Transcript No Slide Title

Introduction to
Oceanography
Dynamic Oceanography:
Atmospheric circulation and Currents
6-1
Atmospheric Processes
Density of air is controlled by temperature, pressure
and moisture content.
• 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 and equals 1.04kg/cm2 (standard air
pressure, one atmosphere) at sea level.
• 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 of cooling or 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.
• Movement of air across a pressure gradient parallel to Earth’s
surface is called a wind and winds are named for the direction
from which they come. In contrast, ocean currents are named
for the direction towards which they travel.
Composition and Properties of the Atmosphere
Ascending air cools as it
expands. Cooler air can
hold less water, so water
vapor condenses into
clouds. Descending air
warms as it compresses
and the clouds
evaporate.
Atmospheric Circulation
Atmospheric circulation is powered by sunlight. Since Earth is in thermal
equilibrium, what assumption can be made about the input and output of
heat on Earth?
Uneven Solar Heating and Latitude
Earth as a whole is in thermal equilibrium, but different latitudes are
not. Moving masses of air and ocean currents transport energy from
locations with a surplus to those with a deficit.
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6-1
Atmospheric Processes
Rotation of the Earth strongly influences winds.
• Global winds blow in response to variation in pressure related
to uneven solar heating (insolation) of Earth’s surface.
• Coriolis deflection is the apparent deflection of objects moving
across Earth’s surface to the right of direction of travel in the
northern hemisphere and to the left of direction of travel in the
southern hemisphere.
6-1
Atmospheric Processes
Three major convection cells are present in each
hemisphere.
• The Hadley cell extends from the Equator to about 30o latitude.
• The Ferrel Cell extends from 30 o to about 50 o latitude.
• The Polar Cell extends from 90 o to about 50o latitude.
The Coriolis Effect
Air moves in cells, influenced by the Coriolis effect.
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Wind Patterns
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Monsoons, Sea Breezes and Land Breezes
Monsoons are patterns of wind circulation that change with the season.
Areas with monsoons generally have dry winters and wet summers.
Sea breeze is cool air from over the water moving toward land. Sea
breezes occur after sunrise.
Land breezes occur after sunset when air warmed by the land blows
toward the water.
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Sea Breezes And Land Breezes
Top: Sea breeze
Bottom: Land breeze
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Storms
The orange shaded areas and arrows designate the location and
movement of most hurricanes.
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Surface Currents
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
solar winds, water often moves in a circular pattern called a gyre.
What are some effects of ocean currents?
•transfer heat from tropical to polar regions
•influence weather and climate
•distribute nutrients and scatter organisms
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6-2
Surface Ocean Currents
Wind-driven currents are produced by the
interaction between the wind and the water.
• As wind moves across the water, collision of air molecules
with water molecules inefficiently transfers energy from the
air to the water.
• Water moves at about 3-4% of the wind speed.
• Zonal wind flow is wind moving nearly parallel to latitude as
a result of Coriolis deflection.
• Westerly-driven ocean currents in the trade winds, easterlydriven ocean currents in the Westerlies and deflection of the
ocean currents by the continents results in gyres, which
occupy most of the ocean basin in each hemisphere.
Surface Currents
Water moves clockwise
in the Northern
Hemisphere gyres and
counterclockwise in the
Southern Hemisphere
gyres.
Flow Within a Gyre
Gyres can be further subdivided into distinct currents. The four
interconnected currents in the North Atlantic Gyre have different flow
characteristics and temperatures.
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6-2
Surface Ocean Currents
With time, wind-driven surface water motion
extends downward into the water column, but speed
decreases and direction changes because of
Coriolis deflection.
• Eckman Spiral is the spiraling pattern described by changes in
water direction and speed with depth.
• Eckman transport is the net transport of water by wind-induced
motion.
• Net transport of the water in an Eckman spiral has a Coriolis deflection of 90o to
the direction of the wind.
• Along coastal areas Eckman transport can induce downwelling
or upwelling by driving water towards or away from the coast,
respectively.
Flow Within a Gyre
The effect of Ekman spiraling and the Coriolis effect cause the
water within a gyre to move in a circular pattern.
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6-2
Surface Ocean Currents
Pressure gradients develop in the ocean because the
sea surface is warped into broad mounds and
depressions with a relief of about one meter.
• Mounds are caused by convergences, places where water flows
together and sinks.
• Depressions are caused by divergences, places from where
water rises to the surface and flows outward.
• Water flowing down pressure gradients on the ocean’s
irregular surface are deflected by Coriolis and the amount of
deflection is a function of location and speed.
Flow Within a Gyre
The surface of the North Atlantic is raised through wind motion and
Ekman transport to form a low hill. Can you determine why the center
of the hill is offset to the west?
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.