Lec 6: Water Masses

• Density Layering


  Stratification is the vertical density layering of a water column

  
• Mixed layer


  • Can vary in depth: winds, stability
• Density Tower

  • Waters of different density at different depths
  • Less dense water tends to float atop more dense water (buoyancy)
• Circulation systems in the ocean

  • Wind-driven surface circulation
  • Deep-water density-driven circulation

  
• Only about 10% of the ocean volume is involved in wind-driven surface currents

  
• The other 90% circulates due to density differences in water masses
• Water mass


  • A body of water with a specific set of characteristics/properties (temperature, salinity, oxygen and/or nutrients)
  • Properties are imprinted by processes at the surface (mixed layer) in specific locations, and are mostly conserved (especially T and S) after the water mass sinks
  • In the interior, the properties will only change by mixing with other waters
  • The properties can be used to trace a water parcel's motion, like a signature
• 1. S plot

  • Used to identify water masses
  • Temperature and salinity combine to form a waters density (ρ)
  • Density is represented by isopycnals - lines of equal density
  • Many combinations of T& S give the same ρ

  
• How a water mass is defined

  1. Identify an extremum or interesting central characteristic

  2. Find the process that created that characteristic

  3. Use additional information about the process to refine the water mass's definition
• Ventilation


  Connection between the surface and the ocean interior
• Motion of water masses

  • Essentially horizontal - along isopycnals
  • A parcel of water moves locally along a path of constant density keeping below less dense water and above more dense water
• Subduction / Deep Convection


  1. Winds at subtropics cause water in the mixed layer to be pumped downwards

  2. This water is NOT denser than the underlying water → it gets injected into intermediate depths, following the isopycnal surface of its own density

  
• Deep Convection


  1. Process that forms deep waters

  2. Occurs on scales of km's to tens of km's

  3. Two types: Near-boundary convection and Open-ocean convection

  
• The Indian Ocean does not reach high latitudes, so there is no Deep Water formation
• The North Pacific is isolated from polar regions by land, island chains and shallow water, and has high precipitation meaning low salinity, so there is no Deep Water formation
• Circumpolar deep water (CDW)

  A mix of NADW and AABW that lines the basins
• Deep Water Formation in the Southern Ocean


  1. Rapid, winter freezing produces very cold, high-density water that sinks down the continental slope of Antarctica to become Antarctic Bottom Water (AABW) - the densest water in the open ocean

  2. AABW sinks beneath the surface and spreads into all the world's ocean basins

  
• Deep Water Formation in the North Atlantic


  1. Intense cooling occurs in the Labrador, Greenland and Norwegian Seas to form North Atlantic Deep Water (NADW)

  2. NADW then sinks and flows south filling the world's oceans (most voluminous water mass)

  3. Higher salinity and temperature than more dense AABW

  
• Pycnocline


  • When sharp, there is little motion across it, as large amounts of energy are needed to move water across strong density gradients
  • Thus water masses trend to flow mainly horizontal
  • Deep waters are effectively isolated from the surface
• High latitude ocean structure


  • Generally no pycnocline because of strong heat loss to the cold atmosphere above the ocean
  • This leads to the surface waters having a high density, causing them to sink and form intermediate and deep waters
  • Since motion is mainly horizontal (except for this sinking) we can determine where water originates from in the ocean by tracing contour of salinity and temperature back to where that water last was in contact with the atmosphere

  
• When water sinks, the water above it has lower density and the water below it has higher density, keeping each layer in the ocean separate as a distinct water mass
• Temperature, salinity and density only vary slightly in a water mass as it moves away from the source, with the main changes due to vertical mixing in the interior