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Wind-driven Currents: Boundary Currents
  • Trade winds set tropical water into west- ward motion --> equatorial currents
  • Deflection by Coriolis Effect + reflection at continents --> western boundary currents = warm water currents
  • West Wind Zone (20°-60°) --> create eastern boundary currents = cold water currents
  • Western boundary currents ca. 10x faster than eastern boundary currents
  • Eastern and western boundary currents enclose subtropical gyre


Wind-driven Currents: Ekman Spiral & Transport

  • Fridjof Nansen (Fram) observed that arctic sea ice moved 20-40º to the right of wind direction. He passed his observations to Wilfrid Ekman
  • Due to Coriolis effect surface water moves 45º to wind direction
  • Dynamic energy is passed to deeper water layers
  • Energy is dissipated due to friction --> depth at which current speed = 0: 
    Ekman depth
  • Coriolis force causes gradual deflection to the right (northern hemisphere) while passing energy further into deeper layers: Ekman spiral
  • Net water transport over Ekman depth is 90º to wind direction