Density Structure of
the Ocean
Density Controls
• Controlled by:
• 1.) Temperature
• 2.) Salinity
• 3.) Pressure
• Density increases with increasing salinity
• Density increases with decreasing temperature.
• Density increases with increasing pressure.
• Density of oceans ranges from 1.020 g/cm3 to 1.030
g/cm3
Three Density Zones
• Surface Zone (mixed layer)-Upper layer of the ocean
• Temperature and salinity are constant because of mixing by wind
and waves.
• Top 100 meters (photic zone)
• 2% of ocean water by volume
• Pycnocline Zone-Middle layer of the ocean
• Density increases rapidly with depth
• 18% of ocean water by volume
• Depth varies with latitude.
• Temperature drops rapidly in this zone (thermocline). Salinity
increases rapidly (halocline).
• Deep Zone-Deepest ocean layer extending to the ocean floor.
• Relatively constant density
• 80% of ocean water by volume
• Temperature and Salinity constant
Density, Temp., Salinity Changes with
Depth
Pycnocline
• Zone in which
density increases
with increasing
depth.
• From 0-1,000 meters
• Increasing density
farther down as
temperature gets
colder.
Halocline
• Zone of rapid salinity
increase or decrease
with depth.
• Often coincides with
thermocline.
Halocline
Thermocline
• Temperature changes
rapidly with depth.
• Both thermocline
and halocline
contribute to form
pycnocline since
temperature and
salinity affect density.
Changes with Latitude
Thermohaline Circulation
• Vertical water circulation in the oceans. Results in top-tobottom exchange of water.
• Driven by density (Density Currents)
• Thermohaline: Thermo (temperature) Haline (Salinity).
Upwelling
• Upwelling zones are areas where thermohaline circulation is
upward toward the ocean surface.
•
•
•
•
Cold water and nutrients are brought to the surface.
Increased diversity of life in upwelling zones
Great fishing in upwelling zone.
Upwelling zones include areas of the west coast of California,
west coast of South America, and South Africa.
• Caused by wind blowing water off the surface and water
moving upward to take its place.
Upwelling
Downwelling
• Downwelling zones are areas where thermohaline circulation
is downward.
Vertical Water Masses
•
•
•
•
•
Surface Water: 0-200 meters
Central Water: To the bottom of the thermocline
Intermediate Water: to about 1,500 meters
Deep water: to about 4,000 meters
Bottom water: in contact with the seafloor
NADW: North Atlantic Deep Water:
-Forms at 50-60 degrees north latitude
-34.9 PPT salinit
-2 to 4 degrees celsius
-Sinks and moves south
AABW: Antarctic Bottom Water (1,000 years to go from south pole to equator)
- 34.8 ppt salinity
- is -0.5 degrees celsius
AAIW: Antarctic Intermediate Water
-Forms at 40 degrees south
- is 5 degrees celcius and salinity of 34.4 ppt