El Niño, La Niña and the Southern Oscillation Spring 2013
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.” Spring 2013
The Southern Oscillation Sir Gilbert Walker (1868-1958) Tahiti Darwin Spring 2013
The Oceanic Connection Jacob Bjerknes (1897-1975) Spring 2013
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) Spring 2013
Equatorial Upwelling Spring 2013
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 Spring 2013
Observed SST Distribution Winds have a major influence on tropical SST pattern. Equatorial Upwelling Coastal Upwelling Spring 2013
SST and Atmospheric Circulation Rising air; clouds and precipitation Sinking air; very little precipitation Walker Circulation Warm Cold Western Equatorial Pacific Eastern Equatorial Pacific Spring 2013
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 Spring 2013
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La Niña conditions: Strong cold tongue El Niño conditions: Cold tongue absent Spring 2013
Recent History of El Niño and La Niña Spring 2013
Recent History of El Niño and La Niña Spring 2013
El Niño is Quasiperiodic Spring 2013
Current SST Anomalies Spring 2013
El Niño’s Life Cycle Initiation Peak Decay Source: IRI
Temporal Evolution of El Niño/La Niña Spring 2013
ENSO Events Can Evolve Differently Spring 2013
Thermocline Temperatures and Anomalies Spring 2013
Thermocline Fluctuations Cross-section of temperatures and currents along Equator Spring 2013
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 and Global Climate Spring 2013
ENSO Teleconnections ENSO 500mb Geopotential Pattern PNA 300mb Geopotential Pattern
ENSO and Global Climate Spring 2013
ENSO and U.S. Climate (Winter Season) El Niño La Niña Temperature Precipitation Spring 2013
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/ Spring 2013