RELATIONSHIP OF NEW-ZEALAND DAILY AND MONTHLY WEATHER PATTERNS TO SYNOPTIC WEATHER TYPES

The effectiveness of the classification of daily mean sea-level pressure patterns over New Zealand into 13 synoptic types by Kidson (1994) has been tested through their relationship with daily and monthly variations in temperature, precipitation, sunshine, and daily wind run over the period 1980-1990. The mean anomalies of these weather elements for each synoptic class often showed spatially consistent patterns, with departures falling in the second and fourth (below normal and above normal) quintiles. These were readily interpretable in terms of the effects of advection, differences in static stability, and the influence of mountain ranges on the low-level circulation. On a monthly basis, 60 per cent of the variance in the frequencies of occurrence of the 13 synoptic types was found to be due to three significant EOFs. These formed the basis of an iterative clustering process to group the flow patterns for the 11-year period into eight sets of monthly analogues or 'regimes', for which the composite patterns of the weather element were obtained. Overall, the results suggest that the synoptic classifications are likely to be helpful in specifying daily values of weather elements, and in interpreting the monthly departures of the New Zealand climate. They should also prove useful in validating climate model performance for the New Zealand region and in providing better detail of the regional climate changes likely to result from the greenhouse effect.