Chapter 6 - Temperature, Salinity, and Density
6.11 Important Concepts
- Density in the ocean is determined by temperature, salinity, and
- Density changes in the ocean are very small, and studies of water
masses and currents require density with an accuracy of 10 parts per
- Density is not measured, it is calculated from measurements of temperature,
salinity, and pressure using the equation of state of sea water.
- Accurate calculations of density require accurate definitions of
temperature and salinity and an accurate equation of state.
- 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,
- A mixed layer of constant temperature and salinity is usually found
in the top 1–100m of the ocean. The depth is determined by wind
speed and the flux of heat through the sea surface.
- 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
- Water parcels below the mixed layer move along neutral surfaces.
- 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.
- Temperature and conductivity are usually measured digitally as a
function of pressure using a CTD. Before 1960–1970 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
- 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.
- 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.