Trend analysis on frequency of New Zealand climate extremes

NIWA publishes monthly climate summaries which report on the occurrence of extreme climate and weather events. These summaries provide context to present-day observations with respect to the historical record at stations located throughout New Zealand. In the last decade, NIWA has reported many record-setting high temperature extremes in the climate summaries, with comparatively few record-setting low temperature extremes. In this study, we have generated ranked temperature and rainfall data iteratively from 1951 to 2020 and assessed if the trend of occurrences of these extremes (high & low) in New Zealand is changing. To do this, we calculated an extremeness ratio, which is derived by normalising the observed extremes with an expected probability of an extreme based on no long-term warming. We found that, in the last decade, on average, high monthly mean temperature extremes in New Zealand are increasing 4-5 times larger than expected in a climate with no long term warming. We also examined New Zealand's homogenised seven-station temperature series and found a similar trend for mean temperature extremes. In addition, we calculated the mean temperature extremes taking the climate warming trend (similar to 1 degrees C per century) into account and found that the rate of increase in extremes is faster than the rate of increase in mean temperature. We also find a positive trend in both high and low rainfall extremes with the low rainfall trend most prominent in eastern New Zealand.