Principles of terrestrial ecosystem ecology.pdf

Hadley cell
Hadley cell
Ferrell cell
Ferrell cell
Westerlies
NE tradewinds
SE tradewinds
Westerlies
Polar cell
ITCZ
lation cell in the Northern Hemisphere and
another in the Southern Hemisphere, driven by
atmospheric heating and uplift at the equator
and subsidence at the poles. Based on observations,
Ferrell proposed in 1865 the conceptual
model that we still use today, although the
actual dynamics are much more complex. This
model describes atmospheric circulation as a
series of three circulation cells in each hemisphere.
(1) The Hadley cell is driven by expansion
and uplift of equatorial air. (2) The polar
cell is driven by subsidence of cold converging
air at the poles. (3) The intermediate Ferrell cell
is driven indirectly by dynamical processes (Fig.
Cold subsiding air
Cold subsiding air
60 o
Subtropical high pressure
Polar cell
Polar front
Subtropical high pressure
Polar front
Figure 2.6. Earth’s latitudinal atmospheric circulations
are driven by rising air at the equator and subsiding
air at the poles. These forces and the Coriolis
forces produce three major cells of vertical atmospheric
circulation (Hadley, Ferrell, and polar cells).
Air warms and rises at the equator due to intense
heating. After reaching the tropopause, the equatorial
air moves poleward to about 30° N and S latitudes,
where it descends and either returns to the
equator, forming the Hadley cell, or moves poleward.
Cold dense air at the poles subsides and moves
toward the equator until it encounters polewardmoving
air at about 60° latitude. There the air rises
30 o
The Atmospheric System 25
Warm rising air
0 o
Cold subsiding air
Warm rising air
Warm
rising
air
Cold subsiding air
and moves either poleward to replace air that has
subsided at the poles (the polar cell) or moves
toward the equator to form the Ferrell cell. Also
shown are the horizontal patterns of atmospheric circulation,
consisting of the prevailing surface winds
(the easterly trade winds in the tropics and the
westerlies in the temperate zones). The boundaries
between these zones are either low-pressure zones
of rising air (the intertropical conversion zone,
ITCZ, and the polar front) or high-pressure zones of
subsiding air (the subtropical high pressure belt and
the poles).
2.6). The Ferrell cell is actually the long-term
average transport caused by weather systems in
the mid-latitudes rather than a stable permanent
atmospheric feature. The chaotic motion
of these mid-latitude weather systems creates a
net poleward transport of heat. These three
cells subdivide the atmosphere into three distinct
circulations: tropical air masses between
the equator and 30° N and S, temperate air
masses between 30 and 60° N and S, and polar
air masses between 60° N and S and the poles
(Fig. 2.6). The latitudinal location of these
cells moves seasonally in response to latitudinal
changes in surface heating by the sun.