[1] The South Pacific Convergence Zone (SPCZ), one of the most extensive features of the global atmospheric circulation, is shown to vary its location according to both the polarity of the El Niño/Southern Oscillation (ENSO), and of the Interdecadal Pacific Oscillation (IPO). We first demonstrate that the IPO can be regarded as the quasi-symmetric Pacific-wide manifestation of the Pacific Decadal Oscillation that has been described for the North Pacific. Shifts in the position of the SPCZ related to ENSO on interannual time scales and to the IPO on decadal time scales appear to be of similar magnitude and are largely linearly independent. A station pressure-based index of variations in SPCZ latitude is shown to be significantly related to the polarity of the IPO when ENSO influences are accounted for. Movements of this sensitive section of the SPCZ have occurred in phase with those of the IPO since the 1890s.

The Interdecadal Pacific Oscillation (IPO) has been shown to be associated with decadal climate variability over parts of the Pacific Basin, and to modulate interannual El Niñ o -Southern Oscillation (ENSO)-related climate variability over Australia. Three phases of the IPO have been identified during the 20th century: a positive phase , a negative phase and another positive phase . Climate data are analysed for the two most recent periods to describe the influence of the IPO on decadal climate trends and interannual modulation of ENSO teleconnections throughout the South West Pacific region (from the equator to 55°S, and 150°E to 140°W). Data coverage was insufficient to include the earliest period in the analysis.Mean sea level pressure (SLP) in the region west of 170°W increased for the most recent positive IPO period, compared with the previous negative phase. SLP decreased to the east of 170°W, with generally more southerly quarter geostrophic flow over the region. Annual surface temperature increased significantly southwest of the South Pacific Convergence Zone (SPCZ) at a rate similar to the average Southern Hemisphere warming. Northwest of the SPCZ temperature increases were less, and northeast of the SPCZ more than the hemispheric warming in surface temperature. Increases of annual precipitation of 30% or more occurred northeast of the SPCZ, with smaller decreases to the southwest, associated with a movement in the mean location of the SPCZ northeastwards. The IPO modulates teleconnections with ENSO in a complex way, strengthening relationships in some areas and weakening them in others. For New Zealand, there is a consistent bias towards stronger teleconnections for the positive IPO period.These results demonstrate that the IPO is a significant source of climate variation on decadal time scales throughout the South West Pacific region, on a background which includes global mean surface temperature increases. The IPO also modulates interannual ENSO climate variability over the region.

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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.

Previous research has highlighted the existence of significant linear correlations of the Southern Oscillation Index (SOI) with mean sea-level pressure in the Australasian region and with patterns of rainfall and temperature anomalies in New Zealand. The issue of the Southern Oscillation influence on New Zealand climate is revisited here, with emphasis on whether climate anomalies during the El Niiio and La Niiia extremes are indeed equal and opposite, as assumed in any linear analysis. The consistency of SOI-climate relationships over time is also assessed by comparing analyses before and after 1950.A linear regression analysis of seasonally stratified rainfall and temperature data from a number of New Zealand sites is carried out, but in addition a test for a simple form of non-linearity (namely bilinearity) is applied. In a separate but complementary analysis, which does not impose a linear structure on the anomalies, climate data are composited for periods representing extremes of the Southern Oscillation. Both analyses show that although it is often reasonable to assume a reversal between El Niiio and La Niiia, non-linearities are also evident and particularly so in southern parts of New Zealand in the winter and spring seasons. At all the central and southern South Island sites analysed, winter temperatures were colder than average at both extremes of the Southern Oscillation.In the earlier record prior to about 1950, the pattern of Sol-related climate anomalies also appears substantially different to more recent teleconnection patterns.