Density currents flowing in the oceans can be vertical as well as
horizontal. Examples of vertical currents are turbidity currents (discussed
in Section 23.2). Sand, silt, and other particles mix with water to form
dense water masses that sweep down continental slopes onto the abyssal
plains. Turbidity currents are just one type of density currents. Cooling,
freezing, and evaporation are other common causes of changes in density
that can form deep ocean density currents.
Density Currents from Polar Water
The densest water in the oceans comes from polar regions, due to the
intense cooling and freezing that occur there. Cooling causes sea water to
contract, hence the water takes up less space. As water molecules become
packed closely together, the water’s density increases. Freezing also affects
the density of polar water. When seawater freezes, most of the salt is left
behind. As salinity increases, the density of the seawater also increases.
Three great water masses are formed as density currents in the polar
regions. They are Antarctic Bottom Water, North Atlantic Deep Water, and
Antarctic Intermediate Water. The coldest, densest water in the oceans is
the Antarctic Bottom Water. This water is produced in large amounts in the
Weddell and Ross seas off the coast of Antarctica. This dense water has an
average temperature of ⫺0.4°C and a salinity of 34.6 psu. The water sinks to
the ocean floor and spreads northward across the equator to 40 degrees
north latitude, which is off the northern coast of California.
30-Minute
Mini
Simulating Ocean
Currents
Atlantic Ocean Currents
North Atlantic Intermediate Water
Materials
Surface Water
• round dish
• drinking straw
• small paper circles
Antarctic
Intermediate Water
Procedure
1
Fill a dish halfway with water,
let stand until the water is
still. Sprinkle paper circles on
the water’s surface.
2
Blow through the straw at a
low angle to the water. What
is the best way to position the
straw if you want to simulate a
clockwise current? Draw a
diagram showing straw’s angle
of position; use arrows to
show air and water motion.
North Atlantic Deep Water
Antarctic Bottom Water
60°
North
40°
LAB
20°
0°
20°
40°
60°
South
DEEP CURRENTS from polar areas flow throughout the Atlantic Ocean. The depth of each
current depends on its density.
The second densest type of seawater is the North Atlantic Deep Water.
Evidence suggests that this water forms near Greenland as higher-saline
Gulf Stream water with cold water from the Arctic Ocean. The water has a
temperature as low as 3°C and a salinity of about 34.5 psu. North Atlantic
Deep Water is less dense than Antarctic Bottom Water and thus flows above
it. It spreads through the Atlantic to about 60 degrees south latitude, which
is around South America’s Cape Horn. There, some of the water returns to
the surface. North Atlantic Deep Water also flows west to form deep water
in the Indian and Pacific oceans.
3 Repeat Step 2, making a
counterclockwise current.
4
Repeat Step 2 again, making
two currents.
Analysis
Relate this simulation to what
actually happens with ocean
currents. How could you make the
simulation more realistic?
Chapter 24 The Moving Ocean
537