Farmers love and hate this swinging weather pattern. It moves erratically between three different phases: El Niño, La Niña and neutral conditions. ENSO brings reliable winter rain to Australia and the west Pacific during La Niña, and increases the risk of drought during El Niño.
But when the pendulum swings too far in either direction, to extreme La Niña or El Niño, it can bring devastating droughts and floods. The 1997-98 El Niño claimed around 23,000 lives worldwide and caused $35-$45 billion in damage. And last week the Australian Bureau of Meteorology officially declared that we’re heading into another major El Niño event.
As recent research shows, we’d better get used to El Niño’s appearance. Until recently, projections for the effect global warming will have on this cycle were all over the place. But climate model projections are now converging: El Niño rainfall changes may intensify and the extremes will become more common.
In the neutral farmer-friendly phase, trade winds blow east to west in a belt across the equator, from Ecuador on the South American coast to Papua New Guinea. The trade winds were named by the 15th-century merchants who used them to sail west across the Pacific. These winds also whisk warm sea surface water west where it evaporates into clouds that deliver winter rain to eastern Australia and PNG.
Meanwhile back east, the warm water blown away by the wind is replaced by cold, nutrient-rich water welling up from the depths off the coast of Ecuador and providing a bounty for fishermen. The air above this patch of cool water becomes cold and dry, creating a high-pressure zone. This boosts the engine of the trade winds.
But every year or three, the frigid surface waters near South America warm, dissipating the high-pressure system above. Without the booster, trade winds stop or even reverse direction in some places. Warm water sloshes back from PNG into the central and eastern Pacific, taking rain-rich clouds with it.
Pacific islands under these clouds suffer calamitous floods with the effects reaching as far east as California and South America, and the fish that would normally come with the cold upwellings off Ecuador stay away. Meanwhile, Australia and Asia are left with heatwaves and droughts. This is an El Niño – Spanish for “the boy child” as early Christian inhabitants of Ecuador and Peru usually noticed its effects around Christmas.
The flipside of the cycle is El Nino’s sister, La Niña. She appears when the trade winds blow more strongly than usual and dump more warm water and more moist air over the east coasts of Australia and Asia. Australia’s 2010-11 La Niña, for instance, brought torrential rains and floods. It was preceded by the warmest northern ocean temperatures on record.
If the cycle is influenced by sea-surface temperatures, what will happen as the planet warms? Until recently climate scientists focused on sea surface temperatures as the “passport to projecting future changes” says climate scientist Scott Power at the Australian Bureau of Meteorology in Melbourne.
But for decades those models came to different conclusions. Some predicted more swinging in the cycle, others less. There is “low confidence in changes in the intensity and spatial pattern of El Niño in a warmer climate”, according to the 2014 Intergovernmental Panel on Climate Change report.
More recently scientists have turned their attention from the sea to the sky and their projections are now converging. Power and colleagues published a 2013 Nature paper predicting that over the next 100 years El-Niño-driven droughts will intensify, leaving Australia and PNG without rain for years while deluging Polynesian nations such as Kiribati.
Read More: Here