# Presentation: 270973

## 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: 	     2005 & 2006
Two Basins (at least):  North Atlantic  &  East Pacific 				      (third :Central Pacific)
Two Outlooks:  May & August for N.Atlantic  and only May Outlook for E. Pacific.
Verification ( #of  Tropical Storms TS,  Hurricanes H,  Major Hurricanes MH and Accumulated Cyclone Energy (ACE) Index.
I will show even other years for comparison and historical behavior. 

**Two Basins (at least):  ****North Atlantic  &  East Pacific**** ****	****	****	****	****      (third :Central Pacific)**
**Two Outlooks:  ****May & August for N.Atlantic****  and only ****May Outlook for E. Pacific****.**
**Verification ( #of  Tropical Storms ****TS****,  Hurricanes ****H****,  Major Hurricanes ****MH**** and Accumulated Cyclone Energy (****ACE****) Index.**
**I will show even other years for comparison and historical behavior. **

**    ****NOAA’s outlooks for N. Atlantic  (since 1998) and E. Pacific (since 2003, Experimental in 2003-2004) have been fairly accurate.   **
**    ****This is a very preliminary analysis of what happened to the forecasts this 2006 season (not over yet, ends in November).**

## All  outlooks are official NOAA outlooks  and are made by teams of scientists from:
Climate Prediction Center (CPC – Lead Center) in Maryland
 National Hurricane Center (NHC) & 
Hurricane Research Division (HRD)  in Florida
Hydrometeorological Prediction Center (HPC) in Maryland

- Climate Prediction Center (CPC – Lead Center) in Maryland
-  National Hurricane Center (NHC) &
- Hurricane Research Division (HRD)  in Florida
- Hydrometeorological Prediction Center (HPC) in Maryland

- _North Atlantic Outlook Team:_
- CPC:*    *Gerald Bell (Lead), Muthuvel Chelliah, Kingste Mo
- NHC:   Christopher Landsea, Eric Blake and Richard Pasch
- HRD:   Stanley Goldenberg
- _East Pacific Outlook Team:_
- CPC:   Muthuvel Chelliah (Lead), Gerald Bell, Kingtse Mo
- NHC:  Christopher Landsea and Eric Blake
- HPC:   Todd Kimberlain

## NOAA’s 2005 Seasonal Hurricane Outlooks Issued 16 May 2005 

## NOAA’s 2005 Seasonal Hurricane Outlooks Issued 22 May 2006

**NOAA’s 2005 Seasonal Hurricane Outlooks**** Issued 22 May **_**2006**_

## 2005 Atlantic Seasonal Activity Outlook  - May

**NOAA’s Accumulated Cyclone Energy (ACE) Index**

## 2005 Atlantic Seasonal Activity Outlook  - August

**2005 **_**Atlantic**_** Seasonal Activity Outlook  - August**

## NOAA’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 2005 

**For 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**

**9**

## East Pacific Hurricane ActivityMay 2005 Outlook

**NOAA’s Accumulated Cyclone Energy (ACE) Index**

## East Pacific Hurricane ActivityMay 2006 Outlook

_**East Pacific**_** Hurricane Activity****May 2006 Outlook**

## North Atlantic Hurricane Season

**North 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 Season

**East 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 Atlantic**

**2004**

**1995**

- -0.5

- ACE >250%

- ACE >250%

## 1994

**1994**

**1993**

- 1993 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

**Basis and Procedures for the ****Seasonal Hurricane Outlooks**

**Interannual and multi-decadal extremes in tropical ****North Atlantic****,  ****East Pacific**** and ****Central Pacific**** hurricane activity are **_**not random**_**.  **
**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.**
**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 with Active Hurricane Season

**Regional North Atlantic Conditions associated with Active Hurricane Season**

**Higher Pressure in **
**Upper Atmosphere**

**Weaker Low-Level Easterly Winds (Blue arrow)**

**Warmer SSTs**
**Lower Wind Shear**
**Lower Surface Pressure**
**(Red Area)**

**Upper-level  Easterlies**
**Expand Westward**
**(Green arrows)**

**Favorable Winds**
**From Africa**

## _

**200-hPa Velocity Potential Loading Patterns during ASO****:**
**Contours shown for**
** “****active” Atlantic **
**hurricane eras.**
**East-west see-saw in tropical convection between West Africa and Amazon basin**

**_**

**_**

**+**

**+**

## Composite Conditions Associated with Active/ Inactive Atlantic Hurricane Seasons

**Composite Conditions Associated with ****Active/ 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.

**   ****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.**

**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 (****o****C)**

**CFS Forecast SST Departures (****o****C)**
**Forecast Issued May 9****th**** 2006**

**The Climate Forecast System (CFS) forecast issued May 9****th**** 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**

**2**

**1**

**During the peak of the 2006 season, tropical **
**Atlantic SSTs were generally 1-2****o****F above **
**Average.  Departures were sometimes** **comparable to last year (2-3****o****F).**

**16**

## Forecasts of current El Ninoin APRIL 2006

**As measured by Nino 3.4 SST Anomaly**

_**ENSO ADVISORY:**_
**La Niña conditions are expected to continue during the next 1-3 months.**

## El Ninoin MAY 2006

**Forecasts of current **
**El Nino****in ****MAY 2006**

_**ENSO ADVISORY:**_
**ENSO-neutral conditions are expected to prevail during the next 3-6 months.**

## El Ninoin JUNE 2006

**Forecasts of current **
**El Nino****in **_**JUNE**_** 2006**

_**ENSO ADVISORY:**_
**ENSO-neutral conditions are expected to prevail during the next 3 months.**

## El Ninoin JULY 2006

**Forecasts of current **
**El Nino****in **_**JULY**_** 2006**

_**ENSO ADVISORY:**_
**ENSO-neutral conditions are expected to prevail during the next 3 months.**

## El Ninoin AUGUST 2006

**Forecasts of current **
**El Nino****in **_**AUGUST**_** 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.**

## El Ninoin SEPTEMBER 2006

**Forecasts of current **
**El Nino****in **_**SEPTEMBER**_** 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.**

## Slide 31

## In Closing.....

- 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 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 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 pretty..
- 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 statistical models. – Needs further  analysis
- 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 debate -  at least demonstrates that just local ocean warm waters alone (thermodynamic argument – GCM style thinking)  is not enough, and that 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 activity.

## Summary

**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.**
**The leading tropical multi-decadal modes of convective variability account for many key circulation and SST anomalies associated with seasonal and multi-decadal extremes in Atlantic hurricane activity since 1950.**
**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 West Africa and Amazon Basin.**
**This phase change accounts for many conditions known to favor active hurricane seasons, and is consistent with transition to warm phase of AMO. **
**Warmer Atlantic SSTs since 1995 are associated with significantly more hurricanes in large part because of their common association with overlying circulation anomalies.**

## 
We see large tropical circulation/ convection/ SST changes occurring between active/ inactive hurricane eras. 
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.
How can minimal impact on winds be maintained if SST increases substantially in a future CO2 world? Is this realistic?
If not, how might CO2 increases affect leading tropical modes. 
Ultimate cause(s) of tropical multi-decadal variability
We should study potential CO2 impacts simultaneous with increasing our understanding of the tropical multi-decadal variability.

**We see large tropical circulation/ convection/ SST changes occurring between active/ inactive hurricane eras. **
**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.**
**How can minimal impact on winds be maintained if SST increases substantially in a future CO2 world? Is this realistic?**
**If not, how might CO2 increases affect leading tropical modes. **
**Ultimate cause(s) of tropical multi-decadal variability**
**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) index

- The 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](http://www.cpc.ncep.noaa.gov/products/outlooks/figure5.gif)__. 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 classifications***_
_*Above-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 Outlook Issued 16 May 2005 

**       ****Season and                    2005                2005                  Climatological**
**     **_**Activity Type               Outlook          Observed           Mean (1971-2000)           **_

**Chance Above Normal        10%        ****Below Normal**
**Chance Near Normal           20%**
**Chance Below Normal         70%**
**                                                 **
**Tropical Storms                   11-15                **** 15 ****                       15.5**
**Hurricanes                              6-8                    ****7****                           9.2**
**Major Hurricanes                  2-4                    ****2****                           4.5**
**ACE % of Median           45% - 95%         ****65%****                      110 %**

## ASO: Tropical Atlantic SSTA

**SST warming in MDR is consistent with phase change in tropical multi-decadal signal and associated weaker trade winds.**

**Observed**

**Regressed**

**Weaker trade winds, along with reduced flow of dry, subsiding northeasterly flow coming into MDR from east side of Bermuda High.**

## Less Activity was Expected and Forecast This Year in 2006 Than 2005 

**Less Activity was Expected and Forecast ****This 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**

## Explained variance = 82%.

**Current active Atlantic hurricane era associated with phase change in tropical multi-decadal signal.**

**Explained variance = 82%.**

**Observed**** ****vs.**** Regressed**

## ASO 5-year LP-Filtered Variance Explained by Leading Tropical Multi-Decadal Modes

**ASO 5-year LP-Filtered Variance Explained by ****Leading Tropical Multi-Decadal Modes**

**LF variability in all KEY circulation features strongly related to leading tropical multi-decadal modes.**

**+**

## February-March 2005 SST Departures

**February-March 2005**** 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.

**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.**

## Slide 54

**Anti-Cyclonic****                            ****Cyclonic**

**AEJ**

## Less 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 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?

**L**

**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 Surface Temperature Variance (%)

**Leading tropical multi-decadal modes capture much surface temperature variability associated with AMO and WAFR monsoon.**

## August Outlook Called for Fewer Hurricanes Than May Outlook 

**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 Temperatures between 2006 and 2005 

**Cause of the Apparent Change in Atlantic Ocean 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)**

**14**