Download printer-friendly version

Performance Benchmark E.12.A.5
Students know Earth’s rotation affects winds and ocean currents. I/S
Winds and ocean currents both distribute energy (in the form of heat) received from the sun.
Additionally, winds and atmospheric circulation are highly effective at transporting water vapor,
either away from an area as it evaporates, or bringing it to an area in moisture-laden air masses
which yield precipitation. Thus, it is important to understand the forces driving the circulation of
Earth’s atmosphere and waters to aid us in predicting weather and climate patterns.
The circulation of the oceans is affected by variations in atmospheric circulation. Surface
currents are driven by the force of the wind pushing on the ocean surface. The frictional drag of
the wind on the surface layer of the ocean creates currents. Conversely, because of their ability
to store huge quantities of heat and moisture, the oceans alter atmospheric conditions and the
weather. For example, tropical storms form over warm ocean waters, which supply the energy
for hurricanes and typhoons. The winter storms that bring precipitation to the western U.S.
originate over the North Pacific. It is clear that forces at work in the atmosphere affect the
ocean, just as forces at work in the ocean affect the atmosphere. If Earth was smooth as a
marble, and was not rotating, the motions of atmospheric and oceanic currents would be quite
simple. But, the earth is far from smooth, and it rotates at a speed of 1670 km/hr (at the equator).
Earth’s rotation has a profound influence on patterns of circulation within both the atmosphere
and the oceans.
Figure 1: Global Convection Cells (from National Center for Atmospheric Research,
http://www.ucar.edu/learn/1_1_1.htm and http://www.ucar.edu/learn/1_1_2_7t.htm)
Ideally, warm air at the equator rises and cold air at the poles descend, creating two hemispheric
circulation cells as depicted on the left of Figure 1. But, the trip between equator and poles is
lengthy, yielding to the development of smaller convection cells in each hemisphere, as seen to
the right of Figure 1. Initially, Hadley cells are created as warm air at the equator rises - creating
a region of low pressure. The warm air begins it poleward trip, but becomes cold enough at
around 30N and 30S latitude that the air descends, creating a region of high pressure. When
the cold air reaches the surface at those latitudes, it travels along the surface back towards the
equator. Similarly, Ferrel cells develop as comparatively warmer air ascends around 60N and
60S latitude and travels through the upper troposphere towards the equator to approximately
30N and 30S latitude where the air is cold enough to descend and complete the circulation cell.
Polar cells also originate from rising air at 60N and 60S latitude, but in this case, the rising air
moves towards the poles until it is cold enough to descend and complete that cell.
The Coriolis Effect prevents the air in the convection cells from traveling straight north and
south paths. Earth rotates counterclockwise (as viewed from above the axis at the North Pole).
In effect, Earth is rotating from west to east as the atmosphere is attempting to circulate from
pole to equator. The result is a frictional drag that deflects the motion of air in the convection
cells. In the northern hemisphere, the Coriolis Effect deflects air to the right of the direction it
was traveling, while in the southern hemisphere, air is deflected to the left of the direction it was
traveling.
For a demonstration on how the Coriolis Effect works, go to
(http://uwf.edu/atc/projects/coriolis/main.swf).
Though Earth as a whole completes one rotation each 24 hour period, the speed of the rotation
varies from equator to poles. Earth’s greatest circumference (40,000 km) is around the equator,
and its rotational speed there is 1670 km/hr. At 40 N (or S) latitude, the circumference is 30,600
km, and the rotational speed is 1280 km/hr. A line of longitude intersecting these two lines of
latitude will make one complete rotation in 24 hours, but the point of intersection at the equator
must travel 9,400 km further during those 24 hours than the point of intersection at 40N (or S)
latitude, which means the point on the equator traveled at a faster speed. The difference in Earth
surface speed exacerbates (greater near equator) or diminishes (lesser near poles) the Coriolis
Effect. Earth’s uneven surface further complicates the deflection caused by the Coriolis Effect.
In the plains regions (and across the oceans), there are few disruptions due to the relatively
smooth surface. But, in mountainous regions, the land itself can deflect the moving air into more
convoluted patterns.
For further information on how Earth’s topography influences wind direction and speed, visit the
following websites:
http://www.stanwellcommunity.org/pdfs/wind/W09_Factors_Affecting_Wind.pdf
http://www.disaster-info.net/viento/english/presentations/structural2.ppt
http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/circulation/coriolis_and%20friction.ht
ml
In general, between 30 N latitude and 30 S latitude, the winds blow from the east, and are
called “trade winds”. Between 30 N and 60 N (and between 30 S and 60 S), the winds blow
from the west, and are called “westerlies”. From 60N to 90N (and 60S to 90S) the winds
again blow from the east, and are termed “easterlies” (Figure 2).
In the upper atmosphere (10km to 15 km in altitude), narrow bands of high velocity winds (jet streams)
form, flowing from west to east. The polar jet stream occurs above the rising air between the Polar cells
and the Ferrel cells. The subtropical jet stream occurs above the descending air between the Ferrel cells
and the Hadley cells (Figure 2).
Figure 2: Convection Cells and Jet Streams (from Tulane University, The Ocean-Atmosphere System,
http://www.tulane.edu/~sanelson/geol204/oceanatmos.htm)
Jet streams affect the weather as they meander above Earth's surface. In the northern hemisphere, when
the jet stream meanders to the south, it brings low pressure centers (and associated storms) further to the
south. When it meanders to the north, the high pressure centers (and clear skies) move to the north.
High pressure and low pressure centers are also influenced by the Coriolis Effect. In the
Northern Hemisphere, as winds rush inward toward a low pressure center, they are deflected
toward the right (Figure 3), creating a counterclockwise rotation (known as “cyclonic winds”).
Both tornados and hurricanes are cyclonic. Winds rushing outward from a high pressure center
are also diverted to their right (Figure 3), creating a clockwise rotation (known as “anticyclonic
winds”). In the Southern Hemisphere, due to a deflection toward the left, the reverse is true
(cyclones rotate clockwise and anticyclones counterclockwise).
Figure 3: Wind Deflection Around Low Pressure Centers and High Pressure Centers in the Northern Hemisphere
(from Tulane University, The Ocean-Atmosphere System,
http://www.tulane.edu/~sanelson/geol204/oceanatmos.htm)
As previously discussed, the winds have an affect on ocean currents. Frictional drag between
surface ocean water and the winds in the six major convection cells around the Earth create
currents. Water accumulates (piles up) in the direction the wind is blowing, while gravity works
to counteract this accumulation and pull the water down. The Coriolis Effect then works to
deflect the water to the right (or left, depending on which hemisphere) of the direction it was
going, and induces large, circular currents called “gyres” in the ocean basins. Figure 4 depicts
the build-up of water under the influence of wind and how the Coriolis Effect impacts the current
(left drawing) as well as the generalized shape and location of gyres (right drawing).
Figure 4: The Effect of Wind and the Coriolis Effect on the Ocean Currents and the Production of Gyres (from
Department of Earth Sciences, University of Southern California, Ocean Currents and Climate,
http://earth.usc.edu/~stott/Catalina/Oceans.html)
On a global scale, the Coriolis Effect, in conjunction with winds in convection cells create
several oceanic currents, as seen in Figure 5.
Figure 5: Global Ocean Currents (from CUNY Geography, Ocean Currents,
http://www.geography.hunter.cuny.edu/~tbw/wc.notes/3.temperature/ocean_currents.htm)
As an example of how ocean currents affect the weather, a warm ocean current flows along the
coast of Peru every year around late December to early January. In some years, strong westerly
trade winds in the equatorial Pacific push warm water toward Indonesia. This produces a large
pool of warm water in the Western Pacific, and an event known as “El Niño” occurs. During an
El Niño, the trade winds weaken and a wave of warm water moves eastward along the equator
toward South America (Figure 6). This warms the central and eastern equatorial Pacific. When
this happens, some parts of the globe experience drought, while others are deluged with rain.
For more information on El Niño, visit
http://www.learningdemo.com/noaa/lesson08/l8la1.htm
Figure 6: A Comparison of Oceanic and Atmospheric Circulation Between Normal and El Niño Years (from Tulane
University, The Ocean-Atmosphere System, http://www.tulane.edu/~sanelson/geol204/oceanatmos.htm)
Performance Benchmark E.12.A.5
Students know Earth’s rotation affects winds and ocean currents. I/S
Common misconceptions associated with this benchmark:
1. Students mistakenly believe Earth’s Coriolis Effect causes the water in a sink or toilet
to spin counterclockwise in the Northern Hemisphere and clockwise in the Southern
Hemisphere.
Too frequently, during discussions on the Coriolis Effect, students have been told that water
spinning down a toilet or sink will move counterclockwise in the Northern Hemisphere and
clockwise in the Southern Hemisphere. This was even addressed in television animation
(The Simpsons – Bart vs. Australia). In reality, the Coriolis Effect is too weak to affect such
a small basin of water over such a short period of time. The direction the water will spin as it
drains is due to the construct of the sink basin or toilet bowl. In the same building, some
sinks could drain counterclockwise while others drained clockwise. The Coriolis Effect is
more effective on the global scale and on long-lasting events, such as air or water movement
in a constant direction.
The following website addresses this misconception. It, as well, points out examples of the
media propagating the misunderstanding.
http://www.ems.psu.edu/~fraser/Bad/BadCoriolis.html
2. Students incorrectly believe all winds are deflected to the right in the Northern
Hemisphere and all winds are deflected to the left in the Southern Hemisphere.
When students are taught about the direction of deflection in each of Earth’s hemispheres,
they do not always grasp the aspect that the deflection is away from the direction the
substance (water, air, projectile, etc.) was moving. If we were on Earth’s equator, a
projectile launched from the equator toward the North Pole would veer to the right and
appear to be deflected to the east, but a projectile launched from the North Pole towards our
position on the equator would veer to its right and appear to be deflected to the west. The
reverse would hold true between the equator and the South Pole. These animations from
Wiley do a very nice job of discussing and showing how the Coriolis Effect works in both
the Northern and Southern Hemispheres.
http://www.wiley.com/college/strahler/0471480533/animations/ch07_animations/animation1
.html
http://www.wiley.com/college/strahler/0471480533/animations/ch07_animations/animation2
.html
3. Students mistakenly believe the Coriolis Effect affects the air of the atmosphere, but not
the waters of the ocean.
Atmospheric motions due to the Coriolis Effect are more readily apparent than those motions
caused in ocean waters. When students are taught about ocean currents, they often learn
about density currents, but are not always taught that the Coriolis Effect helps drive currents.
The following website provides comprehensive videos which thoroughly address the impact
of wind and the Coriolis Effect on ocean currents. When utilizing this site, view first the
video under the Lesson tab, then follow up with the video under the Global Impact tab.
http://www.learningdemo.com/noaa/lesson08.html
Performance Benchmark E.12.A.5
Students know Earth’s rotation affects winds and ocean currents. I/S
Sample Test Questions
1. Surface currents are created:
a.
b.
c.
d.
as the more dense surface water sinks and less dense deep water rises
by frictional drag of the wind against the surface of the ocean waters
as surface waters squeeze between narrow passages separating ocean basins
when Earth’s magnetic field imparts a charge to the surface waters
2. Based on the graphic of global wind circulation, winds in the United States of America
generally blow:
a.
b.
c.
d.
from the North Pole towards the Equator
from the Equator towards the South Pole
from the east towards the west
from the west towards the east
Figure reference: http://www.homepage.montana.edu/~esci111/111erosion/erosionweb_files/image015.jpg
3. A deflection of winds and currents resulting from Earth’s rotation is the:
a.
b.
c.
d.
Coriolis Effect
Greenhouse Effect
Tectonic Effect
Adiabatic Effect
4. In the Northern Hemisphere, the Coriolis Effect causes masses of air to be deflected to the
__________ of their original direction of motion.
a.
b.
c.
d.
south
left
north
right
5. In ocean basins, the Coriolis Effect creates large, circular currents called:
a.
b.
c.
d.
cyclones
polar cells
gyres
anticyclones
6. In the Northern Hemisphere, air flowing out of a high pressure system:
a.
b.
c.
d.
moves in a clockwise pattern
moves in a counterclockwise pattern
moves straight upward because the pressure is so high
moves due South as the air slides downslope
7. All of the following statements are true regarding atmospheric convection EXCEPT:
a.
b.
c.
d.
Hadley cells circulate air between about 30N latitude and 30S latitude
trade winds can be found between 30N latitude and 60N latitude
the polar jet stream is located between the Ferrel cells and Polar cells
easterlies can be found between 60N latitude and 90N latitude
8. When an El Niño occurs:
a.
b.
c.
d.
Earth enters an eleven-year long cycle of global cooling
the Coriolis Effect works in reverse in the Northern and Southern Hemispheres
a large pool of warm water in the Pacific can cause drastic changes in weather
anticyclones capture and contain the La Niña for periods lasting several months
9. By interpreting the accompanying graphic, we see that in the Northern Hemisphere, the
major surface ocean currents rotate in a __________ direction and that in the Southern
Hemisphere, the major surface ocean currents rotate in a __________ direction.
a.
b.
c.
d.
counterclockwise / clockwise
clockwise /counterclockwise
counterclockwise / counterclockwise
clockwise / clockwise
Figure reference: http://www.nmm.ac.uk/upload/img_400/currents.gif
Performance Benchmark E.12.A.5
Students know Earth’s rotation affects winds and ocean currents. I/S
Answers to Sample Test Questions
1. (b)
2. (d)
3. (a)
4. (d)
5. (c)
6. (a)
7. (b)
8. (c)
9. (b)
Performance Benchmark E.12.A.5
Students know Earth’s rotation affects winds and ocean currents. I/S
Intervention Strategies and Resources
The following is a list of intervention strategies and resources that will facilitate student
understanding of this benchmark.
1. National Oceanic and Atmospheric Administration (NOAA)
NOAA has lessons to help students understand the Coriolis Effect as well as movement of
ocean currents.
Coriolis Force
http://oceanexplorer.noaa.gov/explorations/03edge/background/edu/media/coriolis.pdf
Motion in the Ocean
http://www.nos.noaa.gov/education/kits/currents/lessons/ocean_motion.pdf
2. Osher Map Library and Smith Center for Cartographic Education
The University of Southern Maine has several lessons teaching about ocean currents.
Surface Currents
http://www.usm.maine.edu/maps/lessons/nr10.htm
Current and Climate
http://www.usm.maine.edu/maps/lessons/nr12a.htm
3. Science @ NASA
This website helps students better understand the Coriolis Effect. The site links to short
movies which demonstrate the Coriolis Effect in a way which makes sense to students.
Spinning Brains
http://science.nasa.gov/headlines/y2004/23jul_spin.htm
4. Eastern Illinois University Department of Geology & Geography
The site presents colorful, detailed graphics and explanations which help students visualize
atmospheric processes and motions.
Atmospheric Circulation
http://www.ux1.eiu.edu/~cfjps/1400/circulation.html
5. National Oceanic and Atmospheric Administration (NOAA)
NOAA provides activities to help teach students more about El Niño.
El Niño
http://www.oar.noaa.gov/k12/html/elnino2.html