Download The North-South Circulation

The North-South Circulation
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In terms of winds, the big story is the jet streams, which blow west
to east, mostly in the upper troposphere
But on top of this, we also have a weaker north-south (or
meridional) part of the winds as well, consisting of three
overturning circulation cells
tropopause
Polar
cell
North Pole
Ferrel
cell
Hadley
cell
Equator
The North-South Circulation
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In terms of winds, the big story is the jet streams, which blow west
to east, mostly in the upper troposphere
But on top of this, we also have a weaker north-south (or
meridional) part of the winds as well, consisting of three
overturning circulation cells
Of the three, the Hadley cell is by far the strongest
- i.e., the Ferrel and polar cells are relatively weak
warm air
rising
cold air
sinking
Polar cell
Ferrel cell
Hadley cell
Definition: a circulation cell in which warmer air rises and colder
sinks is called a thermally direct circulation
- a cell in which colder air rises and warmer air sinks is
thermally indirect
Definition: a circulation cell in which warmer air rises and colder
sinks is called a thermally direct circulation
- a cell in which colder air rises and warmer air sinks is
thermally indirect
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A thermally direct cell is the natural state of affairs, so it's easy
to explain (warm air rises, cold air sinks)
By contrast, a thermally indirect cell goes against the grain, so
it needs some kind of extra forcing to keep it going
What about our three cells? Which kind are they?
Definition: a circulation cell in which warmer air rises and colder
sinks is called a thermally direct circulation
- a cell in which colder air rises and warmer air sinks is
thermally indirect
Polar
cell
Hadley
cell
Ferrel
cell
Equator
North Pole
Definition: a circulation cell in which warmer air rises and colder
sinks is called a thermally direct circulation
- a cell in which colder air rises and warmer air sinks is
thermally indirect
colder air
sinking
Polar
cell
Hadley
cell
Ferrel
cell
Equator
North Pole
warmer air
rising
Definition: a circulation cell in which warmer air rises and colder
sinks is called a thermally direct circulation
- a cell in which colder air rises and warmer air sinks is
thermally indirect
The Hadley and Polar cells are thermally direct!
Definition: a circulation cell in which warmer air rises and colder
sinks is called a thermally direct circulation
- a cell in which colder air rises and warmer air sinks is
thermally indirect
And what about the Ferrel cell?
Definition: a circulation cell in which warmer air rises and colder
sinks is called a thermally direct circulation
- a cell in which colder air rises and warmer air sinks is
thermally indirect
thermally
direct
thermally
direct
Ferrel
cell
Equator
North Pole
Definition: a circulation cell in which warmer air rises and colder
sinks is called a thermally direct circulation
- a cell in which colder air rises and warmer air sinks is
thermally indirect
colder part
rising
thermally
direct
warmer part
sinking
thermally
direct
Ferrel
cell
Equator
North Pole
Definition: a circulation cell in which warmer air rises and colder
sinks is called a thermally direct circulation
- a cell in which colder air rises and warmer air sinks is
thermally indirect
The Ferrel cell is thermally indirect!
Definition: a circulation cell in which warmer air rises and colder
sinks is called a thermally direct circulation
- a cell in which colder air rises and warmer air sinks is
thermally indirect
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Again, a thermally direct cell is the normal state of affairs,
while a thermally indirect cell needs some kind of extra
forcing
In our case, the forcing that drives the Ferrel cell is, well....it's
complicated (and maybe not 100% understood)
- basically, it's driven by lots of high and low pressure systems
acting together, stirring up the atmosphere
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So we've talked about the jet streams and the three circulation
cells, but we're still missing an important part of the story: the
winds at the ground!
For the most part, the winds at the ground are determined by
the three circulation cells, but modified by the Coriolis force
Polar
cell
Ferrel
cell
Equator
North Pole
Hadley
cell
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So we've talked about the jet streams and the three circulation
cells, but we're still missing an important part of the story: the
winds at the ground!
For the most part, the winds at the ground are determined by
the three circulation cells, but modified by the Coriolis force
Polar
cell
Ferrel
cell
Hadley
cell
Coriolis turns the
wind toward west
●
●
So we've talked about the jet streams and the three circulation
cells, but we're still missing an important part of the story: the
winds at the ground!
For the most part, the winds at the ground are determined by
the three circulation cells, but modified by the Coriolis force
Polar
cell
Ferrel
cell
Coriolis turns the
wind toward east
Hadley
cell
Coriolis turns the
wind toward west
●
●
So we've talked about the jet streams and the three circulation
cells, but we're still missing an important part of the story: the
winds at the ground!
For the most part, the winds at the ground are determined by
the three circulation cells, but modified by the Coriolis force
Polar
cell
Coriolis turns the
wind toward west
Ferrel
cell
Coriolis turns the
wind toward east
Hadley
cell
Coriolis turns the
wind toward west
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●
So we've talked about the jet streams and the three circulation
cells, but we're still missing an important part of the story: the
winds at the ground!
For the most part, the winds at the ground are determined by
the three circulation cells, but modified by the Coriolis force
But remember: toward the west is called easterly (i.e., from
the east)........and toward the east is westerly!
easterlies in
the arctic
westerlies in
mid-latitudes
easterlies in
the tropics
Some terminology:
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The easterly (from the east) winds in the Hadley cell are called
the trade winds
The westerly winds in the Ferrel cell are called the mid-latitude
westerlies
The easterly winds in the polar cell are called the .....(wait for
it)....... polar easterlies
Both at the equator and in between the Hadley and Ferrel cells,
the winds tend to be weak
- called the doldrums in the tropics and the horse latitudes
between the Hadley and Ferrel cells (near 30o)
polar easterlies
mid-latitude
westerlies
horse latitudes
trade winds
doldrums
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Finally, as you might expect, the vertical motions in our three cells
lead to differences in mean precipitation (see next page)
- Remember: upward motions lead to more precipitation,
downward motions less
Polar
cell
Ferrel
cell
Equator
North Pole
Hadley
cell
polar front
subtropical highs
(deserts)
intertropical
convergence zone
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By far, the heaviest precipitation is found in the upward part of
the Hadley cell (near the equator)
- called the intertropical convergence zone, where the
surface winds come together
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The least precipitation is found in the downward part of the
Hadley cell (where it meets the Ferrel cell) near 30o
- this is where most of the world's deserts are found
Polar
cell
North Pole
Ferrel
cell
Hadley
cell
Equator
polar front
intertropical
convergence zone
polar front
intertropical
convergence zone
So we have the east-west jets and the north-south circulation
cells. But is that the whole story?
So we have the east-west jets and the north-south circulation
cells. But is that the whole story? Of course not.....
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On top of the jets and cells, we have a series of semipermanent high and low pressure regions caused by
mountains and land-sea contrasts
Semi-permanent high and low pressure regions in winter......
......and in summer
So we have the east-west jets and the north-south circulation
cells. But is that the whole story? Of course not.....
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And then we also have our day-to-day weather systems
So to summarize, we have (from big to small)......
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the jet streams in the upper troposphere. And on top of that,
we have.........
So to summarize, we have (from big to small)......
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the jet streams in the upper troposphere. And on top of that,
we have.........
the three north-south circulation cells. And on top of that,
we have..........
So to summarize, we have (from big to small)......
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the jet streams in the upper troposphere. And on top of that,
we have.........
the three north-south circulation cells. And on top of that,
we have..........
a series of semi-permanent high and low pressure regions.
And on top of that, we have.........
So to summarize, we have (from big to small)......
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the jet streams in the upper troposphere. And on top of that,
we have.........
the three north-south circulation cells. And on top of that,
we have..........
a series of semi-permanent high and low pressure regions.
And on top of that, we have.........
our day-to-day weather systems