tropicalm. d. eastin tropical cyclone climatology where do tcs occur? when? why? how many?
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Tropical M. D. Eastin
Tropical Cyclone Climatology
Where do TCs occur? When? Why? How Many?
Tropical M. D. Eastin
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” N Atlantic, NE Pacific, SE Pacific“Typhoon” NW Pacific“Severe Tropical Cyclone” SW Pacific, SE Indian“Severe Cyclonic Storm” N Indian“Tropical Cyclone” 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. National Hurricane Center (Atlantic / East Pacific) Joint Typhoon Warning Center (Pacific / Indian*)
10-min: Japan Japanese Meteorological Agency (West Pacific)India Indian Meteorological Dept. (North Indian)France MeteoFrance (South Indian) Australia Regional TC Centers (Australian Region)Fiji Regional TC Centers (South Central Pacific)
* Not the official WMO forecast center
22
26
30
34
38
1 61 121 181 241 301 361 421 481 541
Time (s)
Win
d S
pee
d (
m/s
)
1-min Max Mean = 30.6 m/s 10-min Mean = 26.3 m/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-layercharacteristics
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 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)
• 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)
• 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)
• 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)
• 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)
• 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)
• 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)