Tropical Cyclone Structure

Climatic control of the frequency of tropical cyclones remains poorly understood at the global scale. This means that it is still a mystery why the total number of tropical cyclones, hurricanes and typhoons that develop over the world’s oceans each year is about 80 (Table 4.1), or indeed why this number is not much greater or much less (Emanuel 2004). The first of these two observations can be transferred from the global to the regional scale, because at present it is not known why the number of tropical cyclones forming in the South Pacific is about 11% of the world’s total. The second point, however, does not hold for the South Pacific. Based on the 1970–2006 climatic record (Appendix 1), although 9 storms is the long-term mean of annual tropical cyclone occurrence, the actual number forming in individual years ranges widely between a minimum of 3 and a maximum of 17 storms (Figs. 4.1 and 4.2).