Damage

Tropical cyclones bring so much damage from strong winds, flooding rains and 'storm surge'

of the ocean surface that they have to be carefully tracked by means of satellite pictures, radar, weather stations, buoys and reconnaissance flights, to enable adequate warning of the TC's approach. The power of a wind at 50 m/s is a thousand times that of a more normal 5 m/s, creating huge waves at sea. Winds are strongest on the left side of the eye, where the cyclonic winds around the eye are augmented by the speed of advance of the eye itself. There is great danger to small boats in the left-forward quarter,

Zealand

Figure 13-15 The frequency of tropical cyclones and the effect of an El Niño. The dashed lines show the annual number through each 2x2 degree box of latitude and longitude during the period 1969—89, e.g. over ten a year around Fiji. The shading shows the probability of a tropical cyclone during El Niño events (when the SOI exceeds zero), as a fraction of what is normal. For instance, a tropical cyclone is about four times more likely to strike Tahiti in an El Niño year.

Zealand

Figure 13-15 The frequency of tropical cyclones and the effect of an El Niño. The dashed lines show the annual number through each 2x2 degree box of latitude and longitude during the period 1969—89, e.g. over ten a year around Fiji. The shading shows the probability of a tropical cyclone during El Niño events (when the SOI exceeds zero), as a fraction of what is normal. For instance, a tropical cyclone is about four times more likely to strike Tahiti in an El Niño year.

where the circling winds blow the craft into the path of worse conditions. Sometimes tornadoes form in this quarter, within the spiralling rainbands (Figure 13.12).

The winds were responsible for the destruction of Darwin in Australia's Northern Territory on Christmas Day in 1974, with the loss of fifty-five lives and a billion dollars' worth of damage.

Rains due to TCs can be heavy, particularly as the hurricane crosses the coast, where the greater friction of land surfaces causes deceleration and hence increased local convergence and updraught. For instance, a TC affecting Mackay (a port at 21°S) caused precipitation of 1,400 mm within 72 hours. The effects are also felt inland and over a wide area, so that there is considerable flooding of rivers (Section 10.6 and Note 10.Q). About a quarter of a million people drowned in such floods in Bangladesh in 1970, for instance. Precipitation of 750 mm fell in one day on Whim Creek (21°S in Western Australia), which received only 4 mm in the whole of 1924. Even TCs well to the north of New Zealand bring heavy rains to parts of the north island in late summer and autumn, though less if the TC passes quickly.

A storm surge is the coastal wave of high seas created by an offshore TC. The suction of the central low pressure lifts the sea's surface by about a centimetre for each lowering by a hectopascal, e.g. by 0.5 m in a typical cyclone. But strong onshore winds can heap shallow seas onto a beach by several metres (Table 13.2), according to the slope and shape of the shore. For instance, by 2.8 m when Althea struck Townsville (19°S) in 1971. Such surges flood low-lying coastal land if the TC's landfall coincides with the time of high tide.

The authorities broadcast a 'tropical cyclone advisory' warning when conditions favour the development of a cyclone within 800 km of the Australian coast, and a 'cyclone warning' when the TC is confirmed and named. These are widely broadcast and posted on the Internet. Despite this, it is estimated that TCs globally have caused almost 15,000 deaths each year, or far more if storm surges are included (Table 10.5).

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