The Nimble Atmosphere

Forecasters take into account that the interactions between the ocean and atmosphere are not symmetrical: the nimble atmosphere swiftly responds to changes in sea surface temperature, but the sluggish ocean adjusts slowly to changes in the winds. The ocean has the "memory" of the system and needs to be monitored to anticipate El Niño. The large array of instruments that oceanographers have deployed across the equatorial Pacific enabled them to anticipate the intense El Niño of 1997 several months in advance.

Oscillations in sea surface temperatures (in degrees Centigrade) over the past century, in the eastern equatorial Pacific Ocean (after removal of the seasonal cycle.) El Niño conditions prevailed during the periods in lighter gray above the 25 degree C line, La Niña during the periods in darker gray below the 25 degree C line.

Oscillations in sea surface temperatures (in degrees Centigrade) over the past century, in the eastern equatorial Pacific Ocean (after removal of the seasonal cycle.) El Niño conditions prevailed during the periods in lighter gray above the 25 degree C line, La Niña during the periods in darker gray below the 25 degree C line.

The impact that global warming will have on El Niño is a matter of debate. The properties of El Niño, its intensity and frequency of occurrence, for example, depend on the background state, which includes the time-average intensity of the trade winds, and the spatially averaged depth of the thermocline. The factors that determine those conditions today are poorly understood.

sEE ALso: Equatorial Countercurrent; Southern Oscillation; Trade Winds; Walker Circulation.

BIBLIoGRApHY. Argo: Part of an Integrated Global Observation Strategy, www.argo.ucsd.edu (cited July 2007); Michael Glantz, Currents of Change: Impacts of El Niño and La Niña on Climate and Society (Cambridge University Press, 2001); S. George Philander, El Niño, La Niña, and the Southern Oscillation (Academic Press, Inc., 1990); Tropical Atmosphere Ocean Project, www.pmel.noaa.gov/tao (cited July 2007); Richard H. Grove and John Chappell, eds., El Niño: History and Crisis (Whitehorse Press, 1999).

S. George Philander Princeton University

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