Extended seasonal prediction of precipitation in Fiji

A seasonal rainfall forecasting scheme for Fiji is currently operational, taking advantage of strong relationships between the Southern Oscillation Index (SOI) and rainfall in Fiji. In this scheme, the three-month mean of the SOI is used to forecast the immediately following three-month total rainfall (e.g. July-September SOI predicting October-December rainfall). In the present study, exploratory data analysis is undertaken using correlation coefficients to determine whether this forecast lead-time can be increased. Strong correlations are demonstrated out to four months in advance for some periods of the year. Correlation coefficients between equatorial sea-surface temperatures and Fiji rainfall are found to be higher in many periods of the year than those between SOI and rainfall. The hindcast skill of a seasonal prediction scheme was then evaluated for lead-times longer than the currently operational scheme. Strong skill was shown several months in advance for a few periods of the year, for both predictions based on the SOI and on equatorial sea-surface temperatures. This suggests that the current operational scheme could usefully be extended out to longer lead-times. For forecasts of the wet season rainfall as a whole, substantial skill was shown in one region of Fiji for predictions made as much as four months in advance.