Trends in New Zealand daily temperature and rainfall extremes

Trends in daily temperature and rainfall indices are described for New Zealand. Two periods were examined: 1951–1998, to describe significant trends in temperature and rainfall parameters; and 1930–1998, to ascertain the effects of two main circulation changes that have occurred in the New Zealand region around 1950 and 1976. Indices examined included frequencies of daily maximum and minimum temperatures, above and below specified percentile levels and at those levels, as well as frequencies of these above and below fixed temperature thresholds. Extreme daily rainfall intensity and frequency above the 95th percentile and the length of consecutive dry day sequences were the rainfall indices selected. There were no significant trends in maximum temperature extremes (‘hot days’) but a significant increase in minimum temperatures was associated with decreases in the frequency of extreme ‘cold nights’ over the 48-year period. There was a non-significant tendency for an increase in the frequency of maximum temperature extremes in the north and northeast of New Zealand. A decline occurred in frequency of the minimum temperature 5th percentile (‘cold nights’) of 10–20 days a year in many locations. Trends in rainfall indices show a zonal pattern of response, with the frequency of 1-day 95th percentile extremes decreasing in the north and east, and increasing in the west over the 1951–1996 period. Changes in the frequency of threshold temperatures above 24.9°C (25°C days) and below 0°C (frost days) are strongly linked to atmospheric circulation changes, coupled with regional warming. From 1930–1950 more south to southwest anomalous flow occurred relative to later years. In this period, 25°C days were less frequent in all areas except the northeast, and there was markedly more frost days in all but inland areas of the South Island compared with the 1951–1975 period. There was more airflow from the east and northeast from 1951 to 1975, the frequency of 25°C days increased and frost days decreased in many areas of New Zealand. In the final period examined (1976–1998), more prevalent airflow from the west and southwest was accompanied by more anticyclonic conditions. Days with a temperature of 25°C increased in the northeast only. Frost day frequencies decreased between 5 and 15 days a year in many localities, with little change in the west of the South Island and at higher elevation locations. Copyright © 2001 Royal Meteorological Society