reliable satellite intensity estimates began in 1970. Data taken from the National Hurricane Center "best track" database.

The Atlantic contributes only 9% of the world's Category 4 and 5 hurricanes, so is not much of factor when considering global numbers of these storms. Dr. Gray shows that the number of Category 4 and 5 hurricanes in the Atlantic has remained constant when one compares numbers from the past 15 years with an earlier active period from 1950-1964. However, this is a poor comparison. The period 1950-1964 fell entirely within a time when the warm phase of the AMO dominated the Atlantic, and had significantly enhanced intense hurricane activity (see Figure 8). The period 1990-2004 includes five years from the cold phase of the AMO, when intense hurricane activity was significantly down. Thus, comparison of 1950-1964 with 1990-2004 in the Atlantic is poor. One should make the comparison between data from the 11 years from the most recent warm phase of the AMO (1995-2005), and the previous warm AMO period we have good data for (1944-1969). This comparison shows that Category 4 and 5 hurricanes in the Atlantic have increased by 60% in the past 11 years compared to the previous active period 1944-1969. One can make a similar comparison for the cold phase of the AMO, contrasting the years 1970-1982 with 1983-1994. This comparison show no increase in Category 4 and 5 hurricanes for the later period with warmer SSts. I asked Dr. Landsea about the 60% increase in Category 4 and 5 hurricanes during the most recent warm phase of the AMO, and he thought that at least part of the increase could be explained by inadequate information from the Hurricane Hunters during that period. He explained that during that time, it was common in intense hurricanes for the Hurricane Hunters to get close enough to the eye to fix the storm on radar, but not actually penetrate through the eyewall into the eye. Who can blame them! The older aircraft like the DC-6 used during that time period were not safe to fly into Category 4 and 5 hurricanes. Dr. Landsea is working on a re-analysis project for the entire Atlantic hurricane database, but has only made it to the 1930s, and hopes to have a more definitive answer on the intensities of hurricanes during the 1950-1969 period in a few years.

Figure 8. Number of Category 4 and 5 hurricanes in the Atlantic Ocean since reliable aircraft reconnaissance intensity estimates began in 1944. Data taken from the National Hurricane Center "best track" database and not corrected for any suspected biases. The warm AMO periods are associated with enhanced intense hurricane activity, and are thought to be part of a natural decades-long cycle that affects only the Atlantic Ocean (as far as hurricane activity is concerned).

So who's right? Given the uncertainties in estimating tropical cyclone intensity presented by Drs. Gray, Landsea, and Knaff, plus the very large disagreement with the theory of hurricane intensification, it is unlikely that the large 80% increase in Category 4 and 5 hurricanes found by Webster et al. is real. There does appear to be some increase, but it is likely much smaller. Many troubling questions need to be answered, such as why comparison of the most recent ten years (1995-2004) with the previous ten years (1985-1994) shows almost no increase in Category 4 and 5 storms globally, during a period when a substantial increase in SST occurred.

All the scientists involved in this debate have stated the need for a rigorous re-analysis of all historical tropical cyclone data. However, there is currently little funding for such work. Dr. Knaff told me that his typhoon re-analysis work was unfunded, and that he did it because he felt strongly that the results of Emanuel (2005) and Webster et al. were inaccurate and needed to be challenged. Dr. Landsea's reanalysis of Atlantic storms is funded, but something he can only devote time to when his duties at NHC allow him. Dr. Knaff wrote me, "While I realize there are plans to reanalyze the Atlantic, the West Pacific, Southwest Pacific, and Indian Ocean are all being done piece by piece as part of several unfunded projects with little general support. If people are going to use the data for global studies, then NOAA, NSF or some other entity should fund a global reanalysis." I agree completely! Before I am willing to conclude that Category 4 and 5 hurricanes are indeed showing a significant increase, I want to see the science done with a better dataset, and covering a longer period of time. The NOAA Office of Global Programs or National Science Foundation needs to step in and fund this research.

While Category 4 and 5 hurricanes may indeed be increasing in frequency globally, one cannot yet say that global warming is the cause. Webster et al. close with the sentence, "attribution of the 30-year trends to global warming would require a longer global data record and, especially, a deeper understanding of the role of hurricanes in the general circulation of the atmosphere and ocean, even in the present climate state." Furthermore, global warming cannot be cited as the cause of recent intense storms, such a Hurricane Katrina, Hurricane Wilma, or Australia's Cyclone Larry and Cyclone Glenda.

Webster, Holland, and Curry have submitted another paper for publication titled, "Testing the Hypothesis that Greenhouse Warming is Causing a Global Increase in Hurricane Intensity". I'll be sure to review the paper when it comes out. In addition, earlier this month, the authors published another paper linking increasing SSTs to higher numbers of Category 4 and 5 hurricanes. The paper was called, "Deconvolution of the Factors Contributing to the Increase in Global Hurricane Intensity", and I plan to say more about this paper in my next blog on the global warming-hurricane intensification debate. Later on this Spring, I'll also talk about the 2005 paper by Dr. Kerry Emmanuel of MIT in Nature that found increases in global hurricane duration and power dissipated due to increasing SSTs.

Be sure to tune in Tuesday, when the Colorado State University forecasting team founded by Dr. Bill Gray releases their updated 2006 hurricane season forecast.

Jeff Masters

Emanuel, K.A., "The dependence of hurricane intensity on climate", Nature, 326, 483-485, 1987.

Emanuel, K.A., "Increasing Destructiveness of Tropical Cyclones over the past 30 years, Nature, 436, 686-688, 4 August 2005.

Hoyos, C.D., P.A. Agudelo, P.J. Webster, and J.A. Curry, "Deconvolution of the Factors Contributing to the Increase in Global Hurricane Intensity",, 16 March 2006, 10.1126/science.1123560.

Knaff, J.A., and R.M. Zehr, "Reexamination of Tropical Cyclone Wind-Pressure Relationships", accepted to Weather and Forecasting, 2006.

Knutson, T.R., and R.E. Tuleya, "Impact of CO2-Induced Warming on Hurricane Intensity and Precipitation: Sensitivity to the Choice of Climate Model and Convective Parameterization," Journal of Climate 17, 18: 3477-3495, 2004. 04/tk0401.pdf

Virmani, J.I., and R. H. Weisberg, "The 2005 hurricane season: An echo of the past or a harbinger of the future?", Geophysical Research Letters 33, L05707, 2006 doi:10.1029/2005GL025517.

Webster, P.J., G.J. Holland, J.A. Curry, and H.-R. Chang, "Changes in Tropical Cyclone Number, Duration, and Intensity in a Warming Environment", Science, 309, 1844,1846, 16 September 2005.

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5:34 PM GMT on April 02, 2006
did Daylight Savings Time come a little bit eary for any of you to where it sould be for
162. HurricaneMyles
5:32 PM GMT on April 02, 2006
Daylight Savings Time, David.

And Levi, you surely get blizzards up there pretty regularly, dont you?
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5:30 PM GMT on April 02, 2006

MichaelSTL mail for you
159. Levi32
9:19 AM AKDT on April 02, 2006
It's really to bad I have to be all the way up here in Alaska. I don't get the excitement of having bad weather coming my way like all of you do.
Member Since: November 24, 2005 Posts: 635 Comments: 26780
158. HurricaneKing
5:18 PM GMT on April 02, 2006
I get the bad weather tomorrow.
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157. louastu
5:10 PM GMT on April 02, 2006
I don't know. This is the first time ever for me to be on DST.
5:06 PM GMT on April 02, 2006
was it me or did day light saveing time a little bit eary then it did last year at this time i think it sould be the 2nd week not on the frist week
155. louastu
5:02 PM GMT on April 02, 2006
It looks like the worst of the weather will reach my area between 12:00 AM and 2:00 AM.
153. louastu
4:51 PM GMT on April 02, 2006
I am in the 30 percent risk area.
150. louastu
4:32 PM GMT on April 02, 2006
It doesn't look good for people in the St. Louis area today.
4:30 PM GMT on April 02, 2006
: MichaelSTL more mail for you
147. louastu
4:25 PM GMT on April 02, 2006
Ok, thanks.

Still, 100 mph wind gusts are incredible. I certainly would not want to be hit by that.
145. louastu
4:21 PM GMT on April 02, 2006
When they say 100 mph winds, is that sustained or is it gusts?
4:20 PM GMT on April 02, 2006
MichaelSTL mail for you
141. louastu
4:04 PM GMT on April 02, 2006

That was a lot better.
140. ForecasterColby
3:58 PM GMT on April 02, 2006
I think it already has met the warm, wet air, check out the dryline:

8:06 AM PDT on April 02, 2006
not that march
136. DenverMark
2:56 PM GMT on April 02, 2006
I know, he seems to just want attention. It's frustrating.

BTW, the wind is really howling out here in Denver. I can't help but think that with so much cool, dry air headed east, when it smacks into the warm, humid stuff things are going to really fire up. Hope it doesn't get too ugly around STL.
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135. ForecasterColby
2:56 PM GMT on April 02, 2006
If 5 people push the spam button, the post is removed and reviewed by an admin.

System NE of PR is pretty much gone.
7:45 AM PDT on April 02, 2006
cyclonebuster's if you want to talk about tunnels do it in your own blog not in her Please!
132. DenverMark
2:33 PM GMT on April 02, 2006
Well said, David, but I think you've made your point. This is Dr. Masters' blog and it is up to him as to how he wants to handle the situation. In the meantime, please, everyone just ignore this guy (and click on Spam) when he starts up about the tunnels. Don't engage him in any more debate. When he doesn't get any attention, we can hope he will eventually go somewhere else.
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7:29 AM PDT on April 02, 2006
well said or not will said?
7:26 AM PDT on April 02, 2006
your tunnels are getting annoying cyclonebuster Please STOP! talking about them we do NOT! want to her them thank you or we will start hiting SPAM! ever time you are talking about them so Please STOP!
6:35 AM PDT on April 02, 2006
for get it then
127. louastu
7:30 AM GMT on April 02, 2006
No, I don't. I think you really need to slow down on your typing until you are capable of typing faster. Also, while I agree that all these posts about tunnels are getting annoying, I don't think it is your place to tell him to get lost (at least I think that is what that is supposed to say).
9:50 PM PST on April 01, 2006
sorry for my oopps

do any of you think i said that well?
9:47 PM PST on April 01, 2006
cyclonebuster get lots now you are not welcome her on dr m blog you are ddring us all nets on your tunnel and we do not want to her them so get lots or you will be ban
9:44 PM PST on April 01, 2006
i this e mail JeffMasters and ask him to ban himbe for the 2006 hurricane year starts i wounder if he will do it

JeffMasters mail for you
123. DenverMark
5:38 AM GMT on April 02, 2006
Ignore him AND flag his tunnel posts as spam.
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122. HurricaneMyles
5:31 AM GMT on April 02, 2006
yup, tonite at 2am you're supposed to change it to 3am. Of course you can do it at anytime.
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9:29 PM PST on April 01, 2006
is tonight day light saveing time and is it tonight that we set are clocks 1 hour up
120. DenverMark
5:06 AM GMT on April 02, 2006
The best thing to do with trolls is to ignore them.
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8:46 PM PST on April 01, 2006
annoying people he is doing it to me are we going to her about all of hurricane year 2006 that starts vary soon i hop that dr M can banned cyclonebuste so that way we will not her him talking about you no what all hurricane year 2006 if you all no what i mean so keep on hiting spam may be by time the start of hurricane year 2006 he will be out of this blog he will be annoying ever one with his you no what
117. MZT
4:44 AM GMT on April 02, 2006
Myles, you're not the only one clicking the SPAM button.
Member Since: September 24, 2005 Posts: 0 Comments: 812
8:42 PM PST on April 01, 2006
MichaelSTL mail for you
115. HurricaneMyles
4:20 AM GMT on April 02, 2006
No, I wont see it like you. You're like these people on that new reality show American Inventor. Those people are so blinded with the thought of this 'great' idea that is really nothing impressive. No matter what they're told they wont understand that there idea isnt practical.

This idea isnt pratical, either. Not in the math, nor the implementation. And with your seriously lacking argumentation skills, you're not doing anything to change that with me or anyone else here. In fact, the more you try to convince people the more you discredit yourself. Your opinion has no weight. You've made it obvious you lack the knowledge to truely understand what you're trying to do. By spouting things like the Earth will turn on itside if the poles melt, and that we will have year long hurricane seasons because of a 2C rise in SST, you make yourself look like a fool.

So please, stop. You wont succeed, except at really annoying people.
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Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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