Atlantic tropical cyclone

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Tracks of Atlantic tropical cyclones

An Atlantic tropical cyclone is a tropical cyclone that forms in the Atlantic Ocean usually in the Northern Hemisphere summer or autumn. Tropical storms have one-minute maximum sustained winds of at least 39 mph (34 knots, 17 m/s, 63 km/h), while hurricane have one-minute maximum sustained exceeding 74 mph (64 knots, 33 m/s, 119 km/h).^[1] When applied to tropical cyclones, "Atlantic" generally refers to the entire "Atlantic basin", which includes the North Atlantic Ocean, Caribbean Sea and the Gulf of Mexico.

Most Atlantic tropical storms and hurricanes form between June 1 and November 30.^[2] The United States National Hurricane Center monitors the basin and issues reports, watches and warnings about tropical weather systems for the Atlantic Basin as one of the Regional Specialized Meteorological Centers for tropical cyclones as defined by the World Meteorological Organization.^[3]

Tropical disturbances that reach tropical storm intensity are named from a pre-determined list. Hurricanes of unusual destructiveness may have their names retired from the list.^[4] On average, 10.1 named storms occur each season, with an average of 5.9 becoming hurricanes and 2.5 becoming major hurricanes (Category 3 or greater). The climatological peak of activity is around September 10 each season.^[5]

[edit] Steering factors

The subtropical ridge shows up as a large area of black (dryness) on this water vapor satellite image from September 2000

Tropical cyclones are steered by the surrounding flow throughout the depth of the troposphere (the atmosphere from the surface to about eight miles (12 km) high. Neil Frank, former director of the United States National Hurricane Center, used the analogies such as "a leaf carried along in a stream" or a "brick moving through a river of air" to describe the way atmospheric flow affects the path of a hurricane across the ocean. Specifically, air flow around high pressure systems and toward low pressure areas influence hurricane tracks.

In the tropical latitudes, tropical storms and hurricanes generally move westward with a slight tend toward the north, under the influence of the subtropical ridge, a high pressure system that usually extends east-west across the subtropics.^[6] South of the subtropical ridge, surface easterly winds (blowing from east to west) prevail. If the subtropical ridge is weakened by an upper trough, a tropical cyclone may turn poleward and then recurve,^[7] or curve back toward the northeast into the main belt of the Westerlies. Poleward (north) of the subtropical ridge, westerly winds prevail and generally steer tropical cyclones that reach northern latitudes toward the east. The westerlies also steer extratropical cyclones with their cold and warm fronts from west to east. ^[8]

[edit] Climatology

See also: Tropical cyclogenesis

Climatology does serve to characterize the general properties of an average season and can be used as one of many other tools for making forecasts. Most storms form in warm waters several hundred miles north of the equator near the Intertropical convergence zone from tropical waves. The Coriolis force is usually too weak to initiate sufficient rotation near the equator.^[9] Storms frequently form in the warm waters of the Gulf of Mexico, the Caribbean Sea, and the tropical Atlantic Ocean as far east as the Cape Verde Islands, the origin of strong and long-lasting Cape Verde-type hurricanes. Systems may also strengthen over the Gulf Stream off the coast of the eastern United States, wherever water temperatures exceed 26.5 °C (79.7 °F).^[9]

Although most storms are found within tropical latitudes, occasionally storms will form further north and east from disturbances other than tropical waves such as cold fronts and upper-level lows. There is a strong correlation between Atlantic hurricane activity in the tropics and the presence of an El Niño or La Niña in the Pacific Ocean. El Niño events increase the wind shear over the Atlantic, producing a less-favorable environment for formation and decreasing tropical activity in the Atlantic basin. Conversely, La Niña causes an increase in activity due to a decrease in wind shear. ^[10]

[edit] June

Typical locations and tracks of tropical systems in June. Blue is likely, green more likely, and orange most likely.

The beginning of the hurricane season is most closely related to the timing of increases in sea surface temperatures, convective instability, and other thermodynamic factors. ^[11] Although this month marks the beginning of the hurricane season, the month of June generally sees little activity, with an average of about 3 tropical cyclones per 5 years. Tropical systems usually form in the Gulf of Mexico or off the east coast of the United States.^[12]

[edit] July

Typical locations and tracks in July.

Not much tropical activity occurs during the month of July, but the majority of hurricane seasons see the formation of one tropical cyclone during July. Using data from 1944 to 1996, on average, half of the hurricane seasons had their first tropical storm by July 11, with a second having formed by August 8.^[5]

Formation usually occurs in the eastern Caribbean Sea around the Lesser Antilles, in the northern and eastern parts of the Gulf of Mexico, in the vicinity of the northern Bahamas, and off the coast of The Carolinas and Virginia over the Gulf Stream. Storms travel westward through the Caribbean and then either move towards the north and curve near the eastern coast of the United States or stay on a north-westward track and enter the Gulf of Mexico.^[12]

[edit] August

Typical locations and tracks in August.

Decrease in wind shear from July to August produces a significant increase of tropical activity ^[13]. An average of 2.8 tropical storms develop annually in August. On average, four named systems and one hurricane occur by August 30, and by September 4, the Atlantic ocean has spawned its first major hurricane.^[5]

[edit] September

Typical locations and tracks in September.

The peak of the hurricane season occurs in September and corresponds to low wind shear ^[13] and the warmest sea surface temperatures^[14]. The month of September sees an average of 3.6 storms a year. By September 24, the average season sees 7 named systems, 4 of which are hurricanes. In addition, two major hurricanes occur on average by September 28.^[5]

[edit] October

Typical locations and tracks in October.

The favorable conditions found during September begin to decay in October. The main reason for the decrease in activity is increasing wind shear, although sea surface temperatures are cooler than in September. ^[11] Activity falls off markedly, with 1.8 cyclones developing on average, though there is a climatological secondary peak around October 20.^[15]. By October 21, the average season is expected to have 9 named storms with 5 hurricanes. A third major hurricane would be expected sometime between September 28 and the end of the year for half of all seasons.^[5] In contrast to mid-season activity, the mean locus of formation shifts westward to the Caribbean and Gulf of Mexico, a reverse trend to the eastward progression of June through August.^[12]

[edit] November

Typical locations and tracks in November.

Wind shear from westerlies increases substantially through November, generally preventing cyclone formation. ^[11] On average, one storm forms during November every other year. On rare occasions, a major hurricane forms, such as Category 4 Hurricane Lenny in 1999, which formed in mid November, and Kate, a Category 3 which formed in late November.^[12]

[edit] December to May

Probability of a tropical cyclone of tropical storm or hurricane strength at a specific date, expressed as systems per 100 years

Further information: Off-season storms

Few tropical cyclones can be found in the time between seasons, though about one-third of the years since 1944 have experienced an off-season tropical or subtropical cyclone. In the 63 seasons between 1944 and 2008, 9 tropical cyclones of tropical storm strength formed in May, 8 in December, and 4 total for all four months between January and April.^[12] High vertical wind shear and low sea surface temperatures generally preclude formation. Though a tropical cyclone has been observed in the Atlantic basin in every month in the year, no tropical cyclone is officially documented to have initially formed in January. A subtropical cyclone formed in January in the 1978 season, and both Hurricane Alice and Tropical Storm Zeta formed in December and lasted into January.^[12]

[edit] Extremes

Hurricane Katrina was the costliest and one of the five deadliest hurricanes in the history of the United States, costing over 1800 lives and $81.2 billion USD.

See also: List of Atlantic hurricanes

• The season in which the most tropical storms formed on record was the 2005 Atlantic hurricane season (28). That season was also the one in which the most hurricanes formed on record (15).^[12]
• The 1950 Atlantic hurricane season had the most major hurricanes on record (8).^[12]
• The least active season on record since 1944 (when the database is considered more reliable) was the 1983 Atlantic hurricane season, with one tropical storm, two hurricanes, and one major hurricane. Overall, the 1914 Atlantic hurricane season remains the least active, with only one documented storm.^[12]
• The most intense hurricane on record to form in the North Atlantic basin was Hurricane Wilma (2005) (882 mbar).^[12]
• The longest-lasting hurricane was the San Ciriaco Hurricane of 1899 (28 days).^[12]
• The fastest-moving hurricane was Hurricane Emily (1987) at 69 mph.^[12]
• The most tornadoes spawned by a hurricane was 123 by Hurricane Frances (2004 season).^[12]
• The strongest landfalling hurricane was the Labor Day Hurricane of 1935 (892 hPa).^[12]
• The deadliest hurricane was the Great Hurricane of 1780 (22,000 fatalities).^[16]
• The most damaging hurricane (adjusted for inflation) was Hurricane Katrina of the 2005 season which caused $81.2 billion in damages (2005 USD).^[17]

[edit] Trends

See also: Atlantic hurricane reanalysis

History of all recorded tropical cyclones of tropical storms strength or greater to occur in the North Atlantic.

There has been an increasing trend in tropical cyclone numbers over time. Since 1957, it has been recognized that much of this trend was due to better detection of tropical cyclones over time.^[18] The link between Global warming and numbers of tropical storms is unclear, though a recent study suggests that warmer conditions would lead to a fewer number of tropical cyclones.^[19]

[edit] See also

Tropical cyclones Portal

• List of Atlantic hurricanes
• List of Atlantic hurricane seasons
• South Atlantic tropical cyclone
• List of named tropical cyclones
• Mediterranean tropical cyclone
• Pacific hurricane

[edit] References

[edit] External links

• Tropical Cyclone Forecasters' Reference Guide
• NHC Tropical Cyclone Climatology
• Monthly Mean Sea Surface Temperatures
• Hurricane Preparation Guide