Climate Extremes in the New Zealand Region: Mechanisms, Impacts and Attribution

Abstract

As global surface temperatures have increased with human-induced climate change, notable compound climate extremes in the New Zealand (NZ) region associated with atmospheric heatwaves (AHWs) and marine heatwaves (MHWs) have occurred in the past 6 years. Natural modes of variability that also played a key role regionally include the Interdecadal Pacific Oscillation (IPO), El Niño/Southern Oscillation (ENSO) and changes in the location and strength of the westerlies as seen in the Southern Annular Mode (SAM). Along with mean warming of 0.8°C since 1900, a negative phase of the IPO, La Niña phase of ENSO and a strongly positive SAM contributed to five compound warm extremes in the extended austral summer seasons (NDJFM) of 1934/35, 2017/18, 2018/19, 2021/22 and 2022/23. These are the most intense coupled ocean/atmosphere (MHWs/AHWs) heatwaves on record with average temperature anomalies over land and sea +0.8°C to 1.1°C above 1991–2020 averages. The number of days above 25°C and above the 90th percentile of maximum temperature has increased, while the number of nights below 0°C and below the 10th percentile has decreased. Coastal waters around NZ recently experienced their longest MHW in the satellite era (1982-present) of 289 days through 2023. The estimated recurrence interval reduces from 1 in 300-years for the AHW event during the 1930s climate to a 1 in 25-year event for the most recent decade. Consequences include major loss of ice of almost one-third volume from Southern Alps glaciers from 2017 to 2021 with rapid melt of seasonal snow in all four cases. Above-average temperatures in the December/January grape flowering period resulted in advances in veraison (the onset of ripening); and higher-than-average grape yields in 2022 and 2023 vintages. Marine impacts include widespread sea-sponge bleaching around northern and southern NZ.