Chapter 6 - Temperature, Salinity, and Density

Chapter 6 Contents

6.11 Important Concepts

  1. Density in the ocean is determined by temperature, salinity, and pressure.

  2. Density changes in the ocean are very small, and studies of water masses and currents require density with an accuracy of 10 parts per million.

  3. Density is not measured, it is calculated from measurements of temperature, salinity, and pressure using the equation of state of sea water.

  4. Accurate calculations of density require accurate definitions of temperature and salinity and an accurate equation of state.

  5. Salinity is difficult to define and to measure. To avoid the difficulty, oceanographers use conductivity instead of salinity. They measure conductivity and calculate density from temperature, conductivity, and pressure.

  6. A mixed layer of constant temperature and salinity is usually found in the top 1100m of the ocean. The depth is determined by wind speed and the flux of heat through the sea surface.

  7. To compare temperature and density of water masses at different depths in the ocean, oceanographers use potential temperature and potential density which remove most of the influence of pressure on density.

  8. Water parcels below the mixed layer move along neutral surfaces.

  9. Surface temperature of the ocean was usually measured at sea using bucket or injection temperatures. Global maps of temperature combine these observations with observations of infrared radiance from the sea surface measured by an AVHRR in space.

  10. Temperature and conductivity are usually measured digitally as a function of pressure using a CTD. Before 19601970 the salinity and temperature were measured at roughly 20 depths using Nansen bottles lowered on a line from a ship. The bottles carried reversing thermometers which recorded temperature and depth and they returned a water sample from that depth which was used to determine salinity on board the ship.

  11. Light is rapidly absorbed in the ocean. 95% of sunlight is absorbed in the upper 100m of the clearest sea water. Sunlight rarely penetrates deeper than a few meters in turbid coastal waters.

  12. Phytoplankton change the color of sea water, and the change in color can be observed from space. Water color is used to measure phytoplankton concentration from space.

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