Heightened Tropical Cyclone Activity in the North Atlantic: Natural Variability or Climate Trend?

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Heightened Tropical Cyclone Activity in the
North Atlantic Natural Variability or Climate
Trend?
Greg Holland National Center for Atmospheric
Research Peter Webster Georgia Tech
Summary Atlantic Changes and Relationship to
SST Issues on Data Reliability
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Summary of Past Activity

• A marked increasing trend in tropical cyclone
  frequency closely associated with increasing
  eastern North Atlantic SSTs, which indicates a
  substantial contribution by Greenhouse Warming
  (Holland and Webster 2007)
• We emphasize that the SST-TC relationship is not
  entirely direct, but arises from related
  atmospheric environmental changes, such as
  vertical wind shear (e.g. Goldenberg et al 2001
  Delworth 2006 Kossin and Vimont 2007).

150 Increase
5-y running mean
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Summary of Past Activity

• Stable proportions of hurricanes to all tropical
  cyclones over the past 50-100 years (the higher
  earlier proportions are considered due to
  analysis errors)
• Stable major hurricane proportions but with a
  marked, multi-decadal oscillation (peaks
  associated with equatorial developments and
  expansion of the warm pool)
• Thus, the numbers of hurricanes and major
  hurricanes has increased in line with that for
  all tropical cyclones.

Data Issues
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Where have the Increases Occurred?
Increases have occurred in all regions except the
western Caribbean and southern Gulf of Mexico,
but the largest proportional increases have been
in the eastern Atlantic.
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Can Eastern Atlantic trends be due largely to
data errors?

• There is little doubt that the eastern region was
  poorly observed in earlier years, but there are
  other factors several studies have shown
  independent relationships, eg
• We have shown a strong and stable relationship
  between frequency and SSTs
• Kossin and Vitmer (2007) have shown that there is
  a close relationship between positive phases of
  the AMM and the eastward extension of the genesis
  region and
• Mann and Emanuel (2006) have shown that the AMM
  is potentially increasing because of Greenhouse
  Warming
• We have identified other physical processes to
  support this regional aspect of the trend (to be
  reported later)
• But let us look a bit more closely at the data.

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Can Landfalling Storms be used to infer Missing
Oceanic Data?
The high proportion of landfalling storms in the
pre-satellite era has been used to imply poor
observations of cyclones in the eastern Atlantic
(e.g. Solow and Moore, 2002, Landsea 2006).
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Suggested Correction to pre-1970 Numbers Assuming
Landfall Proportion at post 1970 levels
The trend is substantially reduced! However
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Comparison of Landfall and non-Landfall
Most landfall occurs from equatorial or Gulf of
Mexico genesis, but a substantial fraction of
these also do not make landfall, as defined by
HURDAT. Landfall explains lt20 of the post 1970
central and eastern tropical cyclone frequency
(defined by the number of 6-h observations) and
gt75 of the western tropical cyclone
frequency. Landfall proportion depends on many
factors and is definitely not invariant with
time for example.......
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Implied Correction to pre-1970 Western Frequency
Assuming Landfall Proportion at post 1970 levels
HURDAT Landfall observations cannot be used to
infer basin-wide statistics
Implies that storms were over observed by around
20 off the US east coast, in the Gulf of Mexico
and in the western Caribbean prior to 1960!!!
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A more logical consideration
Note that there are essentially no landfalls on
the Windies and that there are a substantial
number of cyclones that form in the Caribbean and
GOM that do not make landfall. It is reasonable
to expect that the bulk of these would have been
noticed, even 100 years ago. Thus the random
nature of landfall combined with the arbitrary
nature of what defines landfall in HURDAT, does
not adequately describe the reliable part of the
data base.
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Proximity to Observing Regions
We assume that all cyclones that cross to the
left of the black line were highly likely to have
been observed in some form. Note that this says
nothing about how accurate the genesis points are
or the observed intensity, just whether they were
known to have existed.
Coast and Major Ship Routes
Bermuda
Proximity to Land, Islands and Ship Traffic
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All Storms
Using the proximity to land and ship route
approach implies that almost all storms were
within a good observing area for some part of
their life. Note the steady increase of
non-landfall storms, implying that this has a
real component.
Note Windies Landfall
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Relationship to HURDAT Landfall
The slope is exactly the same for both. Including
proximity to land and shipping reduced the
scatter and brought the intercept to near zero.
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All Storms 1916-2005, Split at 1960
Maximum Potential Error Prior to 1960 is lt1
storm per year (or around 6 prior to 1960),
which agrees with the assessment by Neumann
(1999).
Additional increase in remote cyclones Total 23
0.5 per year (6 error prior to 1960) Increase
in remote eastern development Total 32 0.7 per
year (17 error prior to 1960).
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Tropical Storms, Minor and major Hurricanes
TS
Min Hurr
Most of the potential error impact is on tropical
storms 0.5 per year (10) prior to 1945 Minor
hurricanes 0 error Major hurricanes lt10 per
year.
Maj Hurr
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Summary

• The current Atlantic data base indicates a
  substantial trend in tropical cyclones and
  hurricanes and both a multidecadal oscillation
  and trend in major hurricanes
• The trend is strongly related to Atlantic SST and
  thus to Greenhouse Warming
• This trend is robust to all known potential
  problems with the data base, which could change
  the century trends by a maximum of
• All Tropical Cyclones150--100 increase
• Minor Hurricanes100--100 increase
• Major Hurricanes100--75 increase