La Niña and El Niño, explained: What are they, and will heavy rain in the North end?
Aotearoa and the Pacific is due to get a dramatic change in weather over the next few months as we shift climate cycles from a triple dip La Nina event to the opposite direction. During the last three years, a significant amount of warm water built up in the western Pacific Ocean and is now sloshing eastward, so El Nino is building once again, scientists warn. New Zealand is forecast to transition to an El Nino event by the end of August if not this month, and scientists predict this could bring drier weather conditions to the North Island. But what exactly are these climate drivers , and how do they influence our weather patterns? READ MORE: * El Nino combined with global warming means big changes for New Zealands weather * Planet-heating El Nino returns, putting 2023 on track to be hottest year on record * What the potential end of La Nina weather pattern means for Marlborough * Global temperatures could hit new high with likely return of El Nino, but NZ may buck trend La Nina is the cooling phase, whereas El Nino is the warming phase of the waters in the eastern Pacific, off the coast of South America. Both climate drivers influence rainfall, temperature, and wind patterns around the world, including New Zealand. La Nina and El Nino episodes occur on average every few years and can last up to a year or two. During a La Nina event, ocean water from the coast of South America to the central tropical Pacific, cools to below average temperatures. This cooling occurs because of stronger than normal easterly trade winds, which churns cooler, deeper seawater up to the oceans surface. Sea temperatures can warm above average in the far western Pacific when this happens. The unusually cool water in the eastern Pacific suppresses cloud, rain, and thunderstorms. During a La Nina event, northeasterly winds tend to become more common bringing moist, r ainy conditions to northeastern areas of the North Island and reduced rainfall to the lower and western South Island. Warmer than average air and sea temperatures can occur around New Zealand during La Nina. In comparison, an El Nino climate cycle is when ocean water from off the coast of South America to the central tropical Pacific warm above average. The warming takes place as trade winds weaken or even reverse, blowing warm water from the western Pacific towards the east. As a result, sea temperatures in the far western Pacific can cool below average. The unusually warm water in the eastern Pacific then acts as a focal point for cloud, rainfall, and thunderstorms. An El Nino climate cycle typically means New Zealand experiences stronger or more frequent winds from the west in summer, which can encourage dryness in eastern areas and more rain in the west. In winter, the winds tend to blow more from the south, causing colder temperatures across the country. In spring and autumn, southwesterly winds are more common. Those crossing their fingers and wishing for less rain particularly in the North Island are in luck as NIWA forecaster Chris Brandolino says the change in air flow will also affect our rainfall patterns bringing in much drier weather conditions. It's not a coincidence that when La Nina was well and truly entrenched as the main climate driver, we were getting a lot of east and northeast winds and there was a lot of rain for those places like the Coromandel , Gisborne, Hawke's Bay , Auckland and Northland , he said. But because our wind is expected to favour the South West over the coming three-month period, rainfall is going to change. We're expecting drier conditions for a good chunk of the country, particularly the upper North Island. Brandolino said regions such as the Bay of Plenty, Northland and Auckland would find reduced amount of rainfall near normal or below normal levels. But there was also a concern the North Island could become too dry as we head into spring and summer, he said. However, the west of the South Island such as Westport and Westland down to Fiordland, are going to experience potentially wetter than usual conditions. Brandolino said the climate cycle typically peaked in December. However, the impacts typically lagged and peaked after that. There is a lag between when the climate driver El Nino reaches its peak intensity and when the impacts can reach their peak intensity. The impacts can last much longer than several weeks, even months beyond that. Just because the rain stops, doesn't mean the water stops rising, he said.