| 英文摘要: | How climate change will impact the natural phenomenon La Niña, the counterpart of El Niño, has been unclear. In spite of uncertainty, now a study shows a large model consensus for an increasing frequency of extreme La Niña events. Every two-to-seven years, cooler than normal conditions in the tropical Pacific Ocean, known as La Niña, drive atmospheric circulations that generate extreme weather events in many parts of the world, such as droughts, floods, and enhanced hurricane activity. These far-reaching impacts are particularly devastating for the strongest La Niña events. During the extreme 1998–1999 event the severity of droughts, floods, mudslides and hurricanes claimed thousands of people's lives, displaced millions, and caused dramatic economic losses in many parts of the world1. The physical mechanism of La Niña in the present climate is relatively well understood. However, there has been no consensus among climate models on how La Niña will change in a warming world2. Writing in Nature Climate Change, Wenju Cai and colleagues3 find a robust agreement among climate models concerning changes in La Niña — extreme events will become more frequent with global warming. Usually, the eastern tropical Pacific is dry and cool, whereas the western Pacific is characterized by the warmest waters of the world ocean, accompanied by prodigious tropical rainfall (Fig. 1a). These average conditions are maintained by winds at the ocean surface which blow from east to west. During normal La Niña events surface easterly winds intensify, causing cooler and dryer conditions to develop in the central equatorial Pacific that lead to an increased temperature difference with the Maritime Continent, the area of the world that comprises parts of southeastern Asia, Indonesia, and the Philippines. Tropical precipitation is shifted westwards, bringing enhanced rainfall over the Maritime Continent. These conditions are further intensified during extreme La Niña events: the central equatorial Pacific is much colder and drier, and rainfall is further enhanced in the far western Pacific (Fig. 1b).
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