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Water Temperature and Cyclone Frequency in the Pacific: Implications for Pearl Farming Final, June 20083.0 TROPICAL CYCLONES3.1 IntroductionTropical cyclones are non-frontal low pressure systems that form over warm tropical waters.They are characterised by organized convection, sustained gale force winds in excess of 63km/h and wind gusts greater than 90 km/h near their centre that persist for more than sixhours (Bureau of Meteorology, Australia 2007a). The gale force winds can extend more thanhalf way around the cyclone near their centre and can extend hundreds of kilometres fromthe cyclone centre. Tropical cyclones derive their energy from the ocean and form only whenthe following conditions are met:• the sea-surface temperature exceeds 26.5 °C and this temperature extends down to adepth of least 50 m;• the atmosphere cools fast enough to encourage thunderstorm activity;• there are relatively moist layers near the mid-troposphere (5 km);• there is sufficient Coriolis force (earth’s spin) to maintain the low pressure of thesystem;• there is a pre-existing disturbance near the surface with sufficient spin and inflow;and• low vertical wind shear (i.e. change in the wind with height that is less than 40 km/hfrom surface to tropopause).They do not occur within 5° north or south of the equator, because the Coriolis Force isweaker at these latitudes.Tropical cyclones vary in intensity, life cycle, pattern of movement, size and impact (wind,storm surge and flooding). They are assigned intensity categories ranging from 1 to 5 on thebasis of the average maximum wind speed, strongest gust and central pressure. Severetropical cyclones are characterized by sustained winds near the centre of 118 km/h andgusts in excess 165 km/h. Most tropical cyclones have a life-cycle of 3-7 days, but they canpersist for several weeks if they remain in a favourable atmospheric environment. Weakcyclones, in contrast, only reach gale force for a brief period of time. These low pressuresystems usually dissipate over land or colder oceans.The activity and intensity of tropical cyclones varies within seasons, between years, decadesand over multi-decadal timescales. In the Australian/Southwest Pacific basin (east of142°E), tropical cyclone activity generally begins in late October/early November, reaches asingle peak in late February/early March, and then fades out in early May (AtlanticOceanography and Meteorological Laboratory 2007). Between 1968 and 1989/1990, the totalnumber of tropical cyclones that occurred in this basin per year varied from 2 in 1981 to 16in 1971 while the number of severe cyclones varied from 1 in 1979 to 11 in 1971 (Neumann1993). The variation in number of cyclones from year to year is correlated strongly withlocal sea surface temperature before and at the start of the cyclone season, particularly withthose in October (Nicholls 1984). The El Niño-Southern Oscillation (ENSO) phenomenonalso affects the number of cyclones that occur per year. During El Niño events, there is apronounced shift back and forth of cyclone activity with fewer tropical cyclones occurringbetween 145°E and 165°E and more across the South Pacific east of 165°E. There is also aThe Ecology Lab Pty Ltd – Marine and Freshwater Studies Page 11