Auckland has recorded its wettest month since records began 170 years ago, according to the National Institute of Water and Atmospheric Research.

New Zealand’s largest city had a total of 539mm of rain in January, smashing the previous monthly record of 420mm from February 1869, NIWA said on Thursday (Feb 2). It cited the Central Auckland Rainfall series based on historical studies by Anthony Fowler, Honorary Associate Professor from the School of Environment at University of Auckland, that dates back to 1853.

“Nothing in the observational record has come close to what we’ve seen in the past few weeks,” Fowler said. “It was already a very wet January before the storm on the 27th, but that single event doubled the total over a single day. Rainfall intensities in the early evening, at the height of the storm, were quite astounding.”

Auckland, home to 1.6 million people, was smashed by a storm on Jan 27 that caused extensive damage due to widespread flooding and landslides. Four people died, hundreds of properties were damaged or destroyed, and thousands or travellers were left stranded after Auckland International Airport was forced to close.

In total, central Auckland experienced more than 45% of its yearly rainfall in just one month, more than 8.5 times the amount of a typical January and over 2.5 times that of an entire summer, NIWA said.

A slew of environmental factors came together in a “perfect storm” to cause the unparalleled deluge, said NIWA meteorologist Ben Noll.

“A formidable La Niña and marine heatwave led to more moisture being available, which was harnessed by an atmospheric river,” he said. “High pressure to the south then blocked it, keeping it in place. The storm was also supported by a unique phenomenon called a low-level jet, as well as converging winds that extended lengthwise across the most populated part of the country.”

Climate change made the event more intense, NIWA climate scientist Sam Dean said.

“The Earth has warmed by about 1.1°C already because of human activity and this extra heat gives more power to extreme rainfall,” he said. “All other things being equal, we would expect climate change to contribute between 10% and 20% more rain in the most intense part of this storm.”

  - Bloomberg