Six months in a (slightly) cooler climate
WEDNESDAY, JULY 5, 2017
Six months in a (slightly) cooler climate
Now the first six months of the year are done and dusted, NIWA forecasters have been analysing the country’s weather statistics to see where we stand compared to last year’s record breaker.
If you’ve been feeling a little cooler, it’s because temperatures are notably down on average. The first six months of last year set up 2016 to become the hottest year on record, with an average temperature of 15.2°C. For January to June this year that figure has dropped to 13.8°C.
NIWA forecaster Ben Noll says while the first six months of 2017 were 0.02 degrees above the long-term (1981-2010) average, that was nothing compared to the same period last year when they were a whopping 1.43 degrees above average.
In spite of the cool-down, January-June 2017 still ranks as the 39th-warmest January-June period in the last 109 years, according to NIWA’s Seven Station Temperature Series.
Mr Noll says the direction from which the air is coming plays an important role in temperatures across New Zealand. Sub-tropical northerlies tend to draw down warm, humid air while southerlies via the Southern Ocean can pack a chilly punch.
Unlike 2016, January-June of 2017 has not had an abundance of northerly winds.
Out of the first six months of 2017, just February and April experienced predominantly northerly winds. Conversely, the first half of 2016 saw five out of the first six months (February through June) have a notable northerly wind bias.
Sea surface temperatures (SSTs) also significantly influence air temperature in an island nation like New Zealand.
During the first half of 2017, SSTs around New Zealand were not nearly as warm as they were during the first half of 2016. And while they were 0.06°C above the long-term average, the figure for the same period last year was 1.22°C above average.
So far this year, neither El Niño nor La Niña conditions have been in play, Mr Noll says. These weather systems play a role in the modulation of the Earth’s average surface temperature and in 2015-16 there was a strong El Niño event. Global temperature data shows that for the past few decades, the Earth has observed temperature increases during and following El Niño events.
Strong El Niño events are associated with significantly warmer than normal conditions in the upper (shallowest) layers of the equatorial Pacific Ocean. This excess heat gets transported from the tropics toward the poles, thus helping to raise global temperatures.
Once the strong El Niño of 2015-16 and associated oceanic heat had dissipated, global temperature anomalies declined somewhat from 2016 into 2017 but, notably, not to their pre-El Niño levels. They still remain above the 30-year (1981-2010) average.
The cooler conditions so far this year are also being influenced by natural variability.
Last year was an exceptional year where several meteorological factors, such as near record warm seas and consistent northerly winds, worked together to produce New Zealand’s warmest year on record.
SSTs during 2017 haven’t been quite so warm, airflows not as northerly, and there has been a different setup across the tropical Pacific Ocean.
Other notable figures
But if there’s one weather feature this year has been known for, it’s heavy rain from the likes of the Tasman Tempest, ex-Tropical Cyclone Debbie, and ex-Tropical Cyclone Cook.
January-June featured an incredible 996 mm of rain at Whangaparaoa (north of Auckland) – the wettest start to the year on record by a staggering 182 mm.
Auckland (Mangere) has had its second-wettest January-June period on record, with 806mm in the bucket, while Hamilton has recorded its fourth-wettest January-June with 807 mm.
On the flip side, Invercargill has had its third-driest January-June with 395 mm of rain.
January-June Highest temperature Lowest
temperature Highest 1-day rainfall Highest wind
2016 35.5°C (Clyde, 3 Feb) -8.2°C (Mt Cook, 7 Jun) 331 mm (North Egmont, 17 Feb) 196 km/h (Cape Turnagain, 10 Mar) 1319 hours (Whakatane)
2017 35.5°C (Wairoa, 6 Feb) -8.0°C (Middlemarch, 26 Jun) 309 mm (Milford Sound, 31 Jan) 198 km/h (Cape Turnagain, 13 Jun) 1440 hours (Richmond)