With the North Atlantic hurricane season officially starting in a couple weeks (June 1), but possibly getting a head start with a developing low-pressure system in the Bahamas, considerable attention will be paid by the media to each and every storm that gets a name. In the North Atlantic, a name is granted to a tropical or subtropical storm with sustained winds of greater than 34 knots AND when the National Hurricane Center declares it is so. It is these storm counts that are used for a variety of purposes including insurance rates and climate change research.
Back in 2007, David Smith described short-lived, generally weak or marginal tropical storms as Tiny Tims. A couple of posts were dedicated to various aspects of the North Atlantic Tiny Tim climatology including here and here.
One is a modern period (the last twenty years, 1988-2007). This is a period of good (and ever-improving) detection tools, like advanced satellites, improved recon devices, denser buoy networks and so forth.
The modern period also matches the 1988-2007 list of Tiny Tim storms. Tiny Tims are storms so weak, small, remote and/or short-lived that there’s no record of ships or land experiencing storm-force winds, yet they were classified as tropical storms. By historical standards these modern Tiny Tims would have been regarded as depressions or disturbed weather, not tropical storms.
In a local Florida newspaper, Chris Landsea describes a new paper he has (re-)submitted to the Journal of Climate along with three other prominent tropical cyclone and/or climate researchers: Gabe Vecchi (NOAA/GFDL), Lennart Bengtsson (Reading, UK), and Thomas Knutson (NOAA/GFDL).
From Kate Spinner’s article online:
Landsea scrutinized the hurricane center’s storm data and corrected for technological advances in hurricane detection and tracking. He concluded that hurricane seasons of the past rivaled today’s activity, suggesting the influence of a periodic climate cycle in the Atlantic, not global warming, is behind the current spike in storms…
Landsea’s new study, currently under review by other scientists, stemmed from his objection to studies in 2006 and 2007 linking the increased number of recorded hurricanes with a rise in global temperatures.
“I did not agree with the studies because I thought their assumption that all the storms were in the database was faulty,” Landsea said.
However, perhaps the most illuminating part of the article includes two quotations from Landsea’s “critics”, including Michael Mann and Kerry Emanuel, both professors who have published various papers on aspects of the Atlantic hurricane climatology. Many of their papers received a high-level of scrutiny here at Climate Audit during the past 4 years.
Mann disputed Landsea’s research, saying that his technology argument ignores the chance that a single storm could have been counted twice before satellite records could show the exact track. He expressed doubt that the study would pass muster to be published.
Kerry Emanuel, a leading hurricane researcher and professor of atmospheric science at Massachusetts Institute of Technology, said Landsea’s work is scientifically robust, but not as important as looking at whether warming causes hurricanes to gain strength.
“I don’t think the number of storms is a terribly interesting thing,” Emanuel said, emphasizing Atlantic storms now rarely exceed Category 2 strength, but that the majority of damage-inflicting storms are Category 3 or higher. “We’re pretty confident that intensity increases with global temperature. There are arguments about the amount.”
Mann helpfully provides an editorial comment on the likelihood of publication, apparently low in his estimation. However, Emanuel (who has collaborated with Mann on several hurricane related papers) finds that the work is scientifically robust but not important to the issue of global warming. While the frequency argument has largely died away with regards to relationship with SST with a few exceptions including Holland and Webster (2007), and remarkably a paper by Mann and Emanuel which does what Emanuel describes as not “terribly interesting”: correlates historical North Atlantic tropical storm frequency with SST warming and tests the hypothesis of multi-decadal oscillation impact on storm activity (AMO).
Here is the abstract of Landsea et al. (submitted)
Records of Atlantic basin tropical cyclones (TCs) since the late-19th Century
indicate a very large upward trend in storm frequency. This increase in documented TCs
has been previously interpreted as resulting from anthropogenic climate change. However,
improvements in observing and recording practices provide an alternative interpretation for
these changes: recent studies suggest that the number of potentially missed TCs is
sufficient to explain a large part of the recorded increase in TC counts. This study explores
the influence of another factor–TC duration–on observed changes in TC frequency, using
a widely-used Atlantic TC database: HURDAT. We find that the occurrence of short-lived
storms (duration two days or less) in the database has increased dramatically, from less
than one per year in the late-19th/early-20th Century to about five per year since about 2000,
while moderate to long-lived storms have increased little, if at all. Thus, the previously
documented increase in total TC frequency since the late 19th Century in the database is
primarily due to an increase in very short-lived TCs.
We also undertake a sampling study based upon the distribution of ship
observations, which provides quantitative estimates of the frequency of “missed” TCs,
focusing just on the moderate- to long-lived systems with durations exceeding two days.
Both in the raw HURDAT database, and upon adding the estimated numbers of missed
TCs, the time series of moderate to long-lived Atlantic TCs show substantial multi-decadal
variability, but neither time series exhibits a significant trend since the late-19th Century,
with a nominal decrease in the adjusted time series.
Thus, to understand the source of the century-scale increase in Atlantic TC counts
in HURDAT, one must explain the relatively monotonic increase in very short duration
storms since the late-19th Century. While it is possible that the recorded increase in short
duration TCs represents a real climate signal, we consider it is more plausible that the
increase arises primarily from improvements in the quantity and quality of observations,
along with enhanced interpretation techniques, which have allowed National Hurricane
Center forecasters to better monitor and detect initial TC formation, and thus incorporate
increasing numbers of very short-lived systems into the TC database.
The first figure from Landsea’s paper shows the unadjusted frequency of tropical storms (and subtropical) from 1878-2008 which demonstrates the significant upward trend. The second figure shows the frequency of storms which last longer than 2-days, which no longer has a significant trend. Maybe it is possible the Tiny Tims were lost? We’ll keep our eyes out for more of these “Baby Whirls” and at the same time see if Landsea’s paper can “pass muster”.
Holland G. J., and P. J. Webster. 2007: Heightened tropical cyclone activity in the North
Atlantic: natural variability or climate trend? Philos. Transact. R. Soc. A. Math. Phys.
Eng. Sci.. 365, 2695-2716.
Landsea, C. W., G. A. Vecchi, L. Bengtsson, and T. R. Knutson: Impact of duration thresholds on Atlantic tropical cyclone counts. Submitted J. Climate, May 7, 2009.
Mann, M., and K. Emanuel, 2006: Atlantic hurricane trends linked to climate change. Eos,
Trans. Amer. Geophys. Union, 87, 233-241.