Links between the Southern Oscillation Index and Hydrological Hazards on a Tropical Pacific Island1

James P. Terry, 2 Rishi Raj, 3 and Ray A. Kostaschuk 4

Abstract: River floods and hydrological droughts (low stream water resources) are a recurrent problem in different parts of Fiji, causing disruption and hard­ ship for many rural communities. These extremes in fluvial behavior are asso­ ciated with large seasonal variability in rainfall, generated by intense tropical storms in the wet season and prolonged rain failure in the dry season. Such conditions are linked to the influence of El Nino/Southern Oscillation (ENSO) in the Southwest Pacific. The Southern Oscillation Index (SOl) is the climatic measure of the strength of ENSO activities and shows good correspondence with (1) tropical cyclone flood magnitude and (2) critically low stream discharges after a 2-month time lag, in two ofFiji's main river systems. IfENSO conditions become more frequent or sustained in the future as some climate models sug­ gest, then the SOl will be a useful tool for projecting in advance the severity of hydrological hazards, which can assist in disaster mitigation and management.

ON THE INTERANNUAL timescale, the El and a high incidence of storms to different Nino/Southern Oscillation (ENSO) is our parts of the tropical Pacific. As sea surface planet's most powerful climatic phenomenon temperatures rise off the western coast of the (Hilton 1998). At intervals of about 5-7 yr, Americas, a tongue of warm water stretches there is a disturbance in the Walker atmo­ backalong the equator. Rainfall becomes abun­ spheric circulation over the Pacific Ocean and dant in this new low-pressure region, whereas a weakening of the southeast trade winds. the western Pacific suffers rainfall failure and This causes a large pool ofwarm ocean water drought. The strength of ENSO activity is usually centered around New Guinea to surge expressed as the Southern Oscillation Index eastward across the equatorial Pacific (SOl), which is a measure of monthly atmo­ (Congbin Fu et al. 1986). This disturbance spheric pressure differences between Tahiti to the Pacific Ocean and atmospheric system, and Darwin (see Ropelewski and Jones 1987, called El Nino, lasts for more than a year and Allan et al. 1991). brings droughts, prolonged wet conditions, ENSO and Stream Behavior

1 Financial support provided by the University of the ENSO clearly has the potential to affect South Pacific and the Natural Sciences and Engineering stream behavior in a major way through its Research Council of Canada. Manuscript accepted 13 October 2000. influence on Pacific-wide climatic patterns. In 2 Department of Geography, University of the recent years there has been a concerted focus South Pacific, Suva, Fiji (fax: +679 301 487; E-mail: in hydrological studies to explore possible [email protected]). links between ENSO and various aspects of 3 Hydrology Section, Public Works Department, P.O. Box 3740, Samabula, Fiji (E-mail: [email protected]). river flow, including seasonality and extremes 4 Department of Geography, University of Guelph, (e.g., Moss et al. 1994, Chiew et al. 1998, Ontario, N1G 2W1, Canada (E-mail: rkostasc@ Waylen and Laporte 1999). Indications have uoguelph.ca). emerged in Costa Rica, eastern Australia, New Zealand, and parts of the Pacific U.S. mainland that valid relationships between Pacific Science (2001), vol. 55, no. 3:275-283 © 2001 by University of Hawai'i Press stream flow and SOl, or other ENSO mea­ All rights reserved sures such as sea surface temperatures, can be

275 276 PACIFIC SCIENCE· July 2001

established (Kahya and Dracup 1993, Moss faces the predominant trade winds and there­ et al. 1994, Cayan et al. 1999, Krasovskaia fore receives much more precipitation than et al. 1999). the northwest side, which is rain-shadowed For the Fiji Islands in the tropical South­ by interior highlands reaching elevations over west Pacific, the most extreme high and low 1300 m. river flows (i.e., floods and droughts [here The climate is distinctly seasonal. The drought refers to hydrological drought: di­ typical wet season extends from November to minished stream water resources after pro­ April and the dry season from May to Octo­ longed no or low rainfall]) are linked to the ber. Tropical cyclones are a common feature response of island fluvial systems to ENSO­ of the wet months. From 1970 to 2000, 40 induced tropical cyclones and periods of cyclones traversed Fiji island waters. These prolonged rain failure and are serious hy­ severe storms often produce extreme rain­ drological hazards because oftheir impacts on falls, especially over the interior of Viti Levu the physical and human environment. These because the volcanic mountains force oro­ impacts include channel erosion and siltation, graphic lifting of the spiraling rain bands. destruction of property, damage to agricul­ Cyclone Gavin in March 1997, for example, ture (threatening food security), and risks to produced a deluge. The highest-elevation human life and health (R.R., J.P.T., and J. weather station at 760 m received a maximum Rokovada, 1998, paper presented to the 27th 24-hr rainfall of 610 mm, at lO-min in­ Annual Conference of the South Pacific tensities reaching 150 mm!hr (Terry and Applied Geoscience Commission, Suva, Fiji; Raj 1999). Widespread soil saturation and Terry and Raj 1999). Such impacts place rapid surface and subsurface runoff cause a a difficult socioeconomic burden on our high degree of hydrological short-circuiting. resource-limited island nation: the costs to Rivers respond with rapid and very large the Fiji Government of the 1997-1998 increases in discharge, and dangerous over­ drought and 1999 floods amounted to F$100 bank floods are the result. million and F$40 million, respectively. Im­ In the dry season, an uneven distribution proved mitigation and management of hy­ of rain days and prolonged lack of moisture drological hazards is therefore a priority in can occur. Rainfall seasonality is more pro­ Fiji, but this requires a better understanding nounced for the island's leeward northwest ofthe links between these hazards and ENSO region, which receives only 20% of the an­ than currently exists. nual total in the dry months, compared with The aim of this study therefore was to ex­ 33% for the windward side (Terry and Raj amine the relationship between the strength 1998). Rivers in the northwest may experi­ ofthe ENSO signal measured by the SOl and ence very low stream baseflows as a result, the magnitude of extreme high and low dis­ which represents a severe depletion of avail­ charges in rivers on the main island of Fiji. able water resources because the rural popu­ The potential use of the SOl as a predictor lation depends mainly on surface water for for hydrological hazards in the future was domestic needs and watering livestock (Terry then considered. and Raj in press). Human suffering and eco­ nomic hardship are the result. Fiji Climate and ENSO Influences In years without strong ENSO activity, Fiji's rainfall benefits from convection along Fiji's main island Viti Levu (Figure 1) is a the low-pressure South Pacific Convergence mountainous volcanic island 17.5° south of Zone (SPCZ) that extends diagonally from the equator in the Southwest Pacific. The near the Solomon Islands, across to Samoa, tropical latitude and influence of the nearby the Cook Islands, and beyond (Salinger et al. warm Southern Equatorial Ocean Current 1995). At the start of El Niiio events, con­ gives Viti Levu a wet/dry tropical climate. A vective storms and tropical cyclones affecting clear geographical distribution in rainfall is the islands are generated as the eastward­ observed. The southeast side of the island migrating pool of warm sea surface tem- The Southern Oscillation Index and Hydrological Hazards . Terry et at. 277

Fiji Islands (I) Gauging station rs--.L-----''iil

o 10 20 Scale (km)

180 0

FIGURE 1. Location of the Rewa and Ba Rivers on Viti Levu island, Fiji.

peratures passes across northern Fiji waters. Both produced damaging floods in many Then, as El Nino conditions develop fully, an places (Terry and Raj 1999). The floods were equatorward shift in the SPCZ (Hay et al. then followed by one ofthe worst droughts to 1993, Vincent 1994) away from the Fiji group affiict Fiji this century (Fiji Meteorological leads to prolonged dry conditions. The ex­ Service 1998). Rain failed across successive ceptional El Nino of 1997-1998 was a good dry seasons and the intervening wet season example of the variability in climatic and hy­ (when precipitation is normally reliable), and drological impacts in Fiji. The event began river flows reached record lows (Public with a succession of severe tropical cyclones: Works Department, Hydrology Section, Gavin in March 1997 and June in May 1997. unpubl. river flow records). 278 PACIFIC SCIENCE· July 2001

5000

4000 Cyclone Gavin Cyclone June - ~ 3000 -o ~ 2000 u.

1000

Jan. Feb. Mar. Apr. May Jun. Jul.

FIGURE 2. Hydrograph for the Rewa River at Navolau during tropical cyclones Gavin and]une in 1997.

MATERIALS AND METHODS slopes. It also has a recorded history of major floods caused by tropical cyclones: flooding In this study, correlation analysis was used to has occurred 15 times during tropical cy­ examine the relationship between SOl and clones between 1970 and 2000 (i.e., one flood extreme river behavior on Viti Levu. SOl every 2 yr on average), and the fluvial system data were obtained from the Fiji Meteoro­ can be described as a flood-dominated re­ logical Service. The Rewa and Ba Rivers gime. The river hydrograph in Figure 2 (Figure 1) were selected for investigation be­ illustrates the characteristics of the high­ cause of the availability of reliable long-term magnitude flood peaks produced by tropical flow records from hydrological gauging sta­ cyclones Gavin and June at the start of the tions operated by the Hydrology Section of 1997-1998 El Nino. the Public Works Department, and because In earlier work we (Kostaschuk et al. in these are the main rivers on the wet (wind­ press) found that tropical cyclones occur ward) and dry (leeward) sides of the island. more often in Fiji waters when the SOl is Topography is steep and rugged in the upper negative, due to the influence ofthe Southern parts of both study basins, which rise to Oscillation on the usual spatial distribution of 1360 m at the Mount Victoria divide, but cyclone origin, with more storms spawned becomes more hilly nearer the coast. farther to the north and east (closer to Fiji) The Rewa River in the wet zone has the during El Nmos (Revell and Goulter 1986, 2 largest drainage basin, covering 2900 km • Basher and Zheng 1995). However, cyclone Vegetation is mostly dense rain forest, with frequency in Fiji waters did not show a good subsistence cultivation of root crops and link to temporal patterns in the SOL Here, cattle pasturing on floodplains and adjacent therefore, we compare flood peak magnitude The Southern Oscillation Index and Hydrological Hazards . Terry et at. 279

8000 ...,....------,

-.!!? Qp = 3295.5e-O.1543S01 • • E 6000 R2 =0.33 -Q.

"~ 4000 ~ CO CI) Q. -g 2000 • o • • u::

O-t----r------r----r----,....----r------l -4 -3 -2 -1 o 1 2 SOl

FIGURE 3. Peak flood discharge (Q) in the Rewa River at Navolau during tropical cyclones versus SOl for the month that the cyclone occurred.

(rather than timing) produced by tropical curve has the best fit to the data (r = -0.57, cyclones in the Rewa River with the SOl P> 0.05, Table 1). For the analysis of low (Figure 3). flows in the Ba River, difficulties arise when The Ba River in the north of Viti Levu is comparing SOl and monthly means of dis­ Fiji's third largest river system, covering 930 charge because of the large influence of sea­ 2 km • It is the most important river in the dry sonality on river behavior. Thus, poor cor­ zone, but experiences very low baseflows in relations were derived between SOl versus periods of drought. Savanna grassland vege­ monthly Q(r = 0.17), prior 3-month mean of tates the steep highland terrain, whereas the SOl versus monthly Q (r = 0.21), SOl versus more gentle coastal slopes are widely used for Q for the month with the lowest flow each growing sugarcane. Figure 4 compares tem­ year (r = 0.18), and lowest monthly SOl in poral patterns in the SOl with mean dis­ year versus lowest monthly Q in year charge (Q) in the Ba River. (r = 0.23). However, once flow seasonality is removed by plotting 13-month running means, low flow trends in the Ba River show RESULTS good correspondence with £1 Nino events, For the Rewa River, Figure 3 indicates that although lagging behind by approximately 2 peak river discharge produced by tropical cy­ months (r = 0.73, P> 0.001, Figure 5). clones is negatively related to the strength of the SOl (i.e., tropical cyclones that occur during £1 Nino events are more likely to DISCUSSION produce larger floods). A curve-fitting exer­ Despite the fact that it remains difficult to cise showed that an exponential regression predict the occurrence of tropical cyclones 280 PACIFIC SCIENCE· July 2001

4.,.------r- SOl monthly values -501 5-month running means -Q monthly values 2 -Q 5-month running means -Q 13-month running means

0 o(J)

-2

-4 100

+--...... -,-----,-.,.-"""T""--,.-,...---.---r-,.-...... -,-----,r--.,.-"""T""--,.-,...---r---r-f- 1

FIGURE 4. A comparison of the temporal patterns in SOl and Ba River flow (Q) behavior at Toge, 1980-2000.

TABLE 1 SOl Relationship with Peak Flood Discharge in the Rewa River

Regression Curve Type Equation Correlation

Linear Q = -531.94S01 + 3529.70 r= -0.49 2 Polynomial Q = -129.33S01 - 670.03S01 + 3719.7 r = -0.54 Exponential Q = 3295.50e-O.154SOI r = -0.57

near the Fiji Islands (as it does elsewhere), our vulnerable sections of the rural community work suggests that the strength of the SOl at who live on low-lying areas near floodplains the time of future storms can give some in advance ofthe expected flood wave. This is warning of the size of the flood in the Rewa important because in the future floods may River, which has the largest and most popu­ become more severe due to increasing tropi­ lated river basin in Fiji. Because tropical cy­ cal cyclone intensity in the Pacific region if clones rarely develop very close to Fiji, but the climate changes toward more sustained instead tend to generate nearer the equator El Niiio-like conditions, as many climate and then approach on a southerly track over 2 scientists now predict (see Holland 1997, or 3 days, this delay will be useful in alerting Trenberth and Hoar 1997, Timmermann The Southern Oscillation Index and Hydrological Hazards . Terry et at. 281

40 -r------,

35 ..• • ••t.••• 30 • • •.. • • •• • 25 • •• ~ ~...... ~.... ." . .. ~. E 20 Q = 6.19801 + 22.94 '...... :. ... U~ o R2 = 0.53 .. • ."'IIf.... 15 • •"-/~.r . • • ... 10 .- .. • ••.••••...• •• 5 ••

0+-----,------r------r----.------r------1 -4 -3 -2 -1 o 1 2 SOl

FIGURE 5. Correlation between 13-month running means of Ba River flow (Q) and 5-month running means of SOl with a 2-month lag.

et al. 1999; P. Whetton, R. Jones, K. Hen­ supply (e.g., carting by truck) before resource nessy, R. Suppiah, K. Walsh, and W. Cai, scarcity becomes critical. Elsewhere in Fiji's 2000, paper presented to the Pacific Islands dry zone, there may be different lag times Climate Change Conference, Rarotonga, between low sal and low river baseflow. Cook Islands). In this scenario, projections of This needs to be investigated so that pro­ the likely flood hazard magnitude shall be­ jected stream water resources in times of rain come mbre crucial for the success of disaster failure can be assessed on a catchment-by­ reduction programs in Fiji. catchment basis. For future droughts that are related to the ENSO phenomenon (rather than random dry ACKNOWLEDGMENTS years), forecasts of the depletion of surface water resources in the Ba River can be made The Fiji Meteorological Service is thanked with a high degree of confidence 2 months for providing the sal record. This paper was ahead on the basis of sal values. The Ba originally presented to the Pacific Islands River has the largest catchment in the Conference on Climate Change, Climate drought-prone northwest of Viti Levu and Variability, and Sea-Level Rise, organized by supports a substantial population ofsugarcane the South Pacific Regional Environmental farmers. Such forecasts will be valuable for Programme, 17-21 April 2000, Rarotonga, government planning of alternative water Cook Islands. The Department for Inter- 282 PACIFIC SCIENCE· July 2001 national Development of the British Govern­ Pacific. School of Social and Economic ment provided generous funding through the Development, University of the South University of the South Pacific for J.P.T. to Pacific, Suva, Fiji. attend this meeting. The written paper was Holland, G. J. 1997. The maximum potential prepared while J.P.T. visited the University intensity of tropical cyclones. J. Atmos. of Sydney School of Geosciences and thanks Sci. 54:2519-2541. are extended to several colleagues there for Kahya, E., and J. A. Dracup. 1993. U.S. their constructive comments on the work. streamflow patterns in relation to the El Two referees also provided useful suggestions Nino/Southern Oscillation. Water Resour. on the original manuscript. Res. 29:2491-2503. Kostaschuk, R, J. Terry, and R Raj. The Literature Cited impact of tropical cyclones on river floods in Fiji. Hydrol. Sci. J. (in press). Allan, R]., N. Nicholls, P. D. Jones, and I. J. Krasovskaia, I., L. Gottschalk, A. Rodridguez, Butterworth. 1991. A further extension and S. Laporte. 1999. Dependence of the of the Tahiti-Darwin SOl, early ENSO frequency and magnitude of extreme events and Darwin pressure. J. Clim. floods in Costa Rica on the Southern 4:743-749. Oscillation Index. Pages 81-89 in L. Basher, R E., and X. Zheng. 1995. Tropical Gottschalk,J.-C. Olivry, and D. 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Jones. 1987. An Congbin Fu, H. F. Diaz, and J. O. Fletcher. extension of the Tahiti-Darwin Southern 1986. Characteristics of the response of Oscillation Index. Mon. Weather Rev. the sea surface temperature in the central 115:2161-2165. Pacific associated with warm episodes of Salinger, M. J., R E. Basher, B. B. Fitzharris, the Southern Oscillation. Mon. Weather J. E. Hay, P. D.Jones,J. P. MacVeigh, and Rev. 114:1716-1738. I. Schmidely-Leleu. 1995. Climate trends Fiji Meteorological Service. 1998. Annual in the south-west Pacific. Int. J. Climatol. weather summary. Climatological Divi­ 15:285-302. sion, Nadi Airport, Fiji. Terry, J. P., and R Raj. 1998. Hydrological Hay, ]., J. Salinger, B. Fitzharris, and R drought in western Fiji and the contribu­ Basher. 1993. Climatological seesaws in the tion of tropical cyclones. Pages 73-85 in Southwest Pacific. Weather Clim. 13 :9-21. J. P. Terry, ed. Climate and environmental Hilton, A. C. 1998. The influence of El change in the Pacific. 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