Earth 540: Essentials of Oceanography for Educators
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Lesson 4

The Global Thermostat. The Ocean-Atmosphere-Climate Connection

About Lesson 4

Contact the instructor if you have difficulty viewing this image
Source: Broecker, 1991, in Climate change 1995, Impacts, adaptations and mitigation of climate change, UNEP and WMO, Cambridge press university, 1996

In this lesson, we have the ambitious task of tackling the heat distrubution on Earth and it's role in dictating ocean circulation, atmospheric circulation, and Earth's climate.

By the end of this lesson you should have a deeper understanding of the role of the oceans and atmosphere in heat transfer on Earth, the structure of the oceans including surface water and deep water, thermo-haline circulation including patterns and the factors that drive flow, the Coriolis effect and Ekman transport, large-scale atmospheric circulation, oceanic gyres, and boundary currents in the ocean basins.

This figure incorporates many of the topics we will cover in Lesson 4. Note the Gulf stream in the North Atlantic and the sea-to-air heat transfer associated with it. Also note the three dimensional nature of ocean current flow (the conveyor), which includes warm surface currents and deep currents.

 

What will we learn in Lesson 4?

By the end of Lesson 4, you should be able to:

  • Describe the solar energy budget for Earth including variations with latitude of energy input and output.
  • Explain how heat is transferred from low to high latitudes by the oceans and the atmosphere.
  • Explain the roles of latent heat and sensible heat in energy transfer by ocean currents.
  • Describe the pattern of global winds and its relationship to heat transfer.
  • Describe the primary structure of ocean waters as a function of depth, including the changes in average temperature and salinity.
  • Explain the main factors that determine surface and deep ocean currents.
  • Explain the Coriolis effect, it's origin, and it's influence on global winds and ocean currents
  • Describe how the Coriolis effect and Ekman transport influence ocean surface currents
  • Describe the 'ocean conveyor' and it's role in global heat transfer and climate regulation
  • Use on-line resources to construct maps of ocean temperature and salinity

What is due for Lesson 4?

The chart below provides an overview of the requirements for Lesson 4. For assignment details, refer to the lesson page noted. See the Course Schedule (located in the Resources menu to the left) for assignment due dates.

REQUIREMENT

LOCATION

SUBMITTED FOR GRADING?

Activity 1: Pressure Belts

page 1 Yes -   Due 5 March

Activity 2: Ocean Currents and Density

page 4 Yes -   Due 5 March

Activity 3: Gyres and Surface Currents

 

page 5

Yes -   Due 5 March

Questions?

Post them to our Questions? discussion forum on ANGEL or get in touch by email.

Overview, Solar Energy, Pressure, & Wind Belts

The connections between atmospheric circulation, ocean currents and Earth's climate are both simple and wonderfully complex. We could easily spend the whole semester studying these topics and developing the background for ocean circulation, but we won't. Instead, we'll hit the high points and build a background for understanding ocean currents and thermo-haline circulation. It makes sense to start with atmospheric circulation because ocean surface currents are driven by wind. The basic pattern of surface ocean circulation, including the large-scale gyres, is caused by wind and the factors that produce large-scale wind patterns. We need to start by seeing how solar energy input on Earth and poleward transport of heat by winds sets up large-scale atmospheric cells that produce Easterly and Westerly winds in three main bands from the equator to the poles. The Coriolis effect, produced by Earth's rotation and conservation of momentum, plays an important role here. Ocean structure also plays an important role and we need to look at density driven circulation in the ocean as produced by temperature and salinity: so-called thermo-haline circulation. Ocean circulation is three-dimensional and both surface currents and deep water currents play an important role in heat transport. Large-scale ocean currents are impacted by the Coriolis effect, and depth-variations in coupling between surface flow and wind stress produce another important effect, known as Ekman transport, which we will discuss. Ultimately, when we put all the pieces together, the goal is to arrive at a coherent picture of how solar energy, winds, and ocean circulation combine to produce a global thermostat for Earth.


It's all about Blowing Hot Air

This image shows large-scale atmospheric circulation on Earth. Note the three-dimensionality of the circulation. On the right-hand side of the image the arrows show vertical and horizontal air flow as part of six convection cells, three in each of the Northern and Southern hemispheres. The central part of the image shows surface winds blowing from the east (the Trade winds) or the west (The Westerlies) --note that the Polar Easterlies are not easily seen in this image.

Contact the instructor if you have difficulty viewing this image
Atmospheric Circulation and the Trade Winds
Source: http://sealevel.jpl.nasa.gov/overview/climate-climatic.html

Hadley circulation  One of the most important things to understand from this image is that everything is driven by unequal solar heating as a function of latitude. The energy from our Sun is focused on the equatorial region and spread comparatively thinly over the polar regions. This is true on an average annual basis, Earth receives more solar energy at the equator than at the poles. As a result, the land, water, and air, over the equator are warm, and air rises over the equator. This warm air rises through the atmosphere and flows poleward as an upper atmosphere wind that is, essentially, an air-mail package of heat known as a Hadley Cell. The image here has arrows showing circulation in a vertical plate along one line of longitude (just to the east of the African continent) but, of course, this circulation takes place everywhere on Earth. Hadley circulation is 3D; heat moves poleward in upper atmosphere winds and there is a return flow from north to south (in the Northern hemisphere) that sets up the Northeasterly Trade winds (with the help of the Coriolis effect, as we'll see below).

 

Pressure belts  The Hadley Cell is set up by rising air over the equator. The warm rising air causes low pressure at Earth's surface in the equatorial region. By the same token, high pressure is set up at roughly 30 deg. North and 30 deg. South. These belts of high pressure are caused by the descending limbs of the Hadley Cell at those latitudes. In approximate terms, you can think of 30 deg. as the point at which upper atmosphere winds of the Hadley Cell have lost their heat. The cold air becomes dense and falls back to Earth's surface, creatinga belt of high pressure. Similar thinking explains the existance and wind circulation in the Ferrel Cell (labeled the 'mid-latitude' cell in this image) and the Polar Cell.

Reading

Click on the links below. I have provided some guidance about what to focus on, but these are excellent resources, so you are encouraged to read beyond my suggestions.

  • Unequal heating. [1]Why is it warmer over the equator compared to the poles? Think about why it is rainy in the Doldrums and arid in the Horse latitudes
  • Hadley Circulation. [2]What causes the primary pattern of winds on Earth? (sorry, you have to scroll to read all of this)

Activity 1: Pressure Belts

Let's think about the global pattern of atmospheric pressure. Hadley circulation implies a systematic pattern of pressure variation as a function of latitute. One question we could ask involves differences in air masses within a Hadley cell. Referring to the figure above: how do air masses differ between the ascending limbs of a Hadley Cell, over the equator, compared to the descending limb at 30 deg. North?

  • Atmospheric Pressure [3]Start by skimming this page. Look at Figures 7d-1 and 7d-4 and read the associated text.
  • Using these data, let's calculate the average density for air at 30 deg. North compared to that at the equator. You can assume that the atmosphere extends to the 70 km, which is within the Mesosphere. A useful formula for the relationship between pressure, p, density, ρ, gravity, g, and height, h, is: p = ρ g h
  • Let's assume that the average air pressure over the equator at Earth's surface is roughly 101.3 kPa (recall: 1 Pa = 1 Newton per square-meter). Does this jive with what you see in figure 7d-4? In this case, what is the average density of air if the column is 70 km tall?
  • Now calculate the density at 30 deg. north, assuming an average surface pressure of 102.5 kPa.
  • Comment on how air pressure varies as a function of time throughout the year (hint, look at the animation on the web page that you just read).
  • Thinking back to what you know about heat capacity and latent heat, do these differences in density make sense relative to your expectations for heat transfer by large-scale winds? Is the air at 30 deg. north likely to be colder/warmer/denser compared to that at the equator?

Save your document as either a Microsoft Word or PDF file in the following format:
L4_Activity1_AccessAccountID_LastName.doc (or .pages or .pdf)
For example, student Elvis Aaron Presley's file would be named "L4_Activity1_eap1_presley.doc".

Put the file in the DropBox under Lesson 4 on Angel.

See the grading rubric [4] for specifics on how this assignment will be graded.

Redistributing Earth's Heat, Wind, & Currents

Heat redistribution

Earth's heat budget exhibits important variations with latitude. Heat input via solar radiation varies systematically with sun angle, and thus when averaged over a year, we find a simple relationship between latitude and heat absorbed, with more heat received in the equitorial region than in polar regions. The figure below shows this variation along with average annual radiation output from Earth. The dashed lines show the points at which input (heat absorbed at Earth's surface) is equal to output (heat lost via long-wavelength radiation).

Here's a useful animation for thinking about sun-angle variations throughout the year [5]. Click on the show Earth profile button, lower left.

The data in this figure have important implications for wind generation and ocean currents because both are driven by temperature differences, and by heat input between the equatorial and polar regions. Winds and ocean currents play a major role in moving the surplus heat from the equatorial regions to the polar regions. Without this heat transfer, the polar regions of Earth would get colder every year and regions between ~ 35 N and 35 S would get warmer every year. Energy transfer occurs via sensible heat and via latent heat. As you'll recall from Lesson 3, latent heat of evaporation for water is very large compared to heat capacity, so it is no surprise that latent heat is the dominant process, with sensible heat transfer accounting for only a few percent of the heat flux, from ocean to atmosphere, associated with latent heat.

 

Contact the instructor if you have difficulty viewing this image
Credit: http://www.aqua.nasa.gov/highlight.php?id=25

 

Wind and ocean currents

Winds and ocean currents play a big role in redistributing heat.

This figure shows sea surface wind speed, as derived from a NASA's satellite (Aqua) for the month of November, 2004. The large-scale pattern of surface ocean currents is set up by winds. Note the variations with latitude. Look back at the Hadley circulation and think about the location of the easterlies/westerlies compared to what you see here.

 

Contact the instructor if you have difficulty viewing this image
Credit: http://www.aqua.nasa.gov/highlight.php?id=25

 

Reading assignment

Read the link below.

  • The Atlantic heat conveyor slows [6] Are we heading toward The Day After Tommorrow?

Coriolis and Ekman Transport

For large scale winds and ocean currents, the Coriolis effect is an important consideration because it changes the net direction of heat transport.

Coriolis Effect  The Coriolis effect is due to an inertial force that is important in a rotating frame of reference. As you know from Newton's laws of motion, bodies in motion stay in motion unless acted upon by external forces. So, a wind blowing from north to south should go in a straight line, and in fact, it does. The problem is that this straight line does not look straight when referenced to a spinning Earth. There are many on-line resources for the Coriolis effect, so I won't try to duplicate visualizations here, but there are a few things to get straight before looking at a few of them.

1. The Coriolis force is given by the Coriolis acceleration times the mass of the object in motion. This is the familiar F = m a.

2. The Coriolis acceleration is proportional to the cross product between the (linear) velocity of the moving object and the angular velocity of the rotating frame of reference. On Earth, the angular velocity is given by 360 deg/24 Hr. or 2 π (radians) per 24 Hrs, which is equal to 1.454e-4 radians per sec. The formula is: ac = - 2ω x v

3. As you recall from plate tectonics and spherical trigonometry, the magnitude of a cross product like this can be obtained from the magnitude of each vector ( ω and v ) times the sin of the angle between them.

Thus, the magnitude of ac = ω v sinΔ where bold indicates magnitude and Δ is the solid angle between the vectors. For a three-dimensional body, this angle is a solid angle, and we can use the formulas developed from plate tectonics and spherical trigonometry to determine the solid angle. But fortunately, we don't need to use these, because the rotation axis corresponds to Earth's north pole, thus, we can just use the latitude.

If you want more background and examples, I can recommend the wikipedia page [7] which is very well done.

I can also recommend: Coriolis explained [8]

 

The left panel of the figure below is is an excellent summary of how the Coriolis effect (red arrows) combines with pressure driven wind (blue arrows) to produce the anticlockwise rotation (black arrows) around a low pressure system in the Northern Hemisphere What is the wind pattern, and sense of rotation, around a low pressure system in the Southern Hemisphere?

The right panel is a wonderful image of the wind pattern around a Low Pressure system over Iceland. Note the counterclockwise nature of the coiling pattern of the clouds. Does this look familiar from images that you've seen for hurricanes?

Contact the instructor if you have difficulty viewing this image
Sources: Left panel: http://en.wikipedia.org/wiki/Coriolis_effect [9]      Right panel: http://visibleearth.nasa.gov/view_rec.php?id=6204

Reading

Read the link below.

  • Coriolis Effect. [10]Thinking about Hadley circulation (and the image above), why does a north wind (i.e., a wind from the north) in the Northern hemisphere, such as in a Hadley Cell, blow toward the southwest?

Ekman Transport  Water in the ocean can be divided roughly into three regions based on density: surface water, the pycnocline, and deep water. The pycnocline (also called the thermocline or halocline: recall that seawater density (pycno) is determined by temperature (thermo) and salinity (halo).
Winds drive surface currents in the ocean, and these currents are effected by the Coriolis effect. But surface flow causes motion in the water below, so things get a bit more complicated.
The physical properties of Surface Waters vary smoothly from the ocean surface to the pycnocline, but for the purposes of this discussion, it's useful to imagine that the surface zone is composed of several layers of water. The top-most layer is driven by wind. Motion in this layer drives flow in the layer below, and so on, such that wind driven motion at the top, ultimately, drives flow in all of the layers below. Now here's the complication: each one of our 'layers' is influenced by the Coriolis effect. This sets up a spiral, with motion in each successively deeper layer bent somewhat to the right (in the Northern Hemisphere) relative to the forcing from above.

Contact the instructor if you have difficulty viewing this image
Sources: NOAA

Read more about Ekman Transport here [11]

Ocean Circulation: Surface Water and Deep Water

Global ocean currents and thermo-haline circulation

Reading assignment

Read the link below.

  • Broecker, GSA today [12] What If the Conveyor Were to Shut Down? Reflections on a Possible Outcome of the Great Global Experiment

Recall from Lesson 3 that water density increases with increasing salinity and decreasing temperature. Water density varies throughout the ocean and the water at the bottom of the ocean is densest, of course. Ocean-atmosphere interactions have important implications for global ocean currents. For example, think about what would happen if a large scale surface current continually lost heat. The colder it got the denser the water would be, and eventually that water would become dense enough to sink, and become deep water. Now lets add in evaporation. Imagine that water was continually evaporated from our surface current. This would make it saltier, which would increase the density. Eventually, the density would increase enough for the water to sink and become deep water.

Contact the instructor if you have difficulty viewing this image
Source:NOAA

 

This is exactly what happens in the North Atlantic. But, if deep water forms in one place, then surface water has to form somewhere else; we can't push water into the deep ocean without something coming back to the surface, and in fact, surface water forms in various places in the ocean.

Look at the image above, and then back at the figure at the top of page 1 of this lesson. On page 1, the arrows are red for surface currents and blue for deep currents. The colors are a bit hard for me to see, but note that in the North Atlantic, a surface current flows north and a deep current flows south. This means that deep water forms in the North Atlantic.

It's important to appreciate that any Figure drawn at this scale, the entire ocean, is approximate. Don't take everything you see in these figures as the gospel. For a different perspective, have a look at this animation. [13]

The figure below shows the general structure of water masses as a function of depth in the ocean.

Contact the instructor if you have difficulty viewing this image
Source:http://www.onr.navy.mil/Focus/ocean/water/density1.htm

 

Activity 2

Why does surface water sink in the North Atlantic? Let's calculate the density differences between surface waters in the tropics vs. where deep water forms in the north.

Go to this web site and read the information there. [14]

Then click on the links for Temperature and Salinity and pick values in the North Atlantic off the coast of Florida and off the coast of Ireland. For temperature, you'll need to click on "the temperature" and then on "varies mainly with latitude" to get to the image. If you have trouble finding it, click here [15]

For salinity, you can just click on "the salinity of the water" and then on the map.

The colors are a bit tricky to distinguish, but do your best and report the values you chose.

  • 1. Calculate the density of the water masses off the coast of Florida and off the coast of Ireland. Make sure to record your values for temperature and salinity. You can use this web site [16] for the density calculation
  • 2. Comment. Is this about what you expected? How are water densities normally determined in the ocean?
  • 3. Check out the activity dealing with density on this page [17] Use the box below to comment on how well this would work in your classes and how you might improve it.

For numbers 1 and 2, please put your answers in a file and drop that in the dropbox for Lesson 4, Activity 2 on Angel.

For number 3, use the comment box below.

Save your document as either a Microsoft Word or PDF file in the following format:
L4_Activity2_AccessAccountID_LastName.doc (or .pages or .pdf)
For example, student Elvis Aaron Presley's file would be named "L4_Activity2_eap1_presley.doc".

See the grading rubric [4] for specifics on how this assignment will be graded.

Gyres and Surface Currents

The global pattern of winds together with the Coriolis Effect and Ekman Transport produce large-scale currents in the world ocan. Ocean surface currents organize into Gyres that are characterized by circulation at the scale of the ocean basin. The figure below shows the basic pattern. Note that gyres circulate clockwise in the northern Hemisphere and counter-clockwise in the Southern Hemisphere.

Contact the instructor if you have difficulty viewing this image
Source:UCAR

The influence of the Coriolis effect on ocean currents increases with increasing latitude, so the equatorial currents are similar in each ocean basin, although their flow direction (east to west) is consistent with the sense of flow in the large-scale gyres within each ocean basin. But the variation of Coriolis forcing as a function of latitude has a pronounced affect on surface currents. The poleward currents on the Western side of each ocean basin are distinctly different from those on the Eastern side of the ocean basin.

Western Boundary Currents are swift, narrow and deep relative to Eastern Boundary Currents, which are slower, broader and shallower than WBC's. Western Boundary Currents tend to carry heat from the equator poleward, so think back to where we started in this Lesson; everything is driven by differential solar heating. The excess heat received in the equatorial regions drives everything, including the strong Western Boundary Currents such as the familiar Gulf Stream and the East Australian Current (from Finding Nemo fame).

 

The figure below shows surface current information for the North Atlantic. The color scale is flow velocity in m/s, and the arrow along the bottom shows a 1 m/s scale bar (vector). Note the narrowness of the Gulf Stream.

Look here for more info. on the Gulf Stream. [18]

Contact the instructor if you have difficulty viewing this image
Source:NOAA

Activity 3

Let's make our own maps and use them to calculate some things about heat transport. Go to this web site [19] Start by using the defaults including "5-day Intervals" and "Vector overlay Speed." You can enter lat/lon's or just mouse-drag over their map.

 

 

  • 1. Make a few maps of surface currents and look at seasonal variations as well as flow directions and the basic pattern of ocean-basin scale gyres.
  • 2. Save your favorite map and tell me why you chose it in a few sentences. What did you learn from it? Put your map in a file, along with your comments. Also, post your comments in the box below.
  • 3. Using this web site, estimate the volume of water that will flow poleward in the Gulf Stream during June 2009. You can assume that the depth of the current is 1 km. (Note that you can get an estimate of the depth by looking at data on ocean water temperature as a function of depth. That information is available at this web site). Estimate the width of the Gulf Stream from your map.
  • 4. How much heat will be transported North by the Gulf Stream in June 2009? Use your estimate of flow along with the heat capacity information from Lesson 3. You can choose to estimate sensible heat and/or latent heat, but make sure to carefully explain your approach and the values you are using.

For numbers 2, 3 and 4, please put your answers in a file and drop that in the dropbox for Activity 3 on Angel.

For number 2, put your comments in your document and also in the comment box below.

Save your document as either a Microsoft Word or PDF file in the following format:
L4_Activity3_AccessAccountID_LastName.doc (or .pages or .pdf)
For example, student Elvis Aaron Presley's file would be named "L4_Activity3_eap1_presley.doc".

See the grading rubric [4] for specifics on how this assignment will be graded.

Additional Resources

Want to explore these topics more? Here are some resources that might interest you.

Various Web site with links to resources aimed at teachers and students:

  • General Oceanographic Data in a form that can be used to make maps and plots [20]
  • University Corporation for Atmospheric Research (UCAR) [21] Note the Teacher Resources link at upper left
  • NASA Earth Science satellite [22]<

 

Tell us about it!

Have another Web site on this topic that you have found useful? Share it in the Comment area below!

Don't see the "Comment" area below? You need to be logged in to this site first! Do so by using the link at the top of the left-hand menu bar. Once you have logged in, you may need to refresh the page in order to see the comment area below.

Summary & Final Tasks


Reminder - Complete all of the lesson tasks!

You have finished Lesson 4. Double-check the list of requirements on the first page of this lesson ("Lesson 4" in the menu bar) to make sure you have completed all of the activities listed there before beginning the next lesson.

Tell us about it!

If you have anything you'd like to comment on, or add to, the lesson materials, feel free to post your thoughts below.  For example, what did you have the most trouble with in this lesson?  Was there anything useful here that you'd like to try in your own classroom?

Don't see the "Comment" area below? You need to be logged in to this site first! Do so by using the link at the top of the left-hand menu bar. Once you have logged in, you may need to refresh the page in order to see the comment area below.

Authors: Michael Arthur and Chris Marone, Department of Geosciences, College of Earth and Mineral Sciences, The Pennsylvania State University.

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Source URL: https://www.e-education.psu.edu/earth540/content/c4.html

Links:
[1] http://sealevel.jpl.nasa.gov/overview/overviewclimate/
[2] http://www.newmediastudio.org/DataDiscovery/Hurr_ED_Center/Easterly_Waves/Trade_Winds/Trade_Winds.html
[3] http://www.physicalgeography.net/fundamentals/7d.html
[4] https://www.e-education.psu.edu/earth540/grading_rubric_problemsets
[5] http://esminfo.prenhall.com/science/geoanimations/animations/01_EarthSun_E2.html
[6] https://courseware.e-education.psu.edu/courses/earth540/priv/AtlanticConveyor.Quadfasel.pdf
[7] http://en.wikipedia.org/wiki/Coriolis_effect#Meteorology
[8] http://abyss.uoregon.edu/~js/glossary/coriolis_effect.html
[9] http://en.wikipedia.org/wiki/Coriolis_effect
[10] http://www.classzone.com/books/earth_science/terc/content/visualizations/es1904/es1904page01.cfm
[11] http://www.windows.ucar.edu/tour/link=/earth/Water/ekman.html&amp;edu=high
[12] https://courseware.e-education.psu.edu/courses/earth540/priv/Broecker_1999.pdf
[13] https://www.youtube.com/watch?v=lazg1F9hE_c&amp;feature=related
[14] http://www.windows.ucar.edu/tour/link=/earth/Water/density.html&amp;edu=high
[15] http://www.windows.ucar.edu/tour/link=/earth/Water/images/ocean_temp.html&amp;edu=high
[16] http://www.csgnetwork.com/h2odenscalc.html
[17] http://www.windows.ucar.edu/tour/link=/teacher_resources/par5_edu.html&amp;edu=high
[18] http://www.windows.ucar.edu/tour/link=/earth/Water/gulf_stream.html&amp;edu=high
[19] http://www.oscar.noaa.gov/datadisplay/
[20] http://www.nodc.noaa.gov/General/getdata.html
[21] http://www.windows.ucar.edu/
[22] http://www.aqua.nasa.gov/