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cyclones occurred in both the South Pacific and South Indian Oceans during El Nino than during La Nina years. On average 25 and 29 tropical cyclones occurred in El Nino and La Nina years respectively in the Southern Hemisphere. Kuleshov et al also found that an area of cyclogenesis located between 60 o E and 85 o E in El Nino years in the Indian Ocean shifted eastwards (i.e. closer to Australia) in La Nina years. In contrast, the focus for cyclogenesis in the South Pacific shifted eastwards (away from Australia) in El Nino years. These two effects explain the predominance of tropical cyclones in the Australian Region during La Nina years. The decline in total number of tropical cyclones affecting Australia in recent years can be explained by the fewer La Nina events compared with El Nino periods. As shown in Figure 7.3, taken from their paper, Kuleshov et al identified statistically significant increasing trends in severe tropical cyclones, defined as those with a central pressure less than 945 hPa in the Southern Hemisphere (SH), southern Indian Ocean (SIO), and in the south Pacific Ocean (SPO). This observation confirms the earlier one by Nicholls et al (Figure 7.1), and is also consistent with predictions of the effect of CO 2 induced global warming discussed in the next chapter. Figure 7.3 Occurrences of tropical cyclones with central pressures of 945 hPa or less in the Southern Hemisphere between 1981/2 and 2005/6 (from Kuleshov et al, 2007). 7.5 Summary and Conclusions Studies of tropical cyclone activity in Australian waters have found that the numbers of tropical cyclones are negatively affected by the Southern-Oscillation Index – El Nino phenomenon. Fewer cyclones occur in the Australian Region when higher sea surface temperatures occur in the central and east Pacific Ocean. However the more intense cyclones (i.e. those of interest for structural design) are much less influenced by the El Nino phenomenon, and there is evidence of increasing trends in the stronger (Category 3-5) events during the last 30-40 years. However, these conclusions should be re-visited in the future to ensure that no fictitious increase in intensity has occurred in the tropical cyclone database (Section 1.2) due to changes in interpretation of satellite data over the years (see also Section 6.5) To the extent that global warming will affect the sea surface temperature in the east and central Pacific and hence the El Nino-La Nina phenomena, it may be expected 40
that there will be changes to the number and intensities of cyclones in the Australian Region. Recent predictions are discussed in the following Chapter. References Y. Kuleshov, L. Qi, R. Fawcett and D. Jones (2008), On tropical cyclone activity in the Southern Hemisphere: trends and the ENSO connection, Geophysical Research Letters, Vol. 35, L14S08. N. Nicholls, C. Landsea and J. Gill (1998), Recent trends in Australian Region tropical cyclone activity, Meteorological and Atmospheric Physics, Vol. 65, pp 197- 205. H.A. Ramsay, L.M. Leslie, P.J. Lamb, M.B. Richman and M. Leplastrier (2007), Interannual variability of tropical cyclones in the Australian Region: Role of largescale environment, submitted to Journal of Climate. 41
Page 2 and 3: IMPACT OF CLIMATE CHANGE ON DESIGN
Page 4 and 5: Executive Summary The objective of
Page 6 and 7: 1. Introduction 1.1 Project scope B
Page 8 and 9: Figure 1.1. Average number of days
Page 10 and 11: 2. History of cyclonic design wind
Page 12 and 13: The ratios of the current (2002) va
Page 14 and 15: Station Table 2.3. Comparison of de
Page 16 and 17: 2.3 Summary and Discussion The chan
Page 18 and 19: 3. Simulation methods for predictio
Page 20 and 21: with the hurricane. An imposed pres
Page 22 and 23: C.M.L. Dorman (1984), Tropical cycl
Page 24 and 25: In the absence of other comparable
Page 26 and 27: averaging of many individual normal
Page 28 and 29: T.F. Hock and J.L. Franklin (1999),
Page 30 and 31: A road-sign failure near the eye wa
Page 32 and 33: α β = cav (5.6) where c av is an
Page 34 and 35: References G.N. Boughton and D. Fal
Page 36 and 37: 6.3 Interpretations by Emanuel (200
Page 38 and 39: IWTC (2006) ‘The scientific debat
Page 40 and 41: 7. Observed trends in tropical cycl
Page 44 and 45: 8. Predicted effects of climate cha
Page 46 and 47: environmental factors (that are kno
Page 48 and 49: 9. Observations and projections for
Page 50 and 51: 9.3 Report by Cook G.D. Cook (2007)
Page 52 and 53: (the ‘TIM’ cyclones). The latte
Page 54 and 55: are slightly conservative but adequ
Page 56 and 57: • Improving the understanding of
Page 58 and 59: Cyclonic wind speeds - Darwin 1960-
Page 60: ACKNOWLEDGEMENTS The support of Dr

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