January's not-so-summery start with the fresh westerlies and cooler temperatures is the result of a rare climate condition – normal weather with no El Nino–Southern Oscillation influences.
ENSO is the irregularly periodical variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean that affects much of the tropics and subtropics. The warming phase is known as El Nino and the cooling phase as La Nina.
“Right now we are in ENSO neutral, what climatologically is normal – and that can allow a mixture of different weather patterns,” says NIWA forecaster Ben Noll.
It means high pressure systems have tended to be favoured to the west of New Zealand and the low pressure systems to the east of the country.
“And then you have those two working together as cogs. Around high pressure you have the counter clockwise airflow and around low pressure you have the clockwise airflow that has created an anomalous southwesterly through the first month, say eleven days of summer here.
“That's also going to be a cool flow but in the Bay of Plenty since you have a lot of mountain ranges off to your south and east when you do have those westerly flows it can often be a bit warmer right along the shoreline and the coastline there in the Bay of Plenty.”
And that produced warmer temperatures this week, though so far this summer has been a bit cool. The first ten days on 2017 have been near to slightly below average in terms of temperatures in the Bay.
Over the next few days winds shift slightly north of west. Thursday should be another pretty warm day, but thing may cool on Friday.
A small front crossing the Bay of Plenty at the beginning of the day is expected to shift the winds around to south west for the afternoon and evening.
“Friday, especially later in the day you will feel the dew point or humidity slide back a bit and it will feel a little bit cooler as you round out the day there on Friday.”
Over the next three months the chances for average or above average temperatures continue to be even. The chances are about equal for rainfall being normal or below normal. Soil moisture which is a drought indicator has an even chance of being below normal.
“That is something we are watching quite closely. Often times dryness can breed more dryness, can lead to positive feedback. It's something we are keeping a really close eye on here through the summer, as it plays a huge role in agriculture and the farmers.”
Although El Niño has an important influence on New Zealand's climate, it accounts for less than 25 per cent of the year-to year variance in seasonal rainfall and temperature at most locations.
Nevertheless, its effects can be significant.
Typically El Nino impacts on New Zealand's climate are stronger or more frequent winds from the west in summer, leading to an elevated risk of drier-than-normal conditions in east coast areas and more rain than normal in the west – due to the barrier effect of the Southern Alps and main North Island ranges.
While east coast drought is not a certainty during every El Nino, and the districts in which drought occurs can vary from one El Nino to another, Niwa says the risk is sufficiently elevated this summer to warrant risk-management actions by farmers and others whose livelihoods are likely to be adversely affected by prolonged dry conditions.