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 Equatorial Current System best defined in the Pacific (basin size, ITCZ) ITCZ is north of the equator, as SE Trades cross hemispheres create divergence just south of the equator and convergence around 4o N Prevailing easterly winds slope the sea surface up to the western end of the basin, creates an eastward directed pressure gradient, counter currents flow eastward in regions of low wind stress (doldrums) This pressure gradient force also drives the Equatorial Undercurrent (wind stress at the surface around the equator is strong but at depth, baroclinic conditions are sufficiently strong to drive the fast flowing current to the east) Coriolis constrains the flow to the equator (meanders can exist)

Key Points Continued: Summary of Chapter 5 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)

Keep In Mind: Currents To Know: 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)

Equatorial Undercurrent (EUC) & Eastward Directed Pressure Gradient 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)

Upwelling In Low Latitudes: Eastern Tropical Atlantic Seasonal variation, migration of ITCZ, strength of SE Trades More permanent upwelling regions associated with westward directed wind stress

Waves: 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) Kelvin

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 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 Surface equatorial kelvin waves travel ~200 m/s Rossby radius of deformation L = c/f High latitudes smaller eddies closely trapped to the coast (increase in planetary vorticity) Low latitudes larger radius

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: 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 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 “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 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 Atmospheric pressure at sea level, zonal and meridional winds, sea-surface temperature, surface air temperature, and the overall cloudiness