The unprecedented coupled ocean-atmosphere summer heatwave in the New Zealand region 2017/18: drivers, mechanisms and impacts

During austral summer(DJF) 2017/18, the New Zealand region experienced an unprecedented coupled
ocean-atmosphere heatwave, covering an area of 4 million km2. Regional average air temperature
anomalies over land were +2.2 °C, and sea surface temperature anomalies reached +3.7 °C in the
eastern Tasman Sea. This paper discusses the event, including atmospheric and oceanic drivers, the role
of anthropogenic warming, and terrestrial and marine impacts. The heatwave was associated with very
low wind speeds, reducing upper ocean mixing and allowing heat fluxesfrom the atmosphere to the
ocean to cause substantial warming of the stratified surface layers of the Tasman Sea. The event persisted for the entire austral summer resulting in a 3.8 ± 0.6 km3 loss of glacier ice in the Southern Alps (the largest annual loss in records back to 1962), very early Sauvignon Blanc wine-grape maturation in Marlborough, and major species disruption in marine ecosystems. The dominant driver was positive
Southern Annular Mode (SAM) conditions, with a smaller contributionfrom La Niña. The long-term
trend towards positive SAM conditions, a result of stratospheric ozone depletion and greenhouse gas
increase, is thought to have contributed through association with more frequent anticyclonic ‘blocking’
conditions in the New Zealand region and a more poleward average latitude for the Southern Ocean
storm track. The unprecedented heatwave provides a good analoguefor possible mean conditions in the
late 21st century. The best match suggests this extreme summer may be typical of average New Zealand
summer climate for 2081–2100, under the RCP4.5 or RCP6.0 scenario.