New Zealand Climate: I. Precipitation Patterns

New Zealand is a country with high relief that protrudes into the southern westerly circulation. Precipitation patterns are identified by two different methods: correlation of precipitation time series at stations with the time series of circulation indices and principal component analysis. Components were rotated to give more definite patterns. Examination of synoptic events and surface pressure anomalies in key months allowed the identification of the synoptic cause of each component pattern. The link between the components and circulation indices was investigated. The first method shows that precipitation is anomalously high on the windward side of the country to any airflow, and low on the leeward side. Component analysis revealed eight distinct patterns. All am related to synoptic events and surface pressure anomalies where humid air advects over a windward coast. Confirmation is provided by the patterns of correlations of precipitation associated with the circulation indices. These bear a close resemblance to component patterns. The results show that precipitation anomalies are strongly localized. suggesting that the axial ranges of both islands play a dominant role in determining precipitation patterns. This result may usefully he exploited for reconstruction of past synoptic patterns.