Relationships between the Southern Oscillation and Rainfall in the Vanuatu Region of the South West Pacific

Abstract

Fresh water is a key climatic resource for the island nations of the tropical Pacific. The availability of the resource is largely a function of atmospheric circulation that determines spatial and temporal patterns of precipitation. Changes in the Walker Cell circulation and associated Southern Oscillation are a manifestation of atmospheric circulation variability, which is known to influence spatial and temporal rainfall patterns. This research examines relationships between the Southern Oscillation Index (SOI) as a surrogate for atmospheric circulation and rainfall in the Vanuatu region of the tropical South Pacific. Monthly rainfall data for the northern, central and southern regions of Vanuatu are analysed alongside SOI data for the period from 1971 to 2009. As the occurrence of tropical cyclones brings intense and often localised rainfall in certain years, rainfall data influenced by these events are removed from the dataset so as to maintain focus on mean synoptic regional-scale circulation. The results of statistical analysis show strong relationships between annual SOI and annual rainfall anomalies in all three regions, with a three-month lag in the southern region, increasing to a six-month lag in the north. Regarding seasonal rainfall predictions, the results indicate that dry season rainfall can be predicted with modest credibility for the central and southern region several months ahead. However, predictability is much better for wet season rainfall in the northern region several months ahead.