Preface and Acknowledgements

The South Pacific is an almost incomprehensibly vast ocean. Within it lie thousands of islands belonging to over 15 developing nations and territories. These islands display enormous physical diversity. They may be large or small, rugged volcanic mountains with high elevations, flat limestone platforms or tiny coral islets resting just above sea level on top of coral reefs. Many islands are remote and uninhabited while others are densely populated with bustling towns and expanding cities. In...

Wind Strength

Wind strength is the main parameter used for describing the intensity of a tropical cyclone Table 5.2 , to complement readings of low barometric pressure at the storm centre. The strength of the winds is determined from the average wind speed, referred to as the sustained wind speed. Sustained winds are recorded over 10-min averaging times, above ground level in an open and flat space. It should be understood that winds in a tropical cyclone are extremely squally in nature, and gusts much...

Cyclone and Coastline Influences

Tropical Cyclone Wind And Pressure Graph

At any particular coastal location, the level and duration of sea flooding by storm surge depends on the following factors, which are discussed below 1. Depth of the horizontal atmospheric-pressure gradient across the eye of the storm, 2. Speed and radius of maximum winds, 3. Direction and speed of cyclone motion in relation to the land, 4. Timing of tropical cyclone landfall in relation to local tides, 5. Shape of the coastline near the place of cyclone landfall, 6. Bathymetry of the nearshore...

Influences on River Responses

Fiji Flooding Viti Levu

High-magnitude rainfalls delivered by tropical cyclones normally produce extraordinarily big discharges in Pacific island rivers Figs. 9.4 and 9.5 . The nature of a river's response depends primarily on three groups of influences, which are discussed below. The first group are the meteorological characteristics of an individual tropical cyclone, such as the organisation of its cloud bands, speed of movement and the corresponding patterns in precipitation. The second set of factors are those...

Case Study Geomorphic Change on Tatafa Cay in Tonga During Tropical Cyclone Isaac in March

Cyclone Isaac Tonga

Tropical Cyclone Isaac in 1982 was one of the worst storms to affect the Kingdom of Tonga in the last century. The storm was relatively small, but traversed directly over the Ha'apai group of islands on 3 March with sustained winds of 148 km h-1 80 knots . It then moved south west towards the main island of Tongatapu Fig. 7.18 , where pressure fell to 976.4 mb and 120 mm of rain was recorded in the capital Nuku'alofa Woodroffe 1983 . The arrival of the storm coincided with a high spring tide of...

Case Study Decay by Vertical Shearing of Tropical Cyclone June in

Island Gau Fiji Before Global Warming

A good example of cyclone decay by vertical shearing is that of Tropical Cyclone June in May 1997. The initial tropical depression developed close to the main Fiji Islands, being officially named TC June at a location of 14 S 174 E early on 3 May. According to historical data since 1840, TC June has the distinction of being only the fourth cyclone to threaten Fiji outside the normal cyclone season between the months of November and April Fiji Meteorological Service 1997a . The life and...

Slope Susceptibility to Failure

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...

Failure Trigger Mechanisms

Samoa Hurricane Photos

A trigger mechanism is a discrete, identifiable event or influence that initiates slope failure. Tropical cyclones and earthquakes are both very effective trigger mechanisms. Tropical cyclones are able to provide the necessary trigger for slope failure in two different ways. First, dynamic loads are increased through the violent swaying of tall trees during cyclone-force winds, because stresses are transferred to the slope materials by agitation of the root networks. A good example of this...

Tropical Cyclone Floods

Floods Solomon Islands

The huge and intense rainfalls brought by tropical cyclones generate very quick responses in Pacific island streams and rivers. Peakflows are often far in excess of maximum channel capacities, leading to severe overbank inundation Fig. 9.3 . River floods are a serious hydrological hazard because of their impacts across both physical and human landscapes. Flood problems include channel erosion and siltation, destruction of homes and infrastructure, contamination of water resources, damage to...

Torrential Rainfall Distribution and Controls

Cyclone Bola

Tropical cyclones normally produce torrential rainfalls, meaning that both the amount of rainfall is large total in mm and the rainfall intensity is high amount per unit time, expressed in mm h-1 . Over oceans the rainfall is of convectional type since there are no fronts associated with tropical cyclones. Converging air rises over warm sea and cools adiabatically, leading to the condensation of water droplets, cloud formation and subsequent rainfall. But precipitation near the centre of...

Case Study Rainfall Distribution Across Fiji During Tropical Cyclone Gavin in March

Fiji Weather Station

The track and behaviour of Tropical Cyclone Gavin through the Fiji Islands from 4 to 11 March 1997 were described earlier in Section 5.3 on storm surge. Meteorological records from Fiji's 22 synoptic climate stations demonstrate the widely variable pattern of rainfall delivered by TC Gavin over the islands. Fig. 5.28. Maximum 1-day rainfall delivered by Tropical Cyclone Gavin across the Fiji Islands in early March 1997. Fig. 5.28. Maximum 1-day rainfall delivered by Tropical Cyclone Gavin...

Flood Hazard Mitigation

Vanua Levu Flood

The historical record of tropical cyclones in the South Pacific indicates that storms rarely develop close to the main island archipelagoes lying south of 10 S. Instead, nascent tropical disturbances tend to form farther north and then approach on southerly tracks over 2 or 3 days as they mature. This crucial delay should prove invaluable for alerting vulnerable populations Fig. 9.15. Maximum flood heights in several rivers on Vanua Levu island in Fiji, generated by Tropical Cyclone Ami in...

Strong Winds Wind Effects

Tropical Cyclone Waka

It is the steep barometric pressure gradient established between a tropical cyclone's outer edge and its central low that causes strong winds to blow inwards. Violent winds are one of the most dangerous and costly aspects of cyclones for people living on Pacific islands Table 5.1 . Strong winds have a number of effects. They generate large waves at sea and drive these onshore, damaging coral reefs and causing severe coastal erosion. They also contribute to storm surge, resulting in widespread...

Introduction to Island Rivers

Samoa Rivers

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...

Storm Longevity

For the purpose of this section, the lifespan refers to the period that a storm remains classified as a tropical cyclone, based on its structure and intensity as described in Chap. 3. This is independent of position, so includes any time that a cyclone spends in extra-tropical waters, for those storms that migrate beyond latitude 25 S. Tropical and extra-tropical depression phases at the start or end of a cyclone's life are not included. Under this system, the modal lifespan of tropical...

Revell And Goulter 1986 South Pacific Tropical Cyclones And The Southern Oscillation Monthly Weather Review

Allan RJ, Nicholls N, Jones PD, Butterworth IJ 1991 A further extension of the Tahiti-Darwin SOI, early ENSO events and Darwin pressure. Journal of Climate 4 743-749 Allison MA, Kuehl SA, Martin TC, Hassan A 1998 The importance of floodplain sedimentation for river sediment budgets and terrigenous input to the oceans insights from the Brahmaputra-Jamuna River. Geology 22 175-178 Anthes RA 1982 Tropical cyclones their evolution, structure and effects. American Meteorological Society,...

Eye of the Storm

A distinctive feature of tropical cyclones is their small, comparatively tranquil, central area of fair weather, around which the storm rotates. This is known as the eye, which usually measures 20-40 km across Fig. 3.3 . The vivid picture of a small quiet area surrounded by a violent tempest so captures our imagination Fig. 3.3. Tropical Cyclone Eseta passing near the islands of Tonga on 13 March 2003. The eyewall area of dense cumulus cloud contrasts markedly with the central, almost cloudless...

The El Nino Southern Oscillation

Ocean Graphers

A third major control on climate of the South Pacific is the El Nino-Southern Oscillation ENSO . At the inter-annual timescale, the ENSO phenomenon is our planet's most powerful climatic influence Hilton 1998 . Under normal conditions, low pressure at the Equatorial Trough and high pressure in the eastern Pacific establish a pressure gradient that keeps the Southeast Trade Winds blowing strongly. The combination of the Southeast Trades and the South Equatorial Ocean Current flowing east to west...

Case Study River Responses in Fiji to a Succession of Tropical Cyclones During the El Nio Event

The Decay Stage Tropical Cyclone

In early 1997, as El Ni o conditions developed across the Pacific region, five tropical cyclones named Freda, Gavin, Hina, Ian and June, traversed Fiji waters within a period of just 4 months. These storms were the first set Fig. 9.7. Partial duration series for peakflows in the Rewa and Tontouta rivers in Fiji and New Caledonia based on mean daily discharge data , generated mainly by tropical cyclones. From Kostaschuk et al. 2001, 2006 . Fig. 9.7. Partial duration series for peakflows in the...

The Southeast Trade Winds

Fiji Meteorological Service Rainfall

Most of the western region of the tropical South Pacific benefits from the Southeast Trade Winds. These are produced by the effect of Coriolis deflection on surface air drawn towards the low pressure region at the Equator, called the Equatorial Trough. The Coriolis deflection of winds, which is to the left in the Southern Hemisphere, is caused by the Earth's rotation. The Southeast Trade Winds are persistent for most of the year, although they tend to be weaker in the summer season from...

Principles and Controls

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...

Valley Aggradation Features and Rates

River floodplains are valuable components of the physical environment on mountainous islands in the South Pacific, where the availability of flat land is otherwise limited. They are favoured for the sites of towns and villages, and their alluvial soils are generally fertile and prized for farming. Floodplains are constructional features in the landscape and one of the chief processes by which they grow is the vertical accumulation of fine deposits laid down during successive overbank flood...

Case Study Rubble Ramparts Created on Upolu Island by Tropical Cyclone Ofa in February

Case Study Guano Islands

Tropical Cyclone Ofa developed over Tuvalu on 27 January 1990 as a depression within the South Pacific Convergence Zone, then rapidly intensified to hurricane intensity as it moved south-southeast. The track was shown in Fig. 5.10. Samoa was severely affected from 2 to 4 February. Very destructive sustained winds greater than 118 km h-1 lasted for almost 24 h Fiji Meteorological Service 1990 . The violent winds, rain deluge and storm surge caused widespread devastation across Samoa, the worst...

River Sediment Transport

Rewa River Floodings

Those people who live in tropical islands with rivers and streams are familiar with one important feature of hydrological behaviour during tropical cyclones - when rivers begin to rise soon after heavy rainfall starts, the water becomes more turbid at the same time. This is because during the rising stage of a storm hydrograph, the sediment concentration carried by a river generally increases. In addition, powerful cyclone-induced floods are among the few occasions when coarse bedload sediments...

Gravel Sheets and Ramparts

Jaluit Typhoon

Abundant amounts of coral-derived sediment in gravel and sand-size fractions are transported by storm waves onto the surface of reefs. The length of time after the last cyclone and before a subsequent event influences the degree of stabilisation of this material, and hence the chances of its preservation Scoffin 1993 . The coralline sediments are not distributed in random fashion, but normally show distinctive patterns of accumulation. Often the debris is built up in new constructional forms in...

Wind Direction

Tropical Cyclones

Winds drawn in towards the centre of a tropical cyclone do not trace the circular or elliptical shape of the isobars shown on synoptic weather charts, but instead follow spiral streamlines Fig. 5.11 . The angle of deviation of wind Fig. 5.11. Surface streamlines for an intense tropical cyclone, drawn with respect to an arbitrary southward direction of motion. Adapted from Simpson and Riehl 1981 . Fig. 5.11. Surface streamlines for an intense tropical cyclone, drawn with respect to an arbitrary...

Environmental Steering

Prevailing Wind Direction Fiji

Nascent tropical disturbances in the South Pacific may stay more or less stationary while developing. Once formed, the majority of tropical cyclones migrate generally eastwards and polewards from their original position. This means that in spite of being relatively small features at synoptic scales, cyclones may cause great damage on any islands they encounter, throughout a belt that is several hundred kilometres wide and more than a thousand kilometres long. Yet predicting the movement of an...

Contents

Preface and Part I Tropical Cyclones in the South Pacific 1.2 The Study 1.3 Regional Climatic 1.3.1 The Southeast Trade Winds 1.3.2 The South Pacific Convergence 1.3.3 The El Nino-Southern Oscillation 2.1 Principles and 2.2 Storm Formation and 2.2.1 Early Stages 2.2.2 Mature Stage 2.3 Storm 2.3.1 Major Influences 2.3.2 Case Study - Decay by Vertical Shearing of Tropical Cyclone June in May 1997 Chapter 3 Tropical Cyclone Structure 3.1 Shape and 3.2 Cloud Patterns 3.3 Eye of the Storm 3.4 Naming...

Wind Distribution

Cyclone Pie Chart

Figure 5.7 demonstrates a simplified horizontal distribution of wind strengths within an idealised and stationary tropical cyclone that has reached hurricane intensity. One of the most remarkable features of tropical cyclones compared to other types of storms is that the strongest winds are felt just outside the central eye, which by comparison is a zone of almost complete calm. Farther away from the central zone of hurricane-force winds, the winds decrease in strength. The boundaries of wind...

Storm Surge and Sea Flooding Wind and Pressure Components

Ciclone Veena Polinesia 1983

Widespread sea flooding by storm surge around the coastlines of South Pacific islands is a serious hazard during tropical cyclones. For example, on 25 February 2005, Tropical Cyclone Percy, a hurricane-intensity system with sustained winds up to 249 km h-1, had a severe impact on the atoll nation of Tokelau Fig. 5.14 . Tokelau consists of three atolls Fakaofo, Nukunonu and Atafu. The storm surge generated by TC Percy inundated all three atolls. The high surge allowed the powerful winds to send...

Case Study the Good Friday Landslides Triggered on Viti Levu Island by Tropical Cyclone Wally in April

Cyclone Causes And Effects

The effects of Tropical Cyclone Wally on landslide activity in the southern part of Fiji were investigated shortly after the event by Crozier et al. 1981 and Howorth and Prasad 1981 . TC Wally bears the distinction of being one of very few cyclones in recorded history to make a complete traverse of Viti Levu island. The track crossed the island directly from north to south over the Easter period of 1980 Fig. 8.6 . A spectacular amount of mass wasting Fig. 8.6. Track of Tropical Cyclone Wally,...

Reef Damage and Recovery

Tropical Cyclone Kina Fiji

Tropical cyclones have a variety of geomorphic impacts on reefs. Delicate corals may be devastated, not just immediately but also in the weeks afterwards from secondary effects. Living corals on the seaward edges of reefs face the full brunt of large waves generated out at sea compared to more sheltered lagoon-side reefs which are exposed to smaller waves with short fetches, agitated by winds over lagoons , and may sustain enormous damage by mechanical destruction. Many coral skeletons are...

Shape and Size

Synoptic Tropical Cyclone

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...

Case Study New Land Created on Funafuti Atoll by Tropical Cyclone Bebe in October

Funafuti Tuvalu Bebe

Funafuti atoll 8.5 S, 179.2 E in the atoll nation of Tuvalu was struck by Tropical Cyclone Bebe on 21 October 1972. TC Bebe approached initially from the east, but the track then followed an unexpected clockwise loop near the southeast coast of the atoll before curving away southwards. This unusual motion led to a high storm surge at Funafuti, and a monstrous storm wave consequently swept across the eastern half of the atoll. The impact on the coastal geomorphology was the construction of an...

Distribution of Origins and Activity

The median position of all tropical cyclone points of origin from 1939 to 1979 in the South Pacific were plotted by Revell and Goulter 1986a , and was determined to be approximately 13.5 S and 170 E. For average years the spatial pattern in the number of storms shows a distinct trend, decreasing from west to east across the southwest Pacific Fig. 4.4 . This is because the primary influence on tropical cyclone incidence to the west of longitude 170 E is ocean-surface temperature, which becomes...

Channel Adjustments

Wainimala Map

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...

Case Study Coastal Erosion on Niue Island During Tropical Cyclone Heta in January

Coastal Erosion Egypt

Tropical Cyclone Heta was the first tropical cyclone to form in the RSMC-Nadi area of responsibility during the 2003-2004 tropical cyclone season Fiji Meteorological Service 2004 . The system was first identified as a tropical depression north of Fiji on 28 December 2003, as it moved northeastwards to reach a position just west of Atafu, the northernmost of the Tokelau atolls. Fig. 7.6. Changes in the geomorphology of a sand spit, formerly connected to the low limestone island of Yanuca in...

Case Study Catastrophic Valley Aggradation on Guadalcanal Island Caused by Tropical Cyclone Namu in

Tropical Cyclone Namu

Guadalcanal island in Solomon Islands is mostly steep and rugged. The southern half of the island is a mountainous zone rising to over 2,300 m with a northwest-to-southeast trending spine. The mountains are flanked on the northern side by foothills that form an intermediate zone of intensely dissected plateaux, hills and rolling ridges Hackman 1980 . Numerous rivers transect this zone, draining generally northwards from the mountains Fig. 10.12 . The northern region of Guadalcanal is known as...

Coastal Erosion Erosional Features

Coastal Erosional Features

Scour and removal of unconsolidated beach materials are commonly observed around the coasts of islands following tropical cyclones. These processes are independent of whether the beaches form shorelines on volcanic, limestone or coral islands. Shorelines in soft sediments may also retreat landwards, with low vertical steps cut as temporary features into the beach. The loss of sandy beaches is a major geomorphic change for a coastline, and one which is considered highly undesirable for South...

Case Study Storm Surge Produced by Tropical Cyclone Gavin in Fiji March

Spiralling Forces

A fine illustration of the effects of storm surge on sea flooding is provided by the example of Vanua Levu island in Fiji during Tropical Cyclone Gavin in early March 1997 Terry and Raj 1999 . TC Gavin, lasting from 3 to 11 March, was the first cyclone to strike the Fiji Islands in the 1997 hot season Fig. 5.23 , and was the severest storm to affect the nation since the early Fig. 5.23. Satellite image of Tropical Cyclone Gavin, 8 a.m. local time on 7 March 1997. Note the well-developed...

Storm Formation and Development Early Stages

South Pacific Convergence Zone

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...

Cloud Patterns

Tropical Cyclones Global Warming

The area of clouds associated with tropical cyclones is much larger than the region influenced by strong winds. This is because the strong winds reflect the area characterised by the steep atmospheric pressure gradient, whereas the Fig. 3.2. Satellite image of Tropical Cyclone Zoe on 27 December 2002, northeast of the main islands of Vanuatu. Note the plumes of high cirrus clouds at the outer storm extremities and the spiralling arms. Base image courtesy of NOAA. Fig. 3.2. Satellite image of...

Case Study Intensity of Tropical Cyclone Ofa February

Tropical Cyclone Ofa in early February 1990 was a devastating event for several South Pacific nations Fig. 5.10 . The storm initially developed over Tuvalu on 27 January as a depression within the South Pacific Convergence Zone. It rapidly intensified to hurricane strength as it moved south southeast near the Samoan islands with sustained winds greater than 118 km h-1. Samoa was severely affected from 3 to 4 February. Winds became very destructive, lasting for almost 24 h, with rain continuous...

Speed of Advance

Cyclone Nancy

Tropical cyclones advance along their track at slow speeds compared to the extremely fast winds described in Chap. 5 revolving around the storm centre. The speed of cyclone movement is usually under 25 km h-1, equivalent to 600 km or less per day. Yet there is great variability in speed, both between separate cyclones and also during the lifespan of an individual system. As a general rule, a cyclone vortex moves more slowly in the early phase of its life, but then gains speed upon reaching...

Storm Decay Major Influences

Cyclone Structure

All tropical cyclones eventually weaken and decay. This is because the maintenance of storm intensity depends on the following essential ingredients 1. Supply of warmth at the sea surface, 2. Supply of moisture at the sea surface, 3. Release of latent heat through condensation in the mid-troposphere, 4. Removal of air aloft by upper-level divergence. If one of these four key ingredients is removed, the storm inevitably dies down. Tropical cyclones may decay over either land or ocean, and in...

The Study Area

The study area for this book is the tropical South Pacific Ocean. This is an enormous expanse of water, stretching across over 20 million square kilometres. Within this area lie thousands of islands belonging to more than 15 developing island nations, states and territories. From west to east these include Solomon Islands, New Caledonia, Vanuatu, Nauru, Kiribati, Tuvalu, Fiji, Wallis and Futuna, Tonga, Tokelau, Samoa, American Samoa, Niue, Cook Islands and French Polynesia, as shown in Fig....

Numbers Timing and Seasonality

Tropical Cyclones Southern Hemisphere

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...

Mature Stage

Cross Section Tropical Cyclone

As the converging air spirals inwards, it becomes organised into bands of cloud that rotate slowly clockwise around the storm centre. The spiral bands of cloud are known as feeder bands because they feed heat and moisture into the central low pressure. This is essential for the continuing development of the cyclone, by providing energy into the storm system. At a certain distance from the centre of the low pressure, typically 20-40 km, the inflowing air suddenly turns upwards in a ring of...

Track Directions

Any map of tropical cyclone tracks covering a number of years, such as those presented in Figs. 4.9, 4.10 and 4.11, appears initially as a jumble of crisscrossing storm paths. Yet closer inspection reveals that underlying the tangle is a major trend in a northwest to southeast orientation. As a result of this trend in track alignment, the north and western flanks of high islands in the South Pacific commonly find themselves on the 'windward' side in relation to tropical cyclones, because they...