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Basic
Structures and Dynamics |
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General
Circulation in the Troposphere |
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General
Circulation in the Stratosphere |
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Thermally Direct Cells (Hadley and Polar Cells) |
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Both
cells have their rising branches over warm temperature zones and sinking
braches over the cold temperature zone. Both cells directly convert thermal
energy to kinetic energy. |
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Thermally Indirect Cell (Ferrel Cell) |
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This
cell rises over cold temperature zone and sinks over warm temperature zone.
The cell is not driven by thermal forcing but driven by eddy (weather
systems) forcing. |
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Yes and
No! |
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(Due
to sea-land contrast and topography) |
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Yes: the
three-cell model explains reasonably well the surface wind distribution in
the atmosphere. |
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No:
the three-cell model can not explain the circulation pattern in the upper
troposphere. (planetary wave motions are important here.) |
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¶U/¶z µ ¶T/¶y |
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The
vertical shear of zonal wind is related to the latitudinal gradient of
temperature. |
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Jet
streams usually are formed above baroclinic zone (such as the polar front). |
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TBO is referred to the tendency that years with
above normal monsoon rainfall tend to be followed by ones with below normal
rainfall and vice versa. |
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The greatest production of ozone occurs in the
tropics, where the solar UV flux is the highest. |
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However, the general circulation in the
stratosphere transport ozone-rich air from the tropical upper stratosphere
to mid-to-high latitudes. |
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Ozone column depths are highest during
springtime at mid-to-high latitudes. |
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Ozone column depths are the lowest over the
equator. |
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Quasi-Biennial Oscillation (QBO) |
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Sudden
Warming: in Northern Pole |
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Ozone
Hole: in Southern Pole |
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Every
other year or so the normal winter pattern of a cold polar stratosphere
with a westerly vortex is interrupted in the middle winter. |
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The
polar vortex can completely disappear for a period of a few weeks. |
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During
the sudden warming period, the stratospheric temperatures can rise as much
as 40°K in a few days! |
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Planetary-scale waves propagating from the troposphere
(produced by big mountains) into the stratosphere. |
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Those
waves interact with the polar vortex to break down the polar vortex. |
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There
are no big mountains in the Southern Hemisphere to produce planetary-scale
waves. |
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No (?)
sudden warming in the southern polar vortex. |
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The decrease in ozone near the South Pole is
most striking near the spring time (October). |
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During the rest of the year, ozone levels have
remained close to normal in the region. |
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In winter the polar stratosphere is so cold (-80°C
or below) that certain trace atmospheric constituents can condense. |
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These clouds are called “polar stratospheric
clouds” (PSCs). |
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The particles that form typically consist of a
mixture of water and nitric acid (HNO3). |
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The PSCs alter the chemistry of the lower
stratosphere in two ways: |
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(1)
by coupling between the odd nitrogen and chlorine cycles |
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(2)
by providing surfaces on which heterogeneous reactions can occur. |
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Long Antarctic winter (May through September) |
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The stratosphere is cold enough to form PSCs |
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PCSs deplete odd nitrogen (NO) |
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Help convert unreactive forms of chlorine
(ClONO2 and HCl) into more reactive forms (such as Cl2). |
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The reactive chlorine remains bound to the
surface of clouds particles. |
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Sunlight returns in springtime (September) |
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The sunlight releases reactive chlorine from the
particle surface. |
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The chlorine destroy ozone in October. |
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Ozone hole appears. |
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At the end of winter, the polar vortex breaks
down. |
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Allow fresh ozone and odd nitrogen to be brought
in from low latitudes. |
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The ozone hole recovers (disappears) until next
October. |
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Notes