Transcript The Coriolis Effect

Why Spinning Matters –
The Coriolis Effect and the World’s Oceans
October 17-21, 2008
Photo Credit: NASA – Earth Observatory
Getting Started
Objectives
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To demonstrate an understanding of
convection.
To demonstrate an understanding of
how a rotating sphere affects the
speed of fluids at different locations
on its surface.
To demonstrate an understanding of
how motion appears to be affected
by the rotating motion of a sphere.
To demonstrate an understanding of
the Coriolis force and how it affects
the trade winds
To demonstrate an understanding of
how the Coriolis force varies with
latitude.
Take the QUIZ
• What Do You Know?
What is Convection?
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Movement that results when heat is
transferred in a fluid
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First, warmed fluids (like water or air)
become less dense and will rise opposite to
the force of gravity.
•
Next, cooler fluid will move to replace the
rising warm fluid and it will be warmed
itself.
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This cycle repeats to mix the fluid.
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Convection model
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Julius Sumner Miller on Convection - a
riot!
What on Earth?
Descriptions of Earth’s Convection
Edmond Halley
1656-1742
• reasoned that intense solar radiation heated the
air near the Equator and caused it to expand and
rise up.
• This rising air is replaced by cooler air converging
on the Equator from the northern and southern
hemispheres.
• Circulation of the air is driven by a pressuregradient force, which causes high-pressure (cooler,
more dense) air to move into regions of lowpressure (warmer, less dense) air.
• predicted a flow of air from the poles to the
Equator where the air masses converge.
Another View
of Convection
Note that it is the sinking of
cold, dense air NOT the rising
of warm air that drives the
circulation pattern.
Image credit: NASA
What on Earth?
Descriptions of Earth’s Convection
George Hadley
1685-1768
• English lawyer and amateur
meteorologist
• First to describe the reason the
equatorial trade winds preferentially
blow westward.
• Recognized that Earth is a rotating
sphere and that sites on its surface travel
with different speeds (travel different
distances in equal times).
• Model of Earth’s convection termed the
‘Hadley cell’ in his honor.
The Coriolis Effect
Gustave Gaspard de Coriolis
1792 - 1843
• French mathematician, mechanical
engineer, and scientist
• Determined simple rules for the direction
of moving objects on the surface of a
rotating sphere, now known as the
Coriolis effect:
– The apparent (Coriolis) force is
perpendicular to the velocity of the
object and the rotation axis.
– A balance of forces causes objects
traveling in the Northern
Hemisphere to curve to the right.
– A balance of forces causes objects
traveling in the Southern
Hemisphere to curve to the left.
Visualizing the Coriolis Effect
• Earth rotates at different speeds at different latitudes.
• v = d/t
– Rotating earth visualization
– The Coriolis Model
A
B
C
Major Wind Belts
Prevailing Wind Belts of Earth
The earth is encircled by several
broad prevailing wind belts, which
are separated by narrower regions
of either subsidence or ascent. The
direction and location of these wind
belts are determined by solar
radiation and the rotation of the
earth. The three primary circulation
cells are known as the: Hadley cell;
Ferrel cell; and Polar cell.
A Horizontal View
Another Look at the Wind Belts
Significance of
Wind Belts?
• Guide weather and
storms
• Jet Stream
–100 mph
–Between 30-60º
–Above friction zone
• Influences Sailing &
Navigation
• Deserts @ 30º
Ekman’s Contribution
Vagn Walfrid Ekman (1874-1954)
• Swedish oceanographer
• First to describe the Ekman spiral – the
movement of ocean currents in response to
the rotation of the Earth (Coriolis effect)
1. direction of wind
2. Effective force of wind on the sea surface
3. Effective direction of the current flow (Ekman
transport)
4. Coriolis force (effect)
• Based on observations by Norwegian Fridtjof
Nansen (1861-1930) on the Fram expedition.
• Extends to a depth of 100-150 meters
– Limited by surface turbulence, diurnal cycles
– Depth of effect is termed Ekman layer
The Ekman Layer
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Background - Ekman Transport
Direction of Ekman transport
varies with ocean depth (15º
in shallow waters)
Pycnocline can be a
boundary
Can pile up water in the
ocean causing pressure
gradients.
Geostrophic Flow
Ekman transport causes
surface waters to move
toward the central region of a
subtropical gyre.
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A gyre is a large, nearly circular system
of wind-driven surface currents that
center around latitude 30º in both
hemispheres.
produces a broad mound of water
Steepening gyre causes a horizontal
pressure gradient. Water flows
“downhill”
The Coriolis effect acts on the “downhill”
sliding parcels of water
When the outward-directed pressure
gradient force balances the apparent
force due to the Coriolis effect then the
water parcels flow around the gyre
along contours of sea surface elevation.
The horizontal movement of surface
water arising from a balance
between the pressure gradient force
and the Coriolis force is known as
geostrophic flow.
Ocean Basin Model
World Subtropical Gyres
Worldwide Surface Currents
• Equatorial currents
– East to west under influence of
tradewinds
• Western Boundary currents
– South to north (Northern
Hemisphere – opposite in the
Southern Hemisphere)
– Coriolis effect piles up water on the
Western sides of oceans
– Moves very fast (25 -75 mi/day)
– Deeper
– Can be influenced by continental
margins
– Transports large amounts of heat.
Pictured above is the East Coast of the United States, in grey,
with the Gulf Stream, in yellow and orange, revealed through Sea
Surface Temperature data (SST), made from the MODIS
instrument on the Terra satellite. In this image, blue represents the
coldest temperatures (between 1-10 °C) and orange and yellow
represent the warmest temperatures (between 19-30°C). The Gulf
Stream is readily visible as the warmest water in the image.
Up and Down in the Oceans
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Coastal upwelling occurs where
Ekman transport moves surface
waters away from the coast; surface
waters are replaced by water that
wells up from below.
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Brings nutrient rich waters to the surface
Increases biological productivity
Influences weather patterns
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i.e California ‘s summer fogs
Coastal downwelling occurs where
Ekman transport moves surface
waters toward the coast, the water
piles up and sinks.
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Sends oxygen-rich waters to the deep sea
Decreases biological productivity
El Niño – Southern Oscillation
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El Nino – Southern Oscillation – periodic event that reduces or reverses the Pacific
Equatorial Current
El Niño
Upwelling and Downwelling
Southern Oscillation
The Atmospheric Component
El Niño
El Nino Animation – from NOAA – Earth System Research Laboratory
El Niño
Effects on Global Weather Patterns
Work Cited
Castro, Peter & Michael E. Huber. Marine Biology. 5th. New York: The McGraw-Hill Companies, 2005.
“Ekman_spirale.svg” Wikimedia commons. 5 Jan 2008. Chabacano. 6 Mar 2008
<http://commons.wikimedia.org/wiki/Image:Ekman_spirale.svg>
“El Nino and La Nina Ocean Temperature Patterns” National Weather Service Climate Prediction Center. 19 Dec 2005
<http://www.cpc.noaa.gov/products/analysis_monitoring/ensocycle/ensocycle.shtml>
Gore, Pamela. "Wind and global wind systems." 01 Apr 2005. Georgia Perimeter College. 5 Mar 2008
<http://gpc.edu/~pgore/Earth&Space/GPS/wind.html>.
"Hadley cell circulation and the trade winds." 5 Mar 2008
<http://www.newmediastudio.org/DataDiscovery/Hurr_ED_Center/Easterly_Waves/Trade_Winds/Trade_Winds.html>.
"In the zone." Getting the global picture. 2003. University of Wisconsin, Board of Regents. 4 Mar 2008
<http://whyfiles.org/174earth_observe/4.html>.
Madl, Pierre. The El Nino (ENSO) Phenomenon. 01 Dec 2000. Environmental Physics , 437-503 retrieved 07 Mar 2008 from
<http://www.sbg.ac.at/ipk/avstudio/pierofun/atmo/elnino.htm>
"Pressure and winds." 5 Mar 2008
<http://atschool.eduweb.co.uk/kingworc/departments/geography/nottingham/atmosphere/pages/pressureandwindsalevel.
html>.
Srinivasan, Margaret. "Ocean surface topography from space." Overview - climate. NASA - Jet Propulsion Laboratory California Institute of technology . 4 Mar 2008 <http://sealevel.jpl.nasa.gov/overview/climate-earth.html>.
"Traveling on a rotating sphere." Ocean motion and surface currents. NASA. 4 Mar 2008
<http://www.oceanmotion.org/guides/cf_2/cf_student_2.htm>.
Tides
November 30, 2010
What are Tides?
• A tide is defined as a periodic rise and fall of the sea
surface
– very-long period waves noticeable only at the shoreline
– originate in the open ocean
• High tide – wave crest reaches the shoreline
• Low tide – wave trough reaches the shoreline
Tidal Range & Tidal Currents
• Tidal range - the difference in water height between high
and low tides
– varies from a few cm to up to 48 feet (14 m) at the Bay of Fundy in
Canada
• Tidal current – horizontal movement of water that
accompanies the rising and falling tide
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incoming current is a flood current
outgoing is an ebb current
strongest during high or low tides, weakest in between the two
Animation
Blue planet video segment
What Causes Tides?
• Result from the gravitational pull
of the moon and sun on the
Earth.
• Newton’s law of universal
gravitation
– More massive an object, greater its
gravitational pull
– Varies inversely as the square of the
distance between objects
• Tide generating force
– vary inversely as the cube of the
distance from the tide generating
object.
• Therefore the moon has a greater
effect on the tides due to its
proximity
Gravity, Inertia, and the Two Bulges
• Two tidal bulges (high tides) form
• Bulge on the moon side
Moon
Gravitational
force
– caused by gravitational attraction of moon
and Earth exerted on the fluid (oceans)
– largest bulge
– overcomes inertia of water
• Bulge on the “far side” of the Earth
opposite the moon
Earth
– caused by inertia – the tendency of moving
objects to continue moving in a straight line
– inertia exceeds gravitational force here
Inertia
Changing Angles and Changing Tides
• Moon revolves around the Earth
– Its declination - angle relative to the equator - increases and
decreases
– Varies the height and intensity of tides
– Monthly variation
• Animation
Changing Angles and Changing Tides
• Because of Earth’s tilted axis, the sun’s relative position
(declination) to the equator changes throughout the year
– Minimum (spring/fall equinoxes)
– Maximum (summer/winter solstices)
Frequency of Tides – The Lunar Day
• Lunar orbit: 29.5 days
• Lunar tide
• Solar tide (1/2 as large
as lunar one)
• When both in
alignment spring tides
occur. When moon
not in alignment,
neap tides.
• Animation
Tidal Variation due to Variation in
Moon and Earth Orbits.
Continents in the Way .. Again
Land masses can create three different tide patterns
• Diurnal
• Semidiurnal
• Mixed semidiurnal
Tidal Cycles of the World
Other factors affecting tides…
• Shoreline and coastline topography
• Shape of bays and estuaries
• Local wind and weather patterns
Tide Resources
McNish, Larry. "RASC Calgary Centre - A Complete Guide to." 17 Dec 2007. 29 Oct 2008
<http://calgary.rasc.ca/radecl.htm>.
"Tides and Water Levels." NOAA Ocean Service Education. 25 Mar 2008. National Oceanic and
Atmospheric Association, U.S. Department of Commerce. 29 Oct 2008
<http://www.oceanservice.noaa.gov/education/kits/tides/welcome.html>.