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The El Nino-Southern Oscillation

The El Nino-Southern Oscillation . Outline. 1. General Circulation of the Atmosphere 2. The Walker Circulation 3. The Coriolis Force 4. Air-sea interactions 5. Thermocline and ocean wave dynamics 6. Societal and ecological impacts. Heat balance of the atmosphere.

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The El Nino-Southern Oscillation

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  1. The El Nino-Southern Oscillation

  2. Outline • 1. General Circulation of the Atmosphere • 2. The Walker Circulation • 3. The Coriolis Force • 4. Air-sea interactions • 5. Thermocline and ocean wave dynamics • 6. Societal and ecological impacts

  3. Heat balance of the atmosphere • More solar energy is absorbed In the tropics • per second • than is emitted back to • space per second. • However: • the tropics are not warming up! • Energy must be transported away from the tropics • What transports away the thermal energy (heat)? Energy per time per area 1 W [unit for power] = 1 Nm/s = 1 J/s; 200 W ~ 1 human climbing stairs Power is the rate at which energy is used

  4. Heat transport is accomplished by atmospheric and oceanic motion • To get rid of the excess heat in the tropics, something must carry the heat • air, water vapor, water can carry heat • 1 PW= • 1,000,000,000,000,000 W = • 10,000,000,000,000 • Light bulbs a 100W [PW] atmosphere ocean Atmosphere and ocean have to be in motion. A dynamic balance.

  5. The meridional atmospheric circulation: trade winds, jet streams, westerlies, Hadley Circulation

  6. The Hadley Circulation Warm moist air rises in the tropical Intertropical Convergence Zone (ITCZ); while it ascends it cools and precipitation forms; cold and dry air flows poleward and sinks in the subtropics; The supply of air in the tropics is maintained by the trade winds; This meridional circulation cell, which exists in the northern and southern hemisphere is called Hadley Cell after George Hadley (1685-1768)

  7. Annual Means (Jan-Dec) Surface Atmospheric Winds and rainfall Aleutian Low Westerlies Trade wind Trade wind Westerlies Trade winds converge in the Intertropical Convergence Zone (ITCZ)

  8. Sea Surface Temperature and rainfall What drives the Hadley circulation? Warmest waters => largest convection => largest rainfall Rainfall means condensated water vapor => condensation energy warms air => rising motion

  9. The zonal tropical Walker Circulation Vertical velocity at equator Height [hPa] West Pacific: Warm ocean Rising air Lots of precipitation East Pacific: Cold ocean descending air dry

  10. The zonal tropical Walker Circulation Westward (easterly) surface trade winds on the equator Low sea level pressure in western Pacific, High sea level pressure in eastern Pacific Walker

  11. Variations of the Walker Circulation • How does a change in western/eastern Pacific sea surface temperature affect the strength of the Walker circulation and the strength of the equatorial trade winds? • How does a change in sea level pressure (SLP) in the western Pacific affect the sea level pressure in the eastern Pacific? • Walker correlated the SLP variations in the western Pacific with SLP variations elsewhere • The result led to an astonishing discovery….

  12. A tropical pressure seesaw Surface Air Pressure Pattern Walker There is an east-west pressure seesaw - called the Southern Oscillation. If SLP is anomalously high (low) in the Western tropical Pacific, it will be anomalously low (high) in the Eastern Tropical Pacific, this varying pressure difference drives changes in the equatorial trade winds

  13. The tropical Pacific pressure seesaw Sea level pressure in west Pacific and eastern tropical Pacific are anti-correlated The strength of this pressure dipole characterizes the strength of the equatorial trade winds

  14. The Southern Oscillation index (SOI) Walker could not explain the 3-4 yr periodicity of the SOI, nor could he explain the spatial pattern. Hence the SOI met with skepticism that it was a real phenomenon, rather than a statistical artifact. Darwin Tahiti The strength of the equatorial trade winds is proportional to the SOI Trade wind changes want to move ocean waters. But how…? Southern Oscillation Index (SOI) =Tahiti - Darwin surface air pressure

  15. The Coriolis Force: an “apparent force”

  16. The Coriolis Force: a real-life experience Counter-clockwise

  17. The Coriolis Force Freely moving objects on the surface of the Earth experience the Coriolis force, And appear to veer to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Rather than flowing directly from high pressure to low pressure their path is bent for the rotating observer. The Coriolis Force can be computed from Fc= 2 x mass x velocity x 2 x π / (rotation period) sin (latitude) Coriolis force is zero on equator or for a non-moving object (for rotating observer)

  18. Equatorial Easterlies drive ocean upwelling: an application of Coriolis Equatorial trade winds proportional to SOI X Ekman divergence Depth Meridional Velocity Cold water is upwelled in eastern equatorial Pacific because of easterly winds Easterly winds are there because of zonal temperature difference

  19. Tropical air-sea coupling • Higher SST in east Pacific • weakening of pressure gradient • Weakening of Walker Circulation • Weakening of trade winds • Weakening of upwelling of cold waters • Higher SST in east Pacific • Lower SST in east Pacific • strengthening of pressure gradient • Strengthening of Walker Circulation • Strengthening of trade winds • Strengething of upwelling of cold waters • Lower SST in east Pacific Winds SST

  20. ENSO: merging the Southern Oscillation and eastern equatorial Pacific ocean warming • El Niño arises from coupled interactions between the ocean and the atmosphere • Involves the entire tropical Pacific, not just coastal SA • Causes shifts in tropical rainfall patterns that can affect mid-latitude climate through atmospheric teleconnections • Might be predictable Strong pressure difference & strong trade winds Weak pressure difference & weak trade winds El Niño La Niña J. Bjerknes, 1966, 1969, 1972 ENSO = El Nino-Southern Oscillation

  21. Definition of El Nino El Niño: A phenomenon in the equatorial Pacific Ocean characterized by a positive sea surface temperature departure from normal (for the 1971-2000 base period) in the Niño 3.4 region greater than or equal in magnitude to 0.5 degrees C (0.9 degrees Fahrenheit), averaged over three consecutive months. La Niña: A phenomenon in the equatorial Pacific Ocean characterized by a negative sea surface temperature departure from normal (for the 1971-2000 base period) in the Niño 3.4 region greater than or equal in magnitude to 0.5 degrees C (0.9 degrees Fahrenheit), averaged over three consecutive months. Nino 3.4 box El Nino is the warm phase, La Nina the cold phase of ENSO

  22. What is happening below the surface? Sea level 30cm higher Thermocline shallow In EEP during normal conditions Sea Surface Height Temperature front In upper ocean = thermocline Temperature Temperature section on equator

  23. What is happening below the surface? Weakening of equatorial trade winds Thermocline deep In EEP during El NIno Temperature Temperature section on equator Temperature section on equator

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