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.