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Summer 2017-18 Climate Summary: NZ’s hottest recorded summer

New Zealand Climate Summary: Summer 2017-18 Issued: 5 March 2018

New Zealand’s hottest summer on record

TemperatureHottest summer on record. The nation-wide average temperature for summer 2017-18 was 18.8°C (2.1°C above the 1981-2010 from NIWA’s seven station temperature series which began in 1909). Summer temperatures were well above average (>1.20°C above the summer average) across all regions.
RainfallHighly variable from month to month and heavily impacted by two ex-tropical cyclones during February. Summer rainfall in the South Island was above normal (120-149%) or well above normal (>149%) over Canterbury, Marlborough, Nelson, and Tasman, and near normal (80-119%) to below normal (50-79%) around Otago, Southland, and the West Coast. North Island summer rainfall was above or well-above normal around Wellington and much of the upper North Island, and near normal or below normal over remaining North Island locations including Taranaki, Manawatu-Wanganui, Hawke’s Bay, and Gisborne.
Soil moistureAs of 28 February, soils were wetter than normal for the time of year across the upper North Island and the central and upper South Island. Soil moisture was near normal elsewhere, although parts of Hawke’s Bay, Gisborne, and Southland had slightly below normal soil moisture.

Overview

Summer 2017-18 was New Zealand’s hottest summer on record. Overall, the season was characterised by mean sea level pressures that were higher than normal to the east and southeast of New Zealand, and lower than normal over and to the west of the country. This pressure pattern delivered more frequent warm northerly and northeasterly winds than normal, consistent with La Niña conditions. Additionally, the persistence of high pressure over the Tasman Sea (another feature of La Niña) during November and early December had prevented the mixing of cool, deeper ocean water with the surface resulting in anomalously warm sea surface temperatures (SSTs) in the region. This combination of high SSTs and an increased frequency of northerly winds delivered unprecedented warmth to the country during summer. The nation-wide average temperature for summer 2017-18 was 18.8°C (2.1°C above the 1981-2010 summer average from NIWA’s seven station temperature series which began in 1909). This makes summer 2017-18 the hottest summer on record for New Zealand, topping the historical record formerly held by the summer of 1934-35 (which was 1.8°C above the 1981-2010 summer average).

The season got off to a hot start with nearly the entire country observing well-above average (>1.20°C of average) warmth during December 2017. This was the second-warmest December on record at 18.1°C (2.4°C above the 1981-2010 December average from NIWA’s seven station temperature series) and 38 locations recorded record high mean December temperatures. Most of the country received below normal (50-79% of normal) or well below normal (<50% of normal) rainfall during December. By the end of the month, soils were significantly drier than normal for the time of year across a large portion of the North Island, Tasman and northern West Coast, interior Canterbury, and much of Southland and interior Otago. Near normal to above normal soil moisture was observed along the east coast of the North Island, coastal Marlborough, and Westland to Fiordland.

January 2018 was the hottest month on record for New Zealand (20.3°C; 3.1°C higher than the 1981-2010 January average). All but a handful of stations recorded well above average temperatures throughout the country during January 2018 and 94 locations observed their highest mean January temperatures on record. Several tropical airmasses in January led to elevated rainfall levels throughout much of the top half of the South Island, as well as many areas of the North Island. Rainfall was below normal (50-79% of normal) or well below normal (<50% of normal) for much of Southland, Otago, and Hawke’s Bay.

February 2018 was characterised by the passage of ex-tropical cyclones Fehi and Gita which brought significant amounts of rainfall to parts of the country. Rainfall was well above normal (>149% of normal) across much of the upper North Island, Wellington-Wairarapa, the upper South Island, Canterbury and Otago. Elsewhere, rainfall was above normal (120-149% of normal) or near normal (80-119% of normal). In terms of temperatures, above average (0.51°C to 1.20°C above average) to well above average temperatures were observed across the North Island and upper South Island, near average (+0.50°C to -0.50°C of average) in the middle and lower South Island, and below average (0.51°C to 1.20°C below average) in parts of Otago and southern Canterbury.


Further Highlights:
• Fifty-four locations across New Zealand recorded their hottest summer on record, thrity-nine their second-hottest and nine their third-hottest.
• The highest temperature was 38.7°C, observed at Alexandra on 30 January.
• The lowest temperature was 0.2°C, observed at Mt Cook (Airport) and Tekapo on 20 December.
• The highest 1-day rainfall was 297 mm, recorded at Upper Takaka on 17 January.
• The highest wind gust was 165 km/hr, observed at Akitio on 19 December.
• Of the six main centres in summer 2017-18, Auckland was the warmest, Dunedin was the coolest and driest, Wellington was the sunniest, and Hamilton was the wettest and least sunny.

Full Summer 2018 Climate Summary: Climate_Summary_Summer_2018.pdf

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