Moving to a new normal: Analysis of shifting climate normals in New Zealand

• Published in: International journal of climatology Vol. 44; no. 10; pp. 3240 - 3263
• Main Authors: Srinivasan, Raghav, Carey‐Smith, Trevor, Wang, Linda, Harper, Andrew, Dean, Sam, Macara, Gregor, Wang, Ruotong, Stuart, Stephen
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01.08.2024; Wiley Subscription Services, Inc
• Subjects: Anthropogenic climate changes; Anthropogenic factors; Atmospheric research; Automation; Climate; climate anomalies; Climate change; climate normals; Climate variability; Climatic analysis; Climatic extremes; Climatological normals; data homogenisation; Extreme values; Global warming; Historic temperatures; Homogeneity; Homogenization; Human influences; New Zealand climate; Precipitation; Precipitation patterns; Precipitation-temperature relationships; Regional analysis; Regional development; Temperature; Variability; New Zealand
• ISSN: 0899-8418; 1097-0088
• DOI: 10.1002/joc.8521

New Zealand's National Institute of Water and Atmospheric Research publishes climate normals for New Zealand that are used for reporting the regional state of the climate, climate extremes and variability. Temperature and precipitation patterns are affected by both anthropogenic climate change and natural climate variability which in turn affect the climatological normal values calculated every decade. This study investigates how New Zealand's normals for temperature and precipitation have shifted over time at the national, regional and seasonal scales from the 1941–1970 period to the 1991–2020 period. Contrary to WMO recommendations, but aligned with many other countries, New Zealand's climate normals have traditionally not undergone homogenisation. The impact of introducing some homogenisation in the latest 1991–2020 normals, in contrast to historical non‐homogenized station normals, has been assessed using a new homogenized “Seventeen‐Station” temperature series from 1941 to 2020. We demonstrate that interpolating sparse non‐homogenized normals spatially to a grid can produce significant erroneous patterns and therefore undermine the accuracy of the conclusions drawn when using such normals. We find that the historical non‐homogenized temperature normals have a consistent negative bias in the long‐term trend at national, regional and seasonal scales. Our analysis of homogeneity tested precipitation showed consistent decreases at a national scale across all normal periods relative to the 1951–1980 precipitation normal. We also highlight how fixed period temperature and precipitation normals do not fully reflect the current state of a climate that is influenced by decadal variability and global warming. To derive normals fit for use in a changing climate it is suggested that automated methods for broad data homogenisation be developed along with alternative methods to derive normals that account for a non‐stationary climate. Climatological normals for minimum, maximum mean temperatures, and total precipitation in New Zealand for 1991–2020 were calculated and compared against the historical normals. Newly derived homogenized set used as ground truth. Importance of homogenisation highlighted. Alternative climate normals needs to be considered to reflect physical processes and climate change.