On the linearity and stability of Southern Oscillation‐climate relationships for New Zealand

Abstract Previous research has highlighted the existence of significant linear correlations of the Southern Oscillation Index (SOI) with mean sea‐level pressure in the Australasian region and with patterns of rainfall and temperature anomalies in New Zealand. The issue of the Southern Oscillation influence on New Zealand climate is revisited here, with emphasis on whether climate anomalies during the El Niño and La Niña extremes are indeed equal and opposite, as assumed in any linear analysis. The consistency of SOI‐climate relationships over time is also assessed by comparing analyses before and after 1950. A linear regression analysis of seasonally stratified rainfall and temperature data from a number of New Zealand sites is carried out, but in addition a test for a simple form of non‐linearity (namely bilinearity) is applied. In a separate but complementary analysis, which does not impose a linear structure on the anomalies, climate data are composited for periods representing extremes of the Southern Oscillation. Both analyses show that although it is often reasonable to assume a reversal between El Niño and La Niña, non‐linearities are also evident and particularly so in southern parts of New Zealand in the winter and spring seasons. At all the central and southern South Island sites analysed, winter temperatures were colder than average at both extremes of the Southern Oscillation. In the earlier record prior to about 1950, the pattern of SOI‐related climate anomalies also appears substantially different to more recent teleconnection patterns.