Understanding past record-shattering rainfall events in New Zealand

Abstract Record-shattering rainfall events are intensifying and projected to occur more frequently in a warming climate, primarily via changes in extreme rainfall variability. However, the dominance of internal variability at local scales is recognized as a limitation in quantifying these historical extremes. New Zealand has experienced record-breaking rainfall, including Cyclone Gabrielle, which brought 300–400 mm of rain to Hawke’s Bay over less than 48 h in February 2023. Yet, it remains unclear whether these events are the most severe in New Zealand’s recorded history and whether locally extreme rainfall events are now occurring more frequently. Using daily rainfall data from 307 long-running weather stations (>70 years), we examine long-term trends in extreme rainfall and assess the relative severity of past events across New Zealand. To circumvent known challenges in fitting extreme value distributions to quantify the rarity of past events, we instead convert annual maximum rainfall values at each station to a normalized anomaly ( R N ) which adjusts for both the mean and year-to-year variations in extreme rainfall, thereby enabling a comparison between stations in different rainfall regimes. Our results yield no clear historical trend in the frequency of extreme daily rainfall events nationwide. A high number of events were recorded during the twenty years spanning 1921–1940, potentially reflecting decadal variability. Based on our methodology, the most extreme rainfall event occurred in May 1923, delivering over 600 mm in two days to the South Island’s east coast ( R N > 10). Other record-shattering rainfall events which have since been followed by modest extremes include the February 1971 floods in Taranaki and the January 1984 event in Invercargill, while the King Country and Central Otago regions showed statistically modest rainfall extremes throughout the observational record.