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Global circulation

The next lectures are on the global circulation of the atmosphere.

The main focus is on two questions:

What drives the atmospheric circulation

What is the large scale structure of the circulation

In the vertical

In the horizontal

What drives the atmospheric circulation ?

The short answer is: Differential heating.

What does that mean?

The heating of the sun is most intensive in the tropics, but least intensive in the polar regions.

This means that the air in the tropics is much warmer than the air in the polar regions.

Since this means that the air at the equator should be less dense than the air at the poles.

But such a situation is unstable

The cold air should flow under the warm air.

What drives it ? (II )

Thus, we might envision an equilibrium situation ....

...where at the surface cold air flows towards the equator ...
...where it is heated and it rises up in the atmosphere,...
... and aloft it flows back to the polar regions.

Flow like this is called a thermally direct circulation .

If you fill a bathtub with water and cool one side of it but heat the other side, you will set up a thermally direct circulation.

What drives it ? (III)

If the earth did not rotate, and had no topography, presumably the global circulation would consist of one thermally direct cell in either hemisphere.

However, the earth does rotate, and it has topography (continents mountain ranges etc)

We will now look at the effects of these factors.

What does rotation do?

Rotation will make the flow go in a zonal direction (along latitude circles, east to west) instead of a meridional direction (north to south)...

But zonal flow will not reduce the north south temperature gradient

If the temperature gradient is strong enough then the zonal circulation will become unstable and break up.

The circulation in each hemisphere breaks into three cells.
The Hadley cell
The Ferrel cell
and the Polar cell

The three cells: the vertical view

The Hadley cell is characterised by rising air at the equator.

Once aloft, the air flows poleward to about latitude 30 (S and N) where it sinks to the surface and there is a return equatorwards surface flow.

This is a thermally direct circulation.

The Ferrel cell has descending at about latitude 30, poleward surface flow to about 60 degrees (the polar front) where the air rises and returns equatorwards aloft.

This is NOT a thermally direct cell.

The polar cell has rising air at the polar front, flow aloft to the pole, where the air sinks and returns at the surface back to the polar front.

The 3 cells: the horizontal view

The windbelts

In the Hadley cell we have surface air flowing to equatorwards.

Deflection by the coriolis force leads to an easterly flow. These are the tradewinds.

Easterly: means FROM east TO west...

Easterly in BOTH hemispheres! (CF is to right in NH and to left in SH)

In the Ferrel cell the surface flow is poleward, and the deflection leads to the westerly winds, the mid-latitude westerlies

In the polar cell the flow is equatorwards at the surface which gives rise to easterly winds. (the polar easterlies)

The surface pressure field

From the Convergence/Divergence discussion the following rule of thumb could be made:

Rising motion (and divergence aloft) leads to low surface pressure.

Descending motion (and convergence aloft) leads to high surface pressure.

The global circulation has:

Rising motion at the equator (we expect low surface pressure)

Descending at latitude 30 (we expect high surface pressure)

Rising motion at the polar front (low pressure)

Sinking motion at the poles (high pressure)

The surface pressure field II

The high pressure systems at latitude 30 are called the sub-tropical highs

An example is the Bermuda-Azores high.

This is a region of clear skies and calm winds (the horse latitudes)

The low pressure system near the polar front are called the sub-polar lows

The Aleutian low and the Icelandic low.

These features are called the semi permanent pressure systems since they vary seasonally.

Lets look at the seasonal variability in more detail.

Seasonal variability of the mean surface pressure field.

WINTER:
Over the continents strong anticyclones develops in the winter time. This is especially true over the Asian continent, where the massive Siberian high can get very strong.

This is due to the cooling in the continental interior.

Also, in the wintertime the sub-polar lows deepen.

This is due to the increase in equator to-pole temperature gradient during the wintertime. This gradient is the "thermal engine" for the development of sub-polar cyclones.

Seasonal variability of the mean surface pressure field II

SUMMER:

In the summer time the continents warm up and the anticyclone vanishes. In many locations thermal lows (cyclones) appear (especially in arid and semi-arid areas)

At the same time the equator to pole temperature gradient is reduced and there is less energy to form sub-polar cyclones on the polar front.

The Aleutian low vanishes in the summer time.

The inter-tropical convergence zone (ITCZ)

Lets go back and look at the tropics and the Hadley cell.

Where the two Hadley cells meet in the tropics there is a zone of very strong convergence at the surface, and subsequently of rising motions, cloud formation and precipitation.

Where the two cells meet is called the "inter tropical convergence zone" or ITCZ for short.

The ITCZ is sometimes called "the meteorological equator", as it is there where southern and northern hemispheric winds come together.

The ITCZ moves with the sun...
it is north of the equator during northern hemispheric summer and south of the equator during northern hemispheric winter. These shifts impact the monsoon (more on this later)..

Mid-latitude westerlies.

Well above the surface the winds in the mid latitudes generally flow from west to east, without any interference from surface friction.

However this flow is not quite stable and generally exhibits a pattern of long waves encircling the globe.

These are the so called Rossby waves
They show up very clearly in the pressure field above the 500 mb level (which is about 5.5 km up).
The flow due to Rossby waves has both a meridional (north-south) and a zonal (east-west) component to it

Meridional vs. Zonal

The upper air winds greatly influence the mid latitude weather systems.
If the flow aloft is zonal, cold air masses to the north and warm air masses to the south do not interact with one another.

If the flow is meridional, the cold air masses to the north will be advected southwards and the warm air masses to the south will be advected northwards.

This will lead to the formation of strong temperature fronts which can lead to cyclone formation.

The upper air flow can sometimes become extremely meridional in character.

This can lead to blocking lows or blocking highs (also known as blocks)

Often they persist for weeks at a time.

The Jets

Think of the polar front...

On the polar side of it the air is cold (and hence dense) , but warmer on the other side (and hence less dense)

Pressure will drop with height more rapidly in a cold air mass than it does in a warm air mass.

Thus the pressure gradient across the front becomes stronger aloft.

That implies a stronger PGF and hence a faster flow.

This leads to the formation of narrow current of rapidly flowing air. This is the polar jet stream.

The polar jet stream

The polar jet stream meanders along the mid latitude Rossby waves.

It is not uniform, but most often consists of jet streaks, areas where the upper level wind speed is greater. The streaks can move rapidly along the Rossby waves.

Where the wind aloft speeds up divergence aloft will occur, but where they slow down convergence occurs.

These will then force rising or sinking motion of air which tends to decrease or increase the surface pressure.

Thus the jet stream can have a strong impact on the formation of weather systems at the surface.

Other jet streams

The polar jet is not the only jet stream

Where the Hadley and Ferrel cells meet another jet stream forms. This one is called the sub-tropical jet.

It is less variable than its polar counterpart.

It is stronger than the polar jet.

There are also other jets, such as the summertime tropical easterly jet in North Africa, India and Southeast Asia

Also, nocturnal jets, form in many places at night (that's what nocturnal means!)

Short and Long waves

Superimposed on the longwave pattern in the mid-latitude westerlies, there are always shorter waves, that travel rapidly along the other longer waves.

Like the polar jet these short waves have a significant impact on the development of weather systems.

Remember from our discussion of the gradient wind that flow around a high pressure system is super-eostrophic (i.e. faster than the geostrophic wind) whereas flow around a low pressure system is sub-geostrophic (i.e. slower than geostrophic).

This indeed is also true for wind flowing along a wave pattern. On crest of the wave the wind behaves as it is flowing around an anti-cyclone (a High) but at the trough of the wave it behaves as flowing around a cyclone (a Low).

So what?

When the upper level winds go from being sub-geostrophic to super-geostrophic they speed up!

Likewise they slow down when they go the other way.

Respectively, this leads to either divergence or convergence aloft.

But convergence aloft leads to sinking motion and a higher surface pressure.

Divergence aloft leads to rising motion and lower surface pressures.