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

Abstract

River floods and hydrological droughts (low stream water resources) are a recurrent problem in different parts of Fiji, causing disruption and hardship for many rural communities. These extremes in fluvial behavior are associated 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 Niño/Southern Oscillation (ENSO) in the Southwest Pacific. The Southern Oscillation Index (SOI) 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 of Fiji's main river systems. If ENSO conditions become more frequent or sustained in the future as some climate models suggest, then the SOI 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 Niño/Southern Oscillation (ENSO) is our planet's most powerful climatic phenomenon (Hilton 1998). At intervals of about 5-7 yr, there is a disturbance in the Walker atmospheric circulation over the Pacific Ocean and a weakening of the southeast trade winds. This causes a large pool of warm ocean water usually centered around New Guinea to surge eastward across the equatorial Pacific (Congbin Fu et al. 1986). This disturbance to the Pacific Ocean and atmospheric system, called El Niño, lasts for more than a year and brings droughts, prolonged wet conditions, and a high incidence of storms to different parts of the tropical Pacific. As sea surface temperatures rise off the western coast of the Americas, a tongue of warm water stretches back along the equator. Rainfall becomes abundant in this new low-pressure region, whereas the western Pacific suffers rainfall failure and drought. The strength of ENSO activity is expressed as the Southern Oscillation Index (SOI), which is a measure of monthly atmospheric pressure differences between Tahiti and Darwin (see Ropelewski and Jones 1987, Allan et al. 1991).

ENSO and Stream Behavior

ENSO clearly has the potential to affect stream behavior in a major way through its influence on Pacific-wide climatic patterns. In recent years there has been a concerted focus in hydrological studies to explore possible links between ENSO and various aspects of river flow, including seasonality and extremes (e.g., Moss et al. 1994, Chiew et al. 1998, Waylen and Laporte 1999). Indications have emerged in Costa Rica, eastern Australia, New Zealand, and parts of the Pacific U.S. mainland that valid relationships between stream flow and SOI, or other ENSO measures such as sea surface temperatures, can be established (Kahya and Dracup 1993, Moss et al. 1994, Cayan et al. 1999, Krasovskaia et al. 1999).

For the Fiji Islands in the tropical Southwest Pacific, the most extreme high and low river flows (i.e., floods and droughts [here drought refers to hydrological drought: diminished stream water resources after prolonged no or low rainfall]) are linked to the response of island fluvial systems to ENSO-induced tropical cyclones and periods of prolonged rain failure and are serious hydrological hazards because of their impacts on the physical and human environment. These impacts include channel erosion and siltation, destruction of property, damage to agriculture (threatening food security), and risks to human life and health (R.R., J.P.T., and J. Rokovada, 1998, paper presented to the 27th Annual Conference of the South Pacific Applied Geoscience Commission, Suva, Fiji; Terry and Raj 1999). Such impacts place a difficult socioeconomic burden on our resource-limited island nation: the costs to the Fiji Government of the 1997-1998 drought and 1999 floods amounted to F$100 million and F$40 million, respectively. Improved mitigation and management of hydrological hazards is therefore a priority in Fiji, but this requires a better understanding of the links between these hazards and ENSO than currently exists.

The aim of this study therefore was to examine the relationship...