Back-to-back La Niñas will be a soggy symptom of climate change – research
Climate change is loading the dice in favour of La Nina, the pattern partly responsible for the run of tropical storms over summer, according to a new study. La Ninas slowly build up warm water, which is pushed via trade winds to the west of the Pacific. When this heated water reaches a peak, its unleashed in an El Nino event rushing eastward towards the Americas. But as the planet heats, the La Nina recharge will take longer meaning the events will more commonly last for at least two years. The Pacific recently came out of a rare triple dip La Nina. Northland, Auckland and Coromandel experienced a run of storms over the summer, dumping tropical moisture. Every few years, the Pacific Ocean cycles through different weather phases, known as El Nino best known for bringing drought to eastern regions and its opposite, La Nina. The new research paper , published in the prestigious journal Nature , is one of the first to analyse the frequency of La Nina patterns as the climate heats due to global greenhouse pollution. Before 2000, back-to-back La Ninas happened once every 12 years on average. A multi-year event would arrive roughly every nine years if humanity continues to produce high levels of emissions, the research found. Current patterns Niwa scientist Daithi Stone who was not involved in the study described La Nina as the first step in a pile up of warm water towards Indonesia, driven across the Pacific by the easterly trade winds. This body of water gathers deep below the surface. Every now and then, the water returns towards central America in what we call an El Nino event, Stone said. Since the hot water rises to the surface and spreads, it releases heat to the atmosphere. As well as bringing thunderstorms to the eastern Pacific, El Nino typically raises the mercury in North, Central and South America and dries out Southeast Asia and north Australia, creating conditions for bushfires. In New Zealand, were a little bit unusual in that were more affected by La Nina, Stone said. This sloshing of warm water back and forth across the equatorial Pacific occurs every few years, Stone said. The pattern is irregular. At best, scientists can predict about 12 months in advance whether the Pacific will be in La Nina, El Nino or a neutral state, he added. What researchers found Back-to-back La Ninas tend to come after a strong El Nino event, noted the researchers including Wenju Cai, the former chief research scientist at CSIRO. The team used 20 computer models to simulate how the Pacific would behave as the world produced very high levels of emissions. The exercise found that heating in the subtropical Northeastern Pacific will enhance the easterly trade winds, which would track further north as an El Nino dies out, Cai said. The specific mechanics are complex, but essentially, these winds would slow the reheating of the deep waters. It would take the Pacific longer to recharge and tip into a different phase of the cycle, he said. The system requires back-to-back La Nina events to do so. Thus, back-to-back La Nina events occur more often under greenhouse warming. Double-dip events will happen 33% more often, in a world that produces high levels of greenhouse gas. But fewer than half the models concluded that triple-year La Ninas would become more common, Cai said. Humanity could still limit the arrival of multi-year La Nina events. In a world that rapidly cuts emissions, the frequency would increase by 18.6%, the research team concluded. Our finding is another line of evidence that extremes are likely to occur more often in a warming climate unless actions are taken to cut emissions, Cai added. Prior research efforts typically focused on how climate change will influence El Nino, Stone said. La Nina was partly responsible for the extreme rainfall the upper North has experienced this year. When in force, the winds typically blow from the northeast. This allows weather systems with tropical moisture to pass over the North Island. This past year, the pattern of rainfall that we got in New Zealand was exactly what youd expect from a La Nina summer but many times stronger than wed expect, Stone said. Other factors including a phenomenon in the Indian Ocean that brews patches of wet air and wind and sends them towards the Pacific also played a role in the summer storms. In addition, warmer air is capable of carrying more moisture, meaning the same storm in a world without humanitys greenhouse emissions would likely hold and dump less rain. The impact Four people died and an estimated $1.7 billion of property damage occurred during the Auckland Anniversary flooding, when a storm from the tropics unleashed its moisture right over the largest city. But rather than being a stand-out event, the storm was just one of many affecting the upper North Island. Research led by climate scientist Jim Salinger found that Aotearoas hottest years coincide with La Nina. If the climate events become more common, the country would consequently experience air and marine heatwaves more frequently. This can be a dangerous combination, he added, boosting glacial melting and threatening little blue penguin populations . Although the Pacific has officially shifted into El Nino, La Ninas effect on our food supply could be longer-lasting. The relentlessness of the rain over summer was disastrous for many farmers, said maize grower Jamie Blennerhassett. The grain suffered damage during storms, but the consistent damp was the last straw, the Federated Farmers representative said. The maize grew poorly in the soggy soils and a fungal disease broke out in many regions. Its very easy to see how well your crop is doing or not doing and day after day of not seeing it do so well can become demoralising. With most maize feeding chickens and pigs, this seasons poor yields could further raise the price of eggs and meat, Blennerhassett warned. Theyre expensive enough already. La Nina isnt always bad for maize farmers, Blennerhassett said. Sometimes a little bit more rain than what we get could be good, he added. But if we got more weather like this on a regular basis, wed have to consider growing different crops. Our weekly email newsletter, by the Forever Project's Olivia Wannan, rounds up the latest climate events. Sign up here .