May 2007

Warmest May on record Indian summer in many parts of New Zealand; flooding in Nelson and Taranaki

• Rainfall: Record low rainfall in the north and east, well above normal in Nelson
• Soil moisture: Significant deficits in the east of the North Island, as well as Otago
• Temperature: New Zealand’s warmest May in over 140 years of temperature measurements
• Sunshine: Above average in many regions, especially in the east

May 2007 produced a truly Indian Summer ^[1] for much of New Zealand with record temperatures for the month and the driest May on record in the north and east. This was produced by warmer than normal seas to the west of New Zealand and anticyclones to the east producing warm north westerlies over the country. The national average temperature was 12.4 °C (1.7 °C above normal); a new record for May, making it the highest in reliable records dating back to the 1860s. Similarly, Australia recorded its warmest equal May on record being 2.0°C above normal. At the same time a mere tenth to quarter of the normal monthly rainfall occurred in parts of the north and east of the North Island and the Kaikoura coast. Rainfall recorded in the east of the North Island and eastern Otago was 10 mm or less. The very dry weather meant that significant soil moisture deficits (of at least 110 mm) occurred in many eastern regions from Gisborne to Wairarapa, as well as Central Otago, unusual for the time of year.

May 2007 topped previous New Zealand temperature records of 12.3 °C in May 1962 and 12.2 °C in May 1999. Mean temperatures were at least 1.5 °C or more above average throughout much of New Zealand, but temperature anomalies were highest (more than 2.5 °C above normal) throughout Marlborough, Canterbury, and Otago, due to very frequent northwesterly winds. The month was the warmest on record averaged over the country as a whole. One third (35) of NIWA’s (104) near real-time New Zealand temperature stations recorded their highest May mean temperature.

Low rainfall was also a feature for May, with record low May totals in many northern and eastern regions. Rainfall was well below normal in the northern half of the North Island, as well as Gisborne, Hawke’s Bay, Wairarapa, along the Kaikoura coast, and in East Otago. In contrast, rainfall totals were 200 percent (twice) of normal in Nelson due to high rainfall over 22/23 May.

May was sunnier than normal in many North Island and eastern South Island locations.

Depressions (‘lows’) were much more frequent than normal south of Australia, while anticyclones (‘highs’) were more frequent than normal well to the east of the North Island. This pattern produced more frequent northwesterlies than usual over much of New Zealand, the strongest and most frequent for May since 1990.

Other Highlights

• Parts of Nelson and Taranaki were hit by heavy rainfall over 22/23 May, with localised severe flooding.
• The highest temperature during May 2007 was 24.8 °C recorded at Whangarei Airport on the 1^st.
• The lowest air temperature during the month was -4.1 °C recorded at Motu on the 31^st.
• A wind gust as high as 156 km/h was recorded from the west at Taiaroa Head on the 27^th.
• Auckland was the warmest, Dunedin the driest, and Christchurch the sunniest, of the five main centres. Rainfall was below normal and temperatures well above normal in all five main centres. Sunshine hours were above normal in Auckland, Wellington, Christchurch, and Dunedin, and near normal in Hamilton.

Rainfall

Rainfall was 25 percent (a quarter) or less of normal in parts of Northland, Auckland, Waikato, Taupo, Gisborne, Hawke’s Bay, Wairarapa, along the Kaikoura coast, and in East Otago, and 50 percent (half) or less of normal in Coromandel, Bay of Plenty, Wanganui, Manawatu, Horowhenua, Kapiti, North Canterbury, and inland Southland, and also below normal in Thames, Taranaki, Wellington, and much of Southland. In contrast, rainfall totals were 200 percent (twice) of normal in Nelson, and also above normal in Fiordland and parts of Marlborough.

Temperature

Mean temperatures were at least 1.5°C or more above average throughout much of New Zealand, and more than 2.5°C above average in parts of Marlborough, Nelson, Canterbury, and Otago.

Sunshine

May sunshine hours and/or solar radiation were more than 110 percent of normal in many North Island and eastern South Island locations, and at least 120 percent of normal in Gisborne, South Taranaki, Marlborough, inland South Canterbury, coastal Otago, and Southland.

Full report

• Full details of the May 2007 Climate Summary

[1] An Indian Summer is a period of very mild and sunny weather occurring in late autumn. The term came from the north east of the USA where the indigenous people (“Indians”) were able to harvest and store nuts and crops for the coming winter.

For further information, please contact:

Dr Jim Salinger – Principal Scientist, Climate NIWA National Climate Centre – Auckland Phone +64 9 375 2053 [email protected]

Stuart Burgess – Climatologist NIWA National Climate Centre – Wellington Phone +64 4 386 0569 [email protected]

Geoff Baird – Communications Manager Phone +64 4 386 0543 [email protected]

Acknowledgement of NIWA as the source is required.