The Interdecadal pacific oscillation and the Southern Oscillation Index: relative merits for anticipating inflows to the Upper Clutha lakes.

Abstract

Spring values of the Southern Oscillation Index (SOI) and the unfiltered Interdecadal Pacific Oscillation (IPO) are compared for anticipating summer inflows to the lakes of the upper Clutha River, New Zealand. Earlier work proposed the spring SOI to forecast summer inflows. However, more recent data, which includes a change of state of the IPO, indicate the spring SOI only has a degree of predictive ability for the low summer flows. Furthermore, this low-flow predictability requires La Niña conditions (spring SOI>5). The spring IPO index, in contrast, can anticipate both high and low summer inflows when the spring SOI >0, and thus has a wider range of applicability for summer inflow forecasting than the SOI. Both the IPO and SOI appear to have no summer inflow forecasting ability when the spring SOI is negative.