Transcript a surface current

Movements of masses of water in the ocean
Formation of Surface Currents
• The forces that move water to produce ocean
currents are caused by:
▫ Rotation of the earth
▫ Winds
▫ Water density Differences
Effect of a Rotating Earth
• Turns from west to
east.
• Velocity of rotation at
its surface is greatest
at the equator and
least at the poles.
• This is called the
Coriolis effect.
Effect of Rotating Earth
• Find city of Quito, capital of
Ecuador
▫ Put a sticker on it
• Find Buffalo, NY
▫ Put a sticker on it
▫ Use a string to measure the distance
around the earth beginning and
ending at the same city (do not cut
the string, just mark with your
finger)
▫ Speed = distance/time (24 hrs)
Understanding the Coriolis Effect
• Read pp. 164 to first paragraph on pp. 167
under “The Coriolis Effect”
• Discuss with your partner until you both
understand.
• Read pp. 189 (Western Boundary Currents)
to pp. 191 and look at Figure 9.8.
• On a piece of paper, describe the coriolis
effect and draw a picture
▫ Which direction does it turn in N. Hemisphere?
▫ Which direction does it turn in the S.
Hemisphere?
Coriolis Effect
• Earth spins counterclockwise
• Deflection will always be to
the right (looking south from
N. Pole)
• Air turns to the right in the
Northern Hemisphere
• Air turns to the left in the
Southern Hemisphere
Circulation
• Global circulation of air
▫ Uneven solar heating
▫ Rotation of the Earth (Coriolis effect)
▫ Areas near equator where
wind converges is known as
intertropical convergence
zone (ITCZ)
 Doldrums
Wind Patterns
• Trade Winds
▫ Surface winds that move from latitudes where dry air
sinks toward latitudes at the equator
▫ ****Winds are named by the direction from which they
blow
▫ In the N. Hemisphere they are the northeast trades
▫ In the S. Hemisphere they are the southeast trades
• Westerlies
▫ Surface winds that flow between areas where dry air
sinks toward the latitudes at polar cells
▫ In N. Hemisphere they approach from the southwest
▫ In the S. Hemisphere they approach from the northwest
Monsoons
• A monsoon is a pattern of wind circulation that
changes with the season
• Areas that experience monsoons typically have wet
summers and dry winters
• Land changes temperature more rapidly than the
ocean
▫ In spring, land heats faster than the ocean
▫ Air above the land becomes warmer and rises
▫ Relatively cool air flows from over the ocean to the
land to take the place of the warm air, which then
heats, rises, and forms clouds and rain
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Breezes
• Breezes are small, daily “mini-monsoons”
• Sea breezes
▫ Morning sunlight warms the land, which then warms the
air above it
▫ Warm air expands and rises
▫ Cooler air from over the sea moves toward land
• Land breezes
▫ After sunset, the land loses heat quickly.
▫ The air over the still-warm ocean will be warmer than air
over the cooling land
▫ The air over the ocean will rise and cooler air from the
land will move over the ocean
Review
1. Differentiate between trade winds and
westerlies. (i.e. Where are they located and in
what direction do they flow...from _____to
_____?)
2. Diagram and label sea and land breezes.
STOP
• Begin and Finish Storm Powerpoint
• Then continue
Ocean Circulation
• Primary forces start water moving and
determine velocity (speed and direction).
▫ Thermal expansion and contraction
▫ Stress of wind blowing over the water
▫ Density differences
• Secondary forces influence the direction and
nature of the flow.
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Coriolis effect
Gravity
Friction
Shape of the ocean basins (floor)
Surface Currents – Solar heating
• Solar (sun) heating causes water to expand
slightly.
• Sea level near equator is about 8 cm higher than
sea level moving toward poles.
• This global difference creates a very slight slope,
and warm equatorial water flows “downhill”
toward poles.
Surface Currents
• Because the earth is turning west to east, the
water is deflected to the west
• Water’s travel is influenced by the Coriolis effect
which starts a circular flow
Surface Currents - Wind
• Primary force responsible for surface currents
• Trade winds and Westerlies “tug” on the surface
of the water
• Water is deflected to the right of the wind
direction (Coriolis Effect)
• As a rule, the friction of wind blowing for at least
10 hours will cause surface water to flow at
about 2% of wind speed – a surface current
Ekman Transport
• Coriolis effect – spinning of
the earth
• Water flows to the right of
the direction of the wind
• Water in each layer flows a
little more to the right as you
go down
• A down flowing spiral occurs
• Eventually water in one layer
will flow in the opposite
direction of the surface water
Gyres – circular flow of water
• Coriolis effect deflects water to
the right of wind direction
• Clockwise in the northern
hemisphere
• Counterclockwise in the southern
hemisphere
• 5 Major Gyres
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North Atlantic
South Atlantic
South Pacific
North Pacific
Indian
Western and
Eastern
Boundary Currents
• 5 major ocean gyres flow in relation to the spin
of the earth – geostrophic currents.
• Currents on the western boundary of the ocean
flow from the equator to the poles
• Currents on the eastern boundary of the ocean
flow from the poles to the equator
Western Boundary Currents
• Fastest and deepest currents
• Move warm water poleward in each hemisphere
• Examples:
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Gulf Stream (largest)
Japan or Kuroshio Current
Brazil Current
Aqulhas Current
East Australian Current
Eastern Boundary Currents
• Carry cold water toward equator
• Shallow, broad, and slower than western boundary
currents
• Examples:
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Canary Current
Benguela Current
California Current
West Australian Current
Peru or Humboldt Current
Countercurrents and Undercurrents
• Countercurrent - Surface water at the equator where
lack of wind allows it to flow in the opposite direction
of the main current.
▫ Without the wind driving the water to the west, some
backward flow of water occurs here
• Undercurrent – water flows beneath and opposite of
the current over it.
▫ Undercurrents are why the Galapagos islands are in cold
water even though they are in the tropics near Equator
Upwellings
• Form when winds parallel to
shore force water away from
shore; west coast of
continents.
• Water from the bottom is
brought up to replace the
moved water
• Water brings with it nutrients
from the ocean bottom
• Cooler water brought upward
can create fog banks and cool
summers (San Francisco)
Downwellings
• Form when winds
parallel to shore force
water into shore; east
coast of continents
• Extra water is forced
down towards the
bottom
• Helps supply deeper
ocean with dissolved
gases and nutrients
Heat transport and Climate
• Currents redistribute heat throughout the globe
• Without these, Earth would have more extreme
weather.
• Cold water from the poles keeps the Galapogos
Islands cool even though they are in the tropics
• Warm water from Gulf Stream, warms the air above
and keeps much of Europe warmer than other
places at similar latitudes.
Galapagos Islands
FROM: serc.carleton.edu
El Niño Southern Oscillation (ENSO)
• ENSO is a reoccurring natural phenomenon in which
the Eastern Pacific is warmer than usual causing
global weather changes.
• Trade winds weaken and warm equatorial water that
would normally flow west-ward “backs up” to flow east
▫ Upwellings in California and Peru decrease or stop
▫ Increases Pacific tropical cyclone activity
▫ Occurs every 3-8 years
La Niña
• La Nina is a intense return to the “normal”
conditions following strong ENSO
▫ Colder than normal conditions in California and
Peru
Deep Ocean Currents
Gravitational Currents
• Two forces that explain vertical movement in the
ocean:
▫ Gravitational Force (G)
▫ Buoyant Force (B)
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G > B – downward movement = sinking
B >G – upward movement = rising
G = B – no movement = floating at surface
G = B – no movement = floating neutrally
bouyant
Thermohaline Circulation
• Water movement caused by differences in
temperature and salinity.
• Processes that decrease salinity:
▫ Precipitation
▫ Freshwater run-off
• Processes that increase salinity:
▫ Evaporation
▫ Freezing
Thermohaline Circulation continued . . .