Life on an Ocean Planet

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Transcript Life on an Ocean Planet

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►Surface Currents
►Deep Currents
Chapter Topic Menu
►Studying Ocean Currents
 Understanding what causes currents and where they flow is fundamental to all
marine sciences. It helps explain how heat, sediments, nutrients, and organisms
move within the seas.
Causes of Currents
 Three major factors drive ocean currents.
 1. Wind.
 If the wind blows long enough in one direction,
it will cause a water current to develop.
 The current continues to flow until internal friction,
or friction with the sea floor, dissipates its energy.
Chapter 9 Pages 9-3 & 9-4
Surface Currents
 2. Changes in sea level.
 Sea level is the average level of the sea’s surface at its mean
height between high and low tide.
 The ocean’s surface is never flat, ocean circulation cause slopes to develop. The steeper the
“mound” of water, the larger and faster the current. The force that drives this current is the
pressure gradient force.
 3. Variations in water density.
 Differences in water density also cause horizontal differences in water pressure. When the
density of seawater in one area is greater than another, the horizontal pressure gradient
between the two areas initiates a current that flows below the surface.
 The combination of westerlies, trade winds, and the Coriolis effect results in a
circular flow in each ocean basin. This flow is called a gyre.
 There are five major gyres – one in each major ocean basin:
Chapter 9 Pages 9-5 & 9-6
Surface Currents
1. North Atlantic Gyre
2. South Atlantic Gyre
3. North Pacific Gyre
4. South Pacific Gyre
5. Indian Ocean Gyre
 The flow of currents in all parts of the
ocean is a balance of various factors,
including the pressure gradient force,
friction, and the Coriolis effect.
Ekman Transport
 The Ekman transport is an interesting phenomenon
discovered in the 1890s by Fridtjof Nansen.
 The wind and the Coriolis effect influences water
well below the surface because water tends to flow
in what can be imagined as layers.
Chapter 9 Pages 9-6 to 9-8
Surface Currents
 Due to friction, the upper water currents push the deep water below it. This deep layer
pushes the next layer below it. The process continues in layers downward. Each water
layer flows to the right of the layer above causing a spiral motion.
 This spiraling effect of water layers pushing slightly to the right from the one above (to
the left in the Southern Hemisphere) is called the Ekman spiral.
 There is a net motion imparted to the water column down to friction depth. This
motion is called the Ekman transport.
 The net effect, averaging of all the speeds and directions of the Ekman spiral, is to
move water 90° to the right of the wind in the Northern Hemisphere, or to the left in the
Southern Hemisphere.
Western and Eastern Boundary Currents
 Satellite images show that the oceans are really “hilly,” not calm or flat.
 These images show that water piles up where currents meet. Where currents diverge,
“valleys” form.
 There is a dynamic balance between the clockwise deflection of the Coriolis
effect (attempting to move water to the right) and the pressure gradient created
by gravity (attempting to move the water to the left).
 The balance keeps the gyre
flowing around the outside of
the ocean basin.
Chapter 9 Pages 9-8 to 9-10
Surface Currents
 Geostrophic currents are
created by the Earth’s rotation.
 This current results from the
balance between the pressure
gradient force and the
Coriolis effect.
Western and Eastern Boundary Currents (continued)
Chapter 9 Pages 9-10 to 9-16
Surface Currents
 Western boundary currents are found on
the east coasts of the continents and are
stronger and faster than eastern boundary
currents due to western intensification.
Western boundary currents flow through
smaller areas than eastern boundary currents.
 Trade winds blow along the equator pushing
water westward, causing it to “pile up” on the western edge of ocean basins before it
turns to the poles. The Earth’s rotation tends to shift the higher surface level in the
center of the gyre westward. The higher surface level is now west of center and forces
the current to “squeeze” through a
narrower area.
 Total water volume balances out.
Western boundary currents handle the
same volume, but through smaller areas,
so water must move more rapidly.
 Countercurrents and undercurrents are water flows that differ from the major
ocean currents.
 Countercurrents are associated with equatorial currents – it runs opposite of its
adjacent current.
 It is hypothesized they develop in equatorial regions because of the doldrums. Without
wind pushing water westward, water driven in from the east enters the basin more
quickly than it exits. This causes a
countercurrent to develop.
Chapter 9 Pages 9-16 & 9-17
Surface Currents
 Undercurrents flow beneath the
adjacent current and are found
beneath most major currents.
 They can significantly
affect land masses
and land temperatures.
Upwelling and Downwelling
 Upwelling is an upward vertical current that brings deep water to the surface.
Downwelling is a downward vertical current that pushes surface water to the
 Coastal upwellings occur when the wind blows offshore or parallel to shore. In
the Northern Hemisphere this wind blowing southward will cause an upwelling
only on a west coast.
Chapter 9 Pages 9-17 to 9-20
Surface Currents
 The same wind on the east coast in
the Northern Hemisphere sends
surface water toward shore causing
a downwelling.
 These currents have strong
biological effects:
 Upwelling tends to bring deepwater
nutrients up into shallow water.
 Upwellings also relate to significant weather patterns.
 Downwellings are important in carrying and cycling nutrients to the deep ocean
ecosystems and sediments.
Heat Transport and Climate
 Currents play a critical role by transporting heat from warm areas to cool
areas and affects climate by moderating temperatures. Without currents
moving heat, the world’s climates would be more extreme.
El Niño Southern Oscillation (ENSO)
Chapter 9 Pages 9-20 to 9-22
Surface Currents
 El Niño tremendously affects world
weather patterns.
 This brings low pressure and high
rainfall in the Western Pacific.
 The opposite happens in the Eastern
Pacific with high pressure and
less rainfall.
El Niño (continued)
For reasons still not clear, every 3 to 8 years a
rearrangement of the high- and low-pressure
systems occur.
High pressure builds in the Western Pacific and low
pressure in the Eastern Pacific. Trade winds weaken
or reverse and blow eastward – the southern
Chapter 9 Pages 9-22 to 9-24
Surface Currents
This causes warm water of the west to migrate east to
the coast of South America. The loss of upwelling
deprives the water of nutrients. A normally
productive region declines with the collapse of
local fisheries and marine ecosystems.
Over the eastern Pacific, humid air rises causing
precipitation in normally arid regions. Flooding,
tornados, drought and other weather events can
lead to loss of life and property damage.
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Thermohaline Circulation and Water Masses
 Thermohaline circulation is water motion caused by differing water densities.
 In the deep-ocean layers, water density variation, not wind, is the primary cause
of current.
 Circulation drives most of the vertical motion of seawater and
the ocean’s overall circulation.
Chapter 9 Pages 9-26 & 9-27
Deep Currents
 Thermohaline circulation works because water
density increases due to cooling, increased
salinity or both.
 When water becomes dense,
it sinks, causing a downward flow.
 This means water in some
other place must rise to replace
it, causing an upward flow.
 Density differences drive the slow
circulation of deep water.
Five distinct water masses
result from density stratification.
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How Deep Water Forms
Chapter 9 Pages 9-28 to 9-30
Deep Currents
 The intermediate, deep, and bottom water
masses form primarily, but not entirely, at
high latitudes (around 70° North and South).
 The densest ocean waters, Antarctic
Bottom Waters form in the
Antarctic in winter, sink to the bottom
and spread along the ocean floor to
about 40° north latitude.
 In the Arctic the North Atlantic Deep
Waters form, but often get trapped
there by the topography of the ocean basin.
 In the Northern Hemisphere along the east coast of the Siberian Kamchatka
Peninsula the Pacific Deep Waters form. Not as dense as bottom water they make up
the deep layers.
 Mediterranean Deep Waters form due to evaporation rather than cooling, with a
salinity of 38‰. Flowing out of the Mediterranean they form the intermediate water
layer resting above the bottom layer and deep layer.
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Deep-Water Flow Patterns
The enormous water quantities sinking at the poles and
in the Mediterranean create the thermohaline
circulation pattern.
Dense water descends into low areas and bottom water
upwell to compensate.
The rising warm water enters wind-driven currents and is
carried to the poles. There it cools, becomes more dense,
and sinks again, repeating the process.
The Ocean Conveyor Belt
Chapter 9 Pages 9-30 to 9-33
Deep Currents
The interconnected flow of currents that redistribute heat
is called the ocean conveyor belt or the Earth’s “air
The ocean conveyor belt is important because it moderates
the world’s climate. This marriage of surface and deep
water circulation carries heat away from the tropics and, in
turn, keeps the tropics from getting too hot.
Some scientists hypothesize that some Ice Ages may have
resulted from a disruption of the conveyor belt.
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Two Distinct Approaches
 There are two main approaches to study currents:
 1. Lagrangian method, also called the float method.
 Studying the current by tracking a drifting object. This involves floating something in the current
that records the information as it drifts.
 2. Eulerian method, also called the flow method.
Instrumentation and Methods
Chapter 9 Page 9-34
Studying Ocean Currents
 Studying the current by staying in one place and measuring changes to the velocity of the water
as it flows past. This method uses fixed instruments that meter/sample the current as it passes.
 There are five examples of instruments or methods
that scientists apply for studying currents.
 For Lagrangian study methods researchers use:
 1. A drogue. The advantage over a simple surface
float is that the “holey sock” ensures that the current and
not the wind determine where it drifts.
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Instrumentation and Methods (continued)
 2. The Argo float drifts at depth before periodically rising to the surface to transmit to
a satellite a temperature and salinity profile of the water it rose through.
Chapter 9 Pages 9-35 to 38
Studying Ocean Currents
 For Eulerian study methods researchers use:
 3. Various types of flow meters. These devices
use impellors and vanes to measure and record
current speed and direction. The information gathered
is either transmitted immediately or stored for
retrieval later.
 4. A more sophisticated device is the Doppler Acoustic
Current Meter. This instrument determines current
direction and speed.
 5. Oceanographers can now use satellites to help them.
Although they are primarily used for studying the surface,
these instruments use laser and photography to study currents.
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