NOAA

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2003 Atlantic Hurricane Season Summary By Gerald
Bell, Muthuvel Chelliah Climate Prediction
Center NOAA/ NWS
And NOAA Atlantic Hurricane forecast team
members Dr. Chris Landsea1, Stanley Goldenberg1,
Dr. Richard Pasch2, Eric Blake2 1 Hurricane
Research Division/ NOAA/ OAR 2
National Hurricane Center/ NOAA/ NWS
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Outline

1. Season statistics
2. Total seasonal activity
3. Rainfall from U.S. landfalling tropical systems
4. Atmospheric circulation
5. Ongoing active decadal signal
6. NOAA Atlantic Hurricane Outlooks
7. Summary

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2003 Atlantic Hurricane Season Statistics
Observed
Above Normal
14
Tropical Storms 6 Hurricanes 3 Major
Hurricanes 180.5 ACE Index ( median)
Observed
NOAA Prediction
Season Type
May 23 August 7 Above Normal
Above Normal 55 60
Near
Normal 35 35
Below Normal
10 5
14 Tropical Storms
11-15
12-15 6 Hurricanes

6-9 7-9 3
Major Hurricanes
2-4 3-4
183 ACE Index ( median)
110-180 120-170
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Main Development Region (MDR)
MDR is defined as tropical Atlantic and Caribbean
Sea between 9o-21.5oN During 1949-2002 Tropical
Systems first named in MDR account for 71 of
the 53-year total activity measured by ACE
index 55 of all hurricanes 79 of all major
hurricanes Nine-fold drop in activity in MDR
between above- and below-normal seasons
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2003 Named Storm Tracks
Main Development Region
Figure Courtesy of Unisys.com
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2003 Atlantic Tropical Systems By Region Where
First Named
Tropical Hurricanes
Major Storms
Hurricanes
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Total Seasonal Activity

• Total seasonal activity Collective strength and
  duration of tropical storms and hurricanes
• More representative of overall activity than
  seasonal numbers systems
• NOAAs Accumulated Cyclone Energy (ACE) Index
• Wind energy index defined as sum of squares of
  6-hourly maximum sustained wind speed for all
  systems while at least tropical storm strength.
  Similar to HDP index ( Gray et al., CSU).
• ACE is highly predictable and dominated by
  activity in MDR
• Key forecast parameter for NOAA hurricane
  outlooks
• Used to classify seasons above normal, near
  normal, below normal

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Historical Perspective ACE Index
Atlantic Basin Main Development
Region
Above Normal Season
103
Near Normal
65
Below Normal Season
2003 Predicted Range
2003
Active Decade ACE 134 13 TS, 8 h, 3.6
MH
IN-active Decades ACE 65 9 TS, 5 H, 1.5 MH
Active Decades ACE 114 10 TS, 6.5 H, 3 MH
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2003 ACE Index for storms first named in
specified region
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2003 ACE Index by Storm
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U.S. Rainfall TotalsLandfalling Tropical Systems
6 named storms made landfall in U.S. during
2003 4 formed over Gulf of Mexico 2 formed in
MDR 1 hit as tropical depression (Henri) 3 hit
as tropical storms (Bill, Grace, Erika) 2 hit as
hurricanes (Claudette, Isabel)
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Storm Precipitation Totals (mm)
Hurricane Claudette (MDR) 15-16 July
TS Bill (G. Mexico) 30 Jun - 1 Jul
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Storm Precipitation Totals (mm)
TS Grace (G. Mexico) 31 August
TS Erika (G. Mexico) 16-17 August
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Storm Precipitation Totals (mm)
Hurricane Isabel (MDR) 18-19 Sep
TS Henri (G. Mexico) 6 Sep
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Precipitation (mm) 30 Jun 9 Oct
Percent of Period Total Precip by landfall days
Total Precipitation during landfall days
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2003 Aug-Sep Mean Atmospheric Conditions
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Atlantic Weekly SST Departures (oC)
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Height and Anomalies (shading)August-September
200-hPa
Stronger Subtropical Ridge
1000-hPa
Lower Surface Pressure
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200-850 hPa Anomalous Vertical Wind Shear
August-September 2003
Anomalous Shear of Zonal Wind
Anomalous Easterly Shear, Stronger Tropical
Easterly Jet
Anomalous Strength of Vertical Shear
Lower Shear
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700-hPa Rel. Vort. (shading), Zonal Wind (contour)
Aug-Sep 2003 Mean
African Easterly Jet axis north of
normal Cyclonic Shear (shading)
Cyclonic Anoms (Red) Westerly Anoms
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Ongoing Decadal Signal
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200-850 hPa Anomalous Vertical Shear of Zonal
Wind Aug-Sep.
Higher Shear
M/S -8 6 4 -2 0 2 4
Lower Shear
1980 1985 1990 1995
2000
Averaging Region
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700-hPa Zonal Wind Anomalies Aug-Sep
M/S 2 1 0 1 2 3 4
1980 1985 1990 1995
2000
Averaging Region
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Anomalous 700-hPa Relative VorticityEquatorward
flank of African Easterly Jet Aug-Sep
Cyclonic
x 10-6 S-1 -4 2 0 2 4 6 8
Anticyclonic
1980 1985 1990 1995
2000
Averaging Region
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NOAA Atlantic Hurricane Outlooks
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Climate Links to Tropics

• Tropical convection is dominant forcing of
  coherent circulation anomalies associated with
  seasonal and decadal extremes in Atlantic
  hurricane activity. (Bell and Chelliah 2004)
• Dominant Climate factors
• ENSO (Gray (1984)
• Tropical multi-decadal mode (TMM) Chelliah and
  Bell (2003)

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NOAAs Atlantic Hurricane Outlooks Forecast
Procedure

• Predict ASO state of the two leading tropical
  modes.
• Use EOF regressions and analogue/ binning
  techniques to
• predict upcoming atmospheric conditions and
  activity in MDR
• Analysis of the combined modes is critical to
  the forecast
• 2. Observed Apr-May, Jun-Jul anomalies to predict
  ASO activity
• Analogue years, bin with respect to leading
  tropical modes.
• Extended CCA forecasts of vertical shear
• Final outlook is subjective blend of items 1-2

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NOAA Seasonal Atlantic Hurricane Outlook August
Forecast vs. Observed 1998-2003
Hurricanes
Tropical Storms
10
15
14
9
14
14
8
8
12
12
6
4
ACE Index
Major Hurricanes
240
206
5
183
4
135
3
122
3
3
2
76
Observed
Forecast Range
Green bars indicate climatological mean and ACE
range for near-normal season (76-120 of Median)
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Summary

• Above-normal 2003 Atlantic hurricane season.
• 1995-2003 most active in record
• 2. Most of 2003 activity (86 of ACE index)
  associated with MDR.
• 3. Ongoing decadal atmospheric signal favors
  active hurricane seasons.
• 4. NOAA outlooks based on prediction of ENSO and
  Tropical Multi-decadal Mode
• 5. Diagnosing and forecasting seasonal Atlantic
  hurricane variability requires an understanding
  of both modes, and their combined impacts.

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2003 Above-normal Hurricane Season Conditions
Amplified Subtropical Ridge
Stronger Tropical Easterly Jet (200-hPa)

Enhanced Tropical Easterly Jet (200-mb)
WET
Favorable African Easterly Jet (700-hPa)
Very Low Vertical Shear

• Favorable African Easterly Jet (700-hPa) reflects
• Northward shift of jet axis
• Strong cyclonic shear along southern flank

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Extras
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700-hPa Rel. Vort., Zonal Wind (contour)
April-May 2003 Anomalies
Cyclonic Westerly
June-July 2003 Anomalies
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700-hPa Potential VorticityAugust-September Mean
2003
High PV in MDR, Gradient Reversal Linear
Instability
1997 (Inactive Season)
Lower PV in MDR, No Gradient Reversal
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Precipitable Water (inches), 1000-hPa
WindAugust-September 2003
High Precipitable water in MDR and West African
monsoon region