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Experiential Science 20­­—Marine Systems
THE OCEAN’S CURRENTS
T
he world’s oceans have currents that move vast
amounts of water. There are two types of ocean
currents: surface currents and deepwater currents
(also known as thermohaline circulation). Surface
currents move relatively fast in the upper 400 m of the
ocean, which accounts for about 10% of all the water in
the ocean. The other 90% of ocean water is deepwater.
Currents in the deepwater creep slowly with great
momentum over distances of thousands of kilometres.
Surface Ocean Currents
When you think about what causes ocean currents, what
is the first thing that comes to mind? Most likely you
will think of the wind. Surface currents are driven by
wind. The winds and the currents they generate are quite
predictable because the wind has a dominant direction in which it usually flows. This is called a prevailing wind. When wind blows over the ocean surface, the
momentum of moving air molecules is transmitted to
water molecules. This transfer of energy is said to be an
inefficient process because the speed of an ocean current
is only approximately 3–4% of the speed of the wind that
generates it. Nonetheless, wind is a major force that creCoriolis
effect
Wind
ates surface ocean currents. If a wind is blowing at 50 km
per hour, at approximately what speed will the resulting
ocean current flow?
Zonal winds
Surface winds blow in a regular pattern in response
to the heating of air across the Earth’s surface and the
Coriolis effect (more on this follows). The movement of
air parallel or nearly parallel to lines of latitude is called
zonal wind flow. This produces easterly trade winds in
the subtropics and westerlies in the mid-latitudes (see
Figure 2.1).
Wind blowing on the ocean surface in the same direction for extended periods creates wind-driven ocean
currents that move great masses of water. The water piles
up against geographic features such as the shores of continents and flows to the downward slope, creating large
circulating currents known as gyres.
Since the winds are only in contact with the surface
of the water, the frictional forces between the air and the
water occur there. As the molecules of water at the surface slide past those below, they cause the lower molecules
to move, but with reduced effect. This means the water
Polar high pressure
Polar easterlies
90˚
60˚
Subpolar lows
Prevailing
westerlies
30˚
Subtropical highs
Northeast
trades
0˚
Equatorial lows
Southeast
trades
30˚
Subtropical highs
Prevailing
westerlies
60˚
Subpolar lows
Polar easterlies
Polar high pressure
90˚
Figure 2.1 The main global winds are the
westerlies, the easterlies, and the trade
winds. They are the result of uneven heating
of the Earth’s surface and the effect of the
Earth’s rotation.
SCIENTIFIC TERMS
Coriolis effect: the tendency of all particles in
motion on the surface of the Earth—including
water—to be deflected to the right in the
northern hemisphere and to the left in the
southern hemisphere because of the Earth’s
rotation.
gyre: a circular or spiral system of movement,
especially a giant circular ocean surface current.
prevailing wind: a surface wind that generally
flows in the same direction for long periods of
time.
thermohaline circulation: the global densitydriven circulation of the oceans, driven by
contrasts in water density, which are caused by
differences in temperature and salinity.
zonal wind flow: the movement of air parallel or
nearly parallel to lines of latitude.
Chapter 2 Ocean Dynamics77
nd
Spiraling
currents
Wi
below moves more slowly than the surface water, causing
it to turn slightly. Overall, the effect of winds moving at
the surface creates a spiral in the water column beneath it,
known as the Ekman spiral (see Figure 2.2).
rrent
ce cu
Surfa
45º
Ekman spiral
The Ekman spiral is named after the work of Swedish
oceanographer V. W. Ekman, who laid the foundation
for dynamic theories of wind-driven ocean circulation.
Here is a simplified explanation of how the Ekman spiral
works, as shown in Figure 2.2. In the open ocean, the
surface layer of water moves with the wind, but it does
not move precisely parallel to the wind. The surface layer
moves at an angle of 45°. This 45° angle of deflection
results from forces acting on a thin slab of water: the
wind-shear acting on top of the layer of water, bottom
drag with the layer underneath, and the Coriolis force
acting on the slab of water.
In the northern hemisphere, surface water is deflected
45° to the right of the driving wind. Below the surface,
each successive water layer is deflected with respect to the
layer above, and the velocity slows down due to friction
as you go deeper. The spiral extends downward about
100–200 m, slowly losing energy. The base of the spiral
is where the water motion is negligible. When you look
at all the layers in the 100–200 m column, the bulk of the
water moves at an angle of 90° to the right of the wind.
This is called Ekman transport. In the southern hemisphere, the process is reversed: surface water is deflected
45° to the left of the driving wind.
Net water
transport
No water motion
Figure 2.2 In the northern hemisphere, the Coriolis deflection
causes a surface current to flow to the right of the generating
wind. In a column of water, the bulk of the water moves 90º to
the right of the wind.
Wind
The Coriolis effect
The Coriolis effect is the result of the Earth rotating
under a surface that is also moving, such as air or water.
The Coriolis effect is not a force, but simply an effect due
to a body rotating under the influence of gravity; nonetheless, anything travelling on air or water will appear
to move to the right (clockwise) in the northern hemisphere. This applies to ocean currents just as it does to
airplanes. Therefore, the Coriolis effect results in currents being established in all of the world’s oceans. In
the southern hemisphere, the Coriolis effect is reversed,
causing currents to move to the left (counter-clockwise).
SCIENTIFIC TERMS
Ekman transport: the overall movement of a mass of water,
resulting from the Ekman spiral, in a direction 90° to the
wind.
45º
90º
Surface current
Ekman transport:
Net water
movement
Figure 2.3 Viewed from above in the northern hemisphere, the
surface layer of water moves at 45 º to the right of the wind.
The net transport of water through the entire wind-driven
column (known as Ekman transport) is 90 º to the right of the
wind.
78
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Experiential Science 20­­—Marine Systems
E
Activity 1
field activity
lab activity
library activity
Point of the
pencil moves up
4 classroom activity
chapter project
4 research team activity
Modelling the Coriolis Effect
W
N
Paper moves to the left
E
Purpose
To demonstrate the Coriolis effect on a moving object.
Background Information
The Coriolis effect occurs when objects not attached to the
Earth move towards the poles. For example, suppose an
airplane is flying from the equator to the North Pole. It aims
straight north according to its compass. However, as it flies
the Earth is rotating below it. The movement of the Earth
underneath it causes its motion to appear to move to the right
in the northern hemisphere.
Materials and Equipment
• 8.5” × 11” paper
•pencil
• diagram of world oceans
Procedure
1. Work on this activity with your research team. One person
holds a piece of paper in such a way that it can be moved
across a desk or a table from right to left. Label the page
north at the top and south, east, and west as appropriate.
This is to simulate the Coriolis effect in the northern
hemisphere.
2. Another person holds the point of a pencil on the bottom
Web Link
Conduct a search using the key words “Coriolis effect”
to find out more about this phenomenon.
S
left-hand corner of the piece of paper.
3. Each person will need to move slowly and steadily. Person
1 moves the paper from right to left as person 2 moves the
pencil straight up using the edge of the desk as a guide by
staying parallel to it.
4. In your notebook, record which direction the pencil mark
takes. Is the track from west to east or east to west? Why?
Reflections and Conclusions
1. If the track were continued, would you describe it as
clockwise or counter-clockwise?
2. How might an aircraft that is travelling due north have to
compensate for the Coriolis effect in order to arrive at its
destination?
3. Ocean currents move from the equator in a northerly
direction in the northern hemisphere. How does the
Coriolis effect cause their track to vary from straight north?
4. On the diagram provided, mark ocean currents flowing
straight north from the equator towards the Arctic Ocean.
Then illustrate how their movement is affected by the
Coriolis effect.
5. Ocean currents in the southern hemisphere flow from the
equator towards the South Pole. How would you modify
this activity to show how the Coriolis effect is different in
the southern hemisphere?
6. When an object moves from north to south in the northern
hemisphere, how is its movement affected by the Coriolis
effect? On the diagram provided, illustrate its effect
on ocean currents that are flowing from the North Pole
towards the equator.