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Water Temperature and Cyclone Frequency in the Pacific: Implications for Pearl Farming Final, June 2008tendency for tropical cyclones to originate a little closer to the equator. During La Niñaevents, the opposite pattern is observed.3.2 Exposure of Key Pacific Regions to Tropical Cyclone ActivityThe South Pacific region (east of 160°) generally experiences fewer tropical cyclones than theAustralian region. Between the 1970/1971 and 2003/2004 seasons, the South Pacificexperienced 8.6 cyclones on average per season, 4.1 of which were classified as severe and1.7 as intense (McInerney et al. 2006). During this period, the South Pacific region wasexposed to tropical cyclones for 31.7 days on average and to severe cyclones on 10.9 days. Inthe Australian region (90°E - 160°E), there were, on average, 12.8 cyclones per season, 6.3 ofwhich were classified as severe (i.e. had maximum wind speeds > 33 m/sec) and 2.5 asintense (i.e. had maximum wind speeds in excess of 44 m/sec. The Australian region wasexposed to tropical cyclones for 50 days on average and to severe cyclones on 15 days.The spatial patterns of occurrence of tropical cyclones in the South-West Pacific Oceanacross latitudes 10º and 22º S between 1969/1970 and 1988/1989 and their dependency onthe Southern Oscillation phenomenon and sea surface temperatures was initially describedby Basher and Zheng (1985). For this report, more up-to-date information on the averageannual occurrence of tropical cyclones and average annual numbers during El Niño, LaNiña and neutral years in the different regions has been obtained by accessing the southernhemisphere tropical cyclone archive (Bureau of Meteorology, Australia, 2007b). This archiveis based on a 2 x 2 degree resolution gridded analysis of cyclone best track data for theperiod 1969/70 to 1998/99. The map showing the average distribution of tropical cyclonesacross the southern hemisphere over this period shows that cyclone activity is greatestbetween latitudes 10° and 20° S, extends over a greater latitudinal range in the South Pacificthan in the Indian Ocean and that numbers decrease from west to east across the southwestPacific (Figure 10). The latter trend reflects the progressive west-east decline in the oceansurfacetemperature to the west of 170° E (Terry 2007). An area to the north of Port Hedlandon the northwest coast of Australia is the most prone to tropical cyclones, experiencing onesuch event per year on average (Bureau of Meteorology, Australia, 2007b). Three other “hotspots”of cyclone activity are evident in the South West Pacific region, two of which arelocated in the Coral Sea between longitudes 148° and 151°E, 156° and 158° E, respectivelywhile the third is to the west to Vanuatu and New Caledonia (between about 163° and 166°E). The latter region experiences 0.8 cyclones per season on average. Figure 10 implies thatthe incidence of cyclones is, on average, greatest around Vanuatu, followed in turn by Fiji,the Cook Islands, French Polynesia and the Solomon Islands. In widespread nations, such asFrench Polynesia, the risk of cyclones varies across the archipelagos, averaging one percentury to the north of the Marquesas, 1-3 per century from the Marquesas to the region tothe north of the Tuamotu group, 4-8 per century from the Tuamoto group to the Gambiersand one every 2-3 years in the Austral areas (Gabriel and Salvat 1985 in Wells and Jenkins1988).During El Niño years, there is a marked shift in the regional pattern of risk with cycloneactivity extending further east towards longitude 120° W, being greater than on average inthe vicinity of Vanuatu, Fiji, the Cook Islands and French Polynesia, of similar incidence inthe Solomon Islands but less frequent off the north Australian coast (Bureau of Meteorology,Australia, 2007b) (Figure 11). During La Niña events, the risk of cyclone activity does notextend beyond longitude 150° W, cyclones are more common than on average in the vicinityof Vanuatu, New Caledonia and South Coral Sea (Figure 12). The incidence of cyclones alsoshows a north-south shift, with activity extending further south during El Niño than LaThe Ecology Lab Pty Ltd – Marine and Freshwater Studies Page 12