Lec 16: Global Ocean Circulation

• Atmospheric Circulation Model


  • Differential solar heating over latitude
  • Cold dense air sinks near poles
  • Warm light air rises near equator
  • Horizontal pressure gradients drive winds
  • Pressure is a 'pushing force' (pressure doesn't suck!)
  • Real picture modified by topography and Coriolis etc.
• Ocean currents

  • Driven by density gradients below the surface layer
  • Ocean density is determined by the Equation of State (temperature, salinity, pressure)
  • Temperature: heat gains and loses mainly occur at the ocean's top surface (air-water interface)
  • Salinity: freshwater/salt gains or loses mainly occur at the ocean's top surface (air-water interface)

  
• Thermohaline Circulation

  • Density-driven flow
  • Formed by cooling of salty water by low air temperatures at high latitudes
  • Also formed by intense evaporation in hot basins (e.g. Mediterranean Sea)
  • Dense water will sink to level of neutral buoyancy and spread out laterally due to horizontal pressure gradients
  • Water properties can only change by molecular diffusion or turbulent mixing
• Thermohaline Circulation


  1. Water is cooled near the poles, where it sinks

  2. It is transported in the deep ocean where it mixes with other waters

  3. It returns to the surface far from where it sank

  4. It is then transported back on the surface to the high latitudes, where it cooled to start the cycle again
• Global Ocean Circulation / Thermohaline Circulation

  • Carries heat and dissolved chemicals (e.g. salt, CO2, O2, nutrients) around the planet
  • Transports properties between the surface and deep ocean (deep ocean ventilation)
  • Transports heat, important in regulating climate and can cause climate variability
• Different names for the circulation

  • Ocean conveyor belt
  • THC - Thermohaline circulation
  • MOC - Meridional circulation
  • AMOC - Atlantic meridional circulation

  
• Complete circuit is estimated to take hundreds of years to ~1000 years
• Transports large amounts of water (order of 20 Sv) but the velocities are very small (order of 10-20 km/year) when compared to surface currents

  
• AMOC

  Atlantic → two cells: NADW cell and AABW cell
• NADW formation

  1. In the North Atlantic (Labrador and Nordic Seas), warm salty water is cooled in winter by cold winds out of the Arctic

  2. Increases the density of the surface water and causes it to sink, forming North Atlantic Deep Water (NADW)

  3. NADW flows south through the deep Atlantic, mainly in the deep western boundary current

  
• AABW formation


  1. Water formed in the Southern Ocean also travels north in the Atlantic's deep western boundary current, below NADW and more offshore

  2. AABW does not make it all the way up to northern North Atlantic; it "upwells" in the NADW layer and disappears in North Atlantic subtropical region

  
• MOC travel path

  1. Once it reaches the ACC, the water flows around (Antarctica) one or more times (part of Circumpolar Deep Water – CDW)

  2. Part of it upwells around Antarctica (Antarctic Divergence)

  3. Upwelled water either goes south and sinks again with AABW or goes north and forms AAIW

  4. AAIW will progress northward as surface water and will sink again as part of NADW

  5. Another part of the water travelling with the ACC goes north into the Pacific and Indian Oceans at depth

  6. As the deep water flows into the Pacific and Indian Oceans, it mixes and upwells back towards the surface or get mixed in the CDW again

  7. Return of surface flow to the Atlantic → from ACC or from Pacific-Indian route

  8. As the water comes north through the sub-tropics, it is exposed to lots of evaporation, giving it a high salinity by the time in reaches the northern regions

  
• The THC cartoon has both advantages and limitations
• Cold water can dissolve more oxygen than warm water
• Deep-water circulation brings dense, cold, oxygen-enriched water from the surface to the deep ocean
• During its time in the deep ocean, deep water becomes enriched in nutrients as well (nutrients are higher in the deep ocean due to decomposition and reduced use)
• If high latitude surface waters did not cool and sink and eventually return from the deep ocean to the surface the distribution of life in the ocean would be very different

  
• There would be very little macroscopic life in the deep ocean because there would be no oxygen for organisms to breathe
• Life in surface waters may be significantly reduced without circulation of deep water that brings nutrients to the surface