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-to-

bottom 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