Hurricanes Past, Present and Future

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HurricanesPast, Present and Future
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Tropical Definitions
Tropical Depression - A closed area of low
pressure formed in the tropics with wind speeds
38mph or less. Tropical Storm A closed area of
low pressure formed in the tropics with wind
speeds between 39mph and 73mph. Hurricane An
intense area of low pressure formed in the
tropics with wind speeds 74mph or higher.
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Saffir/Simpson Scale
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What causes tropical storms and hurricanes to
form?

• These storms form over warm water 80 degrees F
  or warmer. The depth of the warm water layer
  should not be shallow. Hurricanes stir up with
  ocean water and any colder water that may be just
  below the surface will be mixed with the surface
  layer, making the oceans surface temperature too
  cool to sustain development.
• They require light winds at the middle and upper
  layers of the atmosphere so that the growing
  circulation is not interrupted.
• Anticyclonic outflow of air at the top of their
  circulation will aid development.
• Anything that can interrupt any of these three
  will weaken and may eventually dissipate the
  tropical cyclone.
• They usually form between 10 and 35 degrees North
  latitude, or between Trinidad and Cape Hatteras,
  NC and as far east of the Cape Verde Islands.
  Any closer to the equator and they are unable to
  acquire the cyclonic spin and any farther north
  and upper level wind and/or sea surface
  temperatures can be unfavorable.

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Tropical Cyclone tracks Non El Niño year 2005
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Tropical Atlantic Activity Relatively Quiet June
August 2006

• 1 TC (Alberto) formed June 10 and made landfall
  in Floridas Big Bend area.
• 1 TC (Beryl) formed July 10 near the Southeast
  Coast and remained offshore.
• TCs Chris and Debbie, and lead Hurricane Ernesto
  almost evenly spaced at early, mid and late
  August.

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Early Summer 2006 Tropical Atlantic SST Near
Normal

• Strong Bermuda High in 2006 due to very different
  large scale pattern in place across North America
  driven by the strongest North American Monsoon
  Circulation in over 10 years.
• Strong Bermuda High enhances trade wind
  circulation generating greater evaporative
  cooling of ocean.
• 2005 record high Tropical Atlantic SST were 2
  degrees C above average.

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Increasing Risk of El Niño

• Several months of mixed atmospheric indicator
  signals have converged to a more consistent
  pattern in August.
• Continued SST warming over much of the tropical
  Pacific.
• Continued decrease in the Southern Oscillation
  Index (SOI).
• Marked decrease in Trade Wind strength.

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Short term Impact of increased Equatorial Pacific
SST

• Positive SST anomalies still weak and not yet at
  El Niño levels.
• Increased SST warming implies a shift in tropical
  convection towards the dateline.
• Such eastward-shifted convection often increases
  vertical wind shear over the tropical Atlantic.

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Tropical Cyclone Heat Potential

• SST of 80 degrees F (26.5 C) necessary for TC
  development, but hurricanes generally need warm
  conditions to extend to a depth of several
  hundred feet.
• 2005 TCHP capable of rapid hurricane
  intensification were much more extensive than
  early summer 2006.

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Climate Change and Hurricane Intensity

• Perceived linkage receiving increased scientific
  and media attention due to spike in major
  hurricane frequency and intensity observed in
  recent years.
• Major scientific debate concerning quality and
  interpretation of data used in recent studies
  attempting to quantify any correlation.

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Webster Data Considered Unreliable

• Hurricane intensity is measured by maximum
  sustained winds at ten meters above the surface.
• Hurricane intensity estimates throughout the 70s
  and part of the 80s based on satellite estimates
  only.
• Only 2 GOES satellites during this interval makes
  intensity estimates highly uncertain.

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Atlantic Multi-Decadal Oscillation (AMO)

• The Atlantic has seen a very large increase in
  major hurricanes during the last 11-year period
  of 1995-2005 (average 4.0 per year) in comparison
  to the prior 25-year period of 1970-1994 (average
  1.5 per year).
• Similar increases have occurred during peaks in
  the AMO which is defined as an increase in
  strength in the Atlantic Ocean thermohaline
  circulation (THC) and is not directly related to
  global temperature increase.

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Dr Grays linkage position

• Seasonal and monthly variations of sea surface
  temperature (SST) within individual storm basins
  show only very low correlations with monthly,
  seasonal, and yearly variations of hurricane
  activity.
• Other factors such as multi-decadal oscillations
  tropospheric vertical wind shear, surface
  pressure, low level vorticity, mid-level
  moisture, etc. play more dominant roles in
  explaining hurricane variability than do SST.

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