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Strong El Niños Predicted To Become More Common

Aerial view of earth, indicating climate change
During the 1997 El Niño, areas of the ocean near the tropical coasts of South and North America, shown in white, warmed considerably and rose in height, leading to damaging weather events. New research indicates that as climate changes, more of these events can be expected. NASA

Research indicates climate warming will double the occurrence of the destructive weather pattern known as El Niño in the future—but not the intensity of the events.

February 18, 2014—Extreme El Niño events—a destructive weather pattern marked by catastrophic flooding in some areas and extreme drought in others—are likely to occur much more often in the future if global warming projections are correct. A team of researchers examining climate models determined that a warmer atmosphere is likely to produce the weather pattern twice as often as in the past.

The research team was led by Wenju Cai, Ph.D., a climate modeling expert at the Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia’s national science agency, which is based in Aspendale, Victoria. The research findings were published online in January as a letter, “Increasing Frequency of Extreme El Niño Events Due to Greenhouse Warming,” to the journal Nature Climate Change.

“The question of how greenhouse warming will change the frequency of extreme El Niño events has challenged scientists for more than 20 years. This research is the first comprehensive examination of the issue to produce robust and convincing results,” Cai said in written comments to Civil Engineering online.

Extreme El Niño events are marked by dramatically altered weather patterns, with catastrophic flooding on the Pacific coasts of North and South America and deep droughts in such other areas as the east coast of southern Africa. The effects of strong El Niño events are widespread, causing crop failures, disease outbreaks, and economic losses.

The extreme El Niño event of 1997-98 is estimated to have caused $25 billion in total economic impacts in the United States alone, some sectors of the economy benefitting from milder weather and others suffering. The El Niño is blamed for $2.2 billion in property losses, with $1.1 billion of those in California, which was hit by heavy flooding.

The researchers examined 20 climate models used by the Intergovernmental Panel on Climate Change (IPCC). There was a strong intermodel consensus, Cai noted, with 17 out of 20 models producing an increase in occurrences of extreme El Niño events. Each of the 20 models included a range of 200 years, with historical data extending back as far as 1891 and projections forward to 2090, based on expected levels of warming.

“We compare the frequency of extreme El Niño events during the 1891-1990 [period] with that during 1991-2090, and find there is a doubling,” Cai said. The research does not, however, predict an increase in the strength of extreme El Niño events.

“In the tropics, atmospheric convection and rain bands follow the maximum sea surface temperature (SST),” Cai explained. “Under greenhouse warming, the eastern equatorial Pacific warms faster than the surrounding regions, making it easier to have maximum SST in the eastern equatorial Pacific, and hence more occurrences of extreme El Niño events—even if [the] variability of the SST does not change.”

Cai explained that the team’s research indicates that warming is faster in the eastern equatorial Pacific Ocean than in the surrounding regions because the water is colder there to begin with. In the warmer waters of the western Pacific Ocean, greenhouse warming will increase evaporation at a faster rate than the eastern Pacific. “So the eastern equatorial Pacific warms faster under greenhouse warming.”

This projected warming creates anomalies in the SST gradients with profound consequences. These anomalies lead to a reversal of the easterly winds to westerly winds, and the creation of deep convection over the eastern Pacific Ocean.

On the basis of their findings, the team is now examining the effects of global warming on other extreme climate events as well. “We are looking [to determine] if other climate extremes will change—for example, extreme Indian Ocean dipole events in the tropical Indian Ocean,” Cai said. “These often occur in conjunction with extreme El Niño, but can also occur concurrently.”



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