Ocean Currents Unit (Topic 9A-1) – page 1
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Ocean Currents Unit
Ocean Currents
An ocean current is like a river in the ocean: water is flowing – traveling – from place to place.
Historically, ocean currents have been very important for transportation. When crossing the
ocean in a ship powered by the wind (via sails), being carried by an ocean current (or avoiding a
current going the opposite direction) could save a ship more than a week of travel time. Modern
ships are powerful enough to go against most ocean currents, but doing so costs time and fuel
(e.g., oil = money), so knowledge of ocean currents is still very important. (About 40% of all the
goods imported into the United States – worth $200 billion – come through the ports of Los
Angeles and Long Beach. Port activity contributes $39 billion in wages and taxes to the local
economy, and is related to about 800,000 local jobs.) In addition, ocean currents are studied
because they carry things in the water from place to place in the ocean, like ocean pollution.
Knowledge of the local ocean currents, for example, can help us determine where sewage is
leaking into the ocean or predict how far away the pollution from a leaking sewage pipe will
affect the shoreline. Oil companies need to study ocean currents to prepare emergency plans in
case an oil spill occurs. Ocean currents also carry warm and cold water from place to place, and
can have a significant impact on a region’s climate (e.g., the east and west coasts of the United
States are quite different) Marine biologists are interested in ocean currents for several reasons.
Not only do they transport organisms – particularly their larvae (babies who are plankton) – from
place to place, but they also can bring up nutrients from deep in the ocean, fertilizing
phytoplankton (who are the foundation of the food chain).
1. What is an ocean current?
2. Why do we study ocean currents?
How can knowledge of ocean currents lead to practical benefits?
What causes ocean currents?
Ocean currents can be created in several different ways, but most ocean currents at the surface of
the ocean are created by the wind pushing the surface of the water. Waves can be an important
part of this process: the wind causes waves to grow and break, causing water to surge forward
and become an ocean current. Tides are an important cause of ocean currents in shallow coastal
Ocean Currents Unit (Topic 9A-1) – page 2
waters (like estuaries). Density differences can lead to convection cells in the ocean, causing
“thermohaline circulation.”
Oddly, the major ocean currents do not go in the same direction as the wind. At first, the water
does go in the same direction as the wind, but the water tends to bend off to the side owing to
rotation of the Earth beneath it (i.e., the Coriolis effect). This surface water pushes the water
below it, but the water below it tends to bend off to the side owing to the Coriolis effect. The
subsurface water pushes the water beneath it,
but the deeper water tends to bend off to the
Wind
side, etc. Thus, water ends up going in different
directions at different depths. Oceanographers
refer to this current pattern as the “Ekman
surface water
Spiral,” named for the oceanographer who first
deeper water
explained what was happening. Arctic
even deeper
explorers were the first to point out that ocean
water
currents go to the side of the wind (to the right
of the wind in the northern hemisphere) by
observing icebergs floating in this direction.
"Ekman Spiral"
(90% of an iceberg in beneath the surface – like a
nd
cube of ice floating in your drink – so they are
Wi
mainly pushed by the water, not the wind)
3. What causes (pushes)
most ocean currents?
4. True or false? “Currents go in
the same direction as the wind.”
Ekman
Transport
(Overall
Water
Direction)
surface
water
deeper
water
even
deeper
water
5. Why don’t currents typically go in the same direction as the wind pushing them?
6. True or false? “The surface current may be going in a different direction than the current
below it.”
Ocean Currents Unit (Topic 9A-1) – page 3
Ekman Transport: the overall direction water is pushed by the wind
Ekman showed mathematically how most of the water flows approximately 90o to the side of the
wind owing to the Coriolis effect, so oceanographers often refer to the overall motion of the
water as the “Ekman transport.” (70o is probably a better real-world estimate.)
Wind
even
deeper
water
surface water
Ekman
Transport
(Overall
Water
Direction)
deeper water
The figure below shows how the water (dashed blue arrows) moves in response to various winds
(solid green arrows) in both northern and southern hemispheres. Notice that winds can push
water together or apart, and towards land or away from land. This will have important
implications later on.
7. What is Ekman transport?
8. What is the direction of Ekman transport (the overall motion of the water) for the winds
in the map below? Put an arrow in each picture, and write its direction (north, northeast,
east, southeast, south, southwest, west, northwest) next to it.
Northern Hemisphere
B
C
Land
N A
Southern Hemisphere
D
Ocean Currents Unit (Topic 9A-1) – page 4
Overall Ocean Circulation Pattern
Examine the map below showing the large-scale ocean circulation. The dominant current pattern
in most oceans is a gyre. A gyre is a group of ocean currents moving in a huge, horizontal loop
that goes north in some places and south in other places. The ocean has 5 subtropical gyres (red
arrows by the Equator), and one subpolar gyre (blue arrows by northern Europe).
The only place without a gyre is the Southern (or Antarctic) Ocean. Here, the currents go all the
way around the world. The Antarctic Circumpolar Current (or West Wind Drift) goes east
around the continent of Antarctica, and the East Wind Drift circles to the west closer to the coast
of Antarctica. Recall that winds and currents are named for the direction that they come from,
not the direction that they are going to.
Subpolar
60oN
Subtropical
(Clockwise)
Equator
Equator
Subtropical
(Counterclockwise)
60oS
You will need to memorize these currents. Identify the pattern by answering the questions below.
9. Does the currents between Greenland and northern Europe (the northernmost gyre)
go clockwise (turn to their right) or counterclockwise (turn to their left)?
10. Do the currents just north of Equator go clockwise (turn to their right)
or counterclockwise (turn to their left)?
11. Do the currents just south of Equator go clockwise (turn to their right)
or counterclockwise (turn to their left)?
12. What is different about the currents between Antarctica and continents north of it?
Ocean Currents Unit (Topic 9A-1) – page 5
What causes ocean water to move in gyres?
Let’s examine the Northern Pacific Ocean. The trade winds push water west, away from the
coast of North America. The water travels across the Pacific Ocean until it hits Asia, so it cannot
go forward. It flows north along the coast Asia; it cannot stop at the coast of the Asia, because
the trade winds continue to push more water west, and this incoming water pushes the water out
the way and along the coast of Asia. By the time the water flowing north reaches Japan, the
winds have shifted. The westerlies push the water to the east, away from the coast of Japan and
towards California. (Recall that the winds and currents are named for the direction that they
come from, not the direction that they are going to, so the westerlies come from the west and go
to the east.) When the water reaches California, it is forced to stop or turn by the land. The winds
continue pushing more and more water towards the coast of California, and this water pushes the
water already along the coast out of the way and down the coast to the south. This water begins
to leave the coast near the bend in the coast of California (Point Concepcion not far from Santa
Barbara), and is pushed west again, away from the coast by the trade winds.
C
D
B
A
Green Arrows (Arrows with Tails) = Winds
Blue Arrows (Dashed Arrows) = Direction Water is Pushed by the Wind
Purple Arrows (Solid Arrows, No Tails) = Actual Motion of the Water
13. What pushes ocean current A?
14. What pushes ocean current B?
15. What pushes ocean current C?
16. What pushes ocean current D?
Ocean Currents Unit (Topic 9A-1) – page 6
Further Comments about the Causes of Gyres
Overall, the trade winds and westerlies push the ocean water together in the North Pacific (the
blue, dashed Ekman transport arrows). The currents cannot go north and south into one another
(they are in one another’s way), but they can slide west and east, respectively, at these latitudes.
There are other ways to explain why the water flows along the coasts. As more and more water is
pushed into the coast by the winds, sea level rises along the coast (it really does!). As we all
know, water flows downhill (pulled down by gravity). It cannot flow downhill back into the
ocean, because the winds are pushing water towards the coast, so it flows downhill in the only
direction it can: along the coast. Just as water piles up when winds push water into the coast,
winds create a hole or “gap” in the surface of the ocean where they push water away from the
coast. Water further up the coast will flow down the coast (“downhill”) to fill in the gap.
Professional oceanographers have a more detailed and complicated understanding of the causes
of the gyres. Notice that the winds try to push the water together in the center of the oceans,
causing sea level to rise. Gravity pulls the water downhill, away from the center of the gyre, but
the water turns to the side under the influence of the Coriolis effect and ends up going around the
hill in a circle instead of away from the hill. A current is said to be in geostrophic balance when
the pressure to move downhill (due to gravity) is balanced by the Coriolis effect.
There is yet another way to understand the cause of the gyres involving the conservation of
angular momentum – or more precisely, the conservation of potential vorticity – but these
concepts bring us well beyond the bounds of this course.
Notice that the currents of the subpolar gyre flow west and east across the northern Atlantic
Ocean in the directions dictated by the winds. The land forces them to turn, creating a
counterclockwise gyre. The Coriolis effect is not needed to explain the motion of the subpolar
gyre, and in fact would make the gyre go in the other direction, so the Coriolis effect cannot be
one of the most important factors that create gyres. (The Coriolis effect does affect the currents,
but it does not create the gyres.) The key factors are (i) the directions of the winds and (ii) the
presence of land in the way. Where in the world are their no large gyres? In the Southern Ocean,
where there are no continents in the way to force currents to turn.
17. True or false? “The direction of the gyres is determined by the Coriolis effect. In other
words, in the northern hemisphere all currents in gyres turn to their right (go clockwise).”
18. Why isn’t there a gyre in the Southern Ocean (the ocean next to Antarctica)?
Ocean Currents Unit (Topic 9A-1) – page 7
Boundary Currents
The parts of the gyre that flow along the coasts of the continents are called “boundary currents.”
In other words, the boundary currents flow along the edges or “boundaries” of the ocean. There
are two kinds of boundary currents: eastern boundary currents (EBCs) and western boundary
currents (WBCs). Just as the west coast is on the west side of the continent and the east coast is
on the east side of the continent, western boundary currents are found on the western sides of the
oceans and eastern boundary currents are found on the eastern sides of the oceans. This sounds
simple enough, until you
realize that this means
California
that the east coasts have
Gulf
Current
western boundary
Stream
Kuroshio
West
currents next to them,
East
Coast Coast
East
and west coasts have
EBC
Coast
WBC
WBC
eastern boundary currents
next to them! As you can
imagine, this can lead to
some confusion.
In this class, I focus on the boundary currents of the subtropical gyres. Their western boundary
currents are faster, narrower, deeper, and warmer than the eastern boundary currents. (Or, if you
prefer, their eastern boundary currents are slower, wider, shallower, and colder than their western
boundary currents.) The first major ocean current to be measured and charted was the Gulf
Stream, the northward-flowing, warm current off the east coast of the United States. As we noted
earlier, a current is like a river (a stream) and it comes from the Gulf of Mexico, hence the name
“Gulf Stream.” The other two boundary currents that I want you to know the properties of are the
“California Current” and the “Kuroshio.” (“What is the name of the current along the coast of
California?” Don’t you wish that I would ask this on an exam?) The California Current is a slow,
cold water current that flows south along
Western
Eastern
the coast of California. The Kuroshio, like
Boundary
Boundary
West
East
Current
Current
the Gulf Stream, is a fast, warm water
Coast
Coast
current along the east coast of Japan.
“Kuroi” means “black” in Japanese, and
“shio” means river, so “Kuroshio” means
Ocean
“Black Stream.” It is called the “Black
Stream” because warm water tends to have
less life than cold water; it is the “lifeless
river.”
Ocean Currents Unit (Topic 9A-1) – page 8
19. What is a boundary current?
20. Are western boundary currents found next to the west coasts of continents or the east
coasts of continents?
21. Which is faster, a western boundary current or an eastern boundary current?
22. Which is deeper, a western boundary current or an eastern boundary current?
23. Which is wider, a western boundary current or an eastern boundary current?
24. Which is warmer, a western boundary current or an eastern boundary current?
25. What is the name of the boundary current off the east coast of the United States?
26. What is the name of the boundary current off the east coast of Asia?
27. Is the California Current a western boundary current or an eastern boundary current?
Ocean Currents Unit (Topic 9A-1) – page 9
Western Intensification and Sea Level
We say that western boundary currents are “intensified,” because all of their characteristics
(faster, narrower, deeper, warmer) are more “extreme” than those of eastern boundary currents.
The easiest of these characteristics to explain is temperature (think about where the currents
come from). The other characteristics have to do with the Earth’s rotation (the Coriolis effect).
Perhaps the easiest way to explain western intensification is to think about how the Coriolis
effect alters currents as they travel from one side of the ocean to the other side of the ocean.
As the eastward-flowing current in the figure below travels across the North Pacific ocean
towards California, it naturally bends to its right (south) since the Coriolis effect is stronger near
the Poles. By the time it reaches the coast, it has already pretty much turned, so it gently flows
down the coast. On the other hand, the westward-flowing current near the Equator hardly turns at
all, and it runs into the land all at once.
An enormous amount of water builds
up along the coast (raising sea surface
about 3 feet!), creating a pile of warm
water that we call the “Pacific Warm
Pool.” All this water has to flow north
at the same time, so it has to speed up –
rush north – to make room for all the
water coming in behind it.
Northern
Australia
& Indonesia
Land
South
America
Land
Warm
Cold
28. Where is the Coriolis effect stronger, near the Equator or near the Poles?
29. Where is sea level higher at the Equator, on the west side of the Pacific Ocean (by Asia
and Australia) or on the east side of the Pacific Ocean (by South America)?
Ocean Currents Unit (Topic 9A-1) – page 10
Meanders, Eddies, and Other Mesoscale Phenomena
At this point, we will end our discussion of ocean circulation patterns. I have covered the most
important large-scale, surface ocean currents. In reality, ocean currents are enormously complex:
they shift with time (“meander”), grow and shrink, speed up and slow down, twist in upon
themselves and spin off rotating eddies, etc. We do not have the time to go into the details of
meso-scale phenomena like these, but I would like you to be aware that the subject exists. You
can see these complex details in the classic picture of the Gulf Stream below.
Temperature of Ocean Water
Red = Warm, Blue = Cold.
The Gulf Stream is the wiggly red-orange feature extending up into the green water.
Courtesy of SeaWiFS / NASA / NOAA
30. True or false? “Ocean currents change direction and speed over time (e.g., with the
seasons), just like winds. They do not move in straight lines like arrows.”