ESCI 344 – Tropical Meteorology
Lesson 5 – Tropical Cyclones: Climatology
References: A Global View of Tropical Cyclones, Elsberry (ed.)
The Hurricane, Pielke
Tropical Cyclones: Their evolution, structure, and effects, Anthes
Forecasters’ Guide to Tropical Meteorology, Atkinsson
Forecasters Guide to Tropical Meteorology (updated), Ramage
Global Guide to Tropical Cyclone Forecasting, Holland (ed.)
Reading: A Global View of Tropical Cyclones, Chapter 3 (e-reserve)
Tropical Climatology, pp 151 - 160
Global Guide to Tropical Cyclone Forecasting, Chapter 1 (online at http://www.bom.gov.au/bmrc/pubs/tcguide/globa_guide_intro.htm)
Tropical Cyclone Forecasters Reference Guide, Chapter 3 (online at http://www.nrlmry.navy.mil/~chu/)
REQUIREMENTS FOR FORMATION
˜ In order for a tropical cyclone to form, the following general conditions must be present:
¡ Deep, warm ocean mixed layer.
n Sea-surface temperature at least 26.5°C.
n Mixed layer depth of 45 meters or more.
¡ Relative maxima in absolute vorticity in the lower troposphere
n Need a preexisting cyclonic disturbance.
n Must be at least a few degrees of latitude from the Equator.
¡ Small values of vertical wind shear.
n Disturbance must be in deep easterly flow, or in a region of light upper-level winds.
¡ Mean upward vertical motion with humid mid-levels.
GLOBAL CLIMATOLOGY
Note: Most of the statistics given in this section are from Gray, W.M., 1985: Tropical Cyclone Global Climatology, WMO Technical Document WMO/TD-72, Vol. I, 1985.
˜ About 80 tropical cyclones per year world-wide reach tropical storm strength (³ 34 kts).
˜ About 50 – 55 each year world-wide reach hurricane/typhoon strength (³ 64 kts).
˜ The rate of occurrence globally is very steady.
¡ Global average annual variation is small (about 7%).
¡ Extreme variations are in the range of 16 to 22%.
¡ Variability within a particular region is much larger than global variability.
˜ Most (87%) form within 20° of the Equator.
¡ Those that form farther than 20° from the Equator are usually in the Northern Hemisphere.
˜ Vast majority form in or near monsoon troughs.
¡ Most of the remainder form in tropical waves
¡ A few form along old frontal zones or shear lines.
˜ Cyclogenesis tends to cluster in 2-3 week active periods that occur after 2-3 week inactive periods.
NORTH ATLANTIC
˜ 11.6% of global total tropical storm strength and higher.
˜ 12% of global total hurricane strength and higher.
˜ 97% occur between June 1 and November 30
¡ There is no month without at least one tropical cyclone
¡ Peak of season is September 10
˜ Position and strength of subtropical jet is key factor in seasonality, along with variations in SST.
¡ Genesis regions migrate throughout season in response to upper-level winds and SST.
˜ Per year there are on average
¡ 10.1 named cyclones
¡ 5.9 hurricanes
¡ 2.5 category III or greater
˜ Variability
¡ Have been as few as 1, to over 20 cyclones
˜ Modes of genesis different from global averages
¡ More than half form in tropical waves coming off of Saharan Africa.
¡ Less than half form along ITCZ, with a few forming in baroclinic zones associated with old fronts or shear lines.
˜ Possible 25 to 40 year cycle in numbers (not enough data to conclusively determine).
SOUTH ATLANTIC
˜ Tropical cyclones are very rare in the South Atlantic.
¡ No ITCZ
¡ Strong vertical shear
˜ In satellite era there have been two tropical cyclones in the South Atlantic.
¡ One, in March, 2004, becoming a Category I hurricane.
n Formed from upper-level low.
¡ In April, 1991 there was a nominal tropical storm off of Africa.
NORTHEAST AND NORTHCENTRAL PACIFIC
˜ 19.8% of global total tropical storm strength and higher.
˜ 19.9% of global total hurricane strength and higher.
˜ Numbers undercounted in pre-satellite era (not many ships in this area compared to Atlantic).
˜ Season similar to Atlantic.
¡ Peak in late-August
˜ Per year there are on average
¡ 16.4 named cyclones
¡ 9.2 hurricanes
¡ 4.0 category III or greater
˜ Region off of Central America has highest density of genesis points on the globe.
˜ Little genesis south of 10° due to cold upwelling of ocean waters.
˜ Modes of genesis not well documented.
¡ Controversy concerning whether “African” waves propagate across Central America and spawn Pacific cyclones.
WESTERN NORTH PACIFIC
˜ Granddaddy of all basins
¡ 30.7% of global total tropical storm strength and higher.
¡ 35.7% of global total hurricane strength and higher.
˜ Noted for high occurrence of very large and very intense storms.
¡ Lowest SLP on record was 870 mb in Super Typhoon Tip (October, 1979).
˜ Only basin that is active throughout the year.
˜ Highly seasonal, with peak in late-July/early-August
¡ Can occur in any month
˜ Per year the average is
¡ 26 named storms
¡ 16 typhoons
˜ Genesis regions migrate seasonally with position of monsoon trough.
˜ Vast majority of cyclones form in monsoon trough, with a small minority forming in tropical waves.
˜ Tropical upper-tropospheric trough (TUTT) can play significant role in genesis and development.
NORTH INDIAN OCEAN
˜ 6.5% of global total tropical storm strength and higher.
˜ 5.6% of global total hurricane strength and higher.
˜ Deadliest in world.
¡ Low-lying, flood prone areas of Bangladesh with large population density.
¡ 1970 cyclone killed over 300,000 people.
¡ Several cyclones have killed 100,000 to 200,000 people each.
˜ Highly seasonal, with two peaks in May and November.
¡ The bimodal distribution is associated with the transition seasons of the monsoon, as the monsoon trough is over water during these times.
¡ During height of summer monsoon, the monsoon trough is well inland, and there is also strong vertical shear due to the Tropical Easterly Jet, which suppresses cyclogenisis.
˜ November peak is more pronounced than May peak.
˜ Majority occur in the Bay of Bengal, with far fewer in the Arabian Sea.
˜ Per year there are on average
¡ 5 – 6 tropical storm strength cyclones per year
n Range is 1 – 10
¡ 2 to 3 hurricane strength cyclones per year
SOUTHWEST INDIAN OCEAN (West of 100°E)
˜ 12.4% of global total tropical storm strength and higher.
˜ 9.9% of global total hurricane strength and higher.
˜ Seasonal (October through May)
¡ December through March most active
¡ Peak in January.
¡ Can get off-season genesis.
˜ Per year the average is
¡ 10.4 tropical storm strength cyclones
¡ 4.4 hurricane strength cyclones
˜ Monsoon trough is important genesis feature.
SOUTHEAST INDIAN OCEAN (100°E to 142°E)
˜ 8.2% of global total tropical storm strength and higher.
˜ 7.6% of global total hurricane strength and higher.
˜ Seasonal (October through May)
¡ Peaks in January and February/March
˜ Per year the average is
¡ 6.0 tropical storm strength cyclones
¡ 3.4 hurricane strength cyclones
˜ Monsoon trough is important genesis feature.
SOUTHWEST PACIFIC OCEAN (East of 142°E)
˜ 10.8% of global total tropical storm strength and higher.
˜ 9.5% of global total hurricane strength and higher.
˜ Seasonal (October through April)
¡ Peak in February/March
˜ Per year the average is
¡ 9.0 tropical storm strength cyclones
¡ 4.3 hurricane strength cyclones
˜ Monsoon trough and SPCZ are important genesis features.
˜ Twin cyclones (one on each side of the Equator) sometimes form in early and late season.
