Download Atmospheric and Oceanic Circulation

Atmospheric and Oceanic Circulation
Atmospheric circulation transfers energy and mass over the
Earth
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Redistributes surplus energy along the tropics to deficit areas
Generates weather patterns
Produces ocean currents
Spread natural and anthropogenic pollution
What is atmospheric pressure?
Atmospheric Pressure
Air pressure – pressure exerted on the surface of the
earth by the atmosphere.
 Motion, size and number of air molecules within the
atmosphere determine the temperature and density of air
 Temperature and density of air determine the pressure it
exerts on the surface
 Air pressure is caused by the force of gravity pulling the
mass of the atmosphere toward the surface of the earth
What determines atmospheric pressure?
Atmospheric Pressure
-Pressure = force per unit
area
-Due to gravity the
atmosphere exerts a force

According to the Ideal Gas Law: density (ρ) and
temperature (T) control atmospheric pressure (P)
P = ρRT
R = a constant
Pressure, Density & Temperature
Density (ρ)

Amount of matter (mass) per unit volume (kg/m3)

Density (of a gas) is directly proportional to pressure

Density varies with altitude
Pressure, Density & Temperature
Temperature (T)
 Molecules move faster in hot air than cold air

Faster = more collisions (more force) and therefore higher pressure

Temperature is directly proportional to pressure
Pressure, Density & Temperature

In the atmosphere density and temperature do not change
independently
Example:
When air in the atmosphere is heated it expands and causes a decrease
in density and pressure
How is atmospheric pressure measured?
Atmospheric Pressure

Atmospheric pressure is often measured in millibars (mb)

Atmospheric pressure at sea level is 1013.25 millibars (standard
pressure)
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At the earth’s surface, pressure varies from 980 mb to 1030 mb (about
5%)
Atmospheric Pressure:
In 1643, a student of Galileo, Evangelista Torricelli developed a
method for measuring air pressure while trying to drain mines.
 Noticed that water levels within mines varied daily
 Arrived at the theory that the downward force of the atmosphere on
the water surface also fluctuated
→days of higher air pressure caused lower water levels within mines
Measurement of Atmospheric Pressure:

Atmospheric pressure is measured using and instrument called a
barometer

A mercurial barometer measures atmospheric pressure with a column
of mercury
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Sea-level pressure (1013mb) also can be defined as 29.92 inches of
mercury (in. Hg)
Atmospheric Pressure

A more common type of barometer is the aneroid barometer

It uses the pressure exerted against a partial vacuum to measure air
pressure
Note: decrease in air pressure with increase in elevation
-
Decrease boiling point of water
-
-
0 m  100 C
3000 m  90 C
5000 m  80 C
Increase in cooking times at higher elevations
Wind
Wind – horizontal movement of air
 Produced by differences in air pressure from one location to
another
 Air moves from locations of high pressure to locations of low
pressure
Measurement: wind has two principal components
- Speed (anemometer)
- Directions (wind vane)
Wind Vane:
-Points in the direction
air is moving
-Wind is named based
on the direction from
which it originated
-Designated by
compass directions
Forces Driving Atmospheric Motion
Four forces affect the direction and speed of air (wind) as it
moves throughout the atmosphere:
3.
4.
5.
6.
Gravitational force
Pressure gradient force
Coriolis force
Friction
1) Gravity:
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Draws the mass of the atmosphere towards the surface of the earth
Causes decrease in pressure and mass with increasing elevation
Would not have an atmosphere or air pressure without it
2) Pressure Gradient Force: air moves from high pressure
regions to low pressure
 Variations in air pressure are caused by uneven heating of the
earth’s surface.
Uneven heating of the earth’s surface
Warm surfaces encourage upward vertical motion
Upward airflow
Converging
Horizontal Air
Low pressure
area
Warm Surface
Converging
Horizontal Air
Uneven heating of the earth’s surface
As air molecules cool, they condense and decend towards the surface
Air Cools
Diverging
Horizontal Air
High pressure
area
Diverging
Horizontal Air
Lines of equal pressure are isobars:
-
b/c air moves from high to low pressure, the PGF is exerted at 90º
angles from the isobars
When isobars are close together the pressure gradient is higher
→ stronger winds
When isobars are farther apart the pressure gradient is lower
→ weaker winds
3) Coriolis Force: air flow is deflected from a straight path by
the rotation of the earth
 Earth’s rotational speed increases from the poles toward the
equator
0km /hr at the poles, 1675km /hr at equator
 Deflection increases north and south of the poles
 No deflection at the equator
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3) Coriolis Force:
- The earth rotates eastward
- Deflection causes air motion to curve to the right in the N Hem
- Deflection causes air motion to curve to the left in the S Hem
Geostrophic Wind – upper level winds that move parrallel to
isolines b/c the coriolis force are equal
 Rather than air flowing from high to low pressure, air moves around
high and low pressure areas
 Occurs within upper levels of the troposphere
Northern Hemisphere
Northern Hemisphere
Northern Hemisphere
Northern Hemisphere
Northern Hemisphere
4) Frictional force: drag (backward force) on wind as it moves
over the earth’s surface
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Extends to and elevation of about 500m
Decreases with increasing elevation
Varies with surface texture, wind speed, time of day/year, and
atmos conditions
→ Decreases speed of wind
Northern Hemisphere
Summary of Forces:
Gravity  pulls atmosphere toward earth creating air pressure
PGF  air moves from high to low pressure perpendicular to isolines
Coriolis  deflects wind direction due to earth’s rotation
- zero at the equator
- right in N hem; left in S Hem
Friction  backward force on air movement slowing it down
High Pressure
Low Pressure
Anticyclone
Cyclone
CIRCULATION
N Hem : cw circulation
S Hem : ccw circulation
N Hem : ccw circulation
S Hem : cw circulation
SPIN
Air spins out from
center
Air spins in to center
Sinking Air
Rising Air
Surface divergence
Upper-level
convergence
Surface convergence
Upper-level divergence
Clear skies, sunshine
Clouds, precipitation
NAME
VERTICAL
MOTION
HORIZONTAL
MOTION
WEATHER
Global Air Pressure Patterns:
Temperature differences along the equator and at the poles
cause pressure differences:
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Pressure differences cause air to flow
Warm air along the equator rises creating low pressure
Cold, more dense air over the poles sinks creating high pressure
These pressure differences cause air to flow from the poles north
and south to the equator
 This north-south air flow is called meridinal flow
Four primary pressure areas for each hemisphere:
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Equatorial low-pressure trough (10 N & S)
Polar high-pressure cells (90 N & S)
Subtropical high-pressure cells (20-35 N & S)
Subpolar low-pressure cells (60 N & S)
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Equatorial low-pressure trough (10 N & S)
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Caused by intense heating of the earth’s surface
Warm air rises creating low pressure and associated weather
Characterized by warm, wet atmospheric conditions
Polar high-pressure cells (90 N & S)
Subtropical high-pressure cells (20-35 N & S)
Subpolar low-pressure cells (60 N & S)
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Equatorial low-pressure trough (10 N & S)
Polar high-pressure cells (90 N & S)
- Caused by sinking air over cold surface
- Characterized by cold, dry air
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Subtropical high-pressure cells (20-35 N & S)
Subpolar low-pressure cells (60 N & S)
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Equatorial low-pressure trough (10 N & S)
Polar high-pressure cells (90 N & S)
Subtropical high-pressure cells (20-35 N & S)
- Caused by dry, sinking air
- Characterized by hot, dry air
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Subpolar low-pressure cells (60 N & S)
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Equatorial low-pressure trough (10 N & S)
Polar high-pressure cells (90 N & S)
Subtropical high-pressure cells (20-35 N & S)
Subpolar low-pressure cells (60 N & S)
- Caused by the clash of warmer, wet maritime air and cold, dry polar
air
- The warmer marine air rises over colder, heavier polar air
- Characterized by cool, wet air
Equatorial low-pressure trough:
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Long, narrow band of low pressure encircling the earth
Air converges along the surface towards the center
 Creates trade winds (NE in N Hem; SE in S. Hem)
 Air moves toward equator and is deflected wast by coriolis
Warm air rises along equator → Intertropical Convergence Zone
 Surface flow along the ITCZ is minimal → Doldrums
Air Characteristics:
- constant altitude and consistent daylength result in large amounts of
available energy
- Energy warms and lightens air near the surface causing it to rise
- Air is very moist; lots of evaporation from nearby oceans
Equatorial low-pressure trough:
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Air rises into upper atmosphere moving away from equator
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Air sinks and returns to surface and diverges
Some air moves back toward the ITCZ (hadley cell)
Process is repeated
Results in band of tropical weather systems
 Warm, moist air masses
 Precipitation →Amazon rainforest
Subtropical high-pressure cells:
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Broad zone of high pressure along subtropical latitudes
Upper-level air moves north and south of ITCZ and eventually sinks
along subtropical latitudes
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Moisture is removed by precipitation along the ITCZ
Still within the area of energy surpluses
Resulting in anticyclonic conditions characterized by hot, dry air → world’s
deserts
Air circulation:
 Air diverges along surface:
 Moves toward equator → easterly trade winds
 Moves toward poles → westerlies
Subtropical high-pressure cells:
Anticyclonic circulation (cw in N Hem)
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drier land conditions and cool ocean currents along eastern edge
Wetter land conditions and warm ocean currents along wester edge
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surface winds along high pressure zones are calm
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- Horse latitudes (sailing)
Subpolar low-pressure cells:
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Occur along the boundary midlatitude air masses and polar air
 Boundary called the polar front
 Midlatitude air is warm/cool and moist
 Polar air is cold and dry
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Air converges along Polar Front and rises
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Warmer midlatitude air rises above colder polar air
Moisture within warmer air mass is condensed
Results in cyclonic conditions → precipitation
Westerlies and roaring forties
Polar high-pressure cells:
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Occur over poles, poleward of the polar front
Anticyclonic circulation of cold, dry air around poles
 Weak, polar easterlies
Upper Atmospheric Circulation
Upper level atmospheric circulation is monitored on a constant pressure
surface or a constant isobaric surface
The height of the 500mb pressure surface:
- This height varies
- The layer of air under an upper-level high pressure system are
thicker
- 500mb isobar is at a higher elevation
- 500mb isobars bend poleward on map → ridge
- The layer of air under an upper-level low pressure system are
thinner
- 500mb isobar is at a higher elevation
- 500mb isobars bend equatorward on map → trough
Upper atmospheric circulation generate surface pressure systems
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Divergence of upper-level air results in a void
 Surface air will rise to fill upper level void
 Rising air at the surface results in surface low pressure cyclones
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Convergence of upper-level air results in build up of air
 Air will be pushed down toward the surface
 Sinking air at the surface results in surface high pressure
anticyclones
Rossby Waves- undulations within the smooth westerly flow
of geostrophic winds
 Cold air poleward of polar front; warm air equatorward
 Develop along the polar jet stream
 Waves send cold polar air equatorward and warm midlatitude air
poleward
 Distinct cyclonic and anticyclonic systems form as warm air is
pushed closer to the poles and cold air is squeezed toward equator
 Cold, cyclonic systems can be squeezed out and move even
further south
Jet Stream: irregular, concentrated band of wind at several locations that
supports surface weather systems
 160-480 km wide, 900-2150 m thick, with core velocities exceeding
300 kmph
 Weaken during summer and strengthen in winter
Polar jet stream - located along the tropopause along the polar front
 Elevation of 7600-10,700m, between 30-70 N and S
Subtropical jet stream - along the boundary btw tropical and
midlatitide air
 20-50 N and S
The polar and subtropical jet streams can flow over N. America at the
same time and can even merge
Local Winds
A Sea Breeze
Sea breeze - during the day the land heats faster than the ocean
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Air over the land is warmer than that over ocean
Warmer air is less dense and rises leaving a void over land
Cooler ocean air flows laterally over land to fill void
A sea breeze is strongest in mid-afternoon! Why?…..
A Land Breeze
Land Breeze: at night land cools faster than ocean
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Air over land is cooler than that over ocean
Warmer, less dense air over ocean rises leaving a void
Cooler air over land flow laterally over ocean
Mountain-valley breezes - breezes created by the exchange of cool
mountain air and warmer valley air
 Valley air gains heat rapidly during the day
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Warmer valley air rises moving upslope
 Mountain air loses heat rapidly at night
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Cooler mountain air sinks downslope
Katabatic winds - cold dense air from elevated terrain flows downslope
 Larger regional scale than mountain valley breezes
 Generally form along an elevated highland or plateau
 Also very common over ice sheets
Monsoonal Winds - shifting wind system seasonally b/c of regional scale
pressure changes
 Typical of SE Asia but also occurs over SW U.S.
 Large seasonal temp difference occur over SE Asia b/c of its
continentality
 In winter, ITCZ is over Indian Ocean and Asia dominated by
subtropical high pressure cell
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Region characterised by dry, cool air with very little precip
Monsoonal Winds - shifting wind system seasonally b/c of regional scale
pressure changes
 Typical of SE Asia but also occurs over SW U.S.
 Large seasonal temp difference occur over SE Asia b/c of its
continentality
 In winter, ITCZ is over Indian Ocean and Asia dominated by
subtropical high pressure cell
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Region characterised by dry, cool air with very little precip
 In summer, ITCZ is over SE Asia, which is dominated by tropical
low pressure, and subtropical high pressure is centered over Indian
Ocean
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Results in world record rainfall amounts
Ocean Circulation
Ocean currents are driven by the frictional drag of wind and
their direction is affected by:
-
the coriolis force
Water density differences caused by temperature and salinity
Configuration of land masses and the ocean floor
Astronomical forces (tides)
Surface Currents – ocean currents are driven by the
circulation around high pressure systems and deflected by
the coriolis force.
 Gyres: circulation patterns caused by circulation around high
pressure systems
Equatorial Currents – ocean currents that are driven
westward by the trade winds
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Located near the equator
Are not deflected by coriolis force b/c it is zero along the equator
Such currents push water west toward the eastern shores of land
masses
 Causes water to pile up along eastern coasts of continents
Western Intensification (15cm)
The case of the rubber duck!
-
Jan 1994, a large container ship from Hong Kong
Loaded with toys and other goods
One container split off Japanese coast (30,000 rubber ducks, turtles
and frogs)
Similar thing happened with 60,000 pairs of athletic shoes
Trip of the floating ducks took over a decade!
Deep Water Circulation – ocean water also moves along the
ocean floor in distinct patterns resulting from vertical
motion
Upwelling currents: occur where surface water is swept away from the
coast by surface divergence (coriolis and offshore winds)
 this allows cold, deep ocean water to move up from the ocean floor toward
the surface → nutrients for fishing
Downwelling currents: occur where excess water accumulation along a
coast causes surface water to be pushed to greater ocean depths
(west end of the equatorial currents)
 This allows surface ocean water to move down to the ocean floor
 Warmer surface water is cooled
 Water flows along the ocean floor until it is upwelled
This continuous flow of surface water to the bottom and bottom water to
the surface transports heat and energy around the globe:
- Water that moves along the equator is warmed and transports that heat to
the poles where it loses heat as it cools, sinks, and flows back to the ocean
floor
- That cold bottom water eventually upwells and is warmed and continues
circulating
- Process takes about 1,000 years
- Currents have shifted throughout geologic history