Chapter 5: Other Major Current Systems. Key Points: Summary of Chapter 5. Components of the equatorial current system include: westward flowing N,S equatorial currents (driven by trades and geostrophy) eastward flowing counter current (surface/subsurface), and undercurrent
In the Pacific and Atlantic surface divergence south of the equator associated with the South Equatorial Current (SEC) produces regions of upwelling.
Upwelling is also produced by Trade Winds blowing along the eastern side of the basins. This can be seasonal in regions where the ITCZ migrates
The Asian Monsoons influence the circulation of the Indian Ocean. The Equatorial Undercurrent is seasonal, and the surface Somali Current reverses direction becoming an intense western boundary current during the Southwest Monsoon. The North Equatorial Current also reverses direction and become the South-West Monsoon Current
Disturbances in the ocean (such as Monsoons, ENSO, etc..) in part, propagate as Kelvin and Rossby waves both at the surface (barotropic waves) and along density boundaries (baroclinic waves)
Kelvin waves travel eastward along the equator or along the coast (NH coast on right side, SH coast on left side)
Rossby waves (Planetary waves, conservation of potential vorticity) travel westward along lines of latitude (slower)
Coriolis Force on the Equator is zero
Coriolis Force by 0.5o influences flow of water
Currents To Know:
North Equatorial Current (NEC)
South Equatorial Current (SEC)
Equatorial Counter Current (ECC)
Equatorial Under Current (EUC)
Wind driven water from the surface mixed layer piled on the western side of the basin
Wind stress balances the pressure gradient (Coriolis Force ~= 0 at equator)
Adjustment (depression) of the thermocline on the western end
Baroclinic conditions at depth drive a jet-like current eastward eventually balance by friction (eddy viscosity)
Seasonal variation, migration of ITCZ, strength of SE Trades
More permanent upwelling regions associated with westward directed wind stress
Waves: Gradient the ocean can respond to the winds in distant places by means of large-scale disturbances that travel as waves.
Barotropic: surface waves
Baroclinic: density surface (thermocline)
Rossby (Planetary Waves)
Examples of barotropic and baroclinic waves propagating through the ocean
Most tides are barotropic ‘Kelvin’ waves
Think about what would happen if the wind stress was dramatically reduced or changed directions in the case of the Asian Monsoon
Kelvin Waves through the ocean
Travel eastward along the equator as a double wave ‘equatorial wave guide’
Travel along coasts (coast on right in the NH and on the left in the SH)
Balance between pressure gradient force and coriolis force.
Kelvin waves in the thermocline can have dramatic effects, particularly in low latitudes where the mixed surface layer is thin.
Northward migration of ITCZ in western Atlantic generates disturbance that propagates eastward
Splits into two coastal Kelvin waves when hits the eastern boundary
The region of the disturbance where the thermocline bulges upward cold nutrient rich sub-thermocline water can reach the surface
4-6 week travel time
Rossby Waves: particularly in low latitudes where the mixed surface layer is thin.
Propagate from east to west across basin
Travel along lines of latitude
Move slower than Kelvin waves
Conservation of Potential Vorticity
Example: Waves in the jet-stream
Modeled Propagation of Equatorial Kelvin Wave particularly in low latitudes where the mixed surface layer is thin.
At mid latitudes - western sides of ocean basins are more connected to mid ocean disturbances because Rossby Waves can communicate the information
At the equator the ocean can respond quicker to disturbances because both Kelvin and Rossby waves can propagate
ENSO: El Nino – Southern Oscillation particularly in low latitudes where the mixed surface layer is thin.
“Interest in the phenomenon of El Nino goes back to the mid-19th century but it was the El Nino event of 1972-73 that stimulated large-scale research into climatic fluctuations, which began to be seen as a result of the interaction between atmosphere and ocean.”
El Nino events are perturbations of the particularly in low latitudes where the mixed surface layer is thin.ocean-atmosphere system
Disturbance – a depression in the thermocline accompanied by a slight rise in sea-level propagates eastwards along the Equator as a pulse or series of pulses (Kelvin Waves)
SO Index and Multivariate ENSO Index particularly in low latitudes where the mixed surface layer is thin.
Atmospheric pressure at sea level, zonal and meridional winds, sea-surface temperature, surface air temperature, and the overall cloudiness