A potentially "strong" La Niña has developed in the Pacific and may lead to "frequent storm surge events" causing "significant" impacts on New Zealand.
According to the World Meteorological Organization (WMO), a La Niña is the "large-scale cooling of the ocean surface temperatures in the central and eastern equatorial Pacific Ocean, coupled with changes in the tropical atmospheric circulation, namely winds, pressure and rainfall".
The event which has developed over recent months is expected to last into next year and is forecast to be "moderate to strong". The last time there was a "strong event" was in 2010-2011, followed by a moderate event in 2011-2012.
The WMO's Secretary-General, Professor Petteri Taalas, says La Niña "typically has a cooling effect on global temperatures, but this is more than offset by the heat trapped in our atmosphere by greenhouse gases.
"Therefore, 2020 remains on track to be one of the warmest years on record and 2016-2020 is expected to be the warmest five-year period on record."
So how may it affect New Zealand?
NIWA Weather says that La Niña can "cause high pressure to linger to the east of the country during summer causing more northeasterly winds than usual".
"Northern and eastern areas of NZ tend to have a wetter than usual summer, while southern and western areas are often drier and can be prone to drought."
The forecaster says that summers tend to be warmer and more humid than average for most, but eastern coastal areas will experience more onshore winds and cloudy skies.
NIWA on Friday produced a November to January climate outlook. It shows that rainfall is expected to be normal or at below-average levels across the South Island as well as in southern and central parts of the North Island. Most of Waikato, Auckland and Northland will see normal rainfall.
In terms of temperature, the entire country is expected to be above the average with elevated humidity.
MetOceans Solutions' senior oceanography Dr Joao de Souza said in September that typically during La Niña years, the stronger northeasterly winds can whip up "larger waves on the north coast from Cape Reinga to East Cape".
"Increases in the frequency of occurrence of storms were observed in the past for the north coast during La Niña years. The sequence of storms with a short time interval in between significant events can result in increased beach erosion due to the insufficient time for the beach to recover - for example," Dr de Souza said.
"There is also the potential for more frequent storm surge events, particularly along the north-easterly facing coastlines of New Zealand. These events have the potential to cause significant damage to coastal infrastructure around susceptible low-lying areas of New Zealand."
The WMO says countries in the central and eastern Pacific regions "may be more susceptible to below normal rainfall, while countries in the south-west Pacific will become more prone to above normal rainfall".
