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20-02-2015 Oceans

Extreme La Niña events to increase

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 farreaching 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 world.

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 world. Writing in Nature Climate Change, Wenju Cai and colleagues find a robust agreement among climate models concerning changes in La Niña — extreme events will become more frequent with global warming.

Figure 1 | Illustration of extreme La Niña conditions. a, Average sea surface temperature conditions during boreal winter (December–February) from 50 years of observations. Colouration indicates sea surface temperature, at increments of 1 °C. Surface winds (arrow) blow from east to west, causing surface waters to be displaced westward, while colder waters from the deep ocean move closer to the surface in the eastern equatorial Pacific. Temperatures along the equator increase from approximately 24 °C in the east to about 29 °C in the western part of the basin. The black contour encloses waters warmer than 28 °C, the area where deep convection and tropical rainfall primarily occur. b, Boreal winter extreme La Niña conditions. The surface easterly winds intensify, waters warmer than 28 °C retreat westward along the equator, causing deep convection and rainfall to shift toward the Maritime Continent (as indicated by the box on the left). The central equatorial Pacific (box on the right) becomes cooler, and the temperatura gradient between the Maritime Continent and the central equatorial Pacific is enhanced. This temperatura gradient is projected to strengthen, on average, with climate change, due to the relatively greater warming of the Maritime Continent relative to the central Pacific, creating favourable conditions for the development of extreme La Niña events.

Read the article here

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