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A Review of 2005 & 2006 Hurricane Seasons and
Forecasts
NOAA 31st Annual Climate Diagnostics and Prediction Workshop
Earth System Research Laboratory, Boulder, CO. October 23, 2006.
Muthuvel Chelliah
Climate Prediction Center, NCEP/NWS
• Two seasons: Two seasons: 2005 & 20062005 & 2006
• Two Basins (at least): Two Basins (at least): North Atlantic & East PacificNorth Atlantic & East Pacific (third :Central Pacific) (third :Central Pacific)
• Two Outlooks: Two Outlooks: May & August for N.AtlanticMay & August for N.Atlantic and only and only May Outlook for E. PacificMay Outlook for E. Pacific..
• Verification ( #of Tropical Storms Verification ( #of Tropical Storms TSTS, Hurricanes , Hurricanes HH, , Major Hurricanes Major Hurricanes MHMH and Accumulated Cyclone Energy and Accumulated Cyclone Energy ((ACEACE) Index.) Index.
• I will show even other years for comparison and historical I will show even other years for comparison and historical behavior. behavior.
• NOAA’s outlooks for N. Atlantic (since 1998) and E. Pacific NOAA’s outlooks for N. Atlantic (since 1998) and E. Pacific (since 2003, Experimental in 2003-2004) have been fairly accurate. (since 2003, Experimental in 2003-2004) have been fairly accurate.
• This is a very preliminary analysis of what happened to the This is a very preliminary analysis of what happened to the forecasts this 2006 season (not over yet, ends in November).forecasts this 2006 season (not over yet, ends in November).
• All outlooks are official NOAA outlooks and are made by All outlooks are official NOAA outlooks and are made by teams of scientists from:teams of scientists from:
– Climate Prediction Center (CPC – Lead Center)Climate Prediction Center (CPC – Lead Center) in Maryland in Maryland
– National Hurricane Center (NHC) & National Hurricane Center (NHC) &
– Hurricane Research Division (HRD) in FloridaHurricane Research Division (HRD) in Florida
– Hydrometeorological Prediction Center (HPC)Hydrometeorological Prediction Center (HPC) in Maryland in Maryland
North Atlantic Outlook Team:North Atlantic Outlook Team: CPC:CPC: Gerald Bell (Lead), Muthuvel Chelliah, Kingste Mo Gerald Bell (Lead), Muthuvel Chelliah, Kingste Mo NHC: Christopher Landsea, Eric Blake and Richard Pasch NHC: Christopher Landsea, Eric Blake and Richard Pasch HRD: Stanley GoldenbergHRD: Stanley Goldenberg
East Pacific Outlook Team:East Pacific Outlook Team:CPC: Muthuvel Chelliah (Lead), Gerald Bell, Kingtse MoCPC: Muthuvel Chelliah (Lead), Gerald Bell, Kingtse MoNHC: Christopher Landsea and Eric BlakeNHC: Christopher Landsea and Eric BlakeHPC: Todd KimberlainHPC: Todd Kimberlain
NOAA’s 2005 Seasonal Hurricane OutlooksNOAA’s 2005 Seasonal Hurricane Outlooks Issued 16 May 2005 Issued 16 May 2005
NOAA’s 2005 Seasonal Hurricane OutlooksNOAA’s 2005 Seasonal Hurricane Outlooks Issued 22 May Issued 22 May 20062006
2005 2005 AtlanticAtlantic Seasonal Activity Outlook - May Seasonal Activity Outlook - May
NOAA’s Accumulated Cyclone Energy (ACE) IndexNOAA’s Accumulated Cyclone Energy (ACE) Index
2005 2005 AtlanticAtlantic Seasonal Activity Outlook - August Seasonal Activity Outlook - August
NOAA’s Updated 2005 Atlantic Hurricane OutlookNOAA’s Updated 2005 Atlantic Hurricane Outlook
Already during 2005: 7 Tropical Storms, 2 Major Hurricanes (Dennis and Emily)ACE index = 68% of median
95% to 100% Chance of an Above-Normal Season(Measured by forecasted numbers, strength, and duration of named storms)
Forecasted Seasonal Totals 18-21 Tropical Storms 9-11 Hurricanes 5-7 Major Hurricanes 180%-270% of median ACE value
Forecasted for August-November 2005 11-14 Tropical Storms 7-9 Hurricanes 3-5 Major Hurricanes 110%-200% of median ACE value
For 2006 Less North Atlantic Activity Expected Than 2005For 2006 Less North Atlantic Activity Expected Than 2005
2006 Atlantic Hurricane Outlooks: •May Outlook called for less activity than 2005 •August Outlook called for even less activity than May outlook
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East PacificEast Pacific Hurricane Activity Hurricane ActivityMay 2005 OutlookMay 2005 Outlook
NOAA’s Accumulated Cyclone Energy (ACE) IndexNOAA’s Accumulated Cyclone Energy (ACE) Index
East PacificEast Pacific Hurricane Activity Hurricane ActivityMay 2006 OutlookMay 2006 Outlook
North Atlantic Hurricane SeasonNorth Atlantic Hurricane Season
2005
2006
Tracks Graphics Courtesy of Unisys
0.0
0.5?
TS - 27 (18-21) H - 15 (9-11) MH - 7 (5-7)
TS - 9 (12-15) H - 5 (7-9) MH – 2 (3-4)
ACE - 285% (180-270%)
ACE – 90 % ? (110-170%)
NORMAL
ABOVE-NORMAL (Hyper-Active)
East Pacific Hurricane SeasonEast Pacific Hurricane Season
2005
2006
Tracks Graphics Courtesy of Unisys
TS - 15 (11-15) H - 7 (6-8) MH - 2 (2-4)
TS - 16 (12-16) H - 8 (6-8) MH - 5 (1-3)
ACE: 65 % (45-95%)
ACE: 95 % (45-85%)
BELOW NORMAL
NORMAL
2004 North Atlantic2004
1995
-0.5
ACE >250%
ACE >250%
1994
19931993 Aug-Sep-Oct Nino 3.4
Index 0.4 (from CPC) – officially
NEUTRAL
1994 Aug-Sep-Oct Nino 3.4 Index 0.7 (from CPC) – officially El Nino
Basis and Procedures for the Seasonal Hurricane Outlooks
1. Interannual and multi-decadal extremes in tropical North Atlantic, East Pacific and Central Pacific hurricane activity are not random.
2. Instead, they result from a set of coherent and inter-related atmospheric and oceanic conditions controlled by tropical climate patterns, and some possibly related to increases in ocean surface temperatures.
3. NOAA’s makes seasonal hurricane outlooks by first analyzing and predicting these leading recurring patterns of climate variability in the Tropics, and then predicting their impacts on hurricane activity.
The Two dominant climate factors that influence/control seasonal hurricane activity in all three regions are:
El Niño/ Southern Oscillation (ENSO): Gray (1984)
Tropical multi-decadal climate variability: Chelliah and Bell (2004)
Bell and Chelliah (2006)
Regional North Atlantic Conditions associated Regional North Atlantic Conditions associated with Active Hurricane Seasonwith Active Hurricane Season
Higher Pressure in Upper Atmosphere
Weaker Low-Level Easterly Winds (Blue arrow)
Weaker Low-Level Easterly Winds (Blue arrow)
Warmer SSTsLower Wind Shear
Lower Surface Pressure(Red Area)
Upper-level EasterliesExpand Westward
(Green arrows)
Favorable WindsFrom Africa
200-hPa Velocity 200-hPa Velocity Potential Loading Potential Loading Patterns during ASOPatterns during ASO::
Contours shown forContours shown for “ “active” Atlantic active” Atlantic hurricane eras.hurricane eras.
East-west see-saw in East-west see-saw in tropical convection tropical convection between West Africa and between West Africa and Amazon basinAmazon basin
_
_
+
+
Composite Conditions Associated with Composite Conditions Associated with Active/ Inactive Atlantic Hurricane SeasonsActive/ Inactive Atlantic Hurricane Seasons
Regional Anomalies Exhibit Larger-Scale Convective Signature• Inter-hemispheric symmetry•Baroclinic structure to wind anomalies (reversal in anomalies between 200-hPa and 850-hPa).
Anti-Cyclonic
Lower Shear Higher Shear
Enhanced easterly vertical shear produces 3-celled pattern of anomalous shear magnitude
• Since 1995 (thorough 2005) – 9 out of 11 years are above-normal active hurricane seasons in N.Atlantic – except for two El Nino years 1997 and 2002.
• The generally negative correlation between the N.Atlantic and E. Pacific basin hurricane activities has strengthened in the last couple of decades.
Comparison of North Atlantic and East Pacific Hurricane Activities
0
50
100
150
200
250
300
1971
1974
1977
1980
1983
1986
1989
1992
1995
1998
2001
2004
E.Pac. ACE
N.Atl.ACE
Inactive East PacificActive N.Atlantic
E.Pac Hurricane Activity compared with N.Atl.ACE: % of Respective Basin's 1971-2000 Median
0
50
100
150
200
250
300
1951
1954
1957
1960
1963
1966
1969
1972
1975
1978
1981
1984
1987
1990
1993
1996
1999
2002
2005
E.Pac.
N.Atl.
The opposite phase relation between the two basins’ activities seems to extend far back. BUT the East Pacific Data
considered unreliable before ~ 1965/1970
The Transition to Weak El Niño Conditions was not Predicted
Observed SST Departures (oC)CFS Forecast SST Departures (oC)
Forecast Issued May 9th 2006
The Climate Forecast System (CFS) forecast issued May 9th 2006 indicated slightly below-average SST’s over central equatorial Pacific, thus failing to predict the impending El Niño.
15
Very Warm Waters Returned during August 2006
2
1
2
1
21
During the peak of the 2006 season, tropical Atlantic SSTs were generally 1-2oF above Average. Departures were sometimes comparable to last year (2-3oF).
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Forecasts of current Forecasts of current El NinoEl Nino
inin APRILAPRIL 2006 2006
As measured by Nino 3.4 SST Anomaly
ENSO ADVISORY:
La Niña conditions are expected to continue during the next 1-3 months.
Forecasts of current Forecasts of current El NinoEl Nino
in in MAY 2006MAY 2006
ENSO ADVISORY:
ENSO-neutral conditions are expected to prevail during the next 3-6 months.
Forecasts of current Forecasts of current El NinoEl Nino
in in JUNEJUNE 2006 2006
ENSO ADVISORY:ENSO-neutral conditions are expected to prevail during the next 3 months.
Forecasts of current Forecasts of current El NinoEl Nino
in in JULYJULY 2006 2006
ENSO ADVISORY:ENSO-neutral conditions are expected to prevail during the next 3 months.
Forecasts of current Forecasts of current El NinoEl Nino
in in AUGUSTAUGUST 2006 2006
ENSO ADVISORY:ENSO-neutral conditions are expected to continue for the next one to three months, with a 50% chance that weak El Niño conditions will develop by the end of 2006.
Forecasts of current Forecasts of current El NinoEl Nino
in in SEPTEMBERSEPTEMBER 2006 2006
ENSO ADVISORY:ENSO-neutral conditions are expected to continue for the next one to two months, with weak warm episode (El Niño) conditions likely by the end of 2006.
In Closing…..In Closing…..
• So, what happened to the ‘Above Normal’ North Atlantic Hurricane activity AND So, what happened to the ‘Above Normal’ North Atlantic Hurricane activity AND the ‘Below Normal’ East Pacific outlook ? The season is not over yet (ends in the ‘Below Normal’ East Pacific outlook ? The season is not over yet (ends in November)– But, most likely, the season for both basins will end as NORMAL. November)– But, most likely, the season for both basins will end as NORMAL.
• Not just NOAA, but all other outlooks did indicate ‘Above Normal’ activity for the Not just NOAA, but all other outlooks did indicate ‘Above Normal’ activity for the North Atlantic basin during 2006 season. NOAA has been issuing Hurricane North Atlantic basin during 2006 season. NOAA has been issuing Hurricane outlooks since 1998 and this is the first year the outlook may turn out to be not so outlooks since 1998 and this is the first year the outlook may turn out to be not so pretty..pretty..
• Preliminary culprit: The unexpected and sudden emergence of El Nino – right in the Preliminary culprit: The unexpected and sudden emergence of El Nino – right in the middle of the season - which was not forecast by any of the dynamical or the middle of the season - which was not forecast by any of the dynamical or the statistical models. – Needs further analysis statistical models. – Needs further analysis
• After 2005 being such a record active year, to be followed by ‘such a bla..’ 2006 After 2005 being such a record active year, to be followed by ‘such a bla..’ 2006 season, hopefully brings some sanity to the global warming – hurricane connection season, hopefully brings some sanity to the global warming – hurricane connection debate - at least demonstrates that just local ocean warm waters alone debate - at least demonstrates that just local ocean warm waters alone (thermodynamic argument – GCM style thinking) is not enough, and that (thermodynamic argument – GCM style thinking) is not enough, and that atmospheric circulation factors such as vertical wind shear, cyclonic vorticity, etc. atmospheric circulation factors such as vertical wind shear, cyclonic vorticity, etc. (due to local and remote influences) are just as important in deciding the season’s (due to local and remote influences) are just as important in deciding the season’s activity.activity.
SummarySummary
I have not addressed cause(s) of the LF/ multi-decadal variability, nor has this issue been addressed in Bell and Chelliah (2006). Our work focuses on understanding the observations. This is particularly important if we are to understand how combinations of climate factors (multi-decadal variability and GH warming) will impact hurricane activity in the future.
1.1. The leading tropical multi-decadal modes of convective variability account for The leading tropical multi-decadal modes of convective variability account for many key circulation and SST anomalies associated with seasonal and multi-many key circulation and SST anomalies associated with seasonal and multi-decadal extremes in Atlantic hurricane activity since 1950.decadal extremes in Atlantic hurricane activity since 1950.
2.2. The transition to current active Atlantic hurricane era shows strong link to phase The transition to current active Atlantic hurricane era shows strong link to phase change in tropical multi-decadal signal—see-saw in tropical convection between change in tropical multi-decadal signal—see-saw in tropical convection between West Africa and Amazon Basin.West Africa and Amazon Basin.
3.3. This phase change accounts for many conditions known to favor active hurricane This phase change accounts for many conditions known to favor active hurricane seasons, and is consistent with transition to warm phase of AMO. seasons, and is consistent with transition to warm phase of AMO.
4.4. Warmer Atlantic SSTs since 1995 are associated with significantly more Warmer Atlantic SSTs since 1995 are associated with significantly more hurricanes in large part because of their common association with overlying hurricanes in large part because of their common association with overlying circulation anomalies.circulation anomalies.
1. We see large tropical circulation/ convection/ SST changes occurring between active/ inactive hurricane eras.
2. Numerical simulations indicate little to no wind changes across tropical Atlantic with increasing CO2. Seems inconsistent with observations, suggesting weak (at best) CO2 impact at this time.
3. How can minimal impact on winds be maintained if SST increases substantially in a future CO2 world? Is this realistic?
4. If not, how might CO2 increases affect leading tropical modes.
5. Ultimate cause(s) of tropical multi-decadal variability
6. We should study potential CO2 impacts simultaneous with increasing our understanding of the tropical multi-decadal variability.
Some Unresolved Issues of Concern
Tropical disturbances remain in extended region of cyclonic shear while moving westward into low vertical shear, warm SST environment of central/ western MDR.
AEJ
Measuring total seasonal activity: The Accumulated Cyclone Energy (ACE) indexThe phrase "total seasonal activity" refers to the collective intensity and duration of Atlantic named storms and hurricanes occurring during a given season. The measure of total seasonal activity used by NOAA is called the Accumulated Cyclone Energy (ACE) index. The ACE index is a wind energy index, defined as the sum of the squares of the maximum sustained surface wind speed (knots) measured every six hours for all named systems while they are at least tropical storm strength.NOAA uses the ACE index, combined with the numbers of named storms, hurricanes, and major hurricanes, to categorize North Atlantic hurricane seasons as being above normal, near normal, or below normal.
NOAA’s Atlantic hurricane season classificationsAbove-normal season: An ACE index value well above 103 x 104 kt2 (corresponding to 117% of the median ACE value or 110% of the mean), or an ACE value slightly above 103 x 104 kt2 combined with at least two of the following three parameters being above the long-term average: number of tropical storms, hurricanes, and major hurricanes.
Near-normal season: An ACE index value in the range 66-103 x 104 kt2 (corresponding to 75%-117% of the median or 71%-110% of the mean), or an ACE index value slightly above 103 x 104 kt2 but with less than two of the following three parameters being above the long-term average: numbers of tropical storms, hurricanes, and major hurricanes.
Below-normal season: An ACE index value below 66 x 104 kt2, corresponding to below 75% of the median or 71% of the mean.
NOAA’s 2005 East Pacific Hurricane OutlookNOAA’s 2005 East Pacific Hurricane Outlook
Issued 16 May 2005Issued 16 May 2005
Season and 2005 2005 Climatological Activity Type Outlook Observed Mean (1971-2000)
Chance Above Normal 10% Below NormalChance Near Normal 20%Chance Below Normal 70%
Tropical Storms 11-15 15 15.5Hurricanes 6-8 7 9.2Major Hurricanes 2-4 2 4.5ACE % of Median 45% - 95% 65% 110 %
ASO: Tropical Atlantic SSTAASO: Tropical Atlantic SSTA
SST warming in MDR is consistent with phase change in tropical multi-SST warming in MDR is consistent with phase change in tropical multi-decadal signal and associated weaker trade winds.decadal signal and associated weaker trade winds.
Observed
Regressed
Weaker trade winds, along with reduced flow of dry, subsiding northeasterly Weaker trade winds, along with reduced flow of dry, subsiding northeasterly flow coming into MDR from east side of Bermuda High.flow coming into MDR from east side of Bermuda High.
Along 10N-15N, dry northeasterly flow replaced by moist inflow from deep Along 10N-15N, dry northeasterly flow replaced by moist inflow from deep tropics.tropics.
Less Activity was Expected and Forecast Less Activity was Expected and Forecast This Year in 2006 Than 2005This Year in 2006 Than 2005
2006 Atlantic Hurricane Outlooks: •May Outlook called for less activity than 2005 •August Outlook called for even less activity than May outlook
9
5-yr Running Mean ACE Observed vs. Projected
0
50
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200
19
71
19
74
19
77
19
80
19
83
19
86
19
89
19
92
19
95
19
98
20
01A
CE
(%
of
Me
dia
n)
Current active Atlantic hurricane era associated with phase change in Current active Atlantic hurricane era associated with phase change in tropical multi-decadal signal.tropical multi-decadal signal.
Explained variance = 82%.
Observed vs. Regressed
ASO 5-year LP-Filtered Variance Explained by ASO 5-year LP-Filtered Variance Explained by Leading Tropical Multi-Decadal ModesLeading Tropical Multi-Decadal Modes
LF variability in all KEY circulation features strongly related to leading LF variability in all KEY circulation features strongly related to leading tropical multi-decadal modes.tropical multi-decadal modes.
+
February-March 2005February-March 2005 SST Departures SST Departures
Very warm ocean temperatures continue in Tropical Atlantic
Red shading shows lower vertical wind shear between 200-850-hPa in the main hurricane development region (black box). Blue shading shows higher than normal vertical wind shear. The 3-celled pattern of anomalies between the eastern tropical Pacific and Africa has been in place since 1995. This pattern has resulted in more Atlantic hurricanes and fewer eastern Pacific hurricanes.
Anti-Cyclonic Cyclonic
AEJ
Less Activity Expected This Year Than 2005Less Activity Expected This Year Than 2005
May 22, 2006 Seasonal Hurricane Outlook
Fewer hurricanes expected than 2006:
The combinations of conditions was not expected to be the same as in 2005
• La Niña-like rainfall pattern was not expected to be as pervasive as last year
• Seasonal Atlantic SSTs were not expected to be as warm• Upper-level and lower-level wind and air pressure patterns
not as favorable.
10
Less Activity Expected This Year Than 2005Less Activity Expected This Year Than 2005
August 8, 2006 Updated Seasonal Hurricane Outlook
Fewer hurricanes than May outlook:• Transition from La Niña-like rainfall patterns to ENSO-
neutral conditions occurred in June -- more rapidly than expected
• Seasonal sea-surface temperatures in the tropical North Atlantic were not as warm as previously expected.
• Atmospheric wind patterns over the eastern Atlantic were not as conducive as originally expected, partly due to the transition into El Niño conditions.
11
August Outlook Called for Fewer Hurricanes Than May Outlook – Why?
Wet
Wet
L---L
High Shear
Mid-June through mid-July 2006 Conditions
A transition from La Niña-like conditions to ENSO-neutral conditions occurred by mid-June, and El Niño conditions developed by September. Therefore, the critical combination of 1) an active hurricane era and 2) La Niña was no longer possible. Central Pacific became wet and the shear increased over the Caribbean and Western Atlantic (neither supports an active hurricane season)
Transition away from La Niña-like Rainfall Pattern during June
12
ASO: 5-yr LP-Filtered ASO: 5-yr LP-Filtered Surface Temperature Variance (%)Surface Temperature Variance (%)
Leading tropical multi-decadal modes capture much surface temperature variability associated with AMO and WAFR monsoon.
Sea-surface temperatures during July 2006 were 0.5o-2oFahrenheit above average. Significant warmth was confined to Caribbean Sea.
13
Sea-surface temperatures in the tropical North Atlantic were not as anomalously warm as previously expected
August Outlook Called for Fewer Hurricanes Than May Outlook
Cause of the Apparent Change in Atlantic Ocean Cause of the Apparent Change in Atlantic Ocean Temperatures between 2006 and 2005 Temperatures between 2006 and 2005
Differences in tropical easterly trade winds and associated pressure pattern
2006 has weaker easterly trade winds in the hurricane formation areas (green box), and a reduced flow of moist, tropical air. These conditions cause increased evaporative cooling and cooler sea-surface temperatures. Low pressure (like in 2005) favors hurricane development. A detailed understanding of the reasons behind these changes requires more research.
Anomalous Surface Winds and Air Pressure (Lower pressure indicated by blue shading)
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