NIWA's Historic Weather Events website
The website Ian Wishart has based his opinions on is not New Zealand’s national climate database.
Wishart has conflated the Historic Weather Events website with the New Zealand National Climate Database that is used by NIWA and climate scientists nationally and internationally to analyse weather events and trends in the climate. NIWA’s New Zealand National Climate Database includes official observations back to the 1850s, including some descriptions of significant weather events of the time.
The Historic Weather Events website that Wishart has based his study on is a catalogue that was created to collate descriptive details on impacts and hazards from weather events. It is not an official type of meteorological record. It has gaps because it contains information gathered from newspaper articles, journals, books and reports that periodically come to light. Gathering these sources and validating them takes significant time and is an ongoing process.
The New Zealand National Climate Database is a national electronic database of high-quality instrumental observations, including temperature, rainfall, wind, pressure and much more. NIWA holds data from approximately 6500 climate stations across the country, surrounding islands, some Pacific Islands and Antarctica. NIWA also retains paper and digital copies of original historical observations which are classified as a heritage asset.
NIWA holds 10-minute, hourly and daily observations from more than 600 stations currently operated by NIWA, MetService and other organisations. These data are freely available via CliFlo (https://cliflo.niwa.co.nz/), along with about 80 types of monthly and annual statistical summaries, and several 30-year climate normals. This resource has been freely available to the public and researchers externally since 2007 and is frequently accessed.
NIWA’s scientists use the database to rank and quantify how unusual observations are, and report on it. Summaries using these data are published monthly, seasonally and annually at https://niwa.co.nz/climate/summaries.
NIWA released their monthly climate summary with extensive analysis and discussion on Cyclone Gabrielle: https://niwa.co.nz/climate/monthly/climate-summary-for-february-2023. To produce this analysis, NIWA used data from the National Climate Database and our archived material as described above, as well as other tools from international organisations to provide some historical context. The comparisons to contemporary storms for which we have high-quality observations were done using standard practices.
NIWA did not claim that Gabrielle was the strongest ever storm to pass near the North Island, only one of the strongest storms. Similarly, NIWA did not say it was caused by climate change, only that climate change was likely to be a factor in its severity.
Examining the significance of a weather event like Gabrielle extends well beyond a single meteorological metric. Atmospheric pressure, which Wishart focused on, is only one way of measuring a storm’s intensity and wind strength. NIWA typically examines several factors to describe a cyclone’s intensity in its climate summary, including rain, wind and sea-related impacts.