英文摘要: | The El Niño/Southern Oscillation is Earth’s most prominent source of interannual climate variability, alternating irregularly between El Niño and La Niña, and resulting in global disruption of weather patterns, ecosystems, fisheries and agriculture1, 2, 3, 4, 5. The 1998–1999 extreme La Niña event that followed the 1997–1998 extreme El Niño event6 switched extreme El Niño-induced severe droughts to devastating floods in western Pacific countries, and vice versa in the southwestern United States4, 7. During extreme La Niña events, cold sea surface conditions develop in the central Pacific8, 9, creating an enhanced temperature gradient from the Maritime continent to the central Pacific. Recent studies have revealed robust changes in El Niño characteristics in response to simulated future greenhouse warming10, 11, 12, but how La Niña will change remains unclear. Here we present climate modelling evidence, from simulations conducted for the Coupled Model Intercomparison Project phase 5 (ref. 13), for a near doubling in the frequency of future extreme La Niña events, from one in every 23 years to one in every 13 years. This occurs because projected faster mean warming of the Maritime continent than the central Pacific, enhanced upper ocean vertical temperature gradients, and increased frequency of extreme El Niño events are conducive to development of the extreme La Niña events. Approximately 75% of the increase occurs in years following extreme El Niño events, thus projecting more frequent swings between opposite extremes from one year to the next.
During typical La Niña events, the central-to-eastern equatorial Pacific is colder than normal, inhibiting formation of rain-producing clouds there, but enhancing atmospheric convection and rainfall in the western equatorial Pacific. The associated atmospheric circulation generates extreme weather events in many parts of the world, including droughts in the southwestern United States1, 14 and eastern equatorial Pacific regions, floods in the western Pacific and central American countries1, 15, and increased land-falling west Pacific cyclones and Atlantic hurricanes2, 16, 17. La Niña-related sea surface temperate (SST) anomaly patterns, however, differ from event to event (Fig. 1a, b). Compared with the weak event of 1995, cold anomalies of the 1998 extreme event peaked notably farther west, and exerted much greater impacts. During 1998, extreme events occurred, in part linked to the developing 1998–1999 La Niña event. The southwestern United States experienced one of the most severe droughts in history4, 7, 18. Venezuela endured flash flooding and landslides that killed 25,000 to 50,000 people19. In China, river floods and storms led to the death of thousands, and displaced over 200 million people20. Bangladesh experienced one of the most destructive flooding events in modern history, with over 50% of the country’s land area flooded, leading to severe food shortages and the spread of waterborne epidemic diseases, killing several thousand people and affecting over 30 million more21, 22, 23. The 1998 North Atlantic hurricane season saw one of the deadliest and strongest hurricanes (Mitch) in the historical record4, claiming more than 11,000 lives in Honduras and Nicaragua24.
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