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The terms “hurricane” and “typhoon” are regionally specific names for a strong “tropical cyclone.” A tropical cyclone is the generic term for a non-frontal synoptic scale low-pressure system over tropical or subtropical waters with organized convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation. Tropical cyclones with maximum sustained surface winds of less than 17 m/s (34 kt) are called “tropical depressions” (this is not to be confused with the condition mid-latitude people get during a long, cold and grey winter wishing they could be closer to the equator). Once the tropical cyclone reaches winds of at least 17 m/s they are typically called a “tropical storm” and they are assigned a name. If the cyclone’s winds reach 33 m/s (64 kt), it will be classified as a “hurricane” (in the North Atlantic Ocean, in the Northeast Pacific Ocean east of the dateline,or in the South Pacific Ocean east of 160E); a “typhoon” (in the Northwest Pacific Ocean west of the dateline); a “severe tropical cyclone” (in the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E); a “severe cyclonic storm” (in the North Indian Ocean); and a “tropical cyclone” (in the Southwest Indian Ocean). Note that just the definition of “maximum sustained surface winds” depends upon who is taking the measurements. The World Meteorology Organization guidelines suggest utilizing a 10 min average to get a sustained measurement. Most countries utilize this as the standard. However the National Hurricane Center (NHC) and the Joint Typhoon Warning Center (JTWC) of the USA use a 1 min averaging period to get sustained winds. This difference may provide complications in comparing the statistics from one basin to another, since using a smaller averaging period may slightly raise the number of occurrences.
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Cape Verde-type hurricanes are those Atlantic basin tropical cyclones that develop into tropical storms fairly close (
A “super-typhoon” is a term utilized by the U.S. Joint Typhoon Warning Center in Guam for typhoons that reach maximum sustained 1-minute surface winds of at least 130 kt (240 km/h). This is the equivalent of a strong Saffir-Simpson category 4 or category 5 hurricane in the Atlantic basin or a category 5 severe tropical cyclone in the Australian basin.
It has been recognized since at least the 1930s that lower tropospheric (from the ocean surface to about 5 km with a maximum at 3 km) westward traveling disturbances often serve as the “seedling” circulations for a large proportion of tropical cyclones over the North Atlantic Ocean. In 1945, Riehl helped to substantiate that these disturbances, now known as African easterly waves, had their origins over North Africa. While a variety of mechanisms for the origins of these waves were proposed in the next few decades, it was Burpee in 1972, who documented that the waves were being generated by an instability of the African easterly jet. This instability—known as baroclinic-barotropic instability—is where the value of the potential vorticity begins to decrease toward the north. The jet arises as a result of the reversed lower-tropospheric temperature gradient over western and central North Africa due to extremely warm temperatures over the Saharan Desert in contrast with substantially cooler temperatures along the Gulf of Guinea coast. The waves move generally toward the west in the lower tropospheric tradewind flow across the Atlantic Ocean. They are first seen usually in April or May and continue until October or November. The waves have a period of about 3 or 4 days and a wavelength of 2000 to 2500 km, typically. These “waves&lrquo; are actually convectively-active troughs along an extended wave train. On average, about 60 waves are generated over North Africa each year, but it appears that the number that are formed has no relationship to how much tropical cyclone activity there is over the Atlantic each year. While only about 60% of the Atlantic tropical storms and minor hurricanes (Saffir-Simpson Scale categories 1 and 2) originate from easterly waves, nearly 85% of the intense (or major) hurricanes have their origins as easterly waves. It is suggested that nearly all of the tropical cyclones that occur in the Eastern Pacific Ocean can also be traced back to Africa. It is currently completely unknown how easterly waves change from year to year in both intensity and location and how these might relate to the activity in the Atlantic (and East Pacific).