Lecture 18: El Niño Atmosphere, Ocean, Climate Dynamics EESS 146B/246B
El Niño Transient equatorial motions: Kelvin and Rossby waves. Positive feedback mechanism associated with air-sea coupling. Atmospheric teleconnections.
Distribution of the wind-stress and SST in the tropics The easterly trade winds drive a divergent Ekman transport near the Equator. This equatorial upwelling lowers the SST, but primarily in the eastern part of the tropical oceans. The regions of cool temperatures in the east are known as cold tongues.
Zonal pressure gradient at the equator. SSH Near the surface the pressure gradient force opposes the westward frictional force associated with the easterly winds. What would happen if the easterly winds were to relax?
Example of a westerly wind burst
SSH anomalies associated with westerly wind-bursts Animation courtesy of Lee Fu JPL Colors denote anomalies in the sea surface height as measured from space
Equatorial Kelvin wave An equatorial Kelvin wave is a large-scale wave in SSH, thermocline depth, and zonal velocity that is confined to several degrees of the equator. By definition a Kelvin wave has no meridional velocity. Equatorial Kelvin waves can only propagate to the east. Thermocline depth Equatorial Rossby Radius of Deformation ~2 m/s Mean thermocline depth ~400 km
Observed meridional structure of equatorial Kelvin waves Meridional structure of SSH associated with equatorial Kelvin waves with different periods. Figure from Farrar (2008)
Adjustment of thermocline Thermocline mirrors SSH zero pressure gradient no flow at depth. Water waters rush to the east El Niño.
El Niño/Southern Oscillation Oscillation in SST, sea-level pressure, winds, air temperature, cloudiness, etc. in the tropical Pacific. Time scale 3-7 years.
Example of a westerly wind burst Off the equator the westerly wind burst sets up a divergent Ekman transport which lowers the SSH there.
Rossby waves Slow time-dependent motions that satisfy the Taylor-Proudman theorem propagate west Waves that satisfy this equation are known as ROSSBY WAVES, and play a key role in the transient acceleration of the ocean circulation.
Rossby wave propagation April 13, 1993 July 31, 1993 Fig. from Chelton and Schlax 1996 Rossby waves near the equator take ~6 months to cross the Pacific. Rossby waves with negative SSH anomalies that hit the western boundary will reflect as an eastward propagating Kelvin wave of opposite sign to the original Kelvin wave.
Delayed-oscillator theory Upwelling Kelvin wave generated by reflection of Rossby wave cancels warming in the east. The whole cycle takes less than a year much shorter than the 3-7 year time scale of El Nino. What is missing?
Air-sea coupling, deep convection and the Walker circulation The pile up of warm water to the west drives deep convection in the atmosphere setting up the Walker circulation. The lower branch of the Walker circulation is associated with easterlies that thus reinforce the SST gradient positive feedback.
El Niño o begins Another positive feedback is initiated L Trades H
El Niño o begins Another positive feedback is initiated L Trades H
El Niño o begins Another positive feedback is initiated L Trades H
El Niño o begins Another positive feedback is initiated L Trades H
El Niño Eventually, the Warm Pool spreads over Eq. Pacific
Air-sea coupling Coupling modifies the amplitude and propagation speed of the wave.
Atmospheric teleconnections Fig. from Horel and Wallace JAS (1981) The disturbance in SLP in the tropics associated with El Nino triggers Rossby waves in the atmosphere that propagate in the north Pacific to the northeast. The pressure disturbances intensify the Aleutian Low and shift the Jet Stream south, lining up the storm track with California.