Mature Stage

The tracks of tropical cyclones are shown in Figure 13.13. Their movements are irregular and difficult to predict, especially within 15° of the equator. Generally they move with the average wind of the troposphere, i.e. westward north of about 15°S, and eastward south of that latitude (Figure 12.10 and Figure 13.14). Alternatively, they may loop, meander or stall. They usually move away from the equator, come under the influence of the midlatitude westerlies and then accelerate eastwards to about 15 m/s, before decaying over cooler waters. As a result, most of the tropical cyclones affecting Fiji (18°S) come from the northwest, like that shown in Figure 13.14.

The passage of a TC leaves the SST lowered by as much as 3 K in a swath about 100 km wide. This cooling is partly due to the transfer of heat to the atmosphere within the TC, and partly to upwelling induced by the TC, since the circling winds create Ekman transport of the ocean surface outwards from the cyclone. Consequently, a stationary TC automatically dies away, by lowering the SST below the critical 27°C.

The chances of encountering tropical cyclones are about equal on the two sides of Australia, the greatest likelihood being around 20°S. The part of the coast most frequently crossed by cyclones is a stretch of 100 km around Cairns at 17°S in Queensland, with fifteen making landfall there over seventy-one years, i.e. one each five years on average. In the South Pacific as a whole, TC frequency peaks within the South Pacific

Convergence Zone (Section 12.1), which is a spur from the ITCZ, stretching from the Coral Sea (to the north-east of Australia) to the Tropic of Capricorn at 160°W, near New Caledonia. There has been a TC nearly every year (i.e. fifteen in twenty years) within a 200 km square just west of Fiji, at about 18°S, 170°E.

Tropical cyclones happen more often (especially in the central Pacific and between 5-15°S) during an ENSO warm phase, when the Southern Oscillation Index (Section 12.7) is low (Figure 13.15). But there are fewer tropical cyclones in the south-west Pacific area (including Australia). This is because tropical depressions that otherwise would have developed into cyclones in the west have already formed TCs further east because of the higher SST there, and these mature and recurve towards the southeast before reaching Australia.

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