Chapter 9
Circulation of the Ocean
Surface Currents Are Driven by the
Winds
A combination of four forces – surface
winds, the sun’s 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.
Surface Currents Are Driven by the Winds
The westerlies and the
trade winds are two of the
winds that drive the ocean’s
surface currents.
Surface Currents Flow around the
Periphery of Ocean Basins
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 ocean’s
surface, driven mainly by
wind friction.
Surface water blown
by the winds at point
A will veer to the
right of its initial
path and continue to
the east.
(left) Winds, driven by
uneven solar heating and
Earth’s spin, drive the
movement of the ocean’s
surface currents. The prime
movers are the powerful
westerlies and the
persistent trade winds
(easterlies).
Water at point B
veers right and
continues to the west.
Surface Currents
What are some effects of ocean currents?
Friction
Wind
Surface
water
Net
Direction
of Ekman
transport
Transfer heat from tropical to polar regions
Influence weather and climate
Distribute nutrients and scatter organisms
Surface currents are driven by wind:
Most of Earth’s surface wind energy is concentrated in the
easterlies and westerlies.
Due to the forces of gravity, the Coriolis effect, solar energy,
and winds, water often moves in a circular pattern called a gyre.
4
5
°
Direction of
motion
Surface Currents Flow around the Periphery of Ocean Basins
Surface Currents Flow around the Periphery of Ocean Basins
Consider the North Atlantic.
The Ekman spiral and the mechanism by which it operates.
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.
Seawater Flows in Six Great Surface Circuits
Tr
ad
e
w
in
d
90° to the right of wind
direction is up here
5°
–4
30°
At 15°N
Stepped Art
Geostrophic gyres are gyres in balance between the pressure gradient
and the Coriolis effect. Of the six great currents in the world’s ocean,
five are geostrophic gyres. Note the western boundary currents in
this map.
Fig. 9-6, p. 237
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 gravitypowered movement of water
down the pressure gradient.
Ocean Currents
Eddies
Boundary Currents Have Different Characteristics
Western boundary currents – These are narrow, deep,
fast currents found at the western boundaries of ocean
basins.
zThe Gulf Stream
zThe Japan Current
zThe Brazil Current
zThe Agulhas Current
zThe Eastern Australian Current
• Form
• Migrate
• Dissipate
(friction)
Eastern boundary currents – These currents are cold,
shallow and broad, and their boundaries are not well
defined.
zThe Canary Current
zThe Benguela Current
zThe California Current
zThe West Australian Current
zThe Peru Current
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
Boundary Currents Have Different
Characteristics
Water flow in
the Gulf Stream
and the Canary
Current, parts
of the North
Atlantic gyre.
1 sv = 1 million cubic
meters of water per
second
Boundary Currents Have Different
Characteristics
Surface Currents Affect Weather and
Climate
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.)
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.
Surface Currents Affect Weather and
Climate
Wind Can Induce Upwelling near Coasts
Wind induced vertical circulation is vertical movement induced by
wind-driven horizontal movement of water.
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.
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.
Nutrient-Rich Water Rises near the
Equator
Wind Can Also Induce Upwelling Coastal
Downwelling
Equatorial upwelling.
Coastal downwelling.
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.
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.
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.
• Density Structure
– Temperature increase
– Salinity increase
= density decrease
= density increase
• Density Changes
– Evaporation
– Sea ice formation
– Melting
– River influx
– Precipitation
El Nino/Southern Oscillation
Caballing = the mixing and sinking of water masses
from:
http://www.pmel.noaa.gov/tao/elnino/la-nina-story.html#impact
Density Layered System results from variation in temperature and
salinity
La Niña impact on the global climate
In 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.
Thermohaline Circulation Affects All
the Ocean’s 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
= 0-200m
= 200-thermocline
= thermocline-1500m
= 1500-4000m
= 4000-bottom
Equitorial/Tropical water warm less dense = at surface
High Latitudes water cold, dense sinks = deep water formation
• Thermohaline Circulation
– Vertical, density driven circulation, resulting from change in
temperature and salinity
• Continuity of flow
– Water is a relatively fixed quantity in the oceans
– Water can not accumulate in one location or be removed from
another location without movement of water between those
locations
• Vertical movement of water
• Horizontal movement of water
Thermohaline Flow and Surface Flow:
The Global Heat Connection
Pacific Ocean
AAIW
AABW
Circumpolar water
•Low Surface
Convergence
•Uniform deep
water salinity
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.
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.
• 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 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.
NADW
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.
Atlantic Ocean
AAIW
AABW
• 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.
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.
•N-S Extent
•Surface
Convergence
•Clear,
identifiable
patterns
• 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.