It Only Looks like El Nino

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Computer simulations of how the tropical Pacific Ocean responds to our changing climate reminds a lot of people of El Niño — a big wedge of unusually warm surface temperatures that periodically stretches along the equator.  But a new study points out big differences in a globally warming Pacific, especially in the way rainfall is distributed around the world.

No doubt about it, superficially at least, the resemblance is striking.  Along with surface temperatures, other major pieces of the meteorological furniture are rearranged by global warming to give the tropical Pacific an "El Niño-like" appearance.  The easterly trade winds blow less strongly, for instance, and the center of towering rainclouds that fuels a lot of weather drifts from southeast Asia out into the central ocean.

But looks can be deceiving.  Researchers observe in the current issue of Eos, a publication of the American Geophysical Union, that the two scenarios produce very different responses in the global atmosphere.  El Niño bends the jet stream farther south, for instance, which typically has the effect of bringing unusually wet winters to southern California and the rest of the southwestern United States.  But just the opposite happens in simulations of the impacts of the changes wrought by global warming.  Altered atmospheric circulation patterns push the jetstream — and the tracks of winter storms — farther north, bringing extended drought to the region.

Similarly, as the panels above illustrate, very different precipitation patterns emerge across Asia.  El Niño typically brings drought to Australia and to the normally wet regions of southeast Asia.  Usually, El Niño means wet regions get drier and dry regions get soaked.  But under global warming, the opposite pattern emerges.  Look what happens all across the wet regions of Asia and east Africa — warming spreads more rain rather than less.

University of Miami scientist Pedro DiNezio and colleagues compared the El Niño (ENSO) and global warming simulations of 21 general circulation models and suggested that many of the atmospheric responses between the scenarios are different because the temperature structure of the Pacific below the surface is different.  "Moreover, according to GCM projections, the associated precipitation impacts in response to greenhouse forcing are not El Niño-like" they wrote.  "As such, adherence to an ENSO analogy for interpreting tropical Pacific climate change can lead to serious misconceptions."

Image: Eos, American Geophysical Union

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