New rainfall figures from Greater Wellington’s Environmental Science team confirm that last winter was the wettest on record, following the misery of a summer that was also the wettest on record in the Wellington region.

The figures paint a portrait of suburbs soaked and districts drenched by record amounts of rainfall which exceeded many of the previous highest totals recorded across the entire region. On average, June rainfall was approximately 160 percent up on long-term seasonal averages, with July coming in higher at 200 per cent and August at 170 per cent.

“Spare a thought for the standout soaking of Karori, which in August had a nearly 250 per cent increase in its average rainfall,” says Greater Wellington Senior Climate Scientist Dr Alex Pezza. Other notable records were set in Otaki at 236 per cent and Paparangi at 227 per cent.

The main causes of the extremely wet pattern were threefold, says Dr Pezza.

“Background global warming increasing moisture in the air and leading to higher rainfall, a third consecutive year of a developing La Niña and the development of semi-permanent marine heatwaves around New Zealand.

“These factors contributed to an enhanced northerly flow and formation of the phenomenon known as ‘atmospheric rivers’, which brought large amounts of tropical moisture into our region.”

The compounding factor of the wet seasons is important and has led to lasting, disruptive and expensive consequences, says Dr Pezza.

“A chain of significant rain events effectively prevented our saturated soils from drying out, contributing to the severe slips seen in many areas of the capital by the end of winter.”

Downfalls also raised the threat of flooding in the region, resulting in a hands-on winter for Greater Wellington’s Flood Response Team, which responded to eight flooding events in various parts of the region alongside the Wellington Regional Emergency Management Office, including one where the team was in action for over five days. While the heavy rain and high river flows ultimately presented no major flooding issues, they did result in significant erosion in Wairarapa’s rivers.

Additionally, the Waihenga Bridge on SH53 near Martinborough was closed several times this year, a clear indicator of a strong La Niña pattern, which brings early summer floods.

Greater Wellington’s seasonal climate monitoring is updated every season, and is available online from the link below: https://www.gw.govt.nz/environment/environmental-data-and-information/climate-monitoring/

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