BIOL 4120: Principles of Ecology
Lecture 3: Physical
Environment: Climate
Dafeng Hui
Office: Harned Hall 320
Phone: 963-5777
Email: dhui@tnstate.edu
3.3 Air masses circulate globally
The blanket of air surrounds the planet
– atmosphere – is not static
It is in a constant state of movement,
driven by the rising and sinking of air
masses and the rotation of the Earth
on its axis.
Coriolis effect:
Deflection in the
pattern of air flow.
Clockwise
movement in N
hemisphere,
counterclockwise
in S. Hemisphere.
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Three cells and trade
wind belts
These air movements
create global precipitation
pattern
Major latitudinal
displacements of surface air
currents: convection
currents drive Hadley cells,
pulling air at surface into
Inter-Tropical Convergence
Zone, ITCZ); Ferrel Cells
driven by low pressure zone
at 20º-30º lat.; Midlatitude
westerlies converge into jet
stream; polar cells driven by
high pressure (cold) flows
out of polar region along
Earth’s surface towards
south.
The thermal equator, oscillating latitudinally with seasons,
drives low latitude patterns of rainfall by establishing zones of
low pressure (high rainfall) and high pressure (low rainfall).
The hadley cell (centered
on thermal equator)
depends on convection
currents with updrafts that
cause low latitude
rainforests, and downdrafts
that cause subtropical hot
deserts (20º - 30º N, S lat.).
3.4 Global ocean currents
movement
Surface water movements in the ocean
is dominated by the global pattern of
the prevailing winds (and solar
energy)
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Ocean currents also affect climate, sometimes very dramatically
(source of energy movement too)
Each ocean is dominated by great circular water movement, or
gyres. Gyres move clockwise in the N. Hemisphere and
counterclockwise in the S. Hemisphere (Coriolis effect).
Warmer water moves away from equator and cold water moves
towards equator.
Air moisture and temperature
Evaporation: water to vapor
Condensation: from water
vapor to water
Vapor pressure: amount of
pressure water vapor exerts
independent of pressure of
dry air.
Saturated vapor pressure:
vapor pressure of air at
saturation.
Absolute humidity; amount
of water in a given volume
of air.
Relative humidity: RH
3.5 Global pattern of precipitation
Temporal variation in precipitation
(e.g., Intertropical Convergence Zone shift)
Shifts of ITCZ
produce
rainy
seasons and
dry seasons
in the tropics
Patterns of temporal variation in climate at the
Southeast Asia region:
Seasonal changes in T with the rotation of Earth
about the sun, and the migration of the ITCZ
with the resulting seasonality of rainfall in the
tropics and monsoons in southeast Asia.
3.6 Topography influences regional
and local patterns of precipitation
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Rain shadow:
3.7 Irregular variations in climate
occur at the regional scale
Irregular variations (Little Ice Age: cooling
between mid-14 to mid-19th century)
(El Nino and La Nina)
El Nino: an abnormal warming of surface ocean
waters in the eastern tropical Pacific.
El Nino-Southern Oscillation (ENSO): An
oscillation in the surface pressure between the
southeastern tropic Pacific and the AustralianIndonesian regions.
Normal conditions, strong trade
winds move surface water
westward. As the surface
currents move westward, the
water warms. The warmer
water of the western Pacific
causes the moist maritime air
to rise and cool, bringing
abundant rainfall to the region;
ENSO: Trade winds slacken,
reducing the westward flow of
the surface currents. Rainfall
follows the warm water
eastward, with associated
flooding in Peru and drought in
Indonesia and Australia.
La Nina: injection of cold water becomes
more intense than usual, causing the
surface of eastern Pacific to cool. Results
in droughts in South America and heavy
rainfall in Australia.
3.8 Microclimates
Microclimates defines the local, small
scale conditions in which organisms live.
These conditions include: topography
(aspect=direction a slope face, surface or
underground, beneath vegetation or not),
light, temperature, air conditions or wind
movement, moisture etc.
Vegetation also moderate microclimates.
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Most organisms exist in a microclimate
that is optimal
Scale of climate in hundreds of kilometers
Scale of microclimate can vary from
meters to kilometers to tens of kilometers
3.9 Climate and global vegetation
Global pattern of PPT and
vegetation
Conclusions
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With a few basic physical principles (solar radiation as
energy, air movements, convection currents) one can
explain major patterns of temperature, rainfall,
seasonality, ocean currents on Earth’s surface.
These patterns determine global vegetation distribution
No one ecosystem type dominates globe, but instead
different types vegetation adapted to different climatic
conditions
The foregoing principles and forces explain much of
the global patterns in vegetation types (depending on
temperature, moisture): Wetter vegetation (forests)
green, drier (grassland, desert) tan to brown, cold
(arctic, alpine) areas white.
30º N
Equator
30º S