Fluctuations and trends in indices of the southern hemispheric circulation

Indices of the southern hemisphere circulation are derived for the Australasian area using sea level pressure data. As well as considering indices of zonal and meridional flow, indices are derived based on the empirical orthogonal function analysis of Trenberth (1975). The first two patterns, P1 and P2, of this analysis can be approximately represented by only a few station values, and in this way their time series can be extended. ‘Regimes’ of weather are defined based on the short term mean behaviour of P1 and P2. These regimes are also characterized by large changes in the periodicities at which most of the variance occurs. The quasibiennial oscillation shows up as a fairly stable feature of the meridional indices and P2. Trends in both P1 and P2 since the 1940s are related to the rising temperature trend in New Zealand. There has been a trend for less westerly especially between 25 and 45°S, some increase in westerly south of 45°S and less southwesterly or more northeasterly component to the flow across the whole of New Zealand as a result of the increased tendency for anticyclones to persist to the east of New Zealand rather than over Australia and the Tasman Sea, and their preference for higher latitude blocking.