NIWA says 2021 was New Zealand's warmest year on record with annual temperatures well above average in parts of Auckland, Bay of Plenty, Tasman and Fiordland.
Consistent with a pattern of global warming, NIWA says New Zealand's nationwide average temperature for 2021 was 13.56C, which is 0.95C above the 1981-2010 annual average and higher than 2016, the previous record holder when the average temperature was 13.45C.
No month in 2021 recorded below-average temperatures (meaning less than -0.5C of average), while three - January, February and September – were near average (within -0.5C to +0.5C of average) and nine were above average (greater than +0.5C of average).
The country's temperatures were below or well below average for 26 percent of 2021's days, while 19 percent of days had near average temperatures and 55 percent were above or well above average.
While most of the country saw above average annual temperatures (+0.51C to 1.2C above annual average) Auckland, Bay of Plenty, Tasman and Fiordland saw well above average temperatures (more than 1.2C). Western Waikato, coastal Wairarapa and parts of northern Canterbury and Otago saw near average temperatures (within -0.5C and +0.5C of average).
"Frequent high pressure over the North Island and east of New Zealand, which caused more northerly quarter winds than normal, was a contributor to New Zealand’s warmest year on record," NIWA says in its annual climate summary, released Tuesday.
The hottest period of 2021 was between January 25 and 28, when NIWA says a number of locations had record or near-record high daily maximum and minimum temperatures. The highest temperature of the year (39.4C) was recorded on January 26 in Ashburton. That's the 2nd highest January temperature ever and the 10th equal hottest temperature on record.
Summer featured extended dry spells across the North Island with a drought developing in the Far North. There were also very dry to extremely dry conditions in Marlborough and northern Canterbury during February. Water restrictions were in place in Northland, Auckland, Wairarapa and Hastings District.
NIWA says that during January and February, moderate La Niña conditions in the central Pacific began to ease. While the weather system is often associated in New Zealand with north-easterly winds and warmer weather, that isn't what Aotearoa saw in early 2021.
"January featured more south-westerly winds than usual and February featured easterlies. Both months were some of our coolest of the year (relative to the time of year).
"The atypical impacts can be attributed, in part, to a non-traditional central Pacific type of La Niña, whereby the minimum sea surface temperature anomalies are located in the central Pacific rather than the east."
Autumn was "characterised by long dry spells and warmth, interspersed with bursts of heavy rainfall". NIWA notes much of the country had warm days, but chilly nights with some notable rain events such as the Canterbury flooding at the end of May. Research following this event found the extreme rainfall was 10 to 15 percent more intense as a result of human influence on the climate system.
New Zealand also recorded its warmest winter on record in 2021, surpassing that of a year prior. This was due to more northerly quarter winds than normal. Several atmospheric rivers - described as long, narrow regions in the atmosphere that transport water vapour outside the tropics - impacted Aotearoa.
NIWA points out one atmospheric river brought heavy rain to West Coast, Tasman, Nelson and Marlborough last July, reading to the Buller River recording the highest flow rate for a New Zealand river. Flooding caused evacuations and led to about $132.4 million in private insurance damage.
The re-emergence of La Niña conditions in spring resulted in a transition from "westerly air flow, near average temperatures and widespread wet weather during September to more north-easterly winds during October-December". This brought more warm, humid weather.
October last year was the fifth warmest on record, November was the warmest ever and December was the fourth warmest. These warm conditions were driven by increasing coastal temperatures.
Rainfall was near normal for most of New Zealand in 2021, with above-normal rainfall found in parts of Taranaki, Manawatu/Whanganui, western Greater Wellington, parts of Nelson, interior Canterbury and Otago.
Taranaki experienced the country's highest annual sunshine total during 2021 - 2592 hours in New Plymouth.