Unit 3b – Deep Circulation of
Oceans
1/14 (a-day) – 1/15 (b-day)
Unit 3a Vocab Quiz
•Thermocline
•Pycnocline
•Wind-stress
•Coriolis-force
1/14 – 1/15 Bellringer
Answer the following on page 41 of your journal
“What is the natural process that causes hurricanes in
the northern and southern hemisphere to spin in
opposite directions?”
Coriolis effect
Unit 3b – Deep Circulation of
Oceans
1/14 (a-day) – 1/15 (b-day)
Seawater, Temperature, Salinity and Density
• Water in oceans can sink, rise or flow horizontally relative to the
surrounding water.
• The horizontal movement of ocean water is called an ocean current.
• They can be localized or as broad as the oceans themselves.
• Surface currents flow only in
the surface layer of the
ocean.
• Other currents flow in the
deep ocean
Seawater, Temperature, Salinity and Density
• Surface currents flow much faster than deep currents
• Deep currents affect a much greater thickness of the ocean
• The movement of deep ocean water is controlled by the density of
the water. The density is determined by temperature and salinity.
•
•
•
•
Warm = less dense
Cold = more dense
Low Salinity = less dense
High Salinity = more dense
• A combination of warm/cold and low/high salinities (saltiness) can
create a variation in density of the ocean water.
Seawater, Temperature, Salinity and Density
• The sun heats the earth’s surfaces
unequally because of the tilt of the
axis.
• At low latitudes (close to equator)
it is nearly directly overhead
• At high latitudes (further from the
equator) the sun is at a very low
angle.
• During winter the sun does not
shine at all north of the arctic circle
or south of the Antarctic circle.
Seawater, Temperature, Salinity and Density
• At low latitudes, warm water stays
near the surface because it is less
dense than the deeper water.
• At high latitudes, surface seawater
is chilled, especially in winter,
which increases its density.
• When a mass of water is more
dense than the surrounding water,
it sinks due to the force of gravity.
Seawater, Temperature, Salinity and Density
• Ocean water salinity is approximately 35 ppt (parts per thousand).
• The higher the salinity of ocean water, the more dense it is.
• When the salinity of a mass of water increases, the mass sinks
• When evaporation occurs more
than precipitation the water
becomes saltier.
• It becomes less salty if
precipitation occurs more than
evaporation.
• Melting ice decreases salinity
• Growing ice increases salinity
Water Masses
• The oceans are a massive body of water, but it is not the same everywhere.
• Temperature and salinity vary from place to place.
• Large regions of the oceans have water with about the same temperature
and salinity, called a water mass.
• In the surface of the ocean down to depths of
100 to 200 meters, winds mix the water
vertically.
• The surface water is about the same in temp and
salinity
• In deep ocean, there is no vertical mixing, so no
vertical mixing occurs and individual water
masses maintain their properties for a long time.
Water Masses
• Water masses are named after where they are from.
• Antarctic Bottom Water and North Atlantic Deep Water for example.
• Both North Atlantic Deep Water (NADW) and Antarctic Bottom Water
(AABW) play an important role in the circulation of the world’s
oceans.
• NADW forms around Iceland, has a salinity of 34.9 parts per thousand
with a temp of about 3oC. It takes about 1000 years sink, and travel
towards Antarctica, then into the Indian and Pacific Oceans.
• AABW is the most dense of all deep ocean water masses with a
salinity of 34.7 parts per thousand and a temp of -0.5oC. It forms
along Antarctica and flows under all other water masses moving
northward along the deepest part of the ocean.
What causes the Deep Circulation?
• Deep water moves very slowly.
• Very Cold water from the poles, flows towards the equator and will
gradually rise towards the surface as it approaches the equator but
does not breach the surface layer without additional unknown forces
causing this.
• Oceanographers believe the tides are the final step needed to mix
deep cold water and warm surface water along the equatorial regions
of the oceans.
• Remember the tides are caused by the gravitational pull of the moon
on the Earth’s waters.
CLASSWORK
Use the handout and diagrams to complete page 481 Question 1
Unit 3b Vocabulary
Unit 3c Vocabulary
• Density
• Ocean current
• Salinity
• Water mass
 gyres
 upwelling
 downwelling