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The islands of the tropical South Pacific are vulnerable to a variety of natural hazards. Some are associated with the vagaries of climate, such as tropical cyclones (elsewhere called hurricanes or typhoons), droughts and floods, whereas others are geological in origin, such as volcanic eruptions, earthquakes and tsunamis. The main difference between these two groups of hazards concerns the timescale on which they happen. If we decide arbitrarily to deal on the timescale of a human lifespan, then the effects of a major geological event will be experienced perhaps only once or twice in a lifetime, or possibly not at all. In contrast, tropical cyclones and associated hazards are felt more frequently, perhaps every few years.

The initiation, development and subsequent maturation of tropical cyclones is known as tropical cyclogenesis . The detailed nature of tropical cyclogenesis continues to be under investigation by climatologists, and it is apparent that there is still much we need to learn. This section therefore concentrates on the primary reasons why and main processes how tropical cyclones form in the South Pacific region. In order to be able to understand these fundamentals, it is necessary to have a good grasp of how rising air masses undergo adiabatic cooling, which leads to condensation of water vapour and the simultaneous release of latent heat energy. The adiabatic process is not described here because existing texts on meteorology and physical geography give clear and comprehensive explanations (Barry and Chorley 1998, Strahler and Strahler 2006).

Atmospheric pressure at sea level at the centre of a tropical cyclone is frequently as low as 965 mb. Away from the centre the pressure increases to about 1,020 mb at a storm’s outer edge. This spatial variation in atmospheric pressure at sea level means that one way of examining the shape and size of a tropical cyclone (as with other types of weather systems) is to observe the arrangement of isobars on a synoptic weather chart. The isobar pattern displayed when a storm is slow moving or stationary is often a neat, nearly circular arrangement of concentric rings. This is because there are none of the fronts commonly seen in the structure of mid-latitude depressions (McGregor and Nieuwolt 1998). More rapidly moving tropical cyclones commonly show elliptical or pear-shaped patterns in their isobars (Fig. 3.1). Any elongation in shape is normally oriented in the direction of the storm track (Visher 1925) with the ratio of longest to shortest diameter about 3:2. In elliptical and pear-shaped patterns the isobars are not concentric. There tends to be some bunching of isobars in the leading portion of the storm, relative to storm movement along its track, and some spreading of isobars in the wake. Figure 3.1 illustrates this effect in the isobar patterns of Tropical Cyclones Olaf and Nancy near Samoa and the southern Cook Islands in February 2005.

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).

As a tropical cyclone approaches, the barometer falls, at first slowly and then more quickly until it reaches an alarmingly low level. The pressure gradient at sea level near the centre of a mature cyclone is very steep, as shown by closely spaced isobars on a weather chart (Fig. 5.1). It is the steepness of the pressure gradient that accounts for the high wind velocities. Low pressure also has an important influence on storm surge (Section 5.3). The minimum pressure attained is one of two main parameters used to indicate tropical cyclone intensity. The other parameter is wind strength, discussed in Section 5.2.

If we are interested in understanding something about the sensitivity of island environments to the impacts of tropical cyclones, which is the focus of the following chapters of this book, then we must at some point tackle the thorny issue of what, if any, will be the effects of climate change on the future characteristics and behaviour of tropical cyclones. As might be imagined, in all of the world’s major ocean basins, with the South Pacific no exception, questions abound concerning the nature of the tropical-cyclone regime in a projected warmer world. The surfaces of most tropical oceans have warmed up by 0.25–0.5°C during the past few decades (Santer et al . 2006) and it is widely believed that the increase in greenhouse gas concentration is the primary cause of the observed rise in global mean sea-surface temperature over the past 50 years (IPCC 2001). In cyclone research, scientists have to date largely concentrated their attention on assessing the likelihood of any climate change and associated ocean warming effects on tropical cyclone numbers and frequencies, storm intensities, and the locations of storm origins. Tropical cyclone durations and precipitation have also featured as topics for research, albeit not so prominently.

The clear and warm waters of the tropical South Pacific support the abundant growth of both hard (scleractinian) and soft corals. Scleractinian species are corals that secrete hard skeletons of calcium carbonate from seawater. Diverse types of hermatypic species are the colonial corals, sometimes called frame-builders, which are the ones responsible for the growth of fringing, patch and barrier reefs around volcanic and limestone islands, and the growth of atoll reefs overlying subsiding volcanic foundations. Other organisms that make up reef ecosystems are important sand producers. The biogenic sand contributes to reef, lagoon and beach sediments. These organisms include foremost foraminifera, calcareous and coralline algae, as well as shelled molluscs, echinoderms, calcareous worms and bacteria. Reef-dwelling parrot fish and other coral-eroding fish species are also prolific living producers of coral sand. As they feed on coral polyps, parrot fish excrete fine carbonate sand that falls into interstices in the reef structure and washes onto the reef flat.

The rugged terrain of the high volcanic islands in the South Pacific is susceptible to landslides, debris flows and other types of mass movements. Many extensive slope failures are activated during severe tropical cyclones. Slope susceptibility to failure is influenced by the predominance of clay-rich soils, normally humic latosols, overlying residual red/orange saprolite. Saprolite is completely-weathered rock and sediments in situ . The regolith (both soil1 and saprolite material) is formed by chemical weathering of the common types of volcanic rocks and volcanic-derived sediments found in the South Pacific islands, especially andesites and basalts, and associated breccias and conglomerates. The regolith often has structural weaknesses inherited from the bedrock and overlying sediments from which it is derived, such as inclined bedding planes, faults and sediment/bedrock junctions. These zones of weakness may develop into shear planes along which the regolith mass fails in a landslide. If a particular area has a history of past landslides, it means that hillslopes are formed on weak residual materials and are prone to further failures.

The geology of the volcanic islands in the South Pacific generally consists of lava flows, pyroclastics, breccias and conglomerates, all weathered to varying degrees according to the age of the individual island concerned. Geomorphology is often dominated by volcanic mountains forming a central highland area, such as on Rarotonga in the Cook Islands, Viti Levu in Fiji, Ambrym in Vanuatu and Tahiti in French Polynesia. Orientation of the major drainage networks tends to be in a radial fashion outward from the central highlands. If the volcanoes are aligned in a chain, then the volcanic peaks form an elongated mountainous spine to the island. Examples include Savai’i in Samoa, Kadavu in Fiji, Santa Isabel in Solomon Islands and Pentecost in Vanuatu. On such islands, the orientation of the major river networks is controlled by the linear arrangement of the volcanic mountains. Within individual catchments, drainage patterns are typically dendritic because of the lack of geological or structural controls other than the volcanoes. Drainage densities are high. Centripetal drainage patterns are seen where a river drains a breached volcanic caldera, such as the Tavua and Lovoni rivers on the islands of Viti Levu and Ovalau in Fiji, respectively.

River channel morphology and pattern must adjust to major flood events on tropical Pacific islands, generated by extreme rainfall during tropical cyclones. Alluvial rivers often exhibit large changes in their channels, especially in terms of geometry and position, because they are sensitive to the erosive power of huge river discharges during cyclones. Riverbanks are undercut and collapse (Fig. 10.1), meander bends are cut off and abandoned and riverbeds scour and fill. Yet considering the importance of cycloneinduced floods and related river channel instability for human occupancy and activities on floodplains, it is a pity that little quantitative information exists on the nature and rates of river channel change in the South Pacific.