Lec 5: Temp, Salinity, Ocean VD

• Temperature


  The Earth effectively receives its energy from the sun
• Winds

  • Cause currents and waves
• Incoming solar radiation is not evenly divided over the earth

  
• Variations in incoming solar radiation

  • Daytime x Nighttime
  • Seasons
  • Latitude

  
• Reasons for distribution of incoming solar radiation at different latitudes

  • Angle of impact of solar radiation
  • Earth's surface area
  • Reflection by the ocean
  • Atmospheric path length
  • Albedo (reflected/incoming radiation)

  
• Net surplus of radiation in tropics and deficit at high-latitudes
• Net surplus/deficit of radiation at different latitudes

  Leads to a transport of heat by the oceans and atmosphere from the equatorial regions to the poles

  
• This need to redistribute heat is the main driving force the oceanic and atmospheric circulation
• Seawater


  A solution of many different chemical compounds
• Weathering and erosion of continental rocks, that are then transported to the oceans, is all part of a large, slow (millions of years) cycle linking the land and the oceans

  
• Concentration (mass/volume) of each element in ocean


  Rate it enters and rate it is removed from the oceans
• If the rate of input exceeds the rate of output the concentration increases and vice versa

  
• These processes are slow, but have been occurring for billions of years
• We assume the oceans are in steady-state: quantity in = quantity out
• Dissolved chemicals in seawater

  • Major – >100 ppm
  • Minor – 1 to 100 ppm
  • Trace – < 1 ppm
• Salinity

  The total amount of solid material dissolved in sea water
• Salinity

  Originally expressed as grams of dissolved salts/kilograms of seawater, sometimes given in parts per thousand ‰ or PSU (Practical Salinity Units), but it is dimensionless (no units)
• Ocean seawater has ~35 grams of salt for each kilogram of water
• Processes affecting seawater salinity

  • Precipitation
  • Evaporation
  • Runoff
  • Ice melting
  • Sea ice forming
• Sea Surface Salinity pattern is more complex than temperature because evaporation and precipitation have a complex pattern
• SSS generally highest in sub-tropical regions, lower at the equator and in high-latitude regions, and very high in marginal seas where evaporation dominates
• Atlantic's salinity is higher than the Pacific's
• Ocean circulation is important for sea surface salinity
• Factors controlling ocean surface temperature and salinity

  • Solar radiation
  • Transfer of heat and water with the atmosphere
  • Ice formation/melting
  • Ocean currents
  • Runoff
  • Vertical mixing

  
• Density

  How heavy something is for its size, measured in kg/m3
• Pure water density is ~1000 kg/m3, typical surface ocean density is 1026 kg/m3 (1022 to 1030 kg/m3)
• Density dependence on pressure

  Water is incompressible, only small volume and density changes occur as the pressure changes, except in the deep ocean where pressure over 1000 times the atmospheric leads to a 2% increase in density
• Density dependence on temperature


  Increase in temperature increases the distance between water molecules, causing water to expand and density to decrease. Pure water has a maximum density at ~4°C.
• Density dependence on salinity

  Increasing salinity (more dissolved salts) means an increasing density, as ions or molecules of most substances dissolved in seawater have higher density than water molecules
• Combined effects of temperature and salinity

  • Adding salt lowers the freezing point and inhibits clustering of water molecules, lowering the temperature of maximum density
• Density

  Ions or molecules of most substances dissolved in seawater have higher density than water molecules
• Dissolved substances


  Reduce the clustering of water molecules, increasing density
• Especially important at temperatures near the freezing point where changes in temperature have minimal influence on density
• Adding salt

  • Lowers freezing point
  • Inhibits clustering of water molecules, which lowers the temperature of maximum density
• At 24.7, temperature of maximum density and freezing point are the same
• Above 24.7, freezing point is reached before a density maximum occurs
• Seawater density

  • The slope of the density line is greater for large temperature changes
  • Changes in temperature do not have large effect on the density of cold waters
  • The slope of the density line is greater for salinity than for temperature
  • Most of the ocean's water is cold but salinity does not change much
• Seawater varies with TEMPERATURE, SALINITY, and pressure
• As you decrease temperature, increase salinity and increase pressure, you increase DENSITY
• Equation of State

  A complicated equation describes how ocean density depends on temperature and salinity (and also pressure)