Thermal Circulations and the 3-cell Model of the Earth's Global Scale Circulation Pattern

Just as with water draining from a toilet bowl or a sink, there are small scale circulations where the the Pressure Gradient force is so much stronger than the Coriolis force that the CF can be ignored. 

The pressure differences are created by differences in temperature (such as you might find between a coast and the ocean or between a city and the surrounding country side). The horizontal pressure gradient can then produce a wind flow pattern known as a thermal circulation. 

By applying some of the concepts we learned earlier in the semester we can really understand pretty thoroughly how thermal circulations develop.





We'll start here along a sea coast.  In this picture the air temperatures and pressures on both sides of the picture are the same.

Next we'll warm part of the picture and see what effect that has.




Let's warm the left side of the picture.  A beach will often become much warmer than the nearby ocean during the day (the sand gets hot enough that it is painful to walk across in bare feet). 

The air above the warm sand will warm and expand upward.  Note how the 900 mb level has moved upward on the warm side of the picture.  We find 910 mb pressure above the land at the same level where we earlier had 900 mb.

We've left the temperature of the ocean water the same.  The height of the 900 mb level above the ocean hasn't changed.  We have 910 mb pressure over the land at the same altitude as we find 900 mb over the ocean. 
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Here's another way of arriving at the same result.  We need to remember that pressure decreases relatively slowly in warm low density air.  There is only a 90 mb drop between the ground and the green line on the left side of the picture.  Pressure decreases more rapidly with altitude (a 100 mb drop) in the cooler higher density air on the right side.  We end up with the same upper level pressure gradient.




The temperature differences have created an upper level pressure gradient (pressure difference), higher pressure (910 mb) on the left and lower pressure (900 mb) on the right.  The resulting PGF causes air to start to blow from left to right.




Once the air aloft begins to move it will change the surface pressure pattern.  The air leaving the top left side of the picture will lower the surface pressure (from 1000 mb to 990 mb).  Adding air at to the upper right side of the picture will increase the surface pressure (from 1000 mb to 1010 mb).  Air motions above the ground have created a surface pressure gradient.  Surface winds will begin to blow from right to left.




You can complete the circulation loop by adding rising air above the surface low pressure at left and sinking air above the surface high at right.  The surface winds which blow from the ocean onto land are called a sea breeze (meteorologists always try to specify where the wind is coming from).  Since this air is likely to be moist, cloud formation is likely when the air rises over the warm ground.  Rising air expands and cools.  If you cool moist air to its dew point, clouds form.

It is pretty easy to figure the directions of the winds in a thermal circulation without going through a long-winded development like this.  Just remember that warm air rises.  Draw in a rising air arrow above the warm part of the picture, then complete the loop.

At night the ground cools more quickly than the ocean and becomes colder than the water.  Rising air is found over the warmer ocean water (sea below).  The thermal circulation pattern reverses direction.  Surface winds blow from the land out over the ocean.  This is referred to as a land breeze.



I've stuck some additional examples at the end of this section just in case you're interested.  You'd won't need to be familiar with that material in order to answer questions on the Optional Assignment.

Next we will use basic concept of a thermal circulation to learn about global scale pressure and wind patterns.  Ordinarily you couldn't apply a small scale phenomena like a thermal circulation to the much larger global scale.  However if we make some simplifying assumptions, particularly if we assume that the earth doesn't rotate or only rotates slowly, we can ignore the Coriolis force and a thermal circulation could become established.

Some additional simplifications are also made and are listed below.
 


Because the earth isn't tilted, the incoming sunlight shines on the earth most directly at the equator.  The equator will become hotter than the poles.  By allowing the earth to rotate slowly we spread this warmth out in a belt that circles the globe at the equator rather than concentrating it in a spot on the side of the earth facing the sun.  Because the earth is of uniform composition there aren't any temperature differences created between oceans and continents.

If you remember the short cut that applies to thermal circulations "warm air rises" you can drawn a rising arrow above the equator.  Then you just complete the loop.  Actually there are two loops, one in the northern hemisphere and the other in the southern hemisphere.   This is the 1-cell model or 1-cell representation of the earth's global scale circulation pattern.  The term one cell just means there is one complete loop in the northern hemisphere and another in the southern hemisphere.

Next we will remove the assumption concerning the rotation of the earth.  We won't be able to ignore the Coriolis force now.


Here's what a computer would predict you would now see on the earth.  Things are pretty much the same at the equator in the three cell and one cell models: surface low pressure and rising air.  At upper levels the winds begin to blow from the equator toward the poles.  Once headed toward the poles the upper level winds are deflected by the Coriolis force.  There end up being three closed loops in the northern and in the southern hemispheres.  There are surface belts of low pressure at the equator (the equatorial low) and at 60 degrees latitude (the subpolar low). There are belts of high pressure (the subtropical high) at 30 latitude and high pressure centers at the two poles (the polar highs).

We're going to concentrate on just the surface features (pressure belts and the winds that develop in between).  Even with the unrealistic simplifying assumption, much of what is predicted by the 3-cell model is actually found on the earth.

Here's a map view of the region between 30 S and 30 N latitude.  We're looking down from above at the earth's surface.


There's a lot of information on this picture, but with a little study you should be able to start with a blank sheet of paper and reproduce this figure.  I would suggest starting at the equator.  You need to remember that there is a belt of low pressure found there.  Then remember that the pressure belts alternate:  there are belts of high pressure at 30 N and 30 S.

Let's start at 30 S.  Winds will begin to blow from High pressure at 30 S toward Low pressure at the equator.  Once the winds start to blow they will turn to the left because of the Coriolis force.  Winds blow from 30 N toward the equator and turn to the right in the northern hemisphere (you need to turn the page upside down and look in the direction the winds are blowing).  These are the Trade Winds (northeasterly trade winds north of the equator and southeasterly trades south of the equator).  They converge at the equator and the air there rises (refer back to the cross sectional view of the 3-cell model). This is the cause of the band of clouds that you can often see at or near the equator on a satellite photograph.

The Intertropical Convergence Zone or ITCZ is another name for the equatorial low pressure belt.  This region is also referred to as the doldrums because it is a region where surface winds are often weak.  Sailing ships would sometimes get stranded there hundreds of miles from land.  Fortunately it is a cloudy and rainy region so the sailors wouldn't run out of drinking water (they might well have run out of rum though which might have been worse).
  
Hurricanes form over warm ocean water in the subtropics between the equator and 30 latitude.  Winds at these latitudes have a strong easterly component and hurricanes, at least early in their development, move from east to west.  Middle latitude storms found between 30 and 60 latitude, where the prevailing westerly wind belt is found, move from west to east.

You find sinking air, clear skies, and weak surface winds associated with the subtropical high pressure belt.  This is also known as the horse latitudes.  Sailing ships could become stranded there also.  Horses were apparently either thrown overboard (to conserve drinking water) or eaten if food supplies were running low (at least that's the story I've heard, Wikipedia has a different explanation).  Note that sinking air is associated with the subtropical high pressure belt so this is a region on the earth where skies are clear (Tucson is located at 32 N latitude, so we are strongly affected by the subtropical high pressure belt).

The winds to the north of 30 N and to the south of 30 S are called the "prevailing westerlies."  They blow from the SW in the northern hemisphere and from the NW in the southern hemisphere. The 30 S to 60 S latitude belt in the southern hemisphere is mostly ocean.  Because there is less friction over the oceans, the prevailing westerlies there can get strong, especially in the winter.  They are sometimes referred to as the "roaring 40s" or the "ferocious 50s" (the 40s and 50s refer to the latitude belt they are found in).




Here's another surface map, it's a little simpler because we're just looking from about 30 N to a little bit past 60 N.  Winds blowing north from H pressure at 30 N toward Low pressure at 60 N turn to the right and blow from the SW.  These are the "prevailing westerlies."   The polar easterlies are cold winds coming down from high pressure at the north pole.  The subpolar low pressure belt is found at 60 latitude.  This is also a convergence zone where the cold polar easterly winds and the warmer prevailing westerly winds meet.  The boundary between these two different kinds of air is called the polar front and is often drawn as a stationary front on weather maps.  A strong current of winds called the polar jet stream is found overhead.  Strong middle latitude storms will often form along the polar front.

Now a quick comparison of predicted 3-cell model features and corresponding features found in the real world.



The 3-cell model assumes that the earth is of uniform composition and not tilted toward or away from the sun.  It predicts belts of high pressure at 30 N and 30 S latitude as shown above at left.  Because the real world has oceans and continents we find centers of high pressure, not belts, located near 30 latitude.  They move north and south of 30 degrees during the year as the N. Pole tilts toward and away from the sun.

This is shown a little more detail on the figure below.


The high pressure center off the East Coast of the US is called the Bermuda High.  The Pacific High is found off the west coast.  Don't worry about the names of the Highs off the east and west coasts of South America.

Winds blowing around these centers of high pressure create some of the world's major ocean currents. Clockwise winds around the Pacific High create the California current, a cold southward flowing current found off the west coast of the US.  The Gulf Stream is the warm northward flowing current along the east coast.

The El Nino phenomenon is described at the end of this section.  Ocean water in the equatorial Eastern Pacific is normally cold.  The water warms as it moves westward.  This temperature pattern is shown above.  You can now better understand why this is true.  Two cold ocean currents, the California current north of the equator and its analog in the southern hemisphere meet at the equator in the eastern Pacific.  That is why the water there is so cold.  During an El Nino event the two cold ocean currents stop short of the equator.  The ocean water temperature pattern basically reverses.  This has a profound effect on weather around the globe.

The following two pictures show how the 3-cell models features move during the season.



This is a winter picture (northern hemisphere winter, the North Pole is tilted away from the sun).  All of the 3-cell model features have moved south of their nominal locations.  The intertropical convergence zone (ITCZ) which is normally at the equator has moved south of the equator.


Here's the summer picture (North Pole tilted toward the sun).  The ITCZ has moved north of the equator.

The movement of the Pacific High north and south of its nominal position near 30 degrees latitude is part of what causes our summer monsoon in Arizona.



In the winter the Pacific High is found south of 30 N latitude (the bottom of the figure above).  Winds to the north of the high blow from the west.  Air originating over the Pacific Ocean is moist (though the coastal water is cold so this air isn't as moist as it would be if it came off warmer water).  Before reaching Arizona the air must travel over high mountains in California.  The air loses much of its moisture as it does this (remember the rain shadow effect).  The air is pretty dry by the time it reaches Arizona.  Significant winter rains occur in Arizona when storms systems are able to draw moist subtropical air from the southwest Pacific ocean into Arizona.

 During the summer, the Pacific High moves north of 30 N latitude.  Winds on the southern side of the subtropical high have an easterly component.   Moist air originating in Mexico and from over warm water in the Gulf of Mexico blows into Arizona.  The sun heats the ground during the day, warm moist air in contact with the ground rises and produces convective thunderstorms.

Tucson gets about 12 inches of rain in a normal year.  About half of this comes during the "summer monsoon" season.  The word monsoon, as you can learn below, refers to a seasonal change in wind direction.  It is often used incorrectly in S. Arizona to refer to a thunderstorm.

That's the end of the material you'll need to read to be able to answer the questions on the Optional Assignment.  There is some additional material below - examples where remembering the basic concept of a thermal circulation can help you understand a variety of other wind circulation patterns.





Cities are often warmer than the surrounding countryside, especially at night.  This is referred to as the urban heat island effect.  This difference in temperature can create a "country breeze."  This will sometimes carry pollutants from a factory or odors from a sewer treatment plant located outside the city back into town.


The Asian monsoon is a large scale circulation pattern and is much more complex than a simple thermal circulation.  However you can use the thermal circulation concept to get a general understanding of what to expect at different times of the year.  Before looking at that let's be clear about the meaning of the term monsoon.



Monsoon just refers to a seasonal change in the direction of the prevailing winds.  Most of the year in Arizona winds come from the west and are dry.  For 2 or 3 months in the summer winds come from the south and southeast.  This is when we get our summer thunderstorm season or summer monsoon. 



In the summer land masses in India and Asia become warmer than the oceans nearby.  Surface low pressure forms over the land, moist winds blow from the ocean onshore, and very large amounts of rain can follow.  A map view (top view) is shown at left, a cross sectional view is shown at right (it really just resembles a large scale sea breeze).



The winds change directions in the winter when the land becomes colder than the ocean.

You can also use the thermal circulation to understand some of the basic features of the El Nino phenomenon (you find a discussion of the El Nino on pps 135-139 in the photocopied Classnotes). 

First here is what conditions look like in the tropical Pacific Ocean in normal non-El Nino years (top and side views again)




Cold ocean currents along the west coasts of N. America and S. American normally converge at the equator and begin to flow westward (see top view above).  As the water travels westward it warms.  Some of the warmest sea surface waters on earth are normally found in the western Tropical Pacific (this is also where hurricanes are most frequent).  A temperature gradient becomes established between the W. and E. ends of the tropical Pacific. The cross sectional view above shows the normal temperature and circulation pattern found in the equatorial Pacific Ocean.   You would find surface high pressure in the east and low pressure in the west.  Note that the wind circulation pattern is the same as the simple thermal circulation we studied above.

During a La Nina event, waters in the Eastern Pacific are even colder than normal.  This generally produces drier than normal conditions during the winter in the desert SW.  You can read more about La Nina here.

Every few years El Nino conditions occur and the cold currents don't make it to the Equator.  Warm water is carried from the western Pacific to the eastern Pacific.  The temperature and pressure basically reverses itself.



Now surface high pressure is found in the west and surface low pressure and rising air is found in the E. Pacific (the reversal in the surface pressure pattern is referred to as the southern oscillation).  Indonesia and Australia often experience drought conditions (and devastating wildfires) during El Nino years.  In the desert SW we expect slightly wetter than normal conditions (perhaps 20% wetter than normal).  Wetter conditions are also found in California and in the SE US.

Here's a map showing the effects of El Nino and La Nina conditions on winter weather in N. America (source).