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During austral summer (DJF) 2017/18, the New Zealand region experienced an unprecedented coupled ocean-atmosphere heatwave, covering an area of 4 million km 2 . 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 fluxes from 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 km 3 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 contribution from 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 analogue for 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.

The end-of-summer snowline (EOSS) on 47 glaciers distributed throughout the Southern Alps of New Zealand is related to changes in Southern Hemisphere atmospheric circulation and sea surface temperature patterns over a 23 year period. The EOSS provides an index of the glacier mass balance, as high (low) EOSS elevations relative to the steady-state mean equilibrium line altitude (ELA m ) indicate a negative (positive) glacier mass balance. Southern Hemisphere atmospheric circulation and sea surface temperature anomaly maps are produced for the accumulation season (April-October) and ablation season (November-March) for both composite high and low EOSS years.There is a high correlation between the EOSS for individual glaciers and the mean EOSS for the Southern Alps (EOSS Alps ). The highest EOSS Alps occurred in 1978, 1990, 1998, and 1999; the lowest EOSS Alps occurred in 1983, 1992 , 1993 EOSS Alps values are associated with anomalous northerly (southerly) airflows and weaker (stronger) westerly airflows over the Southern Alps. These patterns are associated with positive (negative) 700 hPa geopotential anomalies to the southeast of New Zealand, a weaker (stronger) subtropical jet, and negative (positive) 700 hPa height anomalies over the southeast Pacific Ocean. High (low) EOSS Alps values are also associated with warm (cool) sea surface temperature anomalies near New Zealand and cool (warm) sea surface temperature anomalies in the eastern equatorial region of the Pacific Ocean.

Temperature and precipitation trends are described for newly homogenized historical climate data sets for the South-west Pacific. Regions that exhibit similar temperature and precipitation trends and variability are defined, and the temperature and precipitation time series aggregated according to these regions. Four temperature regions show distinctive trends: two regions south-west of the South Pacific Convergence Zone (SPCZ), which dispiay steady climate warming; two regions north-east of the SPCZ, which cooled during the 1970s, and warmed in the 1980s. Annual anomalies differ in response to the El Nifio-Southern Oscillation (ENSO) phenomena, depending on the region's position with respect to a pivotal line along the SPCZ. The climate warming apparent throughout much of the south-west Pacific comes from sites where there can be no question of any urban influence. Five main South-west Pacific precipitation regions show distinctive trends that are connected to the main climatological features. Four New Zealand precipitation subregions relate to the interaction of the main climatological features with local orography. Annual precipitation anomalies show marked variability and are also affected by ENSO in most regions. The pivotal line for the response of precipitation regions lies just to the north-east of the SPCZ. The ENSO relationships with precipitation appear consistent on both annual and interdecadal time-scales. From these climatic trends four climatic response regions are recognized in the South-west Pacific.

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