El Niño combined with global warming means big changes for New Zealand’s weather
ANALYSIS: El Nino is officially here, according to the US National Oceanic and Atmospheric Administration, and with it comes a change from the La Nina weather patterns New Zealand has experienced for the past three years. In particular, a switch from prevailing northeasterlies to southwesterlies means New Zealand is one of the few countries where cooler conditions are felt during El Nino . But what flavour will this El Nino be? Time will tell, but El Nino has been looming for some time. Evidence of its imminent arrival could be seen last year in subsurface ocean temperatures, with a buildup of warm water in the Coral Sea and western tropical Pacific. Moreover, it was overdue. When La Nina finally gave up the ghost in March this year, global sea surface temperatures were suddenly the highest on record, as the tropical Pacific abruptly began to warm. READ MORE: * Four key reasons to explain New Zealand's sodden 2023 * Looking at the spike in ocean temperatures * What the potential end of La Nina weather pattern means for Marlborough Meanwhile, record high sea surface temperatures in the extratropical North and South Pacific were partly a signature from La Nina and partly a sign of global warming. The resulting atmospheric rivers delivered torrential rains to California in the north and New Zealand in the south. These sea surface temperature changes can be readily seen by comparing variations from mean temperatures for December 2022 versus May 2023. We can see a startling transformation throughout the central tropical Pacific, with a coastal El Nino off Peru and Ecuador strongly evident. Modest cooling in the eastern North Pacific is associated with the train of storms that barrelled into the West Coast of the US and in northwest Australia from Cyclone Ilsa. The weather in the tropics is seldom average, however. It tends to fluctuate more like a roller coaster. In the atmosphere, this is referred to as the Southern Oscillation. The combined atmosphere and ocean phenomenon is often referred to as the El Nino-Southern Oscillation (ENSO). The bottom of the roller coaster is the cold phase: a basin-wide cooling of the tropical Pacific, named La Nina, while the top of the roller coaster is El Nino, which occurs every three to seven years or so. The most intense phase of each event typically lasts half a year. But El Ninos can be very strong, and hence highly anomalous. La Ninas, by comparison, are usually moderate in strength and occur more often. El Ninos tend to peak in December, although their biggest atmospheric impacts may not be until February. The last major El Nino was in 2016-17, while a weak El Nino occurred in 2019-20. In the tropical Pacific Ocean, the atmosphere and ocean are strongly coupled. Surface winds drive surface ocean currents, and largely determine the sea surface temperature distribution, the differential sea levels, and the heat content of the upper ocean . In turn, the sea surface temperatures determine the winds. Cool waters limit atmospheric convection and storm activity, while high sea surface temperatures attract convection, clusters of thunderstorms, and tropical cyclones (off the equator, where Earths rotation comes into play). Heat that was stored up in the tropical western Pacific during La Nina is moved around and into the atmosphere during El Nino, mainly through evaporation. This cools the ocean and moistens the atmosphere. This alters where the main rainfall occurs. In turn, it changes the latent heating of the atmosphere that sets up teleconnections (links between weather phenomena in different parts of the globe) and major changes in the jet streams and extratropical storm tracks in both hemispheres including across New Zealand, especially in winter. Because most action occurs over the tropical Pacific Ocean, more settled weather and dry spells often occur over land. The warmest years in terms of global mean surface temperature are the latter stages of El Nino events. 2016 is the worlds warmest year on record, in part because of the very strong El Nino event. But 2023 could beat that record and odds are that 2024 will beat it by a lot. So far, there is little evidence that climate change has altered ENSO events themselves. But all impacts of El Nino are exacerbated by global warming, including extremes of the hydrological cycle involving floods and droughts, which are already common with ENSO. Of course, major events related to El Nino have serious social and economic impacts, too. Droughts, floods, heatwaves and other changes can severely disrupt agriculture, fisheries, health, energy demand and air quality (mainly from wildfires). Research shows El Nino persistently reduces country-level economic growth, with damage now estimated in the trillions of US dollars. Globally, El Nino is the largest cause of droughts ; they are more intense, set in quicker and increase the risk of wildfires, especially in Australia, Indonesia and Brazil. In the weak 2019-20 El Nino, smoke from fires in eastern Australia affected the southern hemisphere to the extent that it blocked the sun and may have exacerbated the subsequent La Nina conditions. Meanwhile, torrential rains are heavier, with greater risk of flooding, especially in Peru and Ecuador. Very wet conditions can also (though not always) occur in California and the southeast US. New Zealand had its highest annual mean surface temperature on record in 2022. In the past year the preponderance of northeasterlies due to La Nina has seen an unprecedented number of tropical and subtropical storms bombarding the country. The record rain event in Auckland on January 27, and Cyclone Gabrielle just three weeks later, were just two among many such events. By contrast, New Zealand tends to experience stronger and more frequent winds from the southwest in winter and from the west in summer during El Nino. This can encourage dryness in eastern areas and more rain on the West Coast, with generally cooler conditions overall. But El Nino varies , and there have been three super El Ninos: 1982-83, 1997-98 and 2015-16. It remains to be seen whether the latest will join them. But together with the augmenting effects of global warming, any El Nino can be very disruptive. We need to be vigilant. Kevin Trenberth is a distinguished scholar at NCAR and affiliate faculty at the University of Auckland. This article was originally published on The Conversation . Read the original article .