An historical ‘Triple Dip’ La Nina is officially back in place after months of speculation that it would, but for many the weather patterns may not differ to where they have been since mid-winter.

As WeatherWatch.co.nz has repeatedly said, many of the building blocks for La Nina were there since the last one faded out back in Autumn. Our winter months saw our part of the world in an official “La Nina Watch” meaning conditions were always close for La Nina to return and our weather pattern sure felt that way. So don’t expect any sudden dramatic changes just yet – if at all.

Based on various modelling this third La Nina looks similar to the previous two La Nina events – in both duration and strength. La Nina is measured at the equator and New Zealand is located halfway between Antarctica and the equator – meaning we’re not directly impacted by La Nina conditions all the time. In fact, the previous two La Nina events had lots of news headlines in NZ about extra wet weather – but instead widespread droughts kicked in and floods were very isolated (even if severe in some cases).

With NZ so far from the equator La Nina needs to be strong to really engulf our weather pattern more – and this one is not a strong event based on the data.

WHAT DOES LA NINA MEAN FOR NZ?
NZ turns into a ‘traffic light’ with a fairly weak to moderate La Nina – sometimes turning green and waving down humid, wet, La Nina weather and at other times the light is red and we return to the usual Southern Ocean windy westerlies and cooler southerlies. This is precisely what NZ had over the previous two La Nina. The eastern side of Australia will likely be more vulnerable as they share tropical ocean currents which gives a stronger connection to La Nina weather in the tropics and rainmakers sliding down their east coast.

WILL THIS LA NINA BE DIFFERENT TO THE PREVIOUS TWO? MAYBE.
There is one key difference for this La Nina compared to the previous two for New Zealand – we’ve had a big break in the weather pattern for the first time in five years. In the past six months the five-year rainfall deficit in the upper North Island finally broke and now high pressure zones are being more spaced apart, allowing for rainmakers in between. In NZ – as Nelson, Marlborough and Tasman experienced in August – these highs can then slow down big rainmakers over some parts bringing serious rainfall totals and widespread slips and flooding.

Put simply, there may be more gaps in the defensive high pressure zones to allow the odd La Nina fuelled rainmaker to score a try here in NZ and bring rain this spring – but overall this La Nina is not looking especially powerful.

HOW LONG WILL LA NINA LAST?
International modelling suggests it’s mostly a spring event – fading out this summer. It’s likely to peak in Spring (modelling suggests November is the peak – which is also the start date of our Cyclone Season) and be fading back to “neutral” by early 2023. A negative Indian Ocean Dipole (basically the Indian Ocean’s version of “La Nina”) is currently the main driver of rain events into southern and eastern Australia and then on to western NZ (we’re seeing that set up this week, for example – so not all rain we get is connected to La Nina). The addition of La Nina means Australia has extra rainmakers likely this spring from both the west and the east – and NZ is on the edge of this set up, with the Southern Ocean weather systems still likely to dominate our weather pattern at times with the westerly flow (La Nina tends to bring more easterly flows to NZ).

PUT SIMPLY:
A slightly increased chance of rainmakers and easterlies this spring – but still powerful high pressure zones keeping some areas drier than usual. La Nina starts in mid September, peaks in November, fades out around January 2023. The extra warmth in the atmosphere and sea surface conditions in our part of the world can boost rainmakers to our north – which can sometimes directly impact NZ, but not locked in.

WHERE CAN I FIND MORE DETAILS?
Australia’s Bureau of Meteorology (BoM) is the only non-commercial Government forecaster in our part of the world to cover Climate (unlike Niwa) – so we trust BoM for guidance and not clickbait headlines. You can read much more here from BoM.

• WeatherWatch.co.nz / RuralWeather.co.nz