Summer 2018-19

New Zealand’s third-warmest summer on record

Temperature	It was New Zealand’s third-warmest summer on record. Temperatures across the country were either above average (+0.51°C to +1.20°C of the summer average) or well above average (>1.20°C of the summer average). Some of the highest temperatures relative to what is typical for the time of year occurred in the Bay of Plenty, Waikato and the Coromandel.
Rainfall	Rainfall was below normal (50-79% of the summer normal) to well below normal (<50 % of the summer normal) in Northland, Taranaki, Nelson, Tasman and the West Coast as well as parts of Marlborough, Manawatu-Whanganui, Otago and Southland. Above normal rainfall (>120% of the normal) was observed around Hawke’s Bay and parts of Gisborne. Rainfall was near normal elsewhere (80-120% of the summer normal rainfall).
Soil moisture	Summer started off on a wet note for some with wetter than normal soils present for much of the North Island (with the exception of Taranaki where drier than normal soils were present) and eastern South Island. A dry and warm January led to the rapid depletion of soil moisture levels and this continued throughout February. At the end of summer 2018-19, drier than normal soils were present across much the country. Severely dry soils were present across Northland, Auckland, Bay of Plenty, Waikato and Manawatu-Whanganui and extremely dry soils were present in the Taranaki, Tasman and Nelson regions. Meteorological drought conditions (as defined by the NZ Drought Index) were present at the end of summer in Nelson, Tasman and the Buller District.

Overview

The nationwide average temperature for summer 2018-19 was 17.9°C (1.2°C above the 1981-2010 average from NIWA’s seven station temperature series which begins in 1909) making it the third-warmest summer on record in New Zealand. One of the key climate drivers and contributors to this ranking was the presence of above average sea temperatures around our coastlines. Some coastal areas around Hawke’s Bay and Canterbury experienced marine heatwave conditions for a time. According to Australian research (Hobday et al. 2016), warm sea surface temperature events are considered marine heatwaves (MHWs) if they last for five or more days with temperatures warmer than the 90th percentile based on a 30-year historical baseline period.

Marine heatwave conditions also persisted in the Tasman Sea. New Zealand’s warmest summer on record occurred last year (2017-18) with the summer of 1934-35 coming in at second place. Notably, both of these summers were characterised by significant marine heatwave events.

In addition to the warm seas, summer air flow patterns also favoured warm temperatures with a distinct lack of southerlies throughout the season. December was characterised by the prevalence of warm and moist easterly and north easterly winds, while frequents bouts of high pressure occurred in January and February. Most notably, the combination of high pressure and hot air masses originating from Australia led to prolonged hot conditions throughout much of New Zealand to end January. Several locations across the country observed record or near record high daily maximum and minimum summer temperatures during this time. The warmest summer temperature was 38.4°C recorded in Hanmer Forest on 31 January. This was Hanmer Forest’s warmest temperature on record (data since 1906) and the 18th-equal warmest temperature on record for all months in New Zealand.

In terms of rainfall, summer 2018-19 had an unsettled start. Thunderstorms were a frequent occurrence during December and a storm on 24-25 December caused widespread slips and flooding, disrupting holiday plans for many North Islanders. Conversely, January and February saw widespread dry conditions. Nelson observed a 40-day dry spell which was the 4^th-longest dry spell on record there (with records extending back to 1862), while Tauranga and Hamilton had 36 consecutive dry days – their third-longest dry spells on record (records began in 1910 and 1935, respectively). A dry spell is defined as a period of 15 days or more with less than 1mm of rain on any one day.

The dryness went hand in hand with high sunshine hours around the country. The stations in Richmond and Appleby both recorded 355 hours for the month of January which is a new record for the sunniest month in the South Island. The dry and sunny conditions during January and February were influenced by a central Pacific El Niño event.

Despite a wet start to summer for large parts of the country, the dry and warm January and February led to the rapid depletion of soil moisture levels. The Ministry for Primary Industries classified a medium scale adverse event in Tasman on 8 February due to large wild fires and persistent dryness. Continued dry weather led to meteorological drought conditions (as defined by the NZ Drought Index) to appear in Nelson, Tasman and the Buller District.

Further highlights:

• The highest temperature was 38.4°C, observed at Hanmer Forest on 31 January.
• The lowest temperature was -1.4°C, observed at Tara Hills on 25 February.
• The highest 1-day rainfall was 200 mm, recorded at Milford Sound on 19 January.
• The highest wind gust was 182 km/h, observed at Cape Turnagain on 27 January.
• Of the six main centres in summer 2018, Tauranga was the sunniest, wettest and warmest, Dunedin was the coolest and least sunny, and Christchurch was the driest.

Download

Climate Summary for Summer 2018-19 [PDF 630KB]

Contact

Miss Nava Fedaeff, Climate Scientist, NIWA Auckland.
Tel. 09 375 6337