The 2006 Hurricane Season:
Expectations vs. Reality
Steve Letro
Meteorologist in Charge
National Weather Service, Jacksonville
United States Coastal Communities
Probably Entered the 2006 Hurricane
Season With An Unprecedented Level
Of Apprehension and Anxiety…
Based on the Previous Two Seasons
And Anticipated Conditions…There
Was Ample Reason For This Concern!
To Say 2004 and 2005 Were
Two “Rough” Seasons Would
Be a Gross Understatement!
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•
•
•
•
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NAMED STORMS:
HURRICANES:
MAJOR HURRICANES:
U.S. STRIKES
U.S. HURRICANE STRIKES:
MAJOR U.S. HRCN STRIKES
43
24
13
14
9
7
Mother Nature Got Our
Attention in 2004…
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•
•
•
•
•
•
NAMED STORMS:
15
HURRICANES:
9
MAJOR HURRICANES:
6
U.S. STRIKES
6
U.S. HURRICANE STRIKES:
4
MAJOR U.S. HRCN STRIKES 3
Including 4 Hurricane Strikes in Florida!
Just When We Thought It Couldn’t
Get Any Worse Than 2004:
Along Came 2005!
• NAMED STORMS:
28*
* New Record… Old Record 21 in 1933
• HURRICANES:
15**
** New Record… Old Record 12 in 1969
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•
•
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MAJOR HURRICANES:
U.S. STRIKES
U.S. HURRICANE STRIKES:
MAJOR U.S. HRCN STRIKES
7
8
5
4
At The Beginning Of The Season
There Was Every Reason To Believe
2006 Would See This Unfortunate
Trend Continue
The Primary Reason:
We Are Still In The Active Phase Of A
Long Term Cycle Called The Atlantic
Multi-Decadal Cycle
Just What Is A “Multi-Decadal
Cycle”?
• We know that Atlantic Tropical Cyclone frequency
varies in distinct 25-40 year cycles
• There are negative phases of that cycle when we
experience below normal frequencies…
• There are also positive phases where frequencies
are above normal.
• After nearly 30 years of below normal activity we
entered a positive phase of the cycle in 1995.
• So… how does all this work?
Water cools,
sinks, and
flows
southward
Salty, warm
shallow water
flows northward
Strong vs. Weak Thermohaline Circulation
MAJOR HURRICANES IN WEAK VS ACTIVE
PHASES OF THE MULTIDECADAL CYCLE
WEAK
ACTIVE
Strong
Weak
Phase of Thermohaline Circulation
Strong
THE RESULT…
• The 11 year period between 1995 and 2005
was the most active on record in the
Atlantic basin.
• In fact… Between 1970 and 1994… no
season had more than 3 majors! 1995 -2005
produced 6 seasons with more than 3!
THE BAD NEWS…
WE ARE NOW SQUARELY IN
THE MIDDLE OF THE “ACTIVE”
OR “STRONG” PHASE OF THIS
CYCLE!
As A Result, the Early Season 2006
Forecast Looked Grim…
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•
•
•
•
•
NAMED STORMS:
HURRICANES:
MAJOR HURRICANES:
U.S. STRIKES
U.S. HURRICANE STRIKES:
MAJOR U.S. HRCN STRIKES
17
9
5
?
?
?
The 2006 Season So Far…
•
•
•
•
•
•
•
NAMED STORMS:
9
HURRICANES:
5
MAJOR HURRICANES:
2
U.S. STRIKES
2
U.S. HURRICANE STRIKES:
0
MAJOR U.S. HRCN STRIKES 0
Only 2 weak tropical storm landfalls!
So What Happened in 2006???
Most Of The Blame (or credit) For The
Quieter Season Has Gone To The
Unexpectedly Rapid Development of an
El Nino Event In The Pacific.
So…How Does El Nino Affect Tropical
Cyclone Development?
IT STARTS WITH WHICH WAY THE
WIND BLOWS…
THE IMPORTANCE OF EL NINO
• EL NINO PRODUCES WESTERLY
WINDS IN THE MIDDLE ATMOSPHERE
ACROSS THE TROPICS… CREATING
WIND SHEAR WHICH INHIBITS
DEVELOPMENT.
HERE’S HOW IT WORKS…
SO…
JUST HOW DOES EL NINO
PRODUCE THOSE
WESTERLY WINDS ACROSS
THE TROPICS???
EL - NINO CONDITIONS
WARM
NORMAL ATMOSPHERIC AND OCEANIC CURRENTS REVERSE...
BRINGING ABNORMALLY WARM WATER TO THE SOUTH AMERICAN
PACIFIC COAST
Warmer Air and Water
Produces More Rain and
Thunderstorms Which
Release Added Heat into the
Mid and Upper Atmosphere
EL - NINO CONDITIONS
THIS EXTRA HEAT LOWERS PRESSURES IN THE UPPER AIR...
PULLING THE JET STREAM SOUTH... CREATING STRONG
WESTERLY WINDS ACROSS THE TROPICAL ATLANTIC
STRONGER UPPER WINDS
DESTROY SEEDLING STORMS
BEFORE THEY HAVE A
CHANCE TO DEVELOP
UPPER WIND FLOW
UPPER WIND FLOW
R.I.P.
UPPER WIND FLOW
THIS HOSTILE ENVIRONMENT
HAS SIGNIFICANT EFFECTS
ON SEVERAL PARAMETERS:
•
NUMBER OF HURRICANES
•
NUMBER OF MAJOR HURRICANES
•
AREA OF ORIGIN
•
AREAS AFFECTED
The Statistics
Tropical Cyclone Frequency in
El Nino vs La Nina Years
Phase
# Cases
Total Storms
Hurricanes
Strong La Nina
2
10.5
7.5
Mod. La Nina
7
11.1
6.9
Weak La Nina
5
11.4
6.6
Weak El Nino
7
9.6
5.0
10
8.8
4.4
4
6.8
3.0
Mod. El Nino
Strong El Nino
OVERALL THEN… EL NINO
TENDS TO PRODUCE:
• FEWER OVERALL STORMS
• FEWER INTENSE STORMS
• FEWER MAJOR STORMS
AFFECTING THE UNITED
STATES
BUT…
Some Facts To Consider…
• While This El Nino May Continue To Strengthen,
It Was Mainly In The Early Developmental Stage
During the Peak of the 2006 Hurricane Season.
• The Best Correlations Between El Nino and
Decreased Hurricane Activity Usually Occur
Once The Event Is Fully Established…which it
wasn’t!
• This Implies There Were Probably Other Things
Going On Besides Just El Nino!
Let’s Take A Look At An Interesting
Fact About The 2005 Season…
Typically, in very active seasons, the
majority of storms develop in the
deep tropics… an area called the
MDR, or Main Development Region.
This was certainly the case in 2004…
2004
Very Active
MDR (Main
Development
Region)
As Busy As the Following 2005
Season Was However…
We Began To See An Interesting
Change In Atmospheric Conditions
Across That Main Development
Region
2005
Much less
significant
MDR Activity!
September 2005… African
Dust Blankets Much of the
MDR. This continued
through much of 2006
This dust is indicative of stable, sinking air…
a very unfavorable environment for tropical
cyclone development!
The Fact That Significant MDR Activity
Did Not Account For The Significant
U.S. Landfalls Is Not Really A
Surprise…
The Fact Is… The storms that
ravaged the U.S. Coast in 2005
developed mostly within a few
hundred miles of Key Largo!
This Brings Us To Yet Another
Important Factor In The Strange
(but welcome!) Saga Of the
Relatively Benign 2006 Hurricane
Season
To understand it, we must examine
what controls the motion of
tropical cyclones
H
THE SUMMERTIME ATLANTIC (BERMUDA) HIGH PRESSURE
SYSTEM IS PROBABLY THE MOST IMPORTANT FACTOR IN
DETERMINING TROPICAL CYCLONE MOTION!
H
Tropical cyclones are “steered” westward across the
Tropics along the underside of this Atlantic high, before
eventually recurving around the western edge.
H
H
H
H
H
The problem is that the Atlantic High is not constant… either
in strength, size or position… and this leads to great
differences in the motion of individual tropical cyclones
Climatological Tracks of
Atlantic Basin Tropical Cyclones
WHEN…AND IF… A TROPICAL CYCLONE RECURVES
IS LARGELY A FUNCTION OF THE POSITION AND
STRENGTH OF THE BERMUDA HIGH AT ANY GIVEN
TIME.
PREVAILING PATTERN 1995 - 2003
H
DURING THIS PERIOD, THE ATLANTIC HIGH WAS WEAK
AND DISPLACED TO THE EAST. ..
…AS A RESULT, MOST HURRICANES TENDED TO
RECURVE WELL EAST OF FLORIDA.
PREVAILING PATTERN 2004 & 2005
DURING THE SUMMER OF 2004… THE PATTERN
FINALLY CHANGED… AND THE ATLANTIC HIGH WAS
STRONGER AND DISPLACED FURTHER WEST…
…THIS KEPT HURRICANES MOVING FARTHER SOUTH
AND WEST… AND RECURVED THEM RIGHT OVER
FLORIDA!!!
H
THE CHANGE CONTINUED IN 2005 WITH STORMS
FORMING AND MOVING FARTHER WEST… WITH EVEN
MORE DISASTROUS RESULTS!
Thankfully, 2006 Saw A Return to the 1995-2003 Pattern
H
This kept storms from forming close to the coast, and
kept those farther at sea from approaching.
In Summary… The Benign 2006 Season
Was A Result of Several Factors Including:
• Dry, stable air over the central and east Atlantic
preventing devastating MDR storms from
forming.
• The development of El Nino… especially
affecting latter portions of the season
• The return of the deep East Coast trough
preventing development in our back yard and
tracks that threatened the coast.
WHICH LEAVES US WITH THE
BIG QUESTION…
Will 2007 Continue The 2006 Trend, or
Revert Back To the Devastating
Pattern of 2004-2005?
…This depends a lot on how persistent and
strong the El Nino turns out to be.
Unfortunately, El Nino is not nearly as predictable as we
wish it were!
What We DO Know…
We are still in the Active Phase of the
Multi-Decadal Oscillation…
This Will Be Enough To Keep Us On
High Alert For The Next Several
Years!
THOSE WHO DO NOT
REMEMBER THE PAST…
ARE CONDEMNED TO REPEAT IT…
-SANTAYANA
The End!
www.srh.noaa.gov/jax