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The Science Behind Atlantic Hurricanes Hurricanes and Seasonal Hurricane Predictions and Seasonal Hurricane Predictions By By Dr. Gerry Bell Dr. Gerry Bell Head of NOAA’s Seasonal Hurricane Head of NOAA’s Seasonal Hurricane Outlooks Outlooks Climate Prediction Center Climate Prediction Center Camp Springs, MD Camp Springs, MD public affairs: public affairs: [email protected] [email protected] (301) 763-8000 x 7163 (301) 763-8000 x 7163

Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

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Page 1: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

The Science Behind Atlantic HurricanesThe Science Behind Atlantic Hurricanesand Seasonal Hurricane Predictionsand Seasonal Hurricane Predictions

ByBy

Dr. Gerry BellDr. Gerry Bell

Head of NOAA’s Seasonal Hurricane Outlooks Head of NOAA’s Seasonal Hurricane OutlooksClimate Prediction CenterClimate Prediction Center

Camp Springs, MD Camp Springs, MD

public affairs: public affairs: [email protected]@noaa.gov (301) 763-8000 x 7163(301) 763-8000 x 7163

Page 2: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

BackgroundBackground

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 and…also 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

Page 3: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

OutlineOutline

1. Interpreting NHC hurricane forecasts

2. NOAA’s seasonal hurricane outlooks

3. Recipe for hurricane formation

4. Measuring seasonal activity

5. Recipe for an active hurricane season

6. Conditions during 2005

7. Climate patterns controlling hurricane extremes

8. Summary

Page 4: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

Interpretation of the Prediction ConeInterpretation of the Prediction Cone

It is incorrect to show only the black line as the forecast becauseIt is incorrect to show only the black line as the forecast because it is NOT the forecast. The white cone is the forecast. The it is NOT the forecast. The white cone is the forecast. The hurricane is forecast to be somewhere in the white cone area.hurricane is forecast to be somewhere in the white cone area.

Page 5: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

NOAA’s Seasonal Hurricane OutlooksNOAA’s Seasonal Hurricane Outlooks

2 August 2005 2 August 2005 Outlook Observed Outlook Observed Chance Above Normal 95-100%Chance Above Normal 95-100%Tropical Storms 18-21 22 Tropical Storms 18-21 22 Hurricanes 9-11 12 Hurricanes 9-11 12 Major Hurricanes 5-7 6 Major Hurricanes 5-7 6

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

BUT, Less confidence with seasonal landfalling forecasts. A lot of BUT, Less confidence with seasonal landfalling forecasts. A lot of work still needs to be done in this area.work still needs to be done in this area.

Page 6: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)
Page 7: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

Sea Surface Temperatures (F) Departures from Normal (F)

86 8481

23

Page 8: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)
Page 9: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

. ..

Effects of Vertical Wind Shear (V z) on Tropical Cyclones

WEAK SHEAR = FAVORABLE

low clouds

high cloudsSTRONG SHEAR = UNFAVORABLE

EY

E

LOWER-LEVEL WINDS

UPPER-LEVEL WINDS

Neal D

orst/Stan Goldenberg

Hurricane R

esearch Division

AO

ML/N

OA

AH

RD

Page 10: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

Measuring Seasonal ActivityMeasuring Seasonal Activity

Page 11: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

ACE IndexACE Index•Classifying Hurricane Seasons is Challenging

# TS, #H, # MH, # landfalling storms, etc.

•NOAA’s 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

Page 12: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

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.

Page 13: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

Recipe for an Active Hurricane Season?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.

Page 14: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

Conditions Associated withActive Hurricane Seasons

Tropical storms and hurricanes form as disturbances move westward from Africa into the low-shear, warm water environment (red area) of the tropical Atlantic.

Favorable midlevel jet stream

August-October

Higher Pressure AloftMakes easterly winds over Caribbean instead of westerlies

Weaker easterly trade Winds in lower atmosphere

Low Vertical Wind ShearWarmer SSTs

Lower Surface Pressure

Page 15: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

Conditions During 2005Conditions During 2005

Page 16: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

Courtesy of Unisys WeatherCourtesy of Unisys Weather

Page 17: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

Extremely low vertical wind shear typical of September

Conditions During Record Activity: 15 June-31 JulyVertical Wind Shear

Departures from Normal (Shaded)

Air Pressure Departures from Normal (35,000 ft)

Symmetry between Northern and Southern hemispheres indicates link to suppressed convection near date line

Main DevelopmentRegion

Page 18: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

Very low wind shear. Ridge remains over SE U.S.

Major hurricanes form in Gulf of Mexico where wind shear is almost zero.

Conditions During 15 August-30 September

Vertical Wind Shear Departures from Normal (Shaded)Air Pressure at 35,000 feet (Contours)

HL

H

Page 19: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

Landfalling HurricanesLandfalling Hurricanes2 to 3 times more U.S. landfalling hurricanes during active

seasons than inactive seasons.

Page 20: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

Active 24-Year Period 1955-1970, 1995-2002

67 Major Hurricanes

Inactive 24-Year Period 1971-1994

27 Major Hurricanes

MDR

The U.S. averages 2-3 hurricane strikes in above-normalseasons, compared to just one in below-normal seasons.

Tracks of systems that became major hurricanes Tracks of systems that became major hurricanes after forming in Tropical Atlantic or Caribbean Seaafter forming in Tropical Atlantic or Caribbean Sea

Page 21: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

U.S. Landfalling Named Storms 2002-2005

0

10

20

30

Num

ber Total

Gulf CoastEast Coast

Page 22: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

The ongoing active hurricane era means high levels oftropical 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.

U.S. Landfalling Hurricanes 2002-2005

0

5

10

15N

umbe

r

TotalGulf CoastEast Coast

Page 23: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

Coastal PopulationCoastal Population

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

Page 24: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

Florida Coastal Population

Page 25: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

MH Katrina- Hit as Cat. 4 (145 mph winds) MH Katrina- Hit as Cat. 4 (145 mph winds) August 29August 29thth 2005 2005

Hurricane winds extend 125 miles from centerNew Orleans and Biloxi both in Eye Wall

Eye is 30 miles across

New Orleans

Biloxi

Page 26: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

Hurricanes are Not Just a Coastal Event

Leading cause of death is from inland flooding.

Page 27: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

Climate Patterns Controlling Hurricane ActivityClimate Patterns Controlling Hurricane Activity

Page 28: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

1. The El Niño/ Southern Oscillation (El Niño and La Niña)

Hurricane seasons are largely controlled by recurring rainfall patterns along the equator, which are linked to two dominant climate phenomena:

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)•NOAA’s seasonal hurricane outlooks result from analysis and prediction of these two climate phenomena.

Page 29: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

Weaker Tropical Easterly Jet

Stronger EasterlyTrades

El Niño/ La Niña Cycle

Pacific Ocean Temperatures, Rainfall, Winds at 35,000 feet

Warm & Wet

Cool and Dry

La Niña: More Atlantic Hurricanes

El Nino: Fewer Atlantic Hurricanes

H

H

H

HLow Shear

High Shear

L

L

L

L

Page 30: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

Tropical 20-30 Year SignalTropical 20-30 Year Signal Active Atlantic Hurricane Phase Active Atlantic Hurricane Phase

Surface Temperatures

cool coolWarm

warmWarm

Wetter Drie

rDrier

Precipitation

Monsoonal rainfallfluctuations with 20-30 year cycles

Warm

Page 31: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

Observed 20-30 Year CyclesObserved 20-30 Year Cycles

Page 32: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

H

H

Winds and Air Pressure at 35,000 feetWinds and Air Pressure at 35,000 feetDifference Between 1995-2003 and 1971-1994Difference Between 1995-2003 and 1971-1994

Differences between active and inactive periods is caused by anomalous tropical convection with 20-30 year cycles(Bell and Chelliah, 2005, J. Climate)

H

H

Patterns Linked to 20-30 year cyclesObserved

Page 33: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

Multi-Decadal Signal in Multi-Decadal Signal in Atlantic Sea-Surface Temperatures Atlantic Sea-Surface Temperatures

Warmer Atlantic waters since 1995 are very favorable Warmer Atlantic waters since 1995 are very favorable for hurricane formation. Associated with weaker for hurricane formation. Associated with weaker easterly trade winds.easterly trade winds.

Tropical Atlantic Temperatures (Red)

Page 34: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

Atlantic Sea Surface Temperature Departures (Atlantic Sea Surface Temperature Departures (ooC)C)

Strong multi-decadal fluctuations in Atlantic temperatures dating back to the 1870’s.

1970 19951870 1900 1930

Page 35: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

SummarySummary

1. 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 warmingIt 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.

Page 36: Severe Weather: Hurricanes (Gerry Bell, Ph.D.)

Summary: cont.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.