AS GEOGRAPHY - ATMOSPHERE AND WEATHER - OCEAN CURRENTS.

1. ASGEOGRAPHY
ATMOSPHEREANDWEATHER
OCEANCURRENTS

2. OCEANCURRENTSDEFINITION
An ocean current is a continuous, directed movement of sea water
generated by a number of forces acting upon the water, including wind,
the Coriolis effect, breaking waves, cabbeling, and temperature and salinity
differences.
Cabbeling is when two separate water parcels mix to form a third which
sinks below both parents. The combined water parcel is denser than the
original two water parcels.

4. HORIZONTALWATERMOVEMENTS
Depth contours, shoreline configurations, and interactions with other currents
influence a current's direction and strength. Ocean currents are primarily
horizontal water movements.

5. INFLUENCE
Ocean currents flow for great distances, and together, create the global
conveyor belt which plays a dominant role in determining the climate of
many of the Earth’s regions.
Ocean currents influence the temperature of the regions through which they
travel.
For example, warm currents travelling along more temperate coasts increase
the temperature of the area by warming the sea breezes that blow over
them.

6. EXAMPLES
For example, the Gulf Stream makes
northwest Europe much more temperate
than any other region at the same latitude.
Another example is Lima (Peru), where the
climate is cooler, being sub-tropical, than the
tropical latitudes in which the area is located,
due to the effect of the Humboldt Current.
Evolution of the Gulf
Stream to the west of
Ireland continuing as the
North Atlantic Current

7. ECKMANSPIRALEFFECT
Surface oceanic currents are sometimes wind driven and
develop their typical clockwise spirals in the northern
hemisphere and counter-clockwise rotation in the southern
hemisphere due to imposed wind stresses.
In these wind-driven currents, the Ekman spiral effect results
in the currents flowing at an angle to the driving winds.
In addition, the areas of surface ocean currents move
somewhat with the seasons; this is most notable in
equatorial currents.
The Ekman spiral is a structure of currents or winds near a
horizontal boundary in which the flow direction rotates as one
moves away from the boundary.
Ekman spiral effect.
1. Wind
2. force from above
3. Effective direction
of the current
4. Coriolis effect

8. WESTERNBOUNDARYCURRENTS
Deep ocean basins generally
have a non-symmetric surface
current, in that the eastern
equatorward-flowing branch
is broad and diffuse whereas
the western poleward flowing
branch is very narrow.
These western boundary
currents (of which the Gulf
Stream is an example) are a
consequence of the rotation
of the Earth.
The main ocean currents involved with the North Pacific Gyre

9. MEASUREMENT
Ocean currents are measured
in sverdrup (sv), where 1 sv is
equivalent to a volume flow
rate of 1,000,000 m3 per
second.
Surface currents are found on
the surface of an ocean, and
are driven by large scale wind
currents.
They are directly affected by
the wind—the Coriolis effect
plays a role in their behaviours.
The water transport in the Gulf Stream gradually increases from
30 Sv in the Florida Current to a maximum of 150 Sv south of
Newfoundland at 55°W longitude.
The Antarctic Circumpolar Current, at approximately 125 Sv, is the
largest ocean current.
The entire global input of fresh water from rivers to the ocean is
equal to about 1.2 Sv.

10. MAJOROCEANGYRES
There are five major ocean-wide gyres:
• North Atlantic gyre
• South Atlantic gyre
• North Pacific gyre
• South Pacific gyre
• Indian Ocean gyre
Each is flanked by a strong and narrow
“western boundary current,” and a
weak and broad “eastern boundary
current”.

11. TYPESOFOCEANCURRENTS
SURFACE CURRENTS - Surface Circulation
These waters make up about 10% of all the water in the ocean.
These waters are the upper 400 meters of the ocean.
DEEP WATER CURRENTS - Thermohaline Circulation
These waters make up the other 90% of the ocean.
These waters move around the ocean basins by density driven forces and
gravity. The density difference is a function of different temperatures and
salinity.
These deep waters sink into the deep ocean basins at high latitudes where
the temperatures are cold enough to cause the density to increase.

12. TYPESOFFORCESINFLUENCINGOCEANCURRENTS
Ocean Currents are influenced by two types of forces
PRIMARY FORCES - start the water moving.
The primary forces are:
A. Solar Heating
B. Winds
C. Gravity
D. Coriolis
SECONDARY FORCES - influence where the currents flow.

13. A)SOLARHEATING
Solar heating cause water to expand. Near the equator the water is about 8
cm high than in middle latitudes.
This cause a very slight slope and water wants to flow down the slope.
Winds blowing on the surface of the ocean push the water. Friction is the
coupling between the wind and the water’s surface. A wind blowing for 10
hours across the ocean will cause the surface waters to flow at about 2% of
the wind speed.
Water will pile up in the direction the wind is blowing.
Gravity will tend to pull the water down the "hill" or pile of water against the
pressure gradient. But the Coriolis Force intervenes and cause the water to
move to the right (in the northern hemisphere) around the mound of water.
These large mounds of water and the flow around them are called Gyres.
They produce large circular currents in all the ocean basins.

15. B)WINDS
Wind blowing on the surface of the
ocean has the greatest effect on the
surface. However, for the lower layers of
the ocean to move they must be pushed
by the friction between the layers of
water above.
Consequently, the lower layer moves
slower than the layer above. With each
successive layer down in the water
column the speed is reduce.
This leads to the spiral affect seen in the
diagram.

16. CD)GRAVITYANDCORIOLISEFFECT
When the water is pushed to the right it
forms the hill we described above.
So, when water is pushed along by the
wind it wants to be turned to the right
by the Coriolis force (in the northern
hemisphere) but it must fight against
gravity (trying to move up the hill of
water formed by Ekman transport).
A balance is met between the Coriolis
and the gravity (pressure gradient force).
This balance produces a balanced flow
called a Geostrophic current.

17. MAJOROCEANCURRENTS
Major ocean currents of the world
• Arctic Ocean.
• Atlantic Ocean.
• Pacific Ocean.
• Indian Ocean.
• Southern Ocean.

18. WARMCURRENTS
Warm ocean currents flow away from the equatorial region on the western
side of ocean basins. The Gulf Stream in the North Atlantic and the Kuroshio
Current in the North Pacific are examples of warm currents. Of all the warm
currents, the Gulf Stream has been studied most extensively.

19. GULFSTREAM
A narrow, intense flow of warm water,
the Gulf Stream begins in the
Caribbean Sea and follows the east
coast of the United States northward
around Cape Hatteras, North Carolina.
There the current veers northeastward
across the Atlantic Ocean, where it is
called the North Atlantic Drift.
The current carries warm water to
Iceland and the British Isles.
As a result, these places have warmer
climates than they would otherwise.

20. GULFSTREAMSARGASSOSEA
The Gulf Stream forms the western and
northern boundary of the Sargasso Sea,
which is located in the middle of the
North Atlantic Ocean.
An area of warm water and light winds,
the Sargasso Sea has relatively calm
seas. Great amounts of floating brown
seaweed called sargassum are typically
found on the surface water there.
Similar conditions exist in other oceans,
but nowhere are they as well
developed as in the North Atlantic.

21. COLDCURRENTS
Cold currents flow
toward the equator
on the eastern side of
ocean basins.
Examples of cold
ocean currents
include the Canary
Current in the North
Atlantic, the
California Current in
the North Pacific, and
the Benguela Current
in the South Atlantic.

22. THELABRADORCURRENT
Cold currents can also flow out of far northern
regions. The Labrador Current flows out of
Baffin Bay and past Labrador, the coastal part
of the Canadian province of Newfoundland.
The current carries icebergs from Baffin Bay,
creating a hazard for ships in the North
Atlantic.
The Labrador Current meets the Gulf Stream
off the coast of Newfoundland. When warm,
moist air from the Gulf Stream blows over the
cold Labrador Current, water vapor
condenses. This results in some of the
thickest fogs in the world.

23. THEEASTGREENLANDCURRENTANDOYASHIO
Two other important cold currents originate in northern regions.
The East Greenland Current flows into the North Atlantic through the Strait
of Denmark.
The Oyashio Current flows through the Bering Strait between Siberia and
Alaska and into the North Pacific.