Eighty six tropical cyclones form on average each year throughout the World and about two thirds of these reach the stage of a severe tropical cyclone. The climatology of initial detection points of each cyclone for a 20-year period by Gray (1975) remains applicable today (Figure 1).
|
It is significant that preferred regions of formation are over the tropical oceans, coincidind broadly with regions of high sea surface temperatures (SSTs) (Figure 2). The warmest waters occur in the Western Pacific. This region is known as the “warm pool region”. In contrast, ocean temperatures in the Southeast Pacific are relatively cold. Climatological studies have shown that tropical cyclogenesis occurs only in regions where the SST is above 26.5oC (Palmén 1948, 1956, and Gray, 1975).
There are relatively few formation events within about 2.5o latitude of the Equator. The majority of formation events (87%) occur between 20oN and 20oS and nearly two thirds of the yearly total occur over the oceans of the Northern hemisphere. Moreover, the Eastern hemisphere has twice as many tropical cyclones as in the Western hemisphere. This disparity is attributed largely to the absence of tropical cyclones in the South Atlantic and the eastern South Pacific.
Tropical cyclones are generally seasonal weather systems, the largest frequency of formation over most basins occurring during the late summer to early autumn (fall). The peak in the Southern Hemisphere occurs between January and March. The peak in the Northern Hemisphere occurs between July and September. The Northwest Pacific Ocean region is the most active region, where typhoons occur in all seasons.
|
The majority of tropical cyclones develop from pre-existing deep convective cloud systems, essentialy clusters of thunderstorms, in the tropics, but a few originate from the transformation of subtropical weather systems. See Smith and Montgomery, 2023, Section 1.5.
Fig. 1.2 shows the tracks of all tropical cyclones with maximum near-surface winds > 17 m s−1 for the period 1979-1988. It shows also the mean direction of all hurricane strength systems during the period indicated for each ocean basin. After forming over the warm tropical oceans, tropical cyclones subsequently move westwards and polewards, although the tracks of individual storms can be quite erratic.
|
To a first approximation tropical cyclones are steered by a mass-weighted average of the broadscale winds through the depth of the troposphere. It is common for storms that reach sufficiently high latitudes to recurve and move eastwards, carried along by the prevailing mid-latitude westerly winds.
Many basic aspects of tropical cyclone motion, including a theory for the poleward and westward drift, can be illustrated in terms of barotropic dynamics in which the vortex structure is assumed to be independent of height. The barotropic dynamics of tropical cyclone motion is the topic of Chapter 3 in Smith and Montgomery (2023).
Gray, W.M., 1975. Tropical Cyclone Genesis. Bluebook, vol. 234. Colorado State University, Department of Atmospheric Science, pp121.
Palmén, E., 1948. On the formation and structure of tropical hurricanes. Geophysica, 3, 26–38.
Palmén, E., 1956. Formation and development of tropical cyclones. In: Proc., Trop. Cyclone Symp. Brisbane. Australian Bureau of Meteorology, Melbourne, Australia 3001, 213–231.
Smith, R. K., and M. T. Montgomery, 2023: Tropical cyclones: Observations and basic processes. Elsevier, London, 411pp.
WMO, 1993. Global Guide to Tropical Cyclone Forecasting. WMO/TD 560, Report No. TCP-31. World Meteorological Organization, Geneva, Switzerland.
Latest version: Munich 27 December 2024