How wet has it really been?

Te Whanganui a Tara - 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.

It may come as no surprise that the MetService station at Nelson airport decimated its previous winter record of 570mm set in 1970. It ticked up 701mm and exceeded their winter average rainfall in over four consecutive days in August alone.

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 Aotearoa has been flirting with a global climate system called La Niña. When La Niña is active, there is an increased frequency of rain-bringing weather systems approaching from the north.

The climate drivers increase the chances of drenching systems approaching from the north and New Zealand 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 the country.

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 increase in temperature is 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, except for Christchurch.