Lec 8: Ekman, Upwelling, Downwelling

• Upwelling

  Vertical motion of water where cold, nutrient-rich water from the depths is brought to the surface

  
• Downwelling


  Vertical motion of water where surface water is forced downward

  
• Ekman Motion


  The deflection of surface currents to the right (in the Northern Hemisphere) or left (in the Southern Hemisphere) of the wind direction

  
• Geostrophic Currents


  Currents that flow parallel to isobars (lines of equal pressure) due to the balance between the Coriolis force and the pressure gradient force

  
• Gyres


  Large, circular patterns of ocean currents formed by wind patterns and forces created by the rotation of the Earth

  
• Wind generates movement in the ocean in two ways: by transferring its energy to the surface water through friction, and by blowing/pushing the surface waters in a given direction, creating a slope on the sea surface → pressure gradient

  
• Generally, about 1-3% of the wind energy is converted to ocean kinetic energy
• Wind-Driven Circulation


  1. Winds generate surface currents

  2. Currents interact with water below, transferring energy down by friction

  3. Coriolis effect deflects each layer, creating an Ekman spiral

  4. Ekman transport is 90 degrees to the right (NH) or left (SH) of the wind

  
• Ekman Spiral

  • Surface current is 45 degrees to the right (NH) or left (SH) of wind direction
  • Speed decreases with depth, so does the deflection
  • At a certain depth, the current can have an opposite direction to the wind that generated it

  
• Ekman Transport

  The net transport of water is 90 degrees to the right (NH) or left (SH) of the wind

  
• Conditions required for Ekman motion: uniform density, constant wind for 1-2 days, shallow water stops Ekman spiral

  
• Upwelling

  • Occurs when winds blow away from an area, causing divergence and water to upwell from below
  • Brings cold, nutrient-rich water to the surface

  
• Downwelling


  Occurs when winds blow towards an area, causing convergence and water to be forced down

  
• Upwelling regions are rich in biological life, while downwelling regions have low productivity
• Geostrophic Currents

  Flow parallel to isobars (lines of equal pressure) due to the balance between the pressure gradient force and the Coriolis force

  
• Geostrophic currents can flow in the opposite direction to the Ekman transport in deeper layers

  
• Gyres


  • Large, circular patterns of ocean currents formed by wind patterns and the Coriolis effect
  • Subtropical gyres flow clockwise (anticyclonic) in the Northern Hemisphere, and counterclockwise (cyclonic) in the Southern Hemisphere
  • Subpolar gyres have the opposite direction

  
• Gyres spin at a near constant speed that represents the average wind input, and their motion is not dissipated by turbulence

  
• In the subtropical gyre

  Coriolis deflection of boundary currents towards the center + Ekman Transport = sea surface mounding up in the tropics, balanced by a pressure gradient force pointed outwards

  
• Subpolar gyres have a depressed sea surface
• The sea surface height maximum appears shifted to the western side of subtropical gyres

  
• Steps to find the direction of the geostrophic current

  1. Know the wind direction

  2. Determine the Ekman transport direction

  3. Find the pressure gradient direction

  4. Draw the flow according to the pressure gradient force

  5. Deflect the flow 90 degrees to the right (NH) or left (SH) to get the geostrophic current