Surface Current – Geostrophic Gyre
q Mixed Layer
      Currents controlled by frictional force + Coriolis force
      à wind-driven circulation
      à Ekman transport (horizontal direction)
      à convergence/divergence
      à downwelling/upwelling at the bottom of mixed layer
q  Thermocline
      downwelling/upwelling in the mixed layer
      à pressure gradient force + Coriolis force
      à geostrophic current
      à Sverdrup transport (horizontal)