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The North-South Circulation ● ● 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 ● ● ● 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 ● ● 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 ● ● 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 ● ● 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 ● ● 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 ● ● 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: ● ● ● ● 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 ● 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 ● 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 ● 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..... ● 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..... ● And then we also have our day-to-day weather systems So to summarize, we have (from big to small)...... ● the jet streams in the upper troposphere. And on top of that, we have......... So to summarize, we have (from big to small)...... ● ● 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)...... ● ● ● 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)...... ● ● ● ● 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