TropicalM. D. Eastin Tropical Cyclone Climatology Where do TCs occur? When? Why? How Many?

Tropical M. D. Eastin

Tropical Cyclone Climatology

Where do TCs occur? When? Why? How Many?

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

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

1 61 121 181 241 301 361 421 481 541

Time (s)

1-min Max Mean = 30.6 m/s 10-min Mean = 26.3 m/s

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

Tropical Cyclone Formation Locations

Figure from Gray (1968)

What are common features in these areas of the world?

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

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 )

Sea Surface Temperature

200-850 mb Vertical Wind Shear

January

August Figures from Gray (1968)

700-mb Relative Humidity

North Indian

• 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

South Indian

• 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)

NW Australia and South Pacific

• 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°

NW Pacific

• 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

NE Pacific

• 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

North Atlantic

• 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

North Atlantic

• Development limited to western Caribbean and Gulf• SSTs are too cool in the Atlantic

North Atlantic

• Development in Caribbean and Gulf• SSTs begin to warm in the Atlantic• Easterly waves begin to emerge off African coast

North Atlantic

• Development throughout the basin• Easterly waves activity peaks in late August through early September

North Atlantic

• Development throughout the basin• Lowest vertical shear in September• Basin-average SSTs are the greatest

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

North Atlantic

• Development usually limited to the Caribbean• Elsewhere, SSTs are too cool and/or vertical shear is too high

North Atlantic

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

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)

TropicalM. D. Eastin Tropical Cyclone Climatology Where do TCs occur? When? Why? How Many?

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