Transcript Atmospheric Circulation – Chapter 6

Atmospheric Circulation
(Air-Sea Interaction)
• We live at the bottom of an ocean of air, the
atmosphere
• The atmosphere and the ocean are
interdependent; what happens in one system
causes changes in the other
• Surface currents in the oceans are directly
caused by atmospheric winds
Air-Sea Interaction
• Differences in solar energy (heating) across
the Earth – combined with the Earth’s spin –
create winds
• Winds drive surface currents and create waves
• Likewise, certain atmospheric phenomena are
manifested (originate) in the oceans
– El Niño – Southern Oscillation
– Hurricanes, cyclones
Air-Sea Interaction
• Earth’s atmosphere is composed mainly of
Nitrogen, Oxygen, and Water Vapor
– Nitrogen: 78%
– Oxygen: 21%
– Argon, CO2, Neon, Helium, Methane, others: 1%
• Air is never completely dry, however, and
water vapor (H2O) can occupy as much as 4%
of the air’s volume
Air-Sea Interaction
• Temperature and humidity determine the density
of air masses, which in turn determines whether
these air masses will rise or sink
• Air containing water vapor is less dense than dry
air at the same temperature and density
• Also, when heated, air expands and becomes less
dense
• This means that cold air is denser than warm air
and cold, dry air is much denser than warm,
moist air
Say what?
• Warm air is less dense than cold air because
increasing temperature results in greater
molecular dispersion
 Increasing Temperature
Say what? (Continued)
• Moist air is less dense than cold air because
the weight of water vapor (H2O) is less than
that of Nitrogen (N2) and Oxygen (O2)
• When water vapor increases, the amount of
O2 and N2 decreases per unit volume
– Molecular weight of O2 = 16 + 16 = 32
– Molecular weight of N2 = 14 + 14 = 28
– Molecular weight of H2O = 1 + 1 + 16 = 18
Atmospheric Circulation
• Air masses will move from regions of high
pressure (dense air) to regions of low pressure
(less dense air)
• A low pressure zone results from moist
and/or warm air
• A high pressure zone results from dry and/or
cold air
• The flow of air from regions of high to low
pressure create the winds
Atmospheric Circulation
• Because warm and/or moist air is less dense, it
rises (heat rises)
• Likewise, cool and/or dry air is more dense and so
it sinks
• As air rises, it expands and cools; water vapor in
rising, expanding air will condense into clouds
because the cooler air is no longer able to hold as
much water vapor
• Precipitation transfers water vapor (AND HEAT!!)
from low to high latitudes
Solar Heating Varies with Latitude
• The Earth revolves around the sun in an
elliptical path
• The Earth itself is tilted at an angle of 23.5°
• The tilt of Earth’s rotational axis results in the
seasons
Solar Heating Varies with Latitude
• The amount of solar energy reaching the
Earth’s surface varies with latitude and season
• Because of the Earth’s tilt, solar energy
reaching the equator strikes at a low angle,
concentrating the radiation in a small area;
solar energy reaching the poles, however,
does so at a lower angle and so less heat is
absorbed in polar zones
Near the poles, light filters
through more atmosphere and
approaches at a low angle,
favoring reflection rather than
absorption
Got albedo?
• Albedo is the measure of solar radiation that
is reflected back into space
• A high albedo indicates that more energy is
reflected back into space, while a low albedo
indicates that less energy is reflected back to
space
• Ice and snow (even clouds) increases albedo,
and so much of the light that reaches the
polar regions is reflected back into space
Uneven solar heating and atmospheric
circulation
• Air is warmed in the tropics and rises
• Air is cooled near the poles and falls
• It seems logical to suspect then, that air
heated in the tropics expands and becomes
less dense as it moves towards the poles,
where it will cool (and condense) sinking back
towards the poles….
– BUT, THIS IS NOT WHAT HAPPENS!
HYPOTHETICAL
CIRCULATION
ON A NONSPINNING
EARTH
Enter the Coriolis Effect…
• Experience will tell us, however, that winds in
the mid-latitudes of the northern hemisphere
do NOT flow out of the north, but rather the
west
• The hypothetical winds described do not
resemble the actual wind patterns of the
Earth because we have neglected the effect of
the Earth’s rotation
Enter the Coriolis Effect…
• The rotation of the Earth strongly influences
the motion of air and water
• This effect is named the Coriolis effect after
its discoverer, Gaspard Gustave de Coriolis
• The Coriolis effect changes the intended path
of a moving body
– Causes moving objects on Earth to follow curved
paths
Coriolis: The Example
• Imagine you and your friend are on a carousel
• You are sitting on the inside of the carousel and
your friend is sitting on the outside
• You throw a ball to your friend, but are amazed to
find that the ball curves sharply to the right and
your friend is unable to catch it (and this is not
because you throw like a girl…)
• Other friends watching from a hot air balloon
hovering over the carousel confirm that the path
of the ball was in fact straight
Coriolis: The Example
• If we compare our carousel to the Earth, we
know that the Earth will complete a full
rotation every 24 hours
• People living on the equator however must
complete a much larger circumference of
rotation than would people in middle and high
latitudes
• In order for every part of the Earth to
complete a rotation in 24 hours, points on the
equator MUST travel faster than points near
the poles
Back to the carousel
• When you throw the ball to your friend on the
carousel, YOU are traveling slower than
he/she is riding on the outside of the carousel
• In order for you and your friend to complete a
rotation within the same time, the inner riders
on the carousel must travel slower than those
riding on the outside who must cover more
ground in the same amount of time
The Coriolis Effect
• The equator must travel faster
than higher latitudes must travel
in order for all regions of the
Earth to complete 1 full rotation
in 24 hours (people in Anchorage, AK and the
equator all experience the same 24-hour day)
• Therefore as objects travel from one region of
the globe to another, they are subject to
changing speeds of travel
rst.gsfc.nasa.gov/Sect14Sect14_1c.html
The Coriolis Effect Will Keep You Up At
Night….
The Coriolis Effect
15 mph
Your friend,
rotating faster
to cover more
distance (red
line) in same
time
8 mph
You, rotating
slower to cover
less distance
(blue line) in
same time
Carousel rotating
counter-clockwise
15 mph
You throw
ball while
moving at
8mph in
what you
consider to
be a straight
path
8 mph
Carousel rotating
counter-clockwise
15 mph
As ball
travels,
it
carries
with it
this
slower
motion
with it*
8 mph
*The
carousel
beneath
the ball is
traveling
faster
than it
Carousel rotating
counter-clockwise
15 mph
As ball
travels,
it
carries
with it
this
slower
motion
with it*
8 mph
*The
carousel
beneath
the ball is
traveling
faster
than it
Carousel rotating
counter-clockwise
15 mph
As ball
travels,
it
carries
with it
this
slower
motion
with it*
8 mph
*The
carousel
beneath
the ball is
traveling
faster
than it
Carousel rotating
counter-clockwise
My art is WAY better, but just in case
you want the book’s version…
• The rotation of
the Earth is
counterclockwise
• In the northern
hemisphere,
objects are
deflected to the
right relative to
their path of
motion
• In the southern
hemisphere,
objects are
deflected to the
left for the same
reason (poles are
moving slower
than equator)
Let’s return to
our
hypothetical
model…
• Air does
warm, expand
and rise along
the equator
The Coriolis Effect Influences the
Movement of Air in the Atmosphere
• When the air has traveled ~ one third of the
way from the equator to the pole – to about
30°N or 30°S latitude, the air becomes dense
enough to sink back towards the surface,
completing the loop
• But instead of moving directly towards the
equator, the wind is deflected it to the right in
the Northern Hemisphere, and to the left in
the Southern Hemisphere
At the
equator,
warm, moist
air rises,
resulting in a
low pressure
zone
As the rising
air becomes
colder & drier,
its density
increases,
resulting in a high
pressure zone
Deflected
to the
right
The Coriolis Effect Influences the
Movement of Air in the Atmosphere
• Note that the Coriolis effect does not cause
the winds; it only influences the wind’s
direction
• As air rises at the equator, it will lose water
vapor by precipitation caused by expansion
and cooling. This drier air travels north or
south of the equator and grows denser as it
cools
Descending air towards
equator is deflected to the
right of its path of motion
Descending air towards
equator is deflected to
the left (Southern H.) of
its path of motion
Throwing a monkey wrench into the
Coriolis concept…
• The tendency of wind to deflect because of
the Coriolis effect increases with its speed and
with distance from the equator
• This means that winds in high latitudes
deviate much moreso than do tropical winds
occurring at low latitudes moving at the same
speed
• Likewise, faster winds will be deflected
moreso than slower winds in either region
General Wind Patterns
• This means that there is, in fact, no Coriolis
effect at the equator, and hence, no deflection
of wind
• This is because the change in velocity (speed)
of the Earth changes very little near the
equator, but changes muchy more at higher
latitudes  greater Coriolis effect
General Wind Patterns
• A column of warm, low density air rises away
from the surface and creates a band of low
pressure at the equator
• The weather in areas of low pressure is
characterized by cloudy conditions with lots of
precipitation because rising air cools and
cannot retain (hold onto) its water vapor
• This region is clothed in tropical rain forests
General Wind Patterns
• A column of cool, dense air moves towards
the surface and creates high pressure zones.
Descending air is quite dry and so these
regions are characterized by dry, clear, fair
conditions
• Sinking air is very arid (dry) and the great
deserts of the world are centered along this
band of high pressure (30°N and 30°S)
Desert anyone?
General Wind Patterns
• Sailors have a special term for the calm,
equatorial regions where low pressure persists
and little winds exist; the doldrums
• Sailors also have a special term for the regions
within the high pressure band, where winds
are light and variable; the horse latitudes
• Places between the high and low pressure
bands, on the other hand, experience rapidly
moving air, and are characterized by strong,
dependable winds
(Horse latitudes)
Winds are named for the direction in which they originate
Tropical Cyclones (Hurricanes)
• Tropical cyclones are huge rotating masses of
low pressure characterized by strong winds
and torrential rain
• In North and South America, tropical cyclones
are commonly called hurricanes
• In the western North Pacific, they are called
typhoons
• In the Indian Ocean, they are called cyclones
Tropical Cyclones (Hurricanes)
• Tropical cyclones carry tremendous amounts
of heat from one region of the world to
another
• The energy contained in a single hurricane is
greater than that generated by all energy
sources in the United States in one year!
• Hurricanes are powered by the release of
water’s latent heat of condensation
Recipe for a Hurricane:
– Ocean temperature greater than 25°C (77°F),
which provides an abundance of water vapor to
the atmosphere via evaporation (summer and fall)
– Warm, moist air, which supplies vast amounts of
heat as the water vapor condenses and fuels the
storm
– The Coriolis Effect, which causes the hurricane to
rotate counterclockwise in the Northern
Hemisphere, and clockwise in the Southern
Hemisphere
Tropical Cyclones (Hurricanes)
• In fact, there are no hurricanes that can occur
directly over the equator because the Coriolis
Effect is zero there
• Hurricanes can not form below 8° N or S
latitude; Coriolis effect is not strong enough to
cause deflection
• Hurricane season: June 1 – Nov 30: 97% of
cyclones (hurricanes) occur during this time
Tropical Cyclones (Hurricanes)
• As air rises (in the northern hemisphere), it is
deflected to the right
• This results in a counter-clockwise rotation
L
http://en.wikipedia.org/wiki/File:Hurricane_isabel_and_coriolis_force.jpg
Pressure gradient (moving
towards the low pressure
center) is represented by
blue arrows; the Coriolis
deflection is represented
by red arrows
EYE
Sinking of air
occurs at the
eye as dry
air moves in
from the
atmosphere
Storm Surge: Hurricanes’ right hook
The 2005 Hurricane Season
• The 2005 hurricane season was the most
extensive on record, and actually persisted
into January 2006!
• A record 27 tropical storms formed, a record
15 of which became hurricanes
– 5 became Category 4
– 4 became Category 5 (the highest category)
• $100 billion in damages and >2000 deaths
• Coincidentally, 2005 was (at that time) the
hottest year on record!
The 2011 Hurricane Season