Future Trends in the Frequency and Intensity of Hurricanes

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Future Trends in the Frequency and Intensity of
Hurricanes Chip Konrad Deputy
Director Southeastern Regional Climate
Center Associate Professor UNC-Chapel Hill
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Billion dollar hurricanes in the Carolinas during
last 10 years. 2004 Jeanne flooding Mtns- W.
Piedmont Ivan flooding Mountains Frances
flooding Mountains 2003 Isabel flooding/wind NE
NC 1999 Floyd flooding Coastal
Plain 1996 Fran flooding/wind
E.Pied-C.Plain 1995 Opal flooding/wind
Mountains
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• Outline
• Provide a background on the ingredients needed to
  make a hurricane.
• Identify historical trends in hurricane
  frequencies/intensities and relate these to large
  scale climatic trends (e.g. global warming, El
  Nino)
• In light of these climate trends, discuss future
  scenarios and uncertainties.
• Answer questions

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Ingredients for Making a Hurricane

• sea surface temperatures gt 80 F

• low-level convergence (easterly wave)

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Historical Trends in Atlantic Tropical Cyclones
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ACE Quantifies the number, intensity and
duration of hurricanes over an entire hurricane
season
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• What factors control the number of hurricanes and
  the frequency of landfalls?
• 1. Average sea surface temperatures in the
  subtropical Atlantic.

Knutson Tuleya (2004)
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2. The prevailing wind circulation during the
season
Controlled by the position of subtropical high
and the jet stream position
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Examples Hurricane Floyd Bermuda High well
off of the coast
Hurricane Andrew Bermuda centered across SE US.
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During Sept. 2004 within a period of less than
one month, three major tropical cyclones moved
across a region that expects one tropical cyclone
every three years.
Frances
Ivan
Jeanne
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Mean Tropospheric Circulation for September 2004
Middle Troposphere 500 mb height contour 500 mb
height anomaly
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3. Amount of vertical wind shear, which
correlates with El Nino occurrences in the
Equatorial Pacific.
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Hurricane Tracks during El Nino and La Nina Years
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Why were there so few hurricanes and no U.S.
landfalls this past year?

• SST 0.5 C above normal back in May, but there
  was a cooling
• Surface pressures increased
• Subtropical high expanded across SE US keeping
  activity farther south

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What are the future trends in hurricane
activity? 1. Extrapolate past trends
Are past frequencies underestimated?
Landsea, 2007
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The past record may be a reasonable analog for
landfall frequencies Coastal configuration
matters!
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Many uncertainties regarding effects of global
warming
1. Warmer ocean, but how much warmer? -gt Stronger
hurricanes

• Much inter-annual variability.

• Rate of warming or cooling depends on time
  period of analysis.

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2. What about the future frequency and intensity
of El Nino and La Nina events?

• Are there more El Nino events in a greenhouse
  gas-enhanced world? - maybe
• This would suggest greater inter-annual
  variability in the number of hurricanes

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Can we rely on global climate models to
skillfully predict the future frequencies and
intensities of hurricanes?
Shortcomings of models 1. Incomplete
understanding of the earth-atmosphere system
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Questions?