Storm Formation and Development Early Stages

Tropical cyclones develop initially within regions of low pressure and from pre-existing tropical disturbances. In the eastern region of the South Pacific Ocean, the Equatorial Trough of low pressure rarely, if ever, penetrates south of the Equator. This is one constraint preventing the formation of tropical cyclones in this part of the South Pacific. By comparison, in the western South Pacific, the South Pacific Convergence Zone (SPCZ) described in the previous chapter is a persistent and often well-developed regional zone of low pressure. The SPCZ is therefore often responsible for the initiation of tropical depressions (Revell 1981), and incipient tropical cyclones are often observed embedded within it. Figure 2.1 shows an example of a strongly defined SPCZ on 7 March 2000, extending from the Solomon Islands across Tuvalu, Samoa and down to Niue. At the southeastern end of the SPCZ is an embedded low-pressure system and associated cloud mass, which soon afterwards became organised into Tropical Cyclone Mona.

In the formative stage of a cyclone, an unusually active but poorly organised area of disturbance and convection appears on satellite images. The circulation centre is normally ill-defined and the strongest surface winds tend to occur in disorganised squalls (Australian Bureau of Meteorology 2006). A large area of ocean with surface temperatures around 27°C or above is necessary to thermally 'kick start' a better-defined cell of strong convection at the centre of the low-pressure area. This cannot normally occur in the southeast Pacific because along the western coast of the South American continent, and up to a thousand kilometres offshore, the upwelling waters of the cold Humboldt Current originating from the Antarctic Ocean keep the sea-surface temperatures below this threshold, effectively preventing tropical cyclogenesis. Farther west in the South Pacific, however, the water temperatures increase as the South Equatorial Current flows westwards south of the Equator, gradually warming up by insolation.

South Pacific Convergence Zone

Fig. 2.1. Visible image from a geostationary meteorological satellite at 11:30 a.m. on 7 March 2000 (UTC1), showing cloud formation organised along the South Pacific Convergence Zone. The cloud mass just west of Niue is the initial phase of what became Tropical Cyclone Mona on 8 March and affected the Kingdom of Tonga. Base image courtesy of the Japan Meteorological Agency.

Fig. 2.1. Visible image from a geostationary meteorological satellite at 11:30 a.m. on 7 March 2000 (UTC1), showing cloud formation organised along the South Pacific Convergence Zone. The cloud mass just west of Niue is the initial phase of what became Tropical Cyclone Mona on 8 March and affected the Kingdom of Tonga. Base image courtesy of the Japan Meteorological Agency.

Once a tropical low-pressure system has an active convective cell established at its centre, there is a difference in atmospheric pressure between the middle of the depression and the surrounding area of relatively higher pressure. This difference causes a pressure gradient to be set up, and air is drawn inwards. If the minimum surface pressure drops rapidly, convergence intensifies. Strong winds are generated, with the warm ocean surface providing the moving air with heat and moisture through evaporation (Emanuel 1987). The maximum winds are now concentrated in a tight band close to the low-pressure centre, instead of the earlier disorganised pattern.

As the converging air is drawn inwards, the effect on wind direction of the Coriolis Effect, associated with the rotation of the Earth, becomes important. The Coriolis Effect causes the winds to be deflected from a straight line path as they are drawn inwards by the central low pressure. According to Ferrel's Law, the direction of the wind deflection is to the left in the Southern Hemisphere. The Coriolis Effect is therefore responsible for the spiralling motion around the central vortex of the depression (Fig. 2.2) and is the main reason that tropical cyclones develop their classical rotational nature.

1 UTC means Universal Coordinated Time and is equivalent to Greenwich Mean Time or GMT.

Ferrel Law

Fig. 2.2. Coriolis Effect influencing the direction of air movement into the centre of a nascent tropical cyclone, leading eventually to the organisation of the winds into the classical spiral pattern.

The strength of the Coriolis Effect, called the Coriolis Force, is negligible at the Equator and increases towards the poles. In consequence, approximately 74% of tropical cyclones in the South Pacific develop in latitudes 10-20°S (Table 2.1). The weak Coriolis Force near the Equator means that few tropical cyclones can develop in the zone from the Equator to 5°S, in spite of the presence of warm ocean water providing suitable conditions for convection. Beyond 20°S the presence of cooler ocean prevents sea-surface temperatures attaining the 27°C threshold for active convection over a wide enough area.

Table 2.1. Latitudes of tropical cyclone formation in the

South Pacific, east of 160°E (1970-2004 data).

Table 2.1. Latitudes of tropical cyclone formation in the

South Pacific, east of 160°E (1970-2004 data).

Latitudinal zone of origin

Percentage of tropical cyclones

0-5°S

1

5-10°S

19

10-15°S

48

15-20°S

26

20-25°S

6

0 0

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