Circulation of the Ocean

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Chapter 9 Circulation of the Ocean
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Surface Currents Are Driven by the Winds
The westerlies and the trade winds are two of the
winds that drive the oceans surface
currents. About 10 of the water in the world
ocean is involved in surface currents, water
flowing horizontally in the uppermost 400 meters
(1,300 feet) of the oceans surface, driven
mainly by wind friction. (left) Winds, driven by
uneven solar heating and Earths spin, drive the
movement of the oceans surface currents. The
prime movers are the powerful westerlies and the
persistent trade winds (easterlies).
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Surface Currents
What are some effects of ocean currents? Transfer
heat from tropical to polar regions Influence
weather and climate Distribute nutrients and
scatter organisms Surface currents are driven
by wind Most of Earths surface wind energy is
concentrated in the easterlies and
westerlies. Due to the forces of gravity, the
Coriolis effect, solar energy, and solar winds,
water often moves in a circular pattern called a
gyre.
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Surface Currents Are Driven by the Winds

• A combination of four forces surface winds, the
  suns heat, the Coriolis effect, and gravity
  circulates the ocean surface clockwise in the
  Northern Hemisphere and counterclockwise in the
  Southern Hemisphere, forming gyres.

• The North Atlantic gyre, a series of four
  interconnecting currents with different flow
  characteristics and temperatures.

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Surface Currents Flow around the Periphery of
Ocean Basins

• Surface water blown by the winds at point A will
  veer to the right of its initial path and
  continue to the east.
• Water at point B veers right and continues to the
  west.

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Surface Currents Flow around the Periphery of
Ocean Basins

• The Ekman spiral and the mechanism by which it
  operates.

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Water continues clockwise?
Trade wind
Direction of water movement
Stepped Art
Fig. 9-6, p. 237
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Surface Currents Flow around the Periphery of
Ocean Basins

• The effect of Ekman spiraling and the Coriolis
  effect cause the water within a gyre to move in a
  circular pattern.
• The movement of water away from point B is
  influenced by the rightward tendency of the
  Coriolis effect and the gravity-powered movement
  of water down the pressure gradient.

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Surface Currents Flow around the Periphery of
Ocean Basins
Consider the North Atlantic.
The surface is raised through wind motion and
Ekman transport to form a low hill. The
westward-moving water at B feels a balanced
pull from two forces the one due to Coriolis
effect (which would turn the water to the right)
and the one due to the pressure gradient, driven
by gravity (which would turn it to the left).
The hill is formed by Ekman transport. Water
turns clockwise (inward) to form the dome, then
descends, depressing the thermocline.
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Seawater Flows in Six Great Surface Circuits

• Geostrophic gyres are gyres in balance between
  the pressure gradient and the Coriolis effect. Of
  the six great currents in the worlds ocean, five
  are geostrophic gyres. Note the western boundary
  currents in this map.

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Ocean Currents
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Boundary Currents Have Different Characteristics

• Western boundary currents These are narrow,
  deep, fast currents found at the western
  boundaries of ocean basins.
• The Gulf Stream
• The Japan Current
• The Brazil Current
• The Agulhas Current
• The Eastern Australian Current
• Eastern boundary currents These currents are
  cold, shallow and broad, and their boundaries are
  not well defined.
• The Canary Current
• The Benguela Current
• The California Current
• The West Australian Current
• The Peru Current

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Boundary Currents Have Different Characteristics
The general surface circulation of the North
Atlantic.
Unit for measuring flow rates (or volume
transported by ocean currents) sverdrups 1 sv
1 million cubic meters of water per second
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Boundary Currents Have Different Characteristics

• Eddy formation
• The western boundary of the Gulf Stream is
  usually distinct, marked by abrupt changes in
  water temperature, speed, and direction.
• (a) Meanders (eddies) form at this boundary as
  the Gulf Stream leaves the U.S. coast at Cape
  Hatteras. The meanders can pinch off (b) and
  eventually become isolated cells of warm water
  between the Gulf Stream and the coast (c).
  Likewise, cold cells can pinch off and become
  entrained in the Gulf Stream itself (d). (C
  cold water, W warm water blue cold, red
  warm.)

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Boundary Currents Have Different Characteristics

• Water flow in the Gulf Stream and the Canary
  Current, parts of the North Atlantic gyre.

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Surface Currents Affect Weather and Climate

• General summer air circulation patterns of the
  east and west coasts of the United States. Warm
  ocean currents are shown in red cold currents,
  in blue. Air is chilled as it approaches the west
  coast and warmed as it approaches the east coast.

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Surface Currents Affect Weather and Climate

• Wind induced vertical circulation is vertical
  movement induced by wind-driven horizontal
  movement of water.
• Upwelling is the upward motion of water. This
  motion brings cold, nutrient rich water towards
  the surface.
• Downwelling is downward motion of water. It
  supplies the deeper ocean with dissolved gases.

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Nutrient-Rich Water Rises near the Equator

• Equatorial upwelling.
• The South Equatorial Current, especially in the
  Pacific, straddles the geographical equator.
  Water north of the equator veers to the right
  (northward), and water to the south veers to the
  left (southward). Surface water therefore
  diverges, causing upwelling. Most of the upwelled
  water comes from the area above the equatorial
  undercurrent, at depths of 100 meters or less.

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Wind Can Induce Upwelling near Coasts

• Coastal upwelling.
• In the Northern Hemisphere, coastal upwelling can
  be caused by winds from the north blowing along
  the west coast of a ccontinent. Water moved
  offshore by Ekman transport is replaced by cold,
  deep, nutriend-laden water. In this diagram,
  temperature of the ocean surface is shown in
  degrees Celsius.

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Wind Can Also Induce Upwelling Coastal Downwelling

• Coastal downwelling.
• Wind blowing from the south along a Northern
  Hemisphere west coast for a prolonged period can
  result in downwelling. Areas of downwelling are
  often low in nutrients and therefore relatively
  low in biological productivity.

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El Niño and La Niña Are Exceptions to Normal Wind
and Current Flow
An El Niño Year
A Non-El Niño Year

• In an El Niño year, when the Southern Oscillation
  develops, the trade winds diminish and then
  reverse, leading to an eastward movement of warm
  water along the equator. The surface waters of
  the central and eastern Pacific become warmer,
  and storms over land may increase.
• In a non-El Niño year, normally the air and
  surface water flow westward, the thermocline
  rises, and upwelling of cold water occurs along
  the west coast of Central and South America.

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Moist air rises
Rainfall
Surface winds
Warm-water pool
Thermocline
Upwelling
Tropical Pacific Normal Conditions
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El Nino/Southern Oscillation
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http//www.nasa.gov/vision/earth/lookingatearth/la
nina110707.html http//lwf.ncdc.noaa.gov/oa/climat
e/elnino/elnino.html
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Effects of El Nino
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from http//www.pmel.noaa.gov/tao/elnino/la-nina
-story.htmlimpact La Niña impact on the global
climateIn the U.S., winter temperatures are
warmer than normal in the Southeast, and cooler
than normal in the Northwest. Global climate La
Niña impacts tend to be opposite those of El Niño
impacts. In the tropics, ocean temperature
variations in La Niña tend to be opposite those
of El Niño. At higher latitudes, El Niño and La
Niña are among a number of factors that influence
climate. However, the impacts of El Niño and La
Niña at these latitudes are most clearly seen in
wintertime. In the continental US, during El
Niño years, temperatures in the winter are warmer
than normal in the North Central States, and
cooler than normal in the Southeast and the
Southwest. During a La Niña year, winter
temperatures are warmer than normal in the
Southeast and cooler than normal in the
Northwest. An anomaly is the value observed
during El Niño or La Niña subtracted from the
value in a normal year.
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Thermohaline Circulation Affects All the Oceans
Water

• The movement of water due to different densities
  is thermohaline circulation.
• Remember that the ocean is density stratified,
  with the densest water at the bottom. There are
  five common water masses
• Surface water
• Central water
• Intermediate water
• Deep water
• Bottom water

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Water Masses May Converge, Fall, Travel across
the Seabed, and Slowly Rise

• A model of thermocline circulation caused by
  heating in lower latitudes and cooling in higher
  latitudes. The thermocline at middle and low
  latitudes is held up by the slow upward
  movement of cold water.

• The water layers and deep circulation of the
  Atlantic Ocean. Arrows indicate the direction of
  water movement. Convergence zones are areas where
  water masses approach one another.

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Thermohaline Flow and Surface Flow The Global
Heat Connection

• The global pattern of deep circulation resembles
  a vast conveyor belt that carries surface water
  to the depths and back again. Begin with the
  formation of North Atlantic Deep Water north of
  Iceland, which flows south through the Atlantic
  and then flows over (and mixes with) deep water
  formed near Antarctica. The combined mass
  circumnavigates Antarctica and then moves north
  into the Indian and Pacific ocean basins. Diffuse
  upwelling in all of the ocean returns some of
  this water to the surface. Water in the conveyor
  gradually warms and mixes upward to be returned
  to the North Atlantic by surface circulation.

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Chapter 9 - Summary

• Ocean water circulates in currents caused mainly
  by wind friction at the surface and by
  differences in water mass density beneath the
  surface zone.
• Water near the ocean surface moves to the right
  of the wind direction in the Northern Hemisphere,
  and to the left in the Southern Hemisphere.
• The Coriolis effect modifies the courses of
  currents, with currents turning clockwise in the
  Northern Hemisphere and counterclockwise in the
  Southern Hemisphere. The Coriolis effect is
  largely responsible for the phenomenon of
  westward intensification in both hemispheres.
• Upwelling and downwelling describe the vertical
  movements of water masses. Upwelling is often due
  to the divergence of surface currents
  downwelling is often caused by surface current
  convergence or an increase in the density of
  surface water.

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Chapter 9 - Summary

• El Niño, an anomaly in surface circulation,
  occurs when the trade winds falter, allowing warm
  water to build eastward across the Pacific at the
  equator.
• Circulation of the 90 of ocean water beneath the
  surface zone is driven by gravity, as dense water
  sinks and less dense water rises. Since density
  is largely a function of temperature and
  salinity, the movement of deep water due to
  density differences is called thermohaline
  circulation.
• Water masses almost always form at the ocean
  surface. The densest (and deepest) masses were
  formed by surface conditions that caused water to
  become very cold and salty.
• Because they transfer huge quantities of heat,
  ocean currents greatly affect world weather and
  climate.