NIWA: Autumn Climate Summary

NATIONAL CLIMATE CENTRE Thursday 4 June 2009

National Climate Summary – Autumn 2009: Cold and sunny for most places; dry in the north and wet in the south.

• Temperature: Below average for most of the country, except for Southland and Fiordland where autumn temperatures were near average.
• Sunshine: Above normal for most of New Zealand; below normal in parts of Otago.
• Rainfall: Above normal in Canterbury and Otago, the lower North Island, and western Northland; below normal in central and eastern North Island and northern South Island.

Autumn overall was colder than average (by between 0.5 and 1.5°C) for most of the country with the exception of Southland and Fiordland where temperatures were near average. Below average temperatures persisted for the whole period for most areas, while parts of the West Coast, coastal Fiordland, Southland and south Otago had some respite in April with warmer than average temperatures for that month. The national average temperature of 12.5 °C was 0.7 °C below average for autumn.

Autumn rainfall totals were above normal (120 – 150% of normal) in western parts of Northland, Wairarapa, Canterbury, Southern Alps and Otago. However, much of the rain in these areas occurred in May, with March and April being particularly dry for most of the country, particularly Hawke’s Bay, Tararua District, Marlborough, and north Canterbury. Areas that received below normal (between 50 and 80% of normal) rainfall for autumn were Waikato, central and eastern North Island (with the exception of Wairarapa), Tasman District and Marlborough. Severe soil moisture deficits in many eastern areas of the country eased to near normal levels by the end of May.

March and April were dominated by the slow passage of anticyclones (“highs”) coming over New Zealand from the west. In March, these highs tended to stall over the Tasman Sea resulting in more south to south-westerly air flows than normal over New Zealand, while in April they persisted to the east of the North Island resulting in more northerly air flows than normal over the country. In May, the anticyclones tended to persist over southern Australia resulting in more southerly air flows than normal over New Zealand. More depressions (“lows”) than normal passed over and to the east of New Zealand in this month, which contributed to the cold and wet conditions experienced in many eastern areas during May.

Major Highlights:
• On 5–7 March, a storm crossed the upper North Island and moved down to the Bay of Plenty. Winds reached 150 km/h at Cape Reinga. The high winds, heavy rain and lightning strikes caused property damage and brought down trees and power lines. On 17 March, heavy rain caused surface flooding, landslips, and sewer overflows in Tauranga.
• Heavy rain along the West Coast resulted in flooding on 27 April. Homes were evacuated in Greymouth and roads became impassable. Trampers were stranded in the Mueller Hut in Aoraki Mt Cook National Park, and about 120 people were evacuated from the Milford Track by helicopter.
• There were several extreme events during May. Many locations received record low maximum and minimum temperatures, particularly between the 9th and the 11th and on the 21st and 22nd. Heavy rainfall resulted in flooding in south Canterbury on the 17th, and in Otago on the 18th. High winds buffeted Wellington on the 15th, 17th, 23rd, 24th, 30th and 31st. Snow fell to low levels periodically during the month and tornadoes damaged property in Warkworth on the 11th, and in Taranaki on the 17th.

• The lowest temperature during autumn was recorded at Hanmer Forest on the 12th of May, where the minimum temperature was -7.0 °C. The highest temperature for autumn was 30.5 °C recorded at Hastings on 1 March. The highest 1-day rainfall was 341 mm recorded at Mt Cook on 27 April (the highest autumn 1-day total for this location, since records began in 1928).
Of the six main centres, Tauranga was the warmest and sunniest, Wellington the wettest, and Hamilton was the driest.

ENDS

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