GEOL 2503 Introduction to Oceanography
Dr. David M. Bush
Department of Geosciences
University of West Georgia
Topic 11: Global Atmospheric Circulation
POWERPOINT SLIDE SHOW NOTES
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Topic 11: Global Atmospheric Circulation
We talked about Earth’s internal heat which drives differentiation and plate tectonics,
now we focus on external heat from the sun.
All the atmospheric motions on Earth are driven by energy from the sun. We can start by
looking at diagram (a) air circulation near the equator. Not ascending moist air, which
releases moisture (rain), and the circulation of air on either side of the ascending air.
Those are convection cells. Just like in the mantle. Recall that convection is driven by
differences in density. Warm air is less dense than cold air (just like warm versus cold
water) and so warm air rises. Air cools aloft and sinks to complete the convection cell.
On the left side of the diagram you see three of the atmospheric convection cells in each
hemisphere. On the right side you see the surface winds driven by the convection cells.
We’ll get into this in more detail.
All of this is driven by heating of Earth’s surface by the sun. Or actually, by unequal
heating of the surface. Notice the concentration of solar energy at the equator, and
spreading out, or dissipation, of solar energy at higher latitudes.
The difference in the angle of solar rays is why energy is concentrated at the equator,
and dissipated nearer to the poles.
Seasons are not controlled by the amount of incoming solar energy, but by the tilt of
Earth on its axis. Recall Topic 2.
Northern hemisphere summer (the summer solstice), Earth’s axis tilted toward the sun.
Notice the illuminated half of the planet. The sun is directly above the Tropic of Cancer.
From the Arctic Circle to the north pole there are 24 hours of daylight. From the
Antarctic Circle to the south pole there are 24 hours of darkness. Exactly the opposite at
the winter solstice. Earth’s axis tilted away from the sun. The sun will be directly above
the Tropic of Capricorn, and above the Arctic Circle there will be 24 hours of darkness;
and from the Antarctic Circle to the south pole there are 24 hours of daylight. This
spreads some solar energy up into the northern hemisphere in northern hemisphere
summer, and spreads some solar energy down into the southern hemisphere in southern
hemisphere summer. Refer to previous slide, plus Topic 2, slides 17 and 18.
So there is a surplus of heat in the tropics, a deficit toward the poles. Hot near the
equator, cold near the poles. The unbalance of heat drives wind (and ocean currents as
we’ll see later) which carry excess heat away from the equator and toward the poles.
The overall effect is a moderating of our climate, making Earth more hospitable for life.
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A convection current works the same whether the heat imbalance is in the interior of
Earth, in the atmosphere, or in a room with a source of heat and cold.
Atmospheric convection currents. Remember air pressure, the weight of a column of air
from Earth’s surface to outer space? Rising air reduces the pressure creating a low
pressure area at the surface. Sinking air increases the pressure creating a high pressure
area at the surface. This is what you see on the weather news; areas of low and high
pressure are mostly just rising or sinking air. Notice terms of convergence and
divergence. We used similar terms in plate tectonics referring to relative motion at plate
boundaries. Different process here, but relative movement is the same. Air currents
colliding (convergence) or moving apart (divergence). We’ll see this in the ocean as well.
Contour maps of surface air pressure just like you might see on the evening weather
report. High pressure areas (H) are sinking air, low pressure areas (L) are rising air.
Look what happens to air as it rises or sinks. This is why rain is more likely to be
associated with rising air (low pressure systems), and rain is less likely to occur under
high pressure conditions. Watch for this on the next weather forecast.
Hypothetically, Earth should have one giant convection cell in each hemisphere. Warm
air rising at the equator and cold air sinking at the poles, driving surface wind toward the
equator, and upper level winds toward the poles. This would happen if Earth was not
rotating. Of course Earth does rotate so the picture is more complex as we shall see.
Because rising air cools and becomes denser, the convection cells would be thicker near
the equator. Again, this would happen if Earth was not rotating.
Rotation makes things a little more complex. Large scale motions are deflected to the
right in the northern hemisphere and to the left in the southern hemisphere.
The deflection is caused by the Coriolis Effect (often called the Coriolis Force).
Earth completes one rotation in 24 hours. Because Earth’s circumference is greatest at
the equator, a point on the surface in Quito, Ecuador, will move faster than a point on
the surface in Buffalo, New York. Shorter distance, same time, fewer miles per hour.
Looking down on this from above the north pole.
Things moving over long distances (for example wind, ocean currents, planes, missiles)
will be affected by rotation rate. See the missile fired from Quito. It has a firing speed
toward Buffalo, but also an eastward-directed speed because of Earth’s rotation. Fire
the missile and it’s moving north but also east. Buffalo is also moving (rotating) toward
the east, but at a slower speed than the missile. So the missile actually ends up farther
east than Buffalo. It’s a motion that ends up moving the missile to the right, and the
missile lands off course. Fire a missile from Buffalo toward Quito. The missile has a
slower eastward speed than Quito. So Quito ends up further eastward by the time the
missile gets to the equator, and again the missile is deflected to the right. It’s the same
idea, just the opposite direction in the southern hemisphere. The deflection there is to
the left of the direction of motion.
By the way, you may have heard that water drains down a sink or toilet in opposite
directions in the northern and southern hemisphere, and that it is caused by the Coriolis
effect. This is wrong on both counts.
What is wind?
Earth’s rotation forces air movement such that three convection cells are driven in each
hemisphere, not one as in the hypothetical non-rotating Earth. The cells are given
names, but we won’t worry about that. Let’s go through this bit by bit.
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First, the air:
1. Air rises at the equator. There is convergence at the surface, divergence aloft (same
as in slide 11 only here we’re talking about global scale). Rising air means low
pressure at the surface. It also means high potential for clouds and rain, which
creates conditions favorable for growth of tropical rainforests.
2. Rising air is moving vertically so it is technically not wind (wind is the horizontal
movement of air). At the surface the air is calm. If you are in a sailing ship you are
out of luck. Ancient explorers named this calm wind area the “Doldrums.”
3. Air diverges aloft and flows both the north and to the south.
4. Because of cooling effects, air sinks at around 30° N and S latitude. At the surface
there is divergence deflecting air both to the south (to complete the first set of
convection cells—one in each hemisphere) and to the north (to contribute to the
second set of convection cells). Sinking air means high pressure at the surface and
little chance for clouds and rain. This creates dry surface climate zones and is why
the world’s great deserts are centered at about 30° N and S latitude. Check it out on
a map or globe.
5. As with the Doldrums, vertical movement of air (in this case sinking) is not wind. If
you are in a sailing ship here you are also out of luck. Ancient explorers named this
calm wind area the “Horse Latitudes,” supposedly because sailors would toss things
overboard, including horses, to reduce the ships’ weight and try to take advantage
of any potential wind.
6. As the sinking air is warmed, air rises again at about 60° N and S latitude, completing
the second set of convection cells. This also creates another zone of low pressure
and high rainfall. The world’s temperate rainforests are located at about this
latitude, but mostly in the northern hemisphere, only because there is more land
area in the northern hemisphere. The classic example is the Pacific temperate rain
forest system in western North America—Northwestern United States and
Southwestern Canada. The Olympic National Forest in Washington state is part of
this system.
7. A third set of convection cells is driven by the very cold conditions at the north and
south poles, creating sinking air and the Polar highs (high pressure systems). Polar
fronts separate the polar convection cells from the mid-latitude convection cells,
thus separating cold polar air from warm subtropical air. This is also known as the
jet stream. Major storms form along this boundary. The polar fronts move with the
seasons. In the northern hemisphere winter the polar front moves south and
creates conditions conducive to the formation of major winter storms in the United
States.
Now the surface winds:
1. In both hemispheres, winds at the surface move from 30° toward the equator.
Without Earth’s rotation and the Coriolis effect, they would simply flow toward the
south (in the northern hemisphere) or toward the north (in the southern
hemisphere). But the Coriolis effect deflects the wind toward the right in the
northern hemisphere and to the left in the southern hemisphere.
2. In the northern hemisphere, the surface winds blowing from 30° N toward the
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equator are deflected to the right and end up blowing from northeast to southwest.
Because winds are named for the direction from which they blow (see slide 22),
these winds are named the Northeast Trade Winds. A trade wind is a relatively
constant wind, so named because they were important for commerce, or trade,
during the days of sailing ships. In the southern hemisphere these same winds are
blowing from the southeast and are named the Southeast Trade Winds.
3. Where the northeast trade winds and southeast trade winds meet is a major surface
convergence called the Intertropical Convergence Zone or ITCZ. The ITCZ is where
hurricanes form. It is also known as the “meteorological equator.” It migrates
slightly north and south with the seasons.
4. Surface winds from 60° to 30° are blowing from the southwest in the northern
hemisphere and from the northwest in the southern hemisphere. However, in both
hemispheres they are referred to simply as the Westerlies.
5. Winds moving from the poles toward 60° are called the Polar Easterlies in both
hemispheres.
A slightly different view of the global atmospheric circulation system.
A slightly different view of the global atmospheric circulation system.
I have done a lot of research in The Yucatán Peninsula of Mexico, Honduras, the
Dominican Republic, and Puerto Rico, as well as several small islands of the eastern
Caribbean. The trade winds are very important controllers of climate in this area. Notice
the dashed red lines showing the direction of hurricane movements. Ever wonder why
hurricanes move from east to west? It’s because the trade winds are pushing them
toward the west.
Can you point out the Intertropical Convergence Zone? It’s the narrow band of clouds
near the equator extending mostly unbroken around the globe. Notice that it gets quite
wide and indistinct over southeast Asia. This is because of the large amount of land area
and high mountains there, disrupting global wind patterns somewhat.
Ever notice that our weather mostly moves from west to east across the country? Just
the opposite of the direction of hurricane movement? Because the westerlies (trade
winds) are dominant here, they push our weather toward the east.
So that’s the big picture with global atmospheric circulation. The big picture, large scale.
There are many types of modifications to the big picture which occur on smaller scales—
seasonal, daily, and even by topography.
Seasonal changes—summer means a lot more sun and heating of the Earth surface.
Remember heat capacity? Here is a real world application of the concept. Land has a
lower heat capacity than water so it heats up more in the summer. Water’s high heat
capacity means it will absorb a lot of the summer energy and stay warmer than land in
the winter.
Season migration of the Intertropical Convergence Zone. Remember, it’s the trade winds
that are converging.
The ITCZ is a low-pressure zone, so pressure centers also migrate with the seasons.
Which brings us to the monsoon effect. Monsoons are often described as absence or
presence of rain, but technically a monsoon is a change in wind direction caused by
seasonal changes in heating of Earth’s surface. Monsoons are more pronounced in
Southeast Asia, both because the ITCZ migrates so far because of all the land area, and
because of agricultural methods which are dependent of the rains brought by monsoons.
We also have monsoons but they are neither as pronounced nor are we dependent on
them for water for agriculture because of our vast irrigation infrastructure.
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In January, when the ITCZ is at its southernmost location, winds are blowing off of most
of Southeast Asia. This is the winter monsoon.
In July, when the ITCZ is at its northernmost location, winds are blowing onto much of
Southeast Asia from the south. This is the summer monsoon.
Summer or wet monsoon.
Winter or dry monsoon.
Air flows from high pressure to low pressure. Pop a balloon to experience this.
Day/night affect. A smaller-scale change in heating affecting coastal areas. At the coast,
a seabreeze is formed during the daytime.
Likewise, at the coast a landbreeze is formed during the nighttime.
A seabreeze
Another view of seabreeze versus landbreeze. Remember, this is exactly the same
principle as the monsoon effect, only this is a tiny scale compared to the scale of
monsoons.
The topographic effect is important when you have wind that encounters a mountain.
Here, the wind is already created and moving, for example a trade wind. Also known as
the orographic effect. Remember the term “orogeny” from plate tectonics? “Oro”
means mountain. Orogeny is mountain building. Orographic effect is when an air mass
encounters a mountain and is forced to rise.
Orographic effect is common on the west coast of the United States.
Orographic effect setting of the western United States. The mountains block moisture
from the interior of the country, contributing to the very dry conditions.