How Wet Has It Really Been?

The news cycle has been full of record wet months, slips, road closures and flooding events these past three months. MetService has crunched some of the rainfall tallies and numerous locations around the country have broken rainfall records for winter. As with any extended period of weather there are always many factors at play but read on to learn what has influenced our rainfall and who broke records.

It may come as no surprise that our station at Nelson Airport decimated its previous winter record of 570mm set in 1970. They ticked up 701mm and exceeded their winter average rainfall in over four consecutive days in August alone (17th – 20th).

Other central areas also broke winter records: Blenheim Airport, Wellington Airport, Paraparaumu Airport, Levin and our station in Kelburn (which has observations back to 1928).

Moving north; Palmerston North, New Plymouth, Taupō and Rotorua airports all hit their wettest winters on record. Tauranga, Kaitaia and Auckland airports came in second wettest, with Auckland missing out on their wettest winter by just a couple of mm.

It wasn’t just northern regions of the South Island that were unusually wet; Christchurch, Timaru, Wanaka, and Hokitika airports all set new winter records, with Hokitika recording a whopping 1142mm – more than 400mm above their winter average.

But why have we seen so many records fall – what brought the rain?

All winter we have been flirting with a global climate system called La Niña. The key message for New Zealand is that when La Niña is active, there is an increased frequency of rain-bringing weather systems approaching from the north.

In addition to La Niña, a negative Indian Ocean Dipole (IOD) was present throughout winter, but should ease over the summer months. When this occurs alongside La Niña, it further increases the potential of northerly rainfall events over Aotearoa.

The two climate drivers above work in tandem to increase the chances of drenching systems approaching from the north and we saw several heavy rain events which originated in the Tasman Sea, or the warm waters in the Pacific Ocean north of Northland, tracking south and dumping their rain out over New Zealand.

To make matters worse there has been anomalous high pressure this season east of New Zealand. This has been slowing the departure of weather systems away from New Zealand – usually, lows quickly cross New Zealand and move eastward, caught up in the overall westerly atmospheric flow. With stubborn highs east of New Zealand, incoming weather systems have been unable to freely depart our shores, and have ended up lingering, delivering prolonged rainfall to Aotearoa.

Lastly, to address the elephant in the room - the background increase in temperature due to climate change. Warmer air promotes evaporation and can transport more water vapour around the globe. Water vapour is fuel for rain. This means when the atmosphere is warmer, it has the potential to rain more heavily than a cooler atmosphere. Once again, New Zealand has had a very warm winter.

With winter 2022 being one for the record books and spring tending to bring volatile weather around Aotearoa just know that MetService will be there, providing the forecasts and warnings everyday - keeping you in the know.

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