Basic climate change science is simple: rising CO2 levels trap heat and warm the planet. The devil is in the detail – and one of the most devilish is predicting its impact on the El Niño-Southern Oscillation (ENSO).
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.
In a 2014 Nature Climate Change paper, CSIRO climate scientist Wenju Cai and an international team of collaborators narrowed these predictions further, reporting that extreme El Niño events will double in frequency “from one in 20 years to one in 10 years”. The mechanism, Cai says, is relatively simple. Global warming will add heat across the Pacific. In the warm western Pacific near PNG, heated water will evaporate and disappear into the atmosphere, cooling the surface of the sea.
But over the east Pacific and South America it will be a different story. The east Pacific is cooler so as heat is absorbed the water will not evaporate at the same rate and will warm faster than the west. This will reduce the air pressure difference between them, weakening the trade winds and creating conditions for more extreme El Niño events.
So we’ll see the big boy child more often. What about his sister?
In a Nature Climate Change paper published this January, Cai found we should expect more frequent extreme La Niña events too. This may seem counterintuitive. La Niñas occur when warm water is concentrated between East Australia and PNG, whereas that warm water is lapping the South American coast during an El Niño. If El Niños increase, we should see fewer La Niñas, right?
“Well, no,” Cai says. He predicts the pendulum will swing faster from extreme El Niño to extreme La Niña conditions. During an extreme El Niño, Cai explains, the trade winds reverse to blow west to east, bringing deeper cool water closer to the surface and generating a big cold patch in the central Pacific. The resulting high air pressure hovering over the cold water pushes the pendulum back the other way, sending rain-rich storm clouds to PNG, Southeast Asia and northern and central Australia – La Niña. “After a La Niña things kind of go back to normal,” Cai says. “Then we wait for the next El Niño to come.”
Are we already seeing the pendulum speed up? Maybe – University of New South Wales climate scientist Shayne McGregor published a study in the journal Climate of the Past in 2013 showing the swings between the cycles were more severe in the 30 years leading up to 2009 than any time over the past 600 years. McGregor is yet to attribute this to global warming.
We do know the Pacific and Atlantic Oceans are already sucking in vast amounts of atmospheric heat where it is stored in the depths – likely the cause of the decade-long global warming hiatus that started around 1998. The effects of this deep ocean warming on the El Niño are purely speculative and need to be tested, says Cai. If his latest climate predictions are right, though, we’re in for a stormy ride.