AN ANALYSIS OF AUSTRALIAN SEASONAL RAINFALL ANOMALIES - 1950-1987 .1. SPATIAL PATTERNS

A detailed analysis of the variability of Australian district rainfall on seasonal time-scales over the period 1950-1987 is described. This paper, Part I, describes the dominant spatial modes or patterns of variability. The major analysis tool is rotated principal component analysis (RPCA), used in both the S and T modes. Various criteria are examined to determine the number of components to rotate, including conducting trial rotations and comparison of the resulting patterns with the corresponding one-point correlation maps. The stability of the chosen solutions is examined by repeating the analysis on various subsets of the data. The S-mode analysis, which groups districts with similar temporal variation, provides a regionalization of the continent into eight coherent and approximately equally sized regions. The results of this analysis closely resemble those obtained from cluster analysis. The T-mode analysis clusters seasons with similar large-scale spatial variations (anomaly patterns). The similarity measure used in the T-mode analysis is the congruence coefficient, rather than the correlation or covariance. The patterns produced by this analysis consist of continental-scale anomalies, similar in some respects to the unrotated S-mode patterns, but more amenable to meteorological interpretation. In particular the first pattern, which accounts for approximately 25 per cent of the total variance consists of anomalies of the same sign over the entire continent centred on south-east Australia. The relationship between the two modes of representation is also explored. Regression equations are developed to express the spatially complex T-mode patterns in terms of the localized S-mode patterns, and alternatively, to partition the variance of each of the S-mode patterns between the T-mode components. In Part II the temporal variability and the relationship of these patterns to the Southern Oscillation and other large-scale circulation anomalies are examined.