Atmospheric Circulation (Air-Sea Interaction)

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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

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Air-Sea Interaction

• Differences in solar energy (heating) across the
  Earth combined with the Earths 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

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Air-Sea Interaction

• Earths 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 airs
  volume
• Visible as clouds and fog invisible as water
  vapor
• Enters atmosphere via evaporation, exits via
  condensation

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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

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Say what?

• Warm air is less dense than cold air because
  increasing temperature results in greater
  molecular dispersion
• ? Increasing Temperature

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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

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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

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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

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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 Earths rotational axis results in
  the seasons

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Solar Heating Varies with Latitude

• Approximately half of the solar energy reaching
  the Earth is absorbed, but this heat is not
  evenly absorbed
• The amount of solar energy reaching the Earths
  surface varies with latitude and season
• Because of the Earths 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

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Near the poles, light filters through more
atmosphere and approaches at a low angle,
favoring reflection rather than absorption
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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

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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!

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HYPOTHETICAL CIRCULATION ON A NON-SPINNING EARTH
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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 Earths rotation

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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

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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

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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

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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

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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 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
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The Coriolis Effect Will Keep You Up At Night.
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The Coriolis Effect
Your friend, rotating faster to cover more
distance (red line) in same time
15 mph
8 mph
You, rotating slower to cover less distance (blue
line) in same time
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15 mph
8 mph
You throw ball while moving at 8mph in what you
consider to be a straight path
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15 mph
As ball travels, it carries with it this slower
motion with it
The carousel beneath the ball is traveling
faster than it
8 mph
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15 mph
As ball travels, it carries with it this slower
motion with it
The carousel beneath the ball is traveling
faster than it
8 mph
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15 mph
As ball travels, it carries with it this slower
motion with it
The carousel beneath the ball is traveling
faster than it
8 mph
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My art is WAY better, but just in case you want
the books version
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The Coriolis Effect

• Now imagine that you are at the North Pole and
  your friend is in Rio de Janeiro, Brazil near the
  equator
• You toss a ball to your friend (yes, use your
  imagination) and the same principles apply you
  are traveling around the world slower than your
  friend is. The ball will be deflected to the
  right due to the rotation of the Earth

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• The rotation of the Earth is counter-clockwise
• In the northern hemisphere, objects are deflected
  to the right relative to their path of motion

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• In the southern hemisphere, objects are deflected
  to the left for the same reason (poles are moving
  slower than equator)

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Coriolis Effect

• As a plane travels from Antarctica towards the
  equator, it will veer to the left along its path
  (if it did not alter its course) due to Coriolis
  effect
• During its northern journey, the plane is flying
  over land that is rotating eastward at a slower
  and ever decreasing rate compared to that of
  the jet
• Objects are deflected to the right in the
  Northern Hemisphere and to the left in the
  Southern Hemisphere regardless of what direction
  (N,S,E,W) they are moving in

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The Coriolis Effect Influences the Movement of
Air in the Atmosphere

• Lets return to our hypothetic model of
  atmospheric circulation on the Earth
• Air does warm, expand and rise along the equator
• But, instead of traveling continuously from the
  equator to the poles, rising air moves poleward
  and is deflected eastward (to the right) in the
  Northern Hemisphere, and westward (to the left)
  in the Southern Hemisphere

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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 winds direction
• As air rises at the equator, it will lose water
  vapor by precipitation caused by the expansion
  (there is decreasing atmospheric pressure
  w/increasing altitude) and cooling. This drier
  air travels north or south of the equator and
  grows denser as it cools

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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 30N or
  30S latitude, the air becomes dense enough to
  sink back towards the surface, completing the
  loop
• The Coriolis Effect influences the direction of
  the resulting winds

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At the equator, warm, moist air rises, resulting
in a low pressure zone
Deflected to the right
As the rising air becomes colder drier,
its density increases, resulting in a high
pressure zone
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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
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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

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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

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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

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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 (30N and 30S)

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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

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(Horse latitudes)
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Winds are named for the direction in which they
originate
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Storms and fronts

• Different air masses meet at fronts
• When warm air meets cold air, the warm air rises
  gently, resulting in mild precipitation

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Storms and fronts

• When cold air moves into warm, the warm air rises
  quickly, resulting in LOTS of precipitation

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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

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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 waters
  latent heat of condensation (when water
  evaporates, it stores tremendous amounts of heat
  when water condenses into a liquid, it releases
  this stored heat into the surrounding atmosphere)

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Tropical Cyclones (Hurricanes)

• The conditions required to form a hurricane are
  as follows
• Ocean temperature greater than 25C (77F), 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

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More hurricanes in Northern Hemisphere warmer
weather in tropics there b/c greater amount of
land
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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

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Tropical Cyclones (Hurricanes)

• As air rises (in the northern hemisphere), it is
  deflected to the right
• This results in a counter-clockwise rotation

Pressure gradient (moving towards the low
pressure center) is represented by blue arrows
the Coriolis deflection is represented by red
arrows
L
http//en.wikipedia.org/wiki/FileHurricane_isabel
_and_coriolis_force.jpg
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Sinking of air occurs at the eye as dry air moves
in from the atmosphere
EYE
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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 gt2000 deaths
• Coincidentally, 2005 was the hottest year on
  record!

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