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El Ni ñ o, La Ni ñ a and the Southern Oscillation. Some History. In the early 20 th Century, Sir Gilbert Walker discovered a recurrent pattern in sea level pressure data from the Pacific region. He called this pattern “The Southern Oscillation.”. The Southern Oscillation. Sir Gilbert Walker
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Some History • In the early 20th Century, Sir Gilbert Walker discovered a recurrent pattern in sea level pressure data from the Pacific region. • He called this pattern “The Southern Oscillation.”
The Southern Oscillation Sir Gilbert Walker (1868-1958) Tahiti Darwin
The Oceanic Connection Jacob Bjerknes(1897-1975)
Ekman Transport • Balance between surface wind stress and Coriolis force • In the Northern Hemisphere, the Ekman transport is directed to the right of the flow. (To the left in the Southern Hemisphere)
Coastal Upwelling • Motion of surface waters away from coast requires upwelling of water from below to satisfy continuity of mass. Andes Mts. S. Pacific Ocean
Observed SST Distribution Winds have amajor influenceon tropical SSTpattern. Equatorial Upwelling Coastal Upwelling
SST and Atmospheric Circulation Sinking air;very littleprecipitation Rising air;clouds andprecipitation Walker Circulation Warm Cold Eastern Equatorial Pacific Western Equatorial Pacific
In The Tropics… • Sea surface temperatures are strongly influenced by surface winds. • Atmospheric circulation is strongly influenced by the sea surface temperatures. • Therefore… Strong air-sea interactions are possible
La Niña conditions:Strong cold tongue El Niño conditions:Cold tongue absent
Initiation Peak Decay El Niño’s Life Cycle Source: IRI
ENSO Events Can Evolve Differently Spring 2013
Thermocline Fluctuations Cross-section of temperatures and currents along Equator
Subsurface Structure Source: NASA Goddard Space Flight Center
The delayed oscillator The leading theoretical model is the delayed oscillator [see Battisti and Hirst, 1989]: Here, Ts is the temperature in the East Pacific, b and c are positive constants, and is a time-lag determined by equatorial oceanic adjustment. • The first term on the RHS can be thought of a representing a positive feedback associated with the atmosphere, e.g., the large-scale Darwin-Tahiti pressure difference (the SOI). • The second term represents a negative feedback associated with thermocline adjustment via equatorial waves. • The time delay is the time required for Rossby waves to propagate westward, reflect at the boundary, and return to the region of origin.
Equatorial Kelvin & Rossby Waves Surface currents (l) and thermocline displacements (r) for a Gaussian perturbation 2-layer oceanic SWE model Kelvin wave: Non-dispersive, eastward propagating (~2 m/s for H = 150 m) Rossby waves: Dispersive, westward propagating (fastest is 1/3 of Kelvin wave group velocity)
Propagating equatorial waves http://iri.columbia.edu/climate/ENSO/enso.html
ENSO Teleconnections ENSO 500mb Geopotential Pattern PNA 300mb Geopotential Pattern
ENSO and U.S. Climate(Winter Season) El Niño La Niña Temperature Precipitation
More Information • The International Research Institute for Climate Prediction at Columbia University’s Lamont-Doherty Earth Observatory has an excellent web site with information on El Niño. • http://iri.columbia.edu/climate/ENSO/