Despite a sub-tropical storm and two ex-tropical cyclones, this summer is about to become the hottest in history.
The sun rises over Hataitai beach, Wellington, in the last week of the 2017-2018 summer. [Photo: Hugh McCracken]
There are just five days of summer left and unless they are unprecedentedly cold, NIWA climate scientists say this summer will surpass a record that has been held for more than 80 years.
Until now, the hottest summer in historical record is 1934/35 where the temperature was 1.8°C above the 1981-2010 average. This summer is currently running at 2.3°C, 0.5°C above the previous record.
NIWA meteorologist Ben Noll says the driver of this summer’s remarkable warmth has been the marine heatwave.
“This has been a striking feature on both a regional and global climate scale,” Mr Noll says.
“It began at the end of November last year and has now persisted for three months. There have been three distinct peaks when sea surface temperatures were between 2 to 4°C above average: mid-December, late January and mid-late February.”
But Mr Noll says there were even some areas where sea surface temperatures were 6 or 7°C above average.
“This represented some of the largest ocean temperature anomalies anywhere in the world over the last several months.”
Mr Noll says a warmer than average Tasman Sea is a signature of La Niña, as it is associated with higher than normal air pressure over the region during the late-spring and early-summer – this prevents mixing of deeper, cooler sea water to the surface. In addition, warm north-easterly winds pushed warm water toward the country from the sub-tropics.
Meanwhile, NIWA principal climate scientist Dr Brett Mullan has delved into the record books and found that the previous hottest summer on record of 1934/35 was so unusual it prompted New Zealand Meteorological Service director Dr Edward Kidson to report on it in a special Meteorological Office Note.
Dr Mullan says the note shows there were several similarities to this summer, including widespread drought from November to mid-February.
Dr Kidson wrote that a: “feature of the pressure distribution was that the high-pressure belt and tracks of moving anticyclones were unusually far south in the New Zealand area, generally crossing the Dominion instead of passing to the north of it.”
Dr Mullan says the persistence of anticyclones and north-easterly winds have also been a feature of this summer.
Over land, Dr Kidson noted that “in none of the four months November  to February  did any station in New Zealand record a mean temperature which was not above normal”.
NB: NIWA’s official Summer Climate Summary will be released by Friday 2 March.
Chris Brandolino, Ph 09 375 6335
Ben Noll, Ph 09 375 6334