The Science Behind Atlantic Hurricanes and Seasonal Hurricane Predictions By Dr' Gerry Bell Head of

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The Science Behind Atlantic Hurricanesand
Seasonal Hurricane PredictionsByDr. Gerry
Bell Head of NOAAs Seasonal Hurricane
OutlooksClimate Prediction CenterCamp Springs,
MD public affairs carmeyia.gillis_at_noaa.gov
[phone] x 7163
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Background
The Atlantic hurricane season runs from June
through November. However, most of the activity
occurs during August-October. This is called the
peak of the hurricane season. Fortunately, this
is the time when the climate control is very
strong andalso very predictable. So, when we
make seasonal hurricane forecasts, they are
primarily reflecting the expected activity during
August-October.
Main tropical storm and hurricane formation
region
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Outline

• Interpreting NHC hurricane forecasts
• NOAAs seasonal hurricane outlooks
• Recipe for hurricane formation
• Measuring seasonal activity
• Recipe for an active hurricane season
• Conditions during 2005
• Climate patterns controlling hurricane extremes
• Summary

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Interpretation of the Prediction Cone
It is incorrect to show only the black line as
the forecast because it is NOT the forecast. The
white cone is the forecast. The hurricane is
forecast to be somewhere in the white cone area.
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NOAAs Seasonal Hurricane Outlooks

• Began in August 1998
• Issued in mid-to-late May and early August
• Seasonal Forecasts are highly confident

BUT, Less confidence with seasonal landfalling
forecasts. A lot of work still needs to be done
in this area.
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Recipe for a Hurricane
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Warm Ocean
Sea Surface Temperatures (F)
Departures from Normal (F)
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84
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86
2
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A pre-existing "Disturbance"...
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Not too much "Wind Shear"
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Measuring Seasonal Activity
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ACE Index

• Classifying Hurricane Seasons is Challenging
• TS, H, MH, landfalling storms, etc.
• NOAAs ACE index accounts for the combined
  intensity, numbers, and duration of named storms.
• The Accumulated Cyclone Energy (ACE) Index
• An Energy index defined as sum of squares of
  6-hourly maximum sustained wind speed for all
  systems while at least tropical storm strength.
• ACE index is an excellent measure of seasonal
  activity,
• and is also highly predictable

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2005

• Alternating 20-30 year periods of active /
  inactive seasons
• Nine of last 11 years have been active compared
  to only
• 3 active years during 1970-1994 (25 years).
• We are now 11 years into an active hurricane era.

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Recipe for an Active Hurricane Season?
Active/ inactive seasons and decades result from
a coherent set of atmospheric and oceanic
conditions. They are not random events. The set
of conditions that determine whether or not
hurricanes will form is strongly controlled by
two dominant climate phenomena.
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Conditions Associated withActive Hurricane
Seasons
August-October
Higher Pressure Aloft Makes easterly winds over
Caribbean instead of westerlies
Favorable mid level jet stream
Tropical storms and hurricanes form as
disturbances move westward from Africa into the
low-shear, warm water environment (red area) of
the tropical Atlantic.
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Conditions During 2005
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Courtesy of Unisys Weather
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Conditions During Record Activity 15 June-31 July
Vertical Wind Shear Departures from Normal
(Shaded)
Extremely low vertical wind shear typical of
September
Main Development Region
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Conditions During 15 August-30 September
Vertical Wind Shear Departures from Normal
(Shaded) Air Pressure at 35,000 feet (Contours)
H
Very low wind shear. Ridge remains over SE U.S.
Major hurricanes form in Gulf of Mexico where
wind shear is almost zero.
L
H
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Landfalling Hurricanes
2 to 3 times more U.S. landfalling hurricanes
during active seasons than inactive seasons.
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Tracks of systems that became major hurricanes
after forming in Tropical Atlantic or Caribbean
Sea
Active 24-Year Period 1955-1970, 1995-2002
67 Major Hurricanes
The U.S. averages 2-3 hurricane strikes in
above-normal seasons, compared to just one in
below-normal seasons.
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The ongoing active hurricane era means high
levels of tropical storm and hurricane landfalls
for many years to come. Tropical climate
patterns are similar to those seen during the
1950s-1960s. They are very different than during
1970-1994 when fewer hurricanes formed, and even
fewer affected the United States.
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Coastal Population

• The Bureau of Census considers 87 million people
  to be Atlantic and Gulf coast residents. That's
  nearly 30 percent of the US population threatened
  by Atlantic hurricane season.

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Florida Coastal Population
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MH Katrina- Hit as Cat. 4 (145 mph winds) August
29th 2005
Hurricane winds extend 125 miles from center New
Orleans and Biloxi both in Eye Wall
Biloxi
New Orleans
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Hurricanes are Not Just a Coastal Event
Leading cause of death is from inland flooding.
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Climate Patterns Controlling Hurricane Activity
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Hurricane seasons are largely controlled by
recurring rainfall patterns along the equator,
which are linked to two dominant climate
phenomena
1. The El Niño/ Southern Oscillation (El Niño
and La Niña)
2. 20-30 year (multi-decadal) cycles in monsoonal
rains over western Africa and the Amazon Basin.
These monsoons influence the entire set of
conditions that control seasonal hurricane
activity. They are the dominant cause of the
alternating 20-30 year periods of active/
inactive hurricane seasons. (Bell and Chelliah,
In Press Journal of Climate)

• NOAAs seasonal hurricane outlooks result from
  analysis and prediction of these two climate
  phenomena.

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El Niño/ La Niña CyclePacific Ocean
Temperatures, Rainfall, Winds at 35,000 feet
El Nino Fewer Atlantic Hurricanes
Warm Wet
La Niña More Atlantic Hurricanes
Weaker Tropical Easterly Jet
Cool and Dry
Stronger Easterly Trades
L
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Tropical 20-30 Year Signal Active Atlantic
Hurricane Phase
Surface Temperatures
Warm
warm
Warm
Warm
cool
cool
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Observed 20-30 Year Cycles
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Winds and Air Pressure at 35,000 feetDifference
Between 1995-2003 and 1971-1994
Observed
H
H
Differences between active and inactive periods
is caused by anomalous tropical convection with
20-30 year cycles (Bell and Chelliah, 2005, J.
Climate)
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Multi-Decadal Signal in Atlantic Sea-Surface
Temperatures
Tropical Atlantic Temperatures (Red)
Warmer Atlantic waters since 1995 are very
favorable for hurricane formation. Associated
with weaker easterly trade winds.
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Atlantic Sea Surface Temperature Departures (oC)
Strong multi-decadal fluctuations in Atlantic
temperatures dating back to the 1870s.
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Summary

• We are in an active hurricane era that began in
  1995. This is due to naturally occurring 20-30
  year cycles in tropical monsoon rainfall.
• It is NOT greenhouse
  warming
• 2. A major hurricane (cat. 3-4-5) does major
  damage regardless of strength. Therefore, a
  weakening hurricane should not be conveyed to the
  public as having significantly weaker impacts.
  This happened during hurricane Katrina.

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Summary cont.
2. We can expect high levels of hurricanes and
landfalling hurricanes for many years to come
(10-20 years). We have been saying this since
1998. 3. This is quite different than was seen
during 1970-1994, when only 3 in 25 seasons were
above normal and significantly fewer hurricanes
struck the United States. 4. Tropical climate
patterns controlling seasonal hurricane activity
are very predictable. Therefore, seasonal
activity is predictable. 5. Hurricane
preparedness and hurricane smarts are the most
important things we can do as a society to
minimize impacts if a tropical storm or hurricane
strikes.