Transcript AMS Ocean Studies

Ocean Studies
Introduction to Oceanography
American Meteorological Society
Chapter 5
The Atmosphere and Ocean
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Case in Point
– At middle latitudes, prevailing winds
blow from west to east.
– Sea surface temperatures (SST) change
relatively little through the course of a year.
– This stable SST regime dampens the
summer-to-winter temperature contrast of air
flowing over the ocean to downwind Western
Europe.
• In Western Europe, the air temperature contrast
between summer and winter is less than it is over
most of North America.
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The Atmosphere and Ocean
• Driving Question:
– What role does the ocean play in the longterm average state of the atmosphere?
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The Atmosphere and Ocean
• In this chapter, we examine radiational
heating and cooling of the Earthatmosphere system:
– the interaction of incoming solar radiation with
the atmosphere, ocean, and continents,
– the flow of infrared radiation to space, and
– the greenhouse effect.
• We also discuss heat transport by
atmospheric and oceanic circulations
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Weather and Climate
• We can think of weather as the state of the
atmosphere at some place and time
– described in terms of such variables as temperature,
precipitation, cloud cover, and wind speed.
• Climate is popularly defined as weather at a
particular place averaged over a specific interval
of time.
– By international convention, average values of
weather elements such as temperature or
precipitation are computed over a 30-year period
beginning with the first year of a decade.
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Heating and Cooling Earth’s Surface
• As Earth orbits the sun, its atmosphere and
surface are absorbing energy radiated by the
sun (mostly between 0.25 and 2.5 micrometers).
• Absorption of solar radiation heats the Earthatmosphere system.
• At the same time the entire planet is emitting
infrared radiation to space (mostly between 4 and 24
micrometers), which has a cooling effect on the
Earth-atmosphere system.
• Over the long term, radiational cooling of the
planet essentially balances radiational heating of
the planet so that Earth remains in radiative
equilibrium with surrounding space.
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Heating and Cooling Earth’s Surface
• SOLAR RADIATION
– Once every 24 hrs, Earth completes one
rotation on its axis.
– At any instant, half the planet is illuminated by
solar radiation while the other half is in
darkness.
– The tilt of Earth’s spin axis (23 degrees 27 minutes)
is responsible for the seasons.
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• the Northern Hemisphere tilts away from the sun in
fall and winter, and toward the sun in spring and
summer
Heating and Cooling Earth’s Surface
Dates of solstices and equinoxes vary because Earth
completes one orbit of the sun in 365.24 days,
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necessitating the leap year adjustment.
Heating and Cooling Earth’s Surface
• SOLAR RADIATION
– Annual periodic changes in the planet’s
orientation to the sun result in changes in
• solar altitude (the angle of the sun above the horizon) and
• length of daylight (elapsed time between sunrise and sunset)
– Solar altitude varies from 0º (at sunrise or sunset) to
as much as 90º (sun directly overhead)
– At middle and high latitudes, the altitude of
the noon sun is higher, daylight is longer, and
solar radiation is more intense in summer
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than in winter.
Heating and Cooling Earth’s Surface
• SOLAR RADIATION
– The intensity of solar radiation striking Earth’s
surface per unit area varies with the solar
altitude.
– With increasing solar altitude, more solar
energy strikes a unit area of Earth’s surface in
a unit of time
– Greater solar altitudes in the tropics translate
into more intense radiation and higher
temperatures at Earth’s surface.
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Heating and Cooling Earth’s Surface
Consider this analogy:
(A) A flashlight beam shines on a
horizontal surface most
intensely when the flashlight
shines from directly overhead
(analogous to a solar altitude of
90 degrees).
(B) At an angle decreasing from 90
degrees, the flashlight beam
spreads over an increasing
area of the horizontal surface
so that the light is less
concentrated (less radiational
energy received per unit area).
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Heating and Cooling Earth’s Surface
Path of the sun through
the sky on the solstices
and equinoxes at (A) the
equator, (B) a middle
latitude location in the
Northern Hemisphere,
and (C) the North Pole.
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© American Meteorological Society
Heating and Cooling Earth’s Surface
The difference in length of daylight between the summer and winter
solstices increases from zero at the equator to a maximum (24 hrs) at
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the Arctic and Antarctic Circles
Heating and Cooling Earth’s Surface
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Regular variations in maximum solar altitude and length of daylight
through the year are ultimately responsible for changes in the
receipt of solar radiation and monthly average temperatures
Heating and Cooling Earth’s Surface
• SOLAR RADIATION
– Places downwind of the ocean experience a
smaller contrast between average winter and
summer temperatures and have a maritime
climate.
– Places at the same latitude but well inland
experience a greater temperature contrast
between winter and summer and have a
continental climate.
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Heating and Cooling Earth’s Surface
• SOLAR RADIATION
– Proximity to large bodies of water also affects
the timing of the average warmest and coldest
time of the year.
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• Outside of the tropics, the annual temperature
cycles lags the annual solar radiation cycle.
• In the interior United States, the air temperature
cycle lags the solar radiation cycle by an average
of 27 days.
• But in coastal localities having a strong maritime
influence (e.g., coastal California, Florida), the
average lag time is up to 36 days.
Heating and Cooling Earth’s Surface
• SOLAR RADIATION BUDGET
– Solar radiation intercepted by Earth travels
through the atmosphere and interacts with its
component gases and aerosols. These
interactions consist of
• scattering,
• reflection, and
• absorption.
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Heating and Cooling Earth’s Surface
• SOLAR RADIATION BUDGET
– With scattering, a particle disperses radiation
in all directions: up, down, and sideways.
– Reflection is a special case of scattering in
which a large surface area redirects radiation
in a backward direction.
• The fraction of incident radiation reflected by a
surface is known as the albedo of that surface, i.e.,
albedo = [(reflected radiation)/(incident radiation)] × 100%.
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• Within the atmosphere, the tops of clouds are the
most important reflectors of incoming visible
sunlight.
Heating and Cooling Earth’s Surface
• SOLAR RADIATION BUDGET
– Absorption is a process whereby some of the
radiation that strikes an object is converted to
heat energy.
• Oxygen, ozone, water vapor, and various aerosols
(including cloud particles) absorb solar radiation.
– The strong absorption of ultraviolet (UV) radiation by
oxygen and ozone (O3) in the stratosphere shields
organisms from exposure to potentially lethal intensities of
UV.
– These absorption processes create the so-called
stratospheric ozone shield.
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Heating and Cooling Earth’s Surface
• SOLAR RADIATION BUDGET
– Solar radiation not scattered or reflected to
space or absorbed by atmospheric gases or
aerosols reaches Earth’s surface where it is
either reflected or absorbed.
– High-albedo surfaces reflect a considerable
amount of incident solar radiation whereas lowalbedo surfaces reflect much less incident
solar radiation.
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Heating and Cooling Earth’s Surface
The albedo of some
common surfaces is listed
in Table 5.1.
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Heating and Cooling Earth’s Surface
In this visible
satellite image, the
ocean surface
appears dark
because of its low
albedo for visible
solar radiation.
White areas are
clouds.
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Heating and Cooling Earth’s Surface
• SOLAR RADIATION BUDGET
– The albedo of the ocean surface varies with
solar altitude.
• Under clear skies, the albedo of a flat, tranquil water
surface decreases with increasing solar altitude.
• With cloud-covered skies, only diffuse solar radiation
strikes the water surface; the albedo varies little with
solar altitude and is uniformly less than 10%.
– Considering that the ocean covers about 71%
of the surface of the planet, the ocean is the
principal sink (absorber) for solar radiation
striking Earth’s surface.
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Heating and Cooling Earth’s Surface
Under clear skies, the
albedo of a flat and
undisturbed water
surface changes with
solar altitude.
A wave-covered water
surface has a slightly
higher albedo at high
solar altitudes and a
slightly lower albedo at
low solar altitudes.
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Heating and Cooling Earth’s Surface
• SOLAR RADIATION BUDGET
– Earth’s planetary albedo is about 31%.
• The Earth-atmosphere system reflects or scatters
back to space on average about 31% of the solar
radiation intercepted by the planet.
• The atmosphere (i.e., gases, aerosols, clouds)
absorbs only about 20% of the total solar radiation
intercepted.
• The remaining 49% of solar radiation is absorbed by
Earth’s surface—mostly the ocean.
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Heating and Cooling Earth’s Surface
• SOLAR RADIATION AND THE OCEAN
– The ocean’s absorption of the visible portion of
solar radiation is selective by wavelength.
• Water absorbs the longer wavelengths (i.e., reds
and yellows) of visible light more efficiently than the
shorter wavelengths (i.e., greens and blues) so that
green and blue penetrate to greater depths.
– This explains the blue/green color of the open ocean
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Heating and Cooling Earth’s Surface
Visible solar radiation is
selectively absorbed by
wavelength as it
penetrates the surface
waters of the open ocean.
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Heating and Cooling Earth’s Surface
• SOLAR RADIATION AND THE OCEAN
– The sunlit surface layer of the ocean, down to the depth
where light is just sufficient for photosynthesis, is
termed the photic zone
– In clear ocean waters, this depth is usually from 100 to
200 m (330 to 650 ft) but is much shallower in highly
productive or turbid waters.
– As light becomes dimmer with increasing depth, its
color also changes.
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• This color change affects plant production because each plant
pigment is most efficient with a specific color of light.
• The combination of pigments in any type of phytoplankton
determines its optimal depth distribution.
Heating and Cooling Earth’s Surface
• INFRARED RADIATION AND THE
GREENHOUSE EFFECT
– Emission of heat to space in the form of infrared
radiation balances solar radiational heating of the
Earth-atmosphere system.
– This is global radiative equilibrium
– While the clear atmosphere is relatively transparent to
solar radiation, certain gases in the atmosphere known
as greenhouse gases impede the escape of infrared
radiation to space thereby elevating the temperature of
the lower atmosphere. This important climate control is
the so-called greenhouse effect.
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Heating and Cooling Earth’s Surface
• INFRARED RADIATION AND THE
GREENHOUSE EFFECT
– The atmosphere absorbs only about 20% of the
solar radiation intercepted by the planet.
– The atmosphere absorbs a greater percentage
of the infrared radiation emitted by Earth’s
surface
– The atmosphere, in turn, radiates some IR to
space and some to Earth’s surface.
– Hence, Earth’s surface is heated by absorption
of both solar radiation and atmosphere-emitted
infrared radiation.
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Heating and Cooling Earth’s Surface
• INFRARED RADIATION AND THE
GREENHOUSE EFFECT
– The greenhouse effect is responsible for considerable
warming of Earth’s surface and lower atmosphere.
– Viewed from space, the planet (Earth-atmosphere
system) radiates at about –18 °C (0 °F) whereas the
average temperature at Earth’s surface is about 15 °C
(59 °F).
– The temperature difference is due to the greenhouse
effect and amounts to:
[15 °C – (–18 °C)] = 33 Celsius degrees
or
[59 °F – (0 °F)] = 59 Fahrenheit degrees.
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Heating and Cooling Earth’s Surface
• INFRARED RADIATION AND THE
GREENHOUSE EFFECT
– Water vapor is the principal greenhouse gas.
– Other greenhouse gases include
•
•
•
•
•
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carbon dioxide,
ozone,
methane (CH4),
nitrous oxide (N2O), and
halocarbons (once widely used as refrigerants and
aerosol spray propellants).
Heating and Cooling Earth’s Surface
• INFRARED RADIATION AND THE
GREENHOUSE EFFECT
– The percentage of infrared radiation absorbed by
greenhouse gases varies with wavelength.
– An atmospheric window is a range of wavelengths
over which little or no radiation is absorbed.
– The major infrared window is from about 8 to 13
micrometers (µm).
• Significantly, this window includes the wavelength of the
planet’s peak infrared emission (about 10 µm).
• Through this window, most heat from the Earth-atmosphere
system escapes to space as infrared radiation.
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Heating and Cooling Earth’s Surface
Absorption of radiation
by selected gaseous
components of the
atmosphere as a function
of wavelength.
Absorptivity is the
fraction of radiation
absorbed and ranges
from 0 to 1 (0% to 100%
absorption)
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Heating and Cooling Earth’s Surface
• INFRARED RADIATION AND THE
GREENHOUSE EFFECT
– Natural biogeochemical cycles continually transport
greenhouse gases into and out of the atmosphere and
ocean.
– Human activities alter the rate of biogeochemical
cycling.
– The Callendar effect is the theory that global climate
change can be brought about by enhancement of the
natural greenhouse effect by increased levels of
atmospheric CO2 from anthropogenic sources,
principally the burning of fossil fuels.
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Heating Imbalances: Earth’s Surface
versus Atmosphere
• An important aspect of imbalance in rates of
radiational heating and radiational cooling
involves Earth’s surface versus the
atmosphere.
• The global average annual distribution of
incoming solar radiation and outgoing
infrared radiation implies net warming of
Earth’s surface and net cooling of the
atmosphere
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Heating Imbalances: Earth’s Surface
versus Atmosphere
Globally and annually averaged distribution of 100 units of solar radiation
entering the top of the atmosphere. Depicted from left to right are fluxes of
© AMSradiation (yellow), infrared radiation (red), sensible heat (black), and
solar
latent heat (green).
Heating Imbalances: Earth’s Surface
versus Atmosphere
On a global average
annual basis, Earth’s
surface undergoes net
radiational heating and
the atmosphere
undergoes net
radiational cooling.
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Heating Imbalances: Earth’s Surface
versus Atmosphere
– The atmosphere is not actually cooling relative
to Earth’s surface because radiation is not the
only heat transfer mechanism at work.
– In response to the radiationally induced
temperature gradient between Earth’s surface
and atmosphere, heat is transferred from
Earth’s surface to the atmosphere.
– A combination of latent heating (phase changes
of water) and sensible heating (conduction and
convection) is responsible for this heat transfer.
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Heating Imbalances: Earth’s Surface
versus Atmosphere
• LATENT HEATING
– Latent heating refers to the transfer of heat energy
from one place to another as a consequence of phase
changes of water.
– When water changes phase, heat energy is either
absorbed from the environment (i.e., melting,
evaporation, sublimation) or released to the
environment (i.e., freezing, condensation, deposition).
– Ocean water covers a large portion of Earth’s surface
and is the principal source of water vapor that
eventually returns to Earth’s surface as precipitation.
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Heating Imbalances: Earth’s Surface
versus Atmosphere
Percentage of precipitation over land that originated as evaporation on the
continents, annually averaged over 15 years. In many land areas, the
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principal
source of water for precipitation is evaporation from the ocean.
Heating Imbalances: Earth’s Surface
versus Atmosphere
• LATENT HEATING
– The ocean is a major source of salt crystals that spur
condensation and cloud development in the
atmosphere.
• When sea waves break, drops of salt water enter the
atmosphere and evaporate leaving behind sea-salt crystals
that function as cloud condensation nuclei .
– During cloud formation, water changes phase and
latent heat is released to the atmosphere.
– Through latent heating, then, heat is transferred from
Earth’s surface to the troposphere.
– Latent heat transfer is more important than either
radiational cooling or sensible heat transfer in the
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Heating Imbalances: Earth’s Surface
versus Atmosphere
Earth’s surface is cooled through (A) vaporization of water, (B) net
emission of infrared radiation to the sky, and (C) conduction plus
convection. Numbers are global annual averages based on 100 units
of
solar radiation entering the top of the atmosphere.
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Heating Imbalances: Earth’s Surface
versus Atmosphere
• SENSIBLE HEATING
– Heat transfer via conduction and convection can be
monitored (sensed) by temperature changes; hence
the term sensible heating
– Heat is conducted from the relatively warm surface of
the Earth to the cooler overlying air.
– In this way, convection transports heat from Earth’s
surface into the troposphere.
• Because air is a relatively poor conductor of heat, heat
convection is much more important than conduction as a
transfer mechanism within the troposphere
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Heating Imbalances: Earth’s Surface
versus Atmosphere
Heat conducted from the relatively
warm surface of the Earth to the
cooler overlying air reduces the
density of that air, which is forced to
rise by cooler denser air replacing it
at the surface. Thus convection
currents transport heat from Earth’s
surface into the troposphere.
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Heating Imbalances: Earth’s Surface
versus Atmosphere
A
B
Sensible heating often combines with latent heating to channel heat from Earth’s
surface into the troposphere. Updrafts (ascending branches) of vapor-laden air in
convection currents often produce (A) cumulus clouds which may develop further
into cumulonimbus clouds (B).
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Heating Imbalances: Earth’s Surface
versus Atmosphere
• THE BOWEN RATIO
– This ratio describes how the heat energy received at Earth’s
surface (by absorption of solar and infrared radiation) is
partitioned between sensible heating and latent heating.
– Bowen ratio = [(sensible heating)/(latent heating)]
• At the global scale, Bowen ratio = [(7 units)/(23 units)] = 0.3
– The average Bowen ratio varies from one place to another
• The wetter the surface, the less important is sensible heating and
the more important is latent heating.
– The Bowen ratio ranges from about 0.1 (one-tenth as much
sensible as latent heating) for the ocean to about 5.0 (five times
as much sensible as latent heating) in deserts.
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Heating Imbalances: Tropics versus
High Latitudes
– On a global scale, imbalances in radiational heating
and cooling occur between not only Earth’s surface
and atmosphere but also the tropics and higher
latitudes.
– At higher latitudes, solar radiation spreads over a
greater area and is less intense per unit horizontal
surface area than in the tropics.
– At higher latitudes, the rate of infrared cooling to
space over the period of a year exceeds the rate of
warming caused by absorption of solar radiation.
– At lower latitudes the reverse is true
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Heating Imbalances: Tropics versus
High Latitudes
Variation
by latitude of absorbed solar radiation and outgoing infrared radiation
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derived from satellite sensor measurements.
Heating Imbalances: Tropics versus
High Latitudes
– Averaged over the globe, incoming energy (absorbed
solar radiation) must equal outgoing energy (IR
emitted to space)
• global radiative equilibrium is the prevailing condition.
– Lower latitudes do not become progressively warmer
nor do higher latitudes become colder, because heat
is transported poleward from the tropics into middle
and high latitudes.
– Poleward heat transport is brought about by
• air mass exchange,
• storm systems, and
• ocean circulation.
– The atmosphere and ocean share equally in the
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Heating Imbalances: Tropics versus
High Latitudes
• HEAT TRANSPORT BY AIR MASS EXCHANGE
– North-south exchange of air masses transports
sensible heat from the tropics into middle and high
latitudes.
• An air mass is a huge volume of air covering thousands of
square kilometers that is relatively uniform horizontally in
temperature and humidity.
– The properties of an air mass largely depend on the
characteristics of the surface over which the air mass
forms (its source region) or travels.
• Hence, there are four basic types of air masses:
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–
–
–
–
cold and humid,
cold and dry,
warm and humid, and
warm and dry.
Heating Imbalances: Tropics versus
High Latitudes
Source regions of air masses
that regularly move over
North America. Properties of
an air mass largely depend
on the characteristics of the
surface over which it forms
(its source region) or travels.
Warm air masses that form in
lower latitudes flow toward
the pole while cold air masses
flow toward the equator from
source regions at high
latitudes.
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Heating Imbalances: Tropics versus
High Latitudes
• HEAT TRANSPORT BY STORMS
– At low latitudes, water that evaporates from the warm
ocean surface may be drawn into the circulation of a
developing storm system (cyclone or low).
– As the storm travels into higher latitudes, some of that
water vapor condenses into clouds, thereby releasing
latent heat to the troposphere.
– Latent heat of vaporization acquired at low latitudes is
thereby delivered to middle and high latitudes.
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• Because they entrain much more water vapor and latent
heat, tropical storms and hurricanes are greater contributors
to poleward heat transport than ordinary middle latitude
(extratropical) storms.
Heating Imbalances: Tropics versus
High Latitudes
• HEAT TRANSPORT BY OCEAN CIRCULATION
– The ocean contributes to poleward heat transport via:
• wind-driven surface currents
– Warm surface currents flow from the tropics into middle
latitudes, supplying heat to the cooler middle latitude
troposphere.
– Cold surface currents flow from high to low latitudes, absorbing
heat from the relatively warm troposphere and greater solar
radiation in the tropics.
• the deeper thermohaline circulation
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– In the North Atlantic, a warm surface ocean current flows north
and eastward from the Florida Strait. At high latitudes, the
surface waters cool, sink, and flow southward as cold bottom
water.
– This heat transporting circulation is also known as the
meridional overturning circulation (MOC).
Circulation of the Atmosphere:
The Forces
• PRESSURE GRADIENT FORCE
– Air pressure varies with both space and time and this
drives the circulation of the atmosphere.
– A change in air pressure from one place to another is
known as an air pressure gradient.
– Air pressure gradients occur both vertically and
horizontally within the atmosphere.
– A vertical air pressure gradient is a permanent feature
of the atmosphere because air pressure always
decreases with increasing altitude
– In response to a horizontal air pressure gradient, the
wind blows from where the pressure is relatively high
toward where the pressure is relatively low.
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Circulation of the Atmosphere:
The Forces
• PRESSURE GRADIENT FORCE
– Air pressure varies with both space and time and this drives the
circulation of the atmosphere.
– A change in air pressure from one place to another is known as an
air pressure gradient.
– Air pressure gradients occur both vertically and horizontally within
the atmosphere.
– A vertical air pressure gradient is a permanent feature of the
atmosphere because air pressure always decreases with
increasing altitude
– In response to a horizontal air pressure gradient, the wind blows
from where the pressure is relatively high toward where the
pressure is relatively low.
– The force that causes air to move as the consequence of an air
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Circulation of the Atmosphere:
The Forces
• PRESSURE GRADIENT FORCE
– Horizontal air pressure gradients can be determined
on weather maps from patterns of isobars.
– Pressure gradient force is directed across isobars and
toward low pressure.
– The magnitude of the pressure gradient force is
inversely related to the spacing of isobars.
• The wind is relatively strong where the pressure gradient is
steep (closely spaced isobars), and light or calm where the
pressure gradient is weak (widely spaced isobars).
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Circulation of the Atmosphere:
The Forces
• CORIOLIS EFFECT
– Because Earth rotates, anything moving freely over the planet’s
surface, including air and water, is deflected to the right in the
Northern Hemisphere and to the left in the Southern
Hemisphere.
– This deflection is known as the Coriolis Effect
– Reversal in the direction of the Coriolis Effect between the
Northern and Southern Hemispheres is related to the difference
in an observer’s sense of Earth’s rotation.
– The amount of deflection (the magnitude of the Coriolis Effect)
varies significantly with latitude, from zero at the equator to a
maximum value at the poles.
– The magnitude of the Coriolis Effect also varies with wind speed
and spatial scale of atmospheric circulation.
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Circulation of the Atmosphere:
The Forces
The Coriolis Effect arises from
the rotation of Earth on its axis
and causes deflection of winds
(and ocean currents) to the right
in the Northern Hemisphere and
to the left in the Southern
Hemisphere.
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Circulation of the Atmosphere:
Patterns of Motion
Atmospheric scientists subdivide atmospheric circulation into
discrete weather systems operating at various spatial and temporal
scales.
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Circulation of the Atmosphere:
Patterns of Motion
• Scales of Atmospheric Circulation
– Planetary-scale systems
• Large-scale wind belts encircling the planet (e.g., westerlies
of middle latitudes, trade winds in the tropics).
– Synoptic-scale systems
• Continental or oceanic in scale; migrating cyclones are
examples.
– Mesoscale systems
• Circulation systems that are so small and short-lived that
they may influence the weather in only a relatively small
area, for example, thunderstorms and sea breezes.
– Microscale systems
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• The smallest spatial subdivision of atmospheric circulation
covering only a very small area (e.g., a weak tornado).
Circulation of the Atmosphere:
Patterns of Motion
• PLANETARY-SCALE CIRCULATION
– Three broad wind belts encircle both the Northern and
Southern Hemispheres.
• trade winds
– between about 30 degrees N and 30 degrees S,
• westerlies
– between about 30 and 60 degrees N, and between about 30
and 60 degrees S,
• polar easterlies
– between about 60 degrees N and the North Pole and between
about 60 degrees S and the South Pole.
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Circulation of the Atmosphere:
Patterns of Motion
Schematic representation of the planetary-scale surface circulation of the
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atmosphere.
Three broad wind belts encircle both the Northern and Southern
Hemispheres.
Circulation of the Atmosphere:
Patterns of Motion
• PLANETARY-SCALE CIRCULATION
– The planetary-scale atmospheric circulation plays an important
role in the horizontal transport of water vapor between ocean
basins
• trade winds transport water evaporated from the tropical Atlantic
Ocean basin across Central America to the tropical Pacific Ocean
where it condenses into clouds that produce rain.
• This transport of water increases the salinity of Atlantic surface
waters and freshens the Pacific surface waters.
– In both hemispheres, trade winds blow out of the equatorward
flank of the subtropical anticyclones (massive semi-permanent
high pressure systems centered over the ocean basins near 30
degrees N and S)
– Westerlies blow out of the poleward flank of the subtropical
anticyclones.
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Circulation of the Atmosphere:
Patterns of Motion
• PLANETARY-SCALE CIRCULATION
– In the Northern Hemisphere, in the middle and upper
troposphere, the temperature gradient between the relatively
warm tropics and relatively cold polar areas induces a
northward flow of air aloft that is deflected to the right (to the
east) by Earth’s rotation (the Coriolis Effect).
– A similar situation occurs in the Southern Hemisphere where
the Coriolis Effect deflects a southward-directed flow of air aloft
to the left (to the east).
– These phenomena result in belts of westerlies that encircle the
planet and steer air masses, storms, and fair-weather systems
generally from west to east.
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Circulation of the Atmosphere:
Patterns of Motion
• PLANETARY-SCALE CIRCULATION
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At middle and high latitudes of the Northern Hemisphere, prevailing
winds at altitudes from about 5500 to 12,000 m (18,000 to 40,000 ft)
blow generally from west to east in a wave-like pattern of ridges
(clockwise turns) and troughs (counterclockwise turns).
Circulation of the Atmosphere:
Patterns of Motion
• SYNOPTIC-SCALE WEATHER SYSTEMS
– The most important synoptic-scale weather systems
are highs (or anticyclones) and lows (or cyclones).
– Whereas subtropical highs and subpolar lows are
nearly stationary, synoptic-scale highs and lows move
with the prevailing winds blowing at altitudes of
several kilometers above Earth’s surface, generally
eastward across North America.
• Important exceptions are tropical cyclones (e.g., hurricanes)
that are imbedded in the trade wind flow and generally track
from east to west over the tropical Atlantic and Pacific.
– Eventually these systems come under the influence of the
middle-latitude westerlies and turn toward the north and
northeast.
© AMS
Circulation of the Atmosphere:
Patterns of Motion
• SYNOPTIC-SCALE WEATHER SYSTEMS
Viewed from above in the
Northern Hemisphere,
surface winds blow
clockwise and outward in
a high (anticyclone).
Isobars are drawn at 4millibar intervals.
© AMS
Circulation of the Atmosphere:
Patterns of Motion
Viewed from above in the
Northern Hemisphere, surface
winds blow counterclockwise
and inward in a low (cyclone)
(A), bringing together
contrasting air masses to form
fronts (B).
The curved blue line with
spikes is the leading edge of
relatively cold and dry air (a
cold front) while the curved red
line with half circles is the
leading edge of relatively warm
and humid air (a warm front).
© AMS
Circulation of the Atmosphere:
Patterns of Motion
• MESOSCALE WEATHER SYSTEMS
Circulation systems that are so small and short-lived that they may
influence the weather in only a portion of a large city.
A sea breeze is a relatively cool mesoscale surface wind that develops
during daylight hours and blows inland from the ocean in response
to differential heating of land and sea. During the day, the land surface
warms more than the sea surface inducing a horizontal air pressure
gradient with high pressure over the ocean surface and low pressure
over the land surface. At night, land becomes relatively colder than the
sea surface. The circulation reverses and a land breeze blows offshore.
© AMS
Circulation of the Atmosphere:
Patterns of Motion
• An example of MESOSCALE WEATHER SYSTEMS
© AMS
Sea Breezes
Conclusions
– The ocean has a very low average albedo for incident
solar radiation and is the principal sink for incoming
solar radiation.
– The ocean is the chief source of water vapor and
latent heat for the atmosphere.
– Ocean currents contribute to heat transport from the
tropics to higher latitudes; breaking ocean waves are
an important source of cloud condensation nuclei.
– The atmosphere is frictionally coupled to the ocean
surface; winds supply the kinetic energy that drives
surface ocean currents and generates ocean waves.
– Large-scale surface ocean currents mirror the
© AMS prevailing planetary-scale atmospheric circulation.