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Chapter 6
[part I]
Wind and Ocean Circulation
[The atmosphere]
© 2006 Jones and Bartlett Publishers
• Density of air [~1.2 kg/m3] is controlled by:
– Temperature
– Pressure [Is this also true for water?]
– Moisture content [Why? Compare with full fat milk.]
• Warm air is less dense than cold air and moist air
is less dense than dry air.
• Air pressure is the weight of the air from Earth’s
surface to the top of the atmosphere.
– It equals 1.04kg/cm2 (standard air pressure, one
atmosphere) at sea level. [~10 m water, ~10 000kg/m2,
~1000hPa/m2]
6-1 Atmospheric Processes
• Low pressure zone is where air density is
lower than in surrounding areas because the
air:
– is warmer or
– has a higher moisture content
• High pressure zone is where air pressure is
higher than in surrounding area because the
air:
– is cooler or
– has lower moisture content
6-1 Atmospheric Processes
• Fluids (air and water) flow from areas of
high pressure to areas of low pressure.
• Change in pressure across a horizontal
distance is a pressure gradient.
• Greater the difference in pressure and the shorter the
distance between them  the steeper the pressure
gradient and the stronger the wind.
6-1 Atmospheric Processes
Figure 6-1 Air Pressure
6-1 Atmospheric Processes
• Winds are designated according to the direction
from which they come.
• In contrast, ocean currents are designated according
to the direction towards which they travel.
• Global winds blow in response to variation in
pressure related to uneven solar heating (insolation)
of Earth’s surface.
6-1 Atmospheric Processes
Rotation of the Earth strongly influences winds.
Coriolis deflection is the apparent deviation
of objects moving across Earth’s surface.
– Objects are deflected to the right of direction
of travel in the northern hemisphere,
– Objects are deflected to the left of direction of
travel in the southern hemisphere.
– [The amount of deflection is a function of
latitude and current speed.]
6-1 Atmospheric Processes
Figure 6-2 Coriolis Deflection
6-1 Atmospheric Processes
Three major convection cells are present in
each hemisphere
• The Coriolis effect causes wind in these cells to bend
to the east or west to form the (easterlies), westerlies,
and the trade winds.
6-1 Atmospheric Processes
Global [Average] Wind Circulation
Figure 6-3a Global Wind Pattern
6-1 Atmospheric Processes
Global [Average] Wind Circulation
Figure 6-3b Air Pressure and Prevailing Winds
6-1 Atmospheric Processes
Chapter 6
[part II]
Wind and Ocean Circulation
[Surface currents]
© 2006 Jones and Bartlett Publishers
Wind-driven currents are produced by the
interaction between the wind and the water
• As wind moves across the water, air
molecules collide with water molecules.
• When they collide, energy [momentum] is
transferred from the air to the water.
• Water moves at about 3-4% of the wind
speed.
6-2 Surface Ocean Currents
• Zonal wind flow is wind moving nearly
parallel to latitude.
– This is a result of Coriolis deflection.
• A gyre is a circular current caused by:
– Westerly-driven ocean currents in the trade
winds
– easterly-driven ocean currents in the Westerlies
– deflection of the ocean currents by the
continents
6-2 Surface Ocean Currents
Surface Ocean [Average] Currents
Figure 6-4 Surface Ocean Currents
6-2 Surface Ocean Currents
• The sea surface is warped into broad mounds and depressions
with a relief of about one meter.
• They cause pressure gradients to develop in the ocean.
• Mounds on the ocean’s surface are caused by converging
currents.
• Depressions on the ocean’s surface are caused by diverging
currents.
• Water flows down pressure gradients on the ocean’s irregular
surface, from mounds to depressions [as in the atmosphere].
• The flowing water is deflected by the Coriolis effect [as in the
atmosphere].
– The amount of deflection is a function of latitude and current
speed.
6-2 Surface Ocean Currents
• With time, wind-driven surface water
motion extends downward into the water
column.
– speed decreases
– direction changes because of Coriolis deflection
• Ekman Spiral is the pattern caused by
changes in water direction and speed with
depth. [Ekman (1905)]
6-2 Surface Ocean Currents
• Ekman transport is the net transport of water by wind-induced
motion.
• Net transport of the water in an Ekman spiral has a Coriolis deflection of 90o to
the direction of the wind.
• Along coastal areas, Ekman transport can induce:
– downwelling by driving water towards the coast, or
– upwelling by driving water away from the coast
[The angles for surface current and
net transport depend on the circumstances.]
Figure 6-6b Map View
6-2 Surface Ocean Currents
Figure 6-6a Ekman Spiral in the Northern Hemisphere
6-2 Surface Ocean Currents
Convergence & Divergence of Water Currents in the
Northern Hemisphere
[Where are the
richest fishing
waters? Compare
with global wind
pattern!]
Figure 6-7a Coastal Divergence & Convergence
6-2 Surface Ocean Currents
Figure 6-7b Ocean Divergence & Convergence
6-2 Surface Ocean Currents
Geostrophic flow allows currents to flow long
distances with no apparent Coriolis deflection.
• Coriolis effect deflects water into the center of the
gyres, forming a low mound of water.
• As height of the mound increases, the pressure
gradient steepens pushing the water outward to
level the mound.
• When the pressure gradient equals Coriolis
deflection, the current flows parallel to the wind
around the mound.
– This is called geostrophic flow.
6-2 Surface Ocean Currents
Geostrophic Currents
(a) Stacking of water in center of ocean
(b) Effect of pressure gradient
(c) Geostrophic currents
Figure 6-9
6-2 Surface Ocean Currents
• The current flow pattern in gyres is asymmetrical:
– narrow, deep and swift currents along the basin’s western
edge.
– broad, shallow slower currents along the basin’s eastern edge
Figure 6-10 Flow Asymmetry Around Circulation Gyres
6-2 Surface Ocean Currents
• The geostrophic mound is deflected to the
western part of the ocean basin because of
the eastward rotation of the Earth on its
axis.
• The Sargasso Sea is a large lens of warm
water:
– encircled by the North Atlantic gyre
– separated from cold waters below and laterally
by a strong thermocline
6-2 Surface Ocean Currents
• Western boundary currents, such as the
Gulf Stream, form a meandering boundary.
• These boundaries separate coastal waters
from warmer waters in the gyre’s center.
• Meanders can be cut off to form warmcore and cold-core rings.
6-2 Surface Ocean Currents
Gulf Stream Meanders and Rings
Figure 6-12a Heat Photo of Gulf Stream
6-2 Surface Ocean Currents
Figure 6-12b Formation of Rings
6-2 Surface Ocean Currents