El Niño, La Niña and the Southern Oscillation Mechanisms of Past Climate Change (16:375:553) 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.” Mechanisms of Past Climate Change (16:375:553) Spring 2013 The Southern Oscillation Sir Gilbert Walker (1868-1958) Darwin Mechanisms of Past Climate Change (16:375:553) Tahiti Spring 2013 The Oceanic Connection Jacob Bjerknes (1897-1975) Mechanisms of Past Climate Change (16:375:553) 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) Mechanisms of Past Climate Change (16:375:553) Spring 2013 Equatorial Upwelling Mechanisms of Past Climate Change (16:375:553) Spring 2013 Coastal Upwelling • Motion of surface waters away from coast requires upwelling of water from below to satisfy continuity of mass. Mechanisms of Past Climate Change (16:375:553) Andes Mts. S. Pacific Ocean Spring 2013 Observed SST Distribution Winds have a major influence on tropical SST pattern. Equatorial Upwelling Coastal Upwelling Mechanisms of Past Climate Change (16:375:553) 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 Mechanisms of Past Climate Change (16:375:553) 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 Mechanisms of Past Climate Change (16:375:553) Spring 2013 Mechanisms of Past Climate Change (16:375:553) Spring 2013 Mechanisms of Past Climate Change (16:375:553) Spring 2013 Mechanisms of Past Climate Change (16:375:553) Spring 2013 Mechanisms of Past Climate Change (16:375:553) Spring 2013 La Niña conditions: Strong cold tongue El Niño conditions: Cold tongue absent Mechanisms of Past Climate Change (16:375:553) Spring 2013 Recent History of El Niño and La Niña Mechanisms of Past Climate Change (16:375:553) Spring 2013 Recent History of El Niño and La Niña Mechanisms of Past Climate Change (16:375:553) Spring 2013 El Niño is Quasiperiodic Mechanisms of Past Climate Change (16:375:553) Spring 2013 Current SST Anomalies Mechanisms of Past Climate Change (16:375:553) Spring 2013 El Niño’s Life Cycle Initiation Mechanisms of Past Climate Change (16:375:553) Peak Source: IRI Decay Temporal Evolution of El Niño/La Niña Mechanisms of Past Climate Change (16:375:553) Spring 2013 ENSO Events Can Evolve Differently Mechanisms of Past Climate Change (16:375:553) Spring 2013 Thermocline Temperatures and Anomalies Mechanisms of Past Climate Change (16:375:553) Spring 2013 Thermocline Fluctuations Cross-section of temperatures and currents along Equator Mechanisms of Past Climate Change (16:375:553) Spring 2013 Subsurface Structure Source: NASA Goddard Space Flight Center Mechanisms of Past Climate Change (16:375:553) The delayed oscillator The leading theoretical model is the delayed oscillator [see Battisti and Hirst, 1989]: ¶Ts(t) = bTs(t) - cTs(t - t ) ¶t 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 DarwinTahiti 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. Mechanisms of Past Climate Change (16:375:553) Equatorial Kelvin & Rossby Waves 2-layer oceanic SWE model Surface currents (l) and thermocline displacements (r) for a Gaussian perturbation c g [Kelvin] = g¢H ; g¢ = g(r2 / r1 -1) Kelvin wave: Non-dispersive, eastward propagating (~2 m/s for H = 150 m) c g [Rossby] = g¢H /(2l +1) ; l =1,2,... Mechanisms of Past Climate Change (16:375:553) Rossby waves: Dispersive, westward propagating (fastest is 1/3 of Kelvin wave group velocity) Propagating equatorial waves Mechanisms of Past Climate Change (16:375:553) http://iri.columbia.edu/climate/ENSO/enso.html ENSO and Global Climate Mechanisms of Past Climate Change (16:375:553) Spring 2013 ENSO Teleconnections ENSO 500mb Geopotential Pattern Mechanisms of Past Climate Change (16:375:553) PNA 300mb Geopotential Pattern ENSO and Global Climate Mechanisms of Past Climate Change (16:375:553) Spring 2013 ENSO and U.S. Climate (Winter Season) El Niño La Niña Temperature Precipitation Mechanisms of Past Climate Change (16:375:553) 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/ Mechanisms of Past Climate Change (16:375:553) Spring 2013