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Atmospheric Circulation
Moving things around on present day
Earth
Short-term cycles
Long term (organic)
Long term - rock
(inorganic/tectonic)
Global cycles
• Biogeochemical cycles are the major way that elements are
moved on Earth’s surface
– Driven by solar input (primary production)
– Elements cycle between reservoirs that operate on
different time scales
– Cycles have positive and negative feedbacks and
subject to perturbations
• Interaction with physical processes through tectonic/rock
cycles
• Oceans and atmosphere are important conduits
transporting matter and energy
Oceans & Atmosphere
• Shorter timescales of exchange
• Exchange time in atmosphere – hours to
decades
– Mediates rapid cycling between oceans and
continents
• Exchange time in oceans
– Surface and deep water – years
– Deep circulation – 100’s to 1000’s of years
Atmosphere & Ocean
• Gases and water freely exchange at the oceanatmosphere interface
• Movement of air (and water) by wind help
minimize worldwide temperature extremes.
• Weather is influenced by the movement of water
in air (state of the atmosphere at a specific time
and place)
• Climate is the long-term average of the weather in
an area
In general
• Atmosphere exchanges material with biota
and oceans rapidly
• Cycles that include an atmospheric
component tend to have more rapid
recycling (N and C)
• Cycles without an atmospheric component
can be slower (immobile) because tied to
geological cycles (P)
Atmosphere
• Major conduit for transport between oceans
and land
– Major role in controlling climate (heat
transport)
• Composition evolved as a result of
evolution of life
– Changing due to human activities
– Well-mixed so harbinger of global change
• Structure – layered
• Held on earth’s surface by gravity
Mt. Everest
(8,850 m)
Atmosphere structure
0oC
Mesosphere
Pressure decreases with altitude – 1 atmosphere
of pressure at Earth’s surface at sea level.
30 mi
Stratopause
Ozone
40 km
Dry
20 m
Stratosphere
20 km
-55oC
Weather zone
Water
Vapor
80% of atmospheric mass is in the troposphere
10 m
Tropopause
Troposphere
20oC
Structure of the Atmosphere
• Troposphere is densest and is where our weather
occurs
• Substances in the stratosphere persist for long
periods because there are few removal processes
• In troposphere, temperature decreases with
altitude
• In stratosphere, temperature increases with altitude
due to interactions with particles and radiation
from the sun
• The ozone layer is within the stratosphere
– Ozone absorbs UV at top of stratosphere
Troposphere
• Well-mixed
• Limited exchange with overlying
stratosphere
• Heated by long-wave radiation (heat) reradiated from Earth’s surface
– Temperature decreases with altitude in
troposphere
• Heating from below results in convection,
remember?
– Rising warm air creates thermal instability
Composition of the
atmosphere
• 78% nitrogen and 21% oxygen
• Other elements make up < 1%
• Air is never completely dry and water can
be up to 4% of its volume.
• Residence time of water vapor in the
atmosphere is ~10 days.
also H2S, H2, (CH3)S
Atmosphere
• N2 – fairly inert; long residence time (20 my)
• O2 – accumulated over time; complex controls; shorter
residence time (~10,000 years)
• CO2 – trace constituent; complex controls; short residence
time (~3 years)
- Affected by processes with cycles at various timescales (from rock
to seasonal)
- Long-term variations
- Greenhouse
Atmosphere
• Trace constituents – reduced gases
– Microbially produced at present and removed in
rain/oxidation
– Greenhouse gases
• Ozone – stratosphere
– Problematic in troposphere
• Water vapor
– Varies tremendously
– Important in distributing heat
– Greenhouse gas
Properties of the atmosphere
• Air has mass (and density)
• Molecular movement associated with heat causes
the same mass of warm air to occupy more space
than cool air. So, warm air is less dense.
• Humid air is less dense than dry air at the same
temperature because molecules of water vapor
(H2O) weigh less than N2 and O2 molecules
displaced.
Density structure of
troposphere
• Influenced by temperature and water content
• Water vapor is less dense than dry air so causes
density of air to decrease and air to rise
• Warming air makes it less dense so it rises
• Condensation of water vapor releases heat which
warms the air
• Warm air can hold more water vapor than cold air
Air density affected by
pressure
• Air lifted to altitude experiences less
pressure so expands and cools
• Air compressed as it descends from altitude
warms
Air movement
• Water vapor rises, expands and cools
• Condenses into clouds or precipitation
(cooler air can’t hold as much water)
• Atmosphere can lose water by precipitation
• As air loses water vapor it becomes more
dense and air will then fall, compress and
heat
Atmospheric circulation
• Powered by sunlight – uneven solar heating
• About 51% of incoming energy is absorbed
by Earth’s land and water
• Energy absorption varies depending on the
angle of approach, the sea state and the
presence of ice or other covering (e.g.,
foam)
Heat budget
• Energy imbalance – more energy comes in at the
equator than at the poles
• 51% of the short-wave radiation (light) striking land
is converted to longer-wave radiation (heat) and
transferred into the atmosphere by conduction,
radiation and evaporation.
• Eventually, atmosphere, land and ocean radiate heat
back to space as long-wave radiation (heat)
• Input and outflow of heat comprise the earth’s heat
budget
• We assume thermal equilibrium (Earth is not getting
warmer or cooler) or the overall heat budget of the
earth is balanced
Atmospheric circulation
• Uneven solar heating of earth
–
–
–
–
Atm and oceans move heat poleward
Air moves from high pressure to low pressure
Poleward movement of warm air (less dense)
Equatorward movement of cold air (more
dense)
Movement of heat
• Sensible heat
– Transported by a body that has higher
temperature than its surroundings (conduction
and/or convection)
• Latent heat
– Phase changes of water
– Evaporation takes up heat and condensation
releases heat
Uneven solar heating
• Heat budget for particular latitudes is NOT
balanced
• Sunlight reaching polar latitudes is spread over a
greater area (less radiation per unit area)
• At poles, light goes through more atmosphere so
approaches surface at a low angle favoring
reflection
• Tropical latitudes get greater radiation per unit
area and light passes through less atmosphere so
they get more solar energy than polar areas
Solar
radiation
•Radiation hits the earth in
parallel rays
•Incident angle varies with
latitude
•Energy is spread out over more
area
N
–Less heat per area
•Passes through more
atmosphere
–Which absorbs radiation
•Poles are cooler because they
receive lower intensity solar
radiation do to angle of incident
radiation.
S
Solar radiation
•Second reason the poles are
cooler is the tilt of the earth
on its axis
23.5o
N
–Variation in daylength
–Even when poles have long
daylength, the incident angle is
long.
•Third reason is that poles are
farther from the sun
S
Fig. 4-1
Fig. 4-2
Seasons & solar heating
• Mid-latitudes – N Hemisphere receives 3x
the amount of solar energy per day in June
than in December
• Due to the 23.5o tilt of Earth’s rotational
axis
• N Hemisphere tilts toward the sun in June
and away in December
• Tilt causes seasons
Figs. 4-15 and 4-16
Circulation
• Atmospheric and oceanic circulation are
governed by the redistribution of this
energy
• Water moves heat between tropics to poles
• Ocean currents and water vapor move heat.
• Higher latent heat of vaporization means
vapor transfers more heat per unit mass than
liquid water.
Atmospheric circulation
• Warm air rises and cool air sinks
• Warm air expands and rises
• Expansion causes cooling and contraction
causing increasing density and sinking
• Air will rise where its warmer and sink
where its cooler
Convection
Logically on the earth, one can imagine this
Fig. 13.11
Fig. 4-25
Air movement
• Air is warmed at equator so rises
• As it rises, it dumps its moisture because its
expanded and cooled
• Air moves south to replace air that’s risen
• Creates zone of low pressure (sinking air
creates high pressure and rising air creates
low pressure.
Fig. 4-3
Atmospheric circulation
• But, this is NOT what happens
• Atmospheric circulation is governed not
only by uneven solar heating but,
• The Earth’s rotation
• Eastward (CCW) rotation of the Earth on its
axis deflects moving air or water (or any
object with mass).
• CORIOLIS effect (1835)
Coriolis Effect
• Rotation of the Earth CCW
• Relative speeds of sphere at different latitudes
• Caused by an observer’s moving frame of
reference on a spinning Earth
• Curve is slightly to the right of initial path in the
northern hemisphere
• Curve is slightly to the left of initial path in the
southern hemisphere
Relative speeds of objects at different
radii moving at the same angular speed
Airplane
Coriolis
Coriolis effect and atmospheric
circulation
• Coriolis effect influences wind direction
• End up with 3 sets of cells – by 30 deg, flow has been
deflected 90 deg
• Air is deflected before getting all the way from equator
to poles
• Air only makes it about 1/3 of the way to the poles
before it becomes dense enough to sink
• Descending air turns back toward equator when it
reaches the surface because it is again deflected to the
right
• Heats up when it gets back to equator and rises again.
Fig. 4-11
Fig. 4-7
Hadley Cell
Fig. 4-6
Hadley Cell
Fig. 4-18
Fig. 4-19 Pressure at sea-level (mbars)
Atmospheric circulation &
precipitation
• Air carries water vapor
• In general, uplift of air masses induces precipitation
• Conversely, descending air affects distribution of deserts
– Descending arms of Hadley cells
– Continental interiors
– Leeward (downwind) of mountains
– West coasts of major continents – due to upwelling and
offshore currents
Deserts offset from convergence zones where there is high precipitation.
Deserts at divergence zones.
Features of the model
• At boundaries, air is moving vertically
– Surface winds are weak and erratic
• Equatorial region
– Lots of rain as humid air rises and loses moisture (rain
forests)
– Doldrums
– Intertropical convergence zone (ITCZ) – winds
converge
• 30oN and S region
– Sinking air is arid and evaporation >> precipitation
(deserts and high salinity)
– Horse latitudes
Features of the model
• Air moves horizontally within the cells from areas of
high pressure to areas of low pressure
• Tropical areas – Hadley cells
– Surface winds are strong and dependable
– Trade winds or easterlies centered at ~15oN (northeast
trade winds) and ~ 15oS (southeast trade winds)
– Surface wind moves from horse latitudes to doldrums so
come out of northeast in N hemisphere
• Mid-latitude areas – Ferrel cells
– Westerlies centered at ~ 45oN and ~45oS
– Surface wind moves from horse latitudes to polar cells so
comes out of southwest in the N hemisphere
The 6-celled model
• Not exactly correct either
• North - South variation
• Land versus water distribution
– Equator to pole flow of air different depending on
amount of land at a particular longitude
– ITCZ narrower and more consistent over land than
ocean
– Seasonal differences greater in N hemisphere
(remember, more land)
• The ocean’s thermostatic effect reduces
irregularities due to surface conditions at different
longitudes
Distributions of land masses
-Differential heating and cooling
-Land heats up and cools more rapidly
North - South variation (cont)
• Offset at the equator
– Geographical vs. meteorological equator
– Meteorological equator is ~ 5oN of geographical
equator (thermal equilibrium between hemispheres)
– Meteorological equator and ITCZ generally coincide
and change with seasons (moves N in northern
summer)
– Atmospheric and oceanic circulation is symmetrical
around the meteorological equator NOT the
geographical equator!
• Seasonality
Seasonality important
Shifts in polar front and the ITCZ – meteorological equator
West-East variations
• Air over chilled continents becomes cold and dense in
the winter
• Air sinks creating high pressure over continents
• Air over relatively warmer waters rises (possibly with
water vapor) creating low pressure zones over water
• Air flows from high pressure to low pressure
modifying air flow within cells
• Reverse situation in summer
• Effects pronounced in N hemisphere (mid-latitudes)
where there is about the same amount of land & water
Winds over the Pacific
on two days in Sept
1996
Stronger winds in redorange
Notes:
Deviates from 6-cell
model
Strong westerlies hitting
Canada
Strong tradewinds
(easterlies) over Hawaii
Extratropical cyclone
east of New Zealand
Circulation of the Atmosphere
• Most of the variation from the 6-cell model
is due to
– Geographical distribution of landmasses
– Different response of land and ocean to solar
heating
– Chaotic flow
• Over long term – 6-cell model is pretty
good for describing average flow
• Major surface wind and pressure systems of the world
and their weather
• These wind patterns move 2/3 of heat from tropics to
poles.
Monsoons
• Pattern of wind circulation that changes
with the season
• Generally wet summers and dry winters
• Linked to different heat capacities of land
and water and to N-S movement of the
ITCZ
Wet season
• In the spring, land heats (faster than water)
• Warm air over land rises creating low
pressure
• Cool air flows from ocean to land
• This humid air heats and rises (rains form)
Dry Season
• Land cools (faster than ocean)
• Air cools and sinks over land creating high
pressure
• Dry surface wind moves seaward
• Warms and rises over water (with or
without evaporation and rain over water)
Monsoons
• Most intense over Asia where you have a
huge land mass in the N and a huge ocean
to the S
• Monsoon over India causes wet season
(summer) from April – October (up to 10
meters – 425 inches of rain per year)
• Smaller monsoon in N America (Gulf of
Mexico and SE)
Dry season
Wet season
ITCZ
ITCZ
Sea and Land breezes
• Daily changes in wind direction due to unequal
heating and cooling of land versus water
• Warm air during day on land rises and cool air
from sea moves onshore (with or without water
vapor)
• Warmer air over water rises and cool air on land
during the night sinks and moves offshore
Daytime Onshore Breeze
Nighttime Offshore Breeze
Fig. 4-17