New Zealand Climate: The Instrumental Record

Abstract

This thesis describes the variations of the New Zealand climate both spatially and temporally during the period of instrumental record and uses historical data to extend this record. By rigorous procedures of site assessment, errors in the record are determined enabling many of the inhomogeneities to be removed from the data. Removal of the record inhomogeneities and adjustment of the 'signal-to-noise ratio' in the record allows detection of some climatic trends and fluctuations. New Zealand’s climate varies spatially because of the country's geographical position and high axial relief. Regional response areas occur which show homogeneous temperature and rainfall reactions to differing circulation types. By this criterion, New Zealand classifies into six temperature and 21 rainfall response areas using cluster techniques. The synoptic phenomena which cause ‘warm’ and ‘cold’ temperature and ‘wet’ and ‘dry’ rainfall anomalies are identified by correlation of temperature and rainfall time series with time series of circulation indices, principal component analysis and examination of key years and months. Warm temperatures occur when there is higher than normal frequency of anticyclones to the east and blocking anticyclones to the south east concomitant with higher than normal frequency of cyclones in the north Tasman Sea. More cyclones east and south east of New Zealand lead to lower temperature. The high axial relief causes complex patterns of warming and cooling which show great detail in such a small spatial scale. Parallel air flow to the axial range (north east/south west) causes a north to south gradation to occur. Meridional air flow gives large temperature responses east of the main divides while zonal flow usually causes temperature departures west of the main divide. The oceanic locale of New Zealand ensures that any encroaching air mass is moisture-laden. Wet years over many parts of the country relate to periods when there is a higher than normal frequency of air flow from the north east while dry years are associated with stronger westerly and south westerly surface flow situations. The rugged topography and high relief triggers orographic rainfall on windward coasts. The axial ranges spatially determine rainfall in all but two response areas. Zonal flow promotes dry or wet anomalies in the west or east, while meridional flow causes rainfall anomalies in the northern or southern parts of each island. The eight New Zealand rainfall components uncovered by principal component analysis show that rainfall anomalies are strongly localised. Temporal variations in climate are significant. The years 1921 to 1975 show a ½°C warming which occurs about 1950. The last 25 years of this period are accompanied by changes in the ‘centers of action’ of local circulation with a weakening of the west south west airflow over New Zealand accompanied by more frequent winds from the east and northeast. Anticyclones now occur more often in latitudes east of the country and less frequently in the Tasman Sea. Cyclone and tropical cyclone activity in the Tasman Sea shows an increase concomitant with a rise in the Southern Oscillation index. The mountains cause a varying spatial response to the circulation changes. Temperatures record largest increases in response areas sheltered from the increased flow while rainfall decreases occur in the equivalent rainfall response areas. Rainfall increases are noted in response areas exposed to the enhanced north east flow. Since 1853 there has been considerable temperature amelioration. The meteoroloqical record is complimented by proxy data contained in the frequency of severe snowstorms, glacial variation and iceberg irruptions. The strength of the Southern oscillation and frequencies of tropical cyclones in the south west Pacific are closely tied to New Zealand climatic variations. Any change in air flow circulation is detected as a change in the spatial emphasis of rainfall because relief exerts a tight control over the moist air mass behaviour. These results provide a potential for past reconstructions of circulation patterns especially as the boundary conditions on long time scales are most likely intransigent. Climatic variation has had a significant impact on agriculture in New Zealand. 1950 to 1969 can be described as the Green Years as warmer growing seasons and lack of dry growing seasons allowed the greatest growth of farm productivity. In the 1970s agricultural production has slowed when droughtier conditions occurred. The local ocean currents end New Zealand’s axial relief are vital in determining response area climatic anomalies. Finally, significant temporal and spatial changes in climate have been accompanied by movement of those Southern Hemisphere ‘centers of action’ which affect the New Zealand region.