Nov 17, 2014 - language to mean a temporary respite from surrounding mayhem. Near the eye, which is the warm core of the cyclone, atmospheric pressure at ...
Tropical cyclones in the South Pacific Impacts on island communities James P. Terry Tropical cyclones produce extreme weather conditions over warm oceans, and can be particularly damaging for small Pacific islands. But their physical effects are not all destructive. This article describes how such storms form, looks at their negative and positive impacts and considers how climate change may affect their nature. It is relevant to topics on hazards and climate
T
ropical storms are called cyclones in the South Pacific (Table 1), hurricanes in the North Atlantic and typhoons in the North Pacific oceans. They are intense low-pressure systems that form in tropical latitudes, mostly between 5° and 20°, where warm ocean waters (above 26.5°C) extend over a large area. The initiation and development of a cyclone is called tropical cyclogenesis.
The life of a cyclone Formation To begin with, a low-pressure cell forms by convection caused by the thermal imbalance between the warm tropical ocean and the cool upper atmosphere. Moist air is drawn inwards as convection intensifies and strong winds are generated, converging into the centre. The warm ocean provides heat and moisture to fuel the system.
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A flood in the Sigatoka River in Fiji during Cyclone Mick in December 2009 has pushed vegetation debris high up against the old bridge (now broken and abandoned) near Sigatoka town
As a cyclone matures, it becomes arranged into bands of cloud rotating clockwise in the southern hemisphere, anticlockwise in the northern (Figure 1). The Coriolis effect is important here, as it deflects the path of the inwardly-moving air flow. Coriolis is therefore responsible for producing the familiar
spiralling motion around the central vortex and is the main reason that cyclones develop their classic rotational nature.
Maturity In a mature cyclone, fierce winds spiral around a small central zone of clear, calm
Table 1 Tropical cyclones in the South Pacific (based on 1970–2006 data)
Main cyclone season
November–April
Months with highest frequency
January and February
Average frequency
9–10 cyclones per season
Most intense cyclone on record
Cyclone Zoe, December 2002 Minimum pressure: 890 mb Maximum sustained 10-minute winds: 287 km h−1 3-second gusts: up to 350 km h−1
Maximum annual number
18 cyclones in 1997 (El Niño conditions)
Minimum annual number
3 cyclones in 1995
Geography Review
Base image: NASA
Figure 1 Infrared satellite image of the western tropical South Pacific Ocean on 15 March 2010. Two cyclones have formed, each displaying spiral cloud bands rotating around the storm centre. Cyclone Uli moves towards the Australian mainland (left) while Cyclone Tomas threatens islands in the Fiji archipelago (right)
weather known as the ‘eye’. A small calm eye surrounded by a turmoil of rushing air is such a vivid image that the expression ‘eye of the storm’ has become a metaphor in our everyday language to mean a temporary respite from surrounding mayhem. Near the eye, which is the warm core of the cyclone, atmospheric pressure at sea level may fall to well below 980 millibars. In optimal conditions, cyclones form quickly, bring destr uctive winds (Table 2), torrential precipitation, and drive powerful waves against exposed coastlines. Cyclones are not stationary, but typically move away from their place of origin towards the pole. As they move along, cyclones usually follow simple recurving tracks, although they sometimes have sinuous or even looping tracks. The speed of movement is not necessarily constant, and cyclones sometimes decelerate or accelerate along their tracks. On
average, however, cyclones travel at between 5 and 30 km h−1. As you can imagine, any unpredictable change in direction or speed will cause problems for meteorologists at national climate centres trying to give warnings to people in the path of the storm.
Decay All tropical cyclones eventually weaken and die down as a result of migrating into latitudes outside the tropics. There are several reasons why cyclones inevitably decay, and these sometimes work together. Each involves losing one of the key ‘ingredients’ that are needed to keep the cyclonic system alive: ■■ First, by moving over a cooler ocean surface, a cyclone loses its source of heat energy. ■■ Second, if it makes landfall over a significant landmass, this cuts off the source of moisture.
Table 2 Categorising cyclones: the relationship between wind strength, Beaufort category and sustained wind speeds over 10-minute measurement periods. There are five different strengths of hurricane in the top band
Wind speed Cyclone wind strength
Beaufort scale equivalent
knots
km h−1
m s−1
Gale force
8–9
34–47
63–87
17–24
Storm force
10–11
48–63
88–117
25–32
Hurricane force
12
≥64
≥118
≥33
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Third, if a cyclone enters a region where the upper atmosphere is disturbed, then the vertical structure of the system can’t be maintained. Vertical wind shear is a common cause of this type of disruption. ■■
Impacts on island environments The thousands of islands dotted around the vast expanse of the South Pacific Ocean differ in geology, size and geomorphology. The main types are: ■■ mountainous volcanic islands ■■ flat limestone islands ■■ low sandy islands on coral reefs, sometimes called coral islands This variety of landforms means that the impacts of tropical cyclones vary widely. The most common effects can be grouped according to coastal or inland locations.
Coastal impacts Big, powerful waves driven against coastlines can easily break the living framework of coral reefs, especially the more fragile frond, branching and tabular species of corals. Loose sand and gravel can be removed from beaches, resulting in erosion and even shoreline retreat. Cyclone Funa in 2008 caused this effect on Ambae Island in Vanuatu. However, cyclones can also cause deposition, creating coastal features. Box 1 describes this process and explains how this can be a positive impact of cyclones. The temporary rise in sea level associated with cyclones and the funnelling of the sea into bays and estuaries may create a strong
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Box 1 cyclones?
Positive impacts of
Cyclones not only cause erosion, but also deposition. They can leave behind constructional features on coasts such as sand sheets, beach ridges or gravel ramparts. In areas with coral reefs, storm waves transport large quantities of coralderived sediment in gravel and sand-size fractions onto the reef surfaces. This material is not stable, but is continually reworked, even during the quiet periods between major storms. In 1972 and 1990, for example, Cyclones Bebe and Ofa built up extensive gravel ramparts, in some places rising over 2 m high, around the islands of Funafuti in Tuvalu and Upolu in Samoa. Depositional features like this not only add to the coastal landscape, but over the longer term they are crucial for replenishing beaches and low coral islands with new sediments, and increasing their life (see Figure 2). Some geomorphologists believe that many low Pacific atoll islands in the cyclone belts would not exist if it were not for the periodic input of large amounts of storm-generated coarse sediments. In island river systems where cyclone floods occur regularly, silt accretion on floodplains can be rapid. Therefore, although floods may damage existing crops, the thick and fertile alluvium that they build up allows intensive agriculture. For example, sugarcane cropping is carried out on the floodplains of most of the larger rivers in Fiji.
Cyclone ramparts migrate landward Island
Ocean Reef surface Coral reef
Lagoon
Conglomerate platform
Figure 2 Gravel ramparts built on the seaward edge of coral-reef platforms by tropical cyclone waves do not stay put, but instead tend to move landwards over time
A beach made of coarse gravels on Funafuti Atoll in Tuvalu. These coralline deposits originally formed ramparts on the reef, thrown up by Cyclone Bebe in 1972. Since then, the gravels have been redistributed to become part of the fabric of the atoll islands themselves
storm surge. This often leads to sea-water f looding of low-lying coasts, and saline damage to vegetation. Saltwater intrusion contaminating fresh groundwater aquifers is another problem. On remote Pukapuka Atoll in the northern Cook Islands, for example, precious fresh groundwater lenses were
damaged by saline washover and infiltration during Cyclone Percy in 2005 and took more than a year to recover.
Inland impacts On larger islands which have higher ground, intense cyclonic rainfall may be increased by orographic uplift. This can saturate the soil and trigger hillslope failures such as landslides and debris flows, which may be helped by trees being uprooted in the ferocious winds. At the same time runoff from saturated catchments pours into river channels. High river discharges can become serious floods if they exceed channel capacities. In the Tontouta River on Grande Terre in New Caledonia it has been shown that over 60% of all flood peaks are generated by tropical cyclones, and of these three quarters produce overbank floods. On the positive side, however, fine sediments carried onto floodplains bring nutrients that help to sustain soil fertility and therefore promote the long–term viability of agriculture.
Future concerns Intensive sugarcane farming on many of Fiji’s major river floodplains indicates the fertility of the cyclone-prone area. Sugar is Fiji’s second biggest earner of foreign income after tourism
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Cyclones may bring changes in the landscape which are of scientific interest for physical geographers, but of course Geography Review
GeographyReviewExtras Test your knowledge of cyclones with our downloadable quiz (see back cover).
stronger El Niño events in the future in response to global warming. This, along with other factors, could mean: ■ increased intensities of cyclones ■ shifts in the geographical patterns of storm origins ■ cyclones travelling further after maturing However, we have to be cautious when projecting current scenarios into the future. The modern records of cyclone activity on which we base our analysis only go back to the early 1970s, when weather satellites were introduced. There is a lot of variability in the year-to-year data in this relatively short period. This makes it difficult to distinguish real trends in tropical cyclone behaviour.
Conclusions
Beachrock exposed on Mana Island in western Fiji after the removal of overlying calcareous beach sand during Cyclone Evan in December 2012
Glossary Note that these definitions are specific to cyclones. Beachrock A calcareous sandstone (calcarenite) formed by the cementation of beach materials, mainly coral-derived sands and gravels. Exposed beachrock at the surface indicates that overlying loose beach sediment has been washed away. Landfall When the centre (eye) of a cyclone moves across a coastline and on to land. Orographic enhancement The impact of topography on the air mass of a moving cyclone, resulting in additional uplift, condensation and precipitation above that caused by convection alone. Track sinuosity The amount of ‘wandering’ exhibited by a cyclone track compared to the straight line between its origin and decay positions. Tropical cyclogenesis An umbrella term encompassing all the processes that are responsible for the formation of a new tropical low-pressure system and its development into a mature cyclone. Vertical wind shear A change in the direction and/or speed of wind with height through a section of the atmosphere. Strong vertical shear can disconnect the upper circulation of a cyclone from the inflow at its base, leading to storm decay.
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they can have negative impacts on human lives and livelihoods, especially those of poorer people. Most Pacific islands are developing countries with limited financial resources and can ill afford the costs of recovering from the impacts of a cyclone. The geographical remoteness and subsistence-based livelihoods of small populations on many South Pacific islands makes them particularly vulnerable to cyclone impacts. It is therefore important to research cyclones and increase our understanding of the risks so we can help to reduce people’s vulnerability. Some scientists now believe (although this is not certain) that the Pacific region may experience more frequent or
Further reading Australian Bureau of Meteorology, ‘About Tropical Cyclones’: www.bom.gov.au/cyclone/about/ The Fiji Meteorological Service (FMS) operates the RSMC (Regional Specialised Meteorological Centre) Nadi-Tropical Cyclone Centre. This is the official authority in the South Pacific for monitoring cyclone formation, forecast tracks, release warnings and archive historical data: www.met.gov.fj/ Terry, J. P. (2007) Tropical Cyclones: Climatology and Impacts in the South Pacific, Springer.
Tropical cyclones are a major natural hazard in the South Pacific basin. Because they travel great distances along recurving tracks, innumerable islands spread over enormous areas are at risk of cyclone strike each year. High seas, pounding waves, violent winds and intense rains lash coastal and inland areas. Landscape responses to cyclone impacts include marine and river floods, mass movements and various types of erosion. Not all impacts are negative. As described in Box 1, cyclones also cause deposition which can build up beaches and extend the life of some small Pacific islands. The deposition of silt on floodplains can also help to sustain agriculture in the region.
James Terry is a professor at Zayed University, Dubai where he lectures on earth systems and natural hazards. He was formerly head of geography at the University of the South Pacific based in Fiji. His research interests include tropical geomorphology, natural hazards and environmental change in the Asia‑Pacific region.
Key points • Tropical cyclones are intense, rotational, low-pressure systems that travel away from their place of origin. • Powerful waves, heavy rain and river floods all cause geomorphic changes in the natural landscape. • Various constructional features may be built up by storm-generated processes, in addition to the widely recognised erosional effects.
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