Who to believe? One paper/press release says TC’s are occurring more frequently, another says they aren’t. I tend to believe the latter, because there seems to be more supporting data from other sources for it, such as Dr. Ryan Maue’s tropical cyclone research. See below.

First, from the University of Copenhagen

Tropical cyclones are occurring more frequently than before

Are there more tropical cyclones now than in the past? – or is it just something we believe because we now hear more about them through media coverage and are better able detect them with satellites? New research from the Niels Bohr Institute clearly shows that there is an increasing tendency for cyclones when the climate is warmer, as it has been in recent years. The results are published in the scientific journal PNAS.

How can you examine the frequency of tropical cyclones throughout history when they have not been systematically registered? Today cyclones are monitored from satellites and you can follow their progress and direction very accurately. But it is only the last approx. 40 years that we have been able to do this. Previously, they used observations from ships and aircraft, but these were not systematic measurements. In order to get a long-term view of the frequency of cyclones, it is necessary to go further back in time and use a uniform reference. Climate scientist Aslak Grinsted of the Centre for Ice and Climate at the Niels Bohr Institute at the University of Copenhagen therefore wanted to find some instruments that have stood and registered measurements continuously over a long period of time.

Correlation between sea levels and cyclones

“Tropical cyclones typically form out in the Atlantic Ocean and move towards the U.S. East Coast and the Gulf of Mexico. I found that there were monitoring stations along the Eastern Seaboard of the United States where they had recorded the daily tide levels all the way back to 1923. I have looked at every time there was a rapid change in sea level and I could see that there was a close correlation between sudden changes in sea level and historical accounts of tropical storms,” explains Aslak Grinsted.

Aslak Grinsted now had a tool to create statistics on the frequency of cyclones that make landfall – all the way back to 1923. He could see that there has been an increasing trend in the number of major storm surges since 1923.

Correlation between cyclones and climate

Together with colleagues in China and England, he then looked at the global temperatures over the period to see whether there was a trend for a higher frequency of cyclones in a warmer climate. The global temperature has increased 0.7 degrees C since 1923, but there are variations. For example, there was a warm period in the 1940s but the temperature has really risen since 1980.

“We simply counted how many extreme cyclones with storm surges there were in warm years compared to cold years and we could see that there was a tendency for more cyclones in warmer years,” says Aslak Grinsted.

But not all cyclones are equally harmful and those with the highest storm surges tend to cause the most damage. Cyclones with a strength like Katrina, which hit the New Orleans area in 2005 and caused devastating floods and thousands of deaths, make landfall every 10-30 years on average.

“We have calculated that extreme hurricane surges like Katrina are twice as likely in warm years than in cold years. So when the global climate becomes 3 degrees warmer in the future, as predictions show, what happens then?,” reflects Aslak Grinsted.

http://www.nbi.ku.dk/english/

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Then there is this paper just published in GRL:

Decreasing trend of tropical cyclone frequency in 228-year high-resolution AGCM simulations

Masato Sugi and Jun Yoshimura

We conducted 228-year long, three-member ensemble simulations using a high resolution (60 km grid size) global atmosphere model, MRI-AGCM3.2, with prescribed sea surface temperature and greenhouse gases and aerosols from 1872 to 2099. We found a clear decreasing trend of global tropical cyclone (TC) frequency throughout the 228 years of

the simulation. We also found a significant multidecadal variation (MDV) in the long term variation of Northern Hemisphere and Southern Hemisphere TC count in addition

to the decreasing trend. The decreasing trend and MDV in the long term variation of TC count correspond well to a similar decreasing trend andMDV of upward mass flux averaged over the TC genesis region and active TC season. It has been shown that the upward mass flux decreases primarily because the rate of increase of dry static stability, which is close to that of surface specific humidity, is much larger than the rate of increase

of precipitation, which is nearly the same as that of atmospheric radiative cooling. Thus, it is suggested that the decreasing trend of TC count is mainly caused by the decreasing trend of upward mass flux associated with the increasing dry static stability.

Citation: Sugi, M., and J. Yoshimura (2012), Decreasing trend of tropical cyclone frequency in 228-year high-resolution AGCM simulations, Geophys. Res. Lett., 39, L19805, doi:10.1029/2012GL053360.

[…]

Summary

We conducted 228-year long, three-member ensemble simulations using a high resolution (60 km grid size) MRIAGCM3.2 with prescribed SST and GHG and aerosols from 1872 to 1999. We found a clear decreasing trend of global tropical cyclone (TC) count throughout 228 years of the simulation.

We also found a significant MDV in the long term variation of TC count in NH and SH in addition to the decreasing trend. In order to explore the cause of the decreasing trend of

TC count, we examined the long-term variation of upwardmass flux. We can see a decreasing trend of tropical mean annual average upward mass flux during the 21st century, but the decreasing trend is not so clear in the 20th century. However,

the decreasing trend andMDV is clearly seen if the mass flux is averaged over the TC genesis region and active TC season. It has been shown that the decreasing trend andMDV in the long term variation of TC count well correspond to a similar decreasing trend and MDV of upward mass flux averaged over the TC genesis region and active TC season.

[16] We examined a possible reason for the decreasing trend of upward mass flux. It has been shown that the upward mass flux decreases primarily because the rate of increase of dry static stability, which is close to that of surface specific humidity, is much larger than the rate of increase of precipitation, which is the same as that of atmospheric radiative

cooling. Thus, it is suggested that the decreasing trend of TC count is mainly caused by the decreasing trend of upward mass flux associated with the increasing stability.

[17] So far, there is no observational evidence indicating a clear decreasing trend of global or hemispheric TC frequency as simulated by the model. We have noted that in our simulation the decreasing trend of western Northern Pacific TC count is the main contributor to the NH TC count decreasing trend, while the MDV in theNorth Atlantic TC count is mainly contributing to the MDV of NH TC count (figure not shown). Considering the large uncertainties in the regional TC frequency trend in the models as well as the observation, we should first compare the model simulation and observation in western North Pacific and North Atlantic regions, where relatively reliable long historical observation data are available.

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Dr. Ryan Maue comments: Current global TC numbers have remained quite flat at around 85 for the past 40-years but with very large variation year-to-year. e.g. my old FSU website graphic.

They note the current average is 84.8 I’d say this paper is consistent with what many think — fewer storms in future, but more intense. However, a 60-km grid scale model is just a first crack at the problem, hence the GRL publication.

I suspect they will attempt a 12-km Earth simulator type study ASAP.

Their paragraph [17] is spot on — I’d say this paper is quite good overall. ============================================================== Full paper here: http://www.leif.org/EOS/2012GL053360.pdf Thanks to Dr. Leif Svalgaard.

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