Geoscience Reference
In-Depth Information
along the east coast of the United States (Figure 6.33). Currents
in the Southern Hemisphere perform the same function but cir-
culate counterclockwise. A great example of such a current is
the Humboldt Current, which flows northward along the west
coast of South America. This current delivers cold water from
the Antarctic latitudes to the equatorial region where it warms
and then flows west as the South Equatorial Current.
The primary difference between oceanic and atmospheric
circulatory processes is that the continents block the movement
of water. This blockage results in distinct circulatory systems,
called
gyres
, such as the one in the Atlantic basin. In addition to
the oceanic circulation across the surface, a slow mix of water
also occurs vertically between layers of the ocean. These cur-
rents, which together make up the
thermohaline circulation
,
also called the
oceanic conveyor belt
, which links all the
world's oceans, are generated because of regional differences
in water density that evolve through variations in temperature
and salinity. In general, water at the surface of the ocean is typi-
cally warmer and less salty than deeper water. A good place
to witness the beginning of this system is in the western part
of the tropical Atlantic basin, shown in Figure 6.34. Here, as
part of the North Atlantic gyre, warm water flows northward
in the Gulf Stream (as illustrated in Figure 6.33). As this warm
water travels to higher latitudes, extensive evaporation occurs,
which increases the relative salinity of the current. Along with
Figure 6.33 Sea-surface temperatures in the western Atlan-
tic.
NOAA-7 satellite image showing sea-surface temperatures
for a week in April. Red represents the warmest water, which is
flowing northeastward along the east coast of North America in
the Gulf Stream. Greens, blues, and purples are progressively
cooler sea-surface temperatures.
Figure 6.34 Surface and deep
water circulation loop in the
world oceans.
Note the loca-
tions where water upwells and
downwells. These combined
processes drive the circulation
of ocean currents around the
world.
Warm water
loses heat
to air
Cold water
downwelling
Warm water
upwelling
Cold water
absorbs heat
from air
Warm water
upwelling
Shallow current of
warm, less salty water
Deep current of cold, salty water
Gyres
Large oceanic circulatory systems that form because
currents are deflected by landmasses.
Downwelling current
A current that sinks to great depths
within the ocean because water temperature drops and salinity
increases.
Thermohaline circulation
The global oceanic circulatory
system that is driven by differences in salinity.
Upwelling current
A current that ascends to the surface of the
ocean because water temperature increases and salinity decreases.
