Tropical Cyclone Climatology
Where do TCs occur? When? Why? How Many?
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Tropical Cyclone Definitions
Tropical Cyclone (TC)
Generic term for a non-frontal synoptic-scale low pressure
system over tropical or subtropical waters with organized
convection and a definite cyclonic surface wind circulation
Tropical Depression (TD)
A TC with maximum sustained surface winds < 17 m/s
Tropical Storm (TS)
A TC with maximum sustained surface winds > 17 m/s
(> 33 knots or > 37 mph)
Becomes a “named” system
Hurricane (H)
A TC with maximum sustained surface winds > 33 m/s
(> 64 knots or > 74 mph)
“Hurricane”
“Typhoon”
“Severe Tropical Cyclone”
“Severe Cyclonic Storm”
“Tropical Cyclone”
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N Atlantic, NE Pacific, SE Pacific
NW Pacific
SW Pacific, SE Indian
N Indian
SW Indian
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Tropical Cyclone Definitions
What is a sustained wind?
Wind speed determined by averaging observed values
over a given amount of time (either 1-min or 10-min)
1-min:
U.S.
10-min:
Japan
India
France
Australia
Fiji
National Hurricane Center (Atlantic / East Pacific)
Joint Typhoon Warning Center (Pacific / Indian*)
Japanese Meteorological Agency (West Pacific)
Indian Meteorological Dept. (North Indian)
MeteoFrance (South Indian)
Regional TC Centers (Australian Region)
Regional TC Centers (South Central Pacific)
Wind Speed (m/s)
* Not the official WMO forecast center
38
1-min Max Mean = 30.6 m/s
34
10-min Mean = 26.3 m/s
30
26
22
1
61
121
181
241
301
361
421
481
541
Time (s)
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Tropical Cyclone Definitions
What is a surface wind?
Wind speed observed at 10-m above the ground
Observations made at different altitudes can be
adjusted to 10-m assuming certain boundary-layer
characteristics
Typical Boundary Layer Mean Wind Profiles
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Tropical Cyclone Formation Locations
Figure from Gray (1968)
What are common features in these areas of the world?
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Common Factors Favorable for Development
 Warm Sea Surface Temperatures (SSTs)
• Generally > 26.5°C
• TCs are most common over waters > 28ºC
 Low vertical wind shear near the center of the system
• 200 - 850 mb difference; usually < 20 m/s
• TCs are most common in regions with shear < 10 m/s
 Significant planetary vorticity
• Coriolis force; usually poleward of 5°
• Most TCs occur in the 10º-20º latitudinal belts
 Moist troposphere
• Generally > 40-50% at mid-levels
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Limiting Factors
• In much of the Tropics, the SST is generally warm
enough for TC development
• Exceptions: SE Atlantic
SE Pacific
Central Pacific (marginal)
• In those regions with warm SSTs and ample planetary vorticity,
vertical wind shear usually limits TC development
• Dry air at mid-levels can also limit development in some
regions where the SST and vertical shear are favorable
(most evident in the Atlantic → Subsidence in Bermuda High)
(
→ Saharan Air Layer
)
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Sea Surface Temperature
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Sea Surface Temperature
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Sea Surface Temperature
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Sea Surface Temperature
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200-850 mb Vertical Wind Shear
January
August
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Figures from Gray (1968)
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700-mb Relative Humidity
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700-mb Relative Humidity
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700-mb Relative Humidity
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700-mb Relative Humidity
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North Indian
Figure from Gray (1968)
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Average of 5 tropical cyclones each year
Two seasons per year (Apr-Jun and Sep-Dec)
Most occur in the Bay of Bengal and move toward the NW
Development linked to monsoon trough passage
Deadliest TC in history struck Bangladesh in 1970 killing over 300,000 people
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South Indian
Figure from Gray (1968)
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Average of 8 tropical cyclones each year
Season runs from October through May
Most move to the west and southwest
Development linked to the poleward movement of the
equatorial trough (inter-tropical convergence zone; ITCZ)
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NW Australia and South Pacific
Figure from Gray (1968)
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Average of 14 tropical cyclones each year
Season runs from November through June
Most move to the west and southwest
Development limited by high wind shear poleward of 20°
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NW Pacific
Figure from Gray (1968)
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Average of 31 tropical cyclones each year
Season runs year round, but ~70% occur in Jul-Oct
Most move to the west and northwest
Development influenced by the equatorial and monsoon troughs
About 35% of all TCs occur in the NW Pacific
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NE Pacific
Figure from Gray (1968)
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Average of 16 tropical cyclones each year
Season runs from May-October (peak in Aug)
Most move to the west and northwest
Development influenced by the equatorial trough and easterly waves
Development poleward of 20° limited by cold SSTs, dry air,
and high wind shear
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North Atlantic
Figure from Gray (1968)
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Average of 10 tropical cyclones each year
Season runs from June-November
Most move to the west and northwest
Development linked to easterly waves, upper-level lows
and stationary frontal boundaries
• Basin with the greatest diversity in storm formation and behavior
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North Atlantic
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North Atlantic
• Development limited to western Caribbean and Gulf
• SSTs are too cool in the Atlantic
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North Atlantic
• Development in Caribbean and Gulf
• SSTs begin to warm in the Atlantic
• Easterly waves begin to emerge off African coast
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North Atlantic
• Development throughout the basin
• Easterly waves activity peaks in late August through early September
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North Atlantic
• Development throughout the basin
• Lowest vertical shear in September
• Basin-average SSTs are the greatest
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North Atlantic
• Development in Caribbean and along U.S. coast
• Vertical shear begins to increase over Atlantic
• Development from upper-level lows and frontal boundaries
are much more common
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North Atlantic
• Development usually limited to the Caribbean
• Elsewhere, SSTs are too cool and/or vertical shear is too high
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North Atlantic
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North Atlantic
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North Atlantic
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North Atlantic
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North Atlantic
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North Atlantic
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Tropical Cyclone Climatology
Summary
• Tropical cyclones vary in name and definition in different
regions of the Tropics
• Tropical cyclones occur throughout most* tropical regions
• Tropical cyclones form when a number of necessary
(but not sufficient*) conditions are in place:
• Warm SSTs
• Low vertical wind shear
• A moist mid-level troposphere
• Some planetary vorticity
• These conditions most often occur in the NW Pacific where
the most TCs are observed annually
• The North Atlantic sees the greatest shift in development
region as well as diversity in system origin and behavior
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References
Gray, W. M., 1968: Global view of the origin of tropical disturbances and storms.
Mon. Wea. Rev., 96, 669-770.
Tropical Prediction Center (TPC) / National Hurricane Center (NHC) website
(http://www.nhc.noaa.gov/)
NOAA Climate Diagnostic Center (CDC) website
(http://www.cdc.noaa.gov/cgi-bin/data/getpage.pl)
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