Rainfall across Wellington was up 170% last month compared with previous long-term seasonal averages, with some suburbs seeing increases of more than 240%. (File photo)
Wellington has had one of its wettest winters on record following an equally wet summer, according to new rainfall data from the regional council.
The figures released by Greater Wellington Regional Council’s environmental science team on Monday, show rainfall amounts exceeded many of the previous highest totals recorded across the entire region.
The wettest month was July. Rainfall in that month was up 200% on long-term seasonal averages. August was not far behind with rainfall 170% higher than average and June was up 160%.
“Spare a thought for the standout soaking of Karori, which in August had a nearly 250% increase in its average rainfall,” said the council’s senior climate scientist, Dr Alex Pezza.
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Wellington City Council area engineer Richard Davidson talks about a slip on Woodhouse Ave in Karori.
Rainfall records have been collected at Karori reservoir (Zealandia) since the construction of the lower dam for Wellington’s water supply in 1878.
According to the new figures, in August rainfall of 296mm was recorded at the reservoir, the second-highest August total since records began in 1879. Only 1967 had a wetter August.
The total winter rainfall at Karori reservoir of 776mm makes it the wettest winter since records began.
Some places such as Ōtaki and Paparangi saw a 236% and 227% rainfall increase, with Pezza explaining the causes of the extremely wet pattern were threefold.
“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 was important and had led to lasting, disruptive and expensive consequences, Pezza said.
“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.”
The heavy rainfall had also seen the regional council’s flood team respond to eight flooding events across the region alongside the Wellington Regional Emergency Management Office, including one where the team was in action for more than 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, with the Waihenga Bridge on SH53 near Martinborough closed several times this year.
This was a “clear indicator” of a strong La Niña pattern, which brought early summer floods, Pezza said.