This is more a mini lesson than a journal. Embrace diversity!
In order to understand a lot of what we are doing here on the boar you need to have a basic understanding of ocean circulation (how water moves around the ocean). For this, we need to go over the concept of density and how it produces motions in the ocean. There are tons of great books that discuss density and ocean circulation better than what I can do here. I will attempt to give you the most basic information, or at least confuse you enough that you would want to read one of those books.
I have written the text as a casual conversation in which I ask you questions. I provide the answers, but do try to figure the answers on your own as that will help you follow the text. I have also included hypothetical demonstrations as mind exercises. I do recommend performing them at home if you get a chance.
Ocean Circulation
Imagine an ocean with no motion at all. No currents, no waves, no tides, no motion. In order to have a change in the motion of any object we need a force (I will assume all forces are unbalanced unless noted otherwise). Therefore, we need to have at least one force if we want the water in our steady ocean to move . Which do you think are the main forces that stir the oceans?
This is not the ocean I was proposing. There are a lot of motions on this one.
The main forces are: differences in density, winds and tides (I will ignore the effects of Earth's rotation as it modifies motion but does not cause it). Even though winds are essential for understanding ocean circulation, I will focus here only on how density affects the motion in the oceans to describe what is known as thermohaline circulation.
Some of you have enough background on physics to find this redundant, but some others would benefit from a very simple explanation. Here is a menu on what I will be explaining so you can select the topics, if any, that you want to read:
Density
To float or not (buoyancy)
Changes in water density
Density and currents
Ocean thermohaline curcilation
1. DENSITY:
I do not want to get too technical here, but we do need to understand density: Density is a measure of how much matter (mass) is in a particular volume (for the mathematically inclined d=m/v, where d=density, m=mass and v=volume).
Let us see an example. Styrofoam, as in Styrofoam pieces, is made out of a plastic with a lot of air bubbles. Even though air is made out of matter, most of the matter in the styrofoam is given by the plastic, so we will assume that air contributes with no matter to the styrofoam. Given this,
A piece of Styrofoam has low density because it has a little amount of matter (plastic) in a large volume (plastic and lots of air bubbles).
Let us compare the Styrofoam piece with a metal pieces of the same size (solid metal, not the ones with a cavity in the metal to work as a thermos). The metal pieces has a lot more matter than the styrofoam, but the same volume. Which one is more dense?
Ans: The metal is more dense because it has more matter than the Styrofoam for the same volume (size of the pieces).
2. TO FLOAT OR NOT
Objects that float on water are less dense than water, and objects that sink in water have higher density. Objects with the same density as water do not float and do not sink, they stay where we put them in the water.
An iceberg floats over in the water because it is less dense than the water
Imagine you place a Styrofoam piece on a bucket with water. Would it float? What can you tell about the density of the Styrofoam compared to the density of the water?
Ans: the piece will float because it is less dense than the water.
Now place the metal piece on the water. Would it float? What can you tell about the density of the metal compared to the density of the water?
Ans: The metal piece will sink because it is more dense than the water
Liquids follow the same principle (essentially Archimedes principle, but in different words), the denser liquid will be under the less dense liquid. You can see this by adding water and oil to a glass, which one is less dense? How would you know?
The ocean is mostly water, but water that has different densities in different places. We find less dense water at the surface and denser water at the bottom.
3. CHANGES IN WATER DENSITY
Some of you might be asking, how can water have different densities, if it is just water? And if water can have different densities, how can I change the density of water?
Let us try this. Go the the kitchen and get a big pot filled with water. Place gently an un-cracked egg inside the pot. Does the egg float? What does that tell you about the density of the egg with respect to the density of the water?
The egg should sink ( contact your water utility if it does not), which means the density of the egg is higher than the density of the water. If you wanted the egg to float, how could you make it float? You would need to either increase the density of the water so it is higher than the density of the egg, or make the density of the egg lower than that of the water. It is easier to mess with the water, so let us make the water more dense.
Add salt to the pot and stir it gently so you do not crack the egg. Keep adding salt until you see that the egg floats (you will need a lot of salt, by the way). I learned this trick from my aunt was teaching me how to pickle fresh olives. This is her recipe for figuring out how much salt should be added to the water.
What you have discovered is that salt changes the density of water. The more salt you add, the more mass will be in the salty water mixture without changing its volume. More mass in the same volume means higher density.
How could you test if the salty water is more dense than the fresh water? Since you already have the salty water in the pot, take a certain amount with a glass and add a few drops of food coloring to it. Take another large glass for water and fill it half way through with fresh water. Tilt the fresh water glass and slowly pour the colored salty water sliding it on the inside wall of the other glass to prevent mixing both waters. When mixing is avoided, you end up with two separate layers of liquid just like in the water and oil glass. The saltier water is below the less salty tap water , just like in the oceans!
You actually produced a vertical current on the side of the glass, and a horizontal current at the bottom of the glass. You added the salty water at the top and it moved all the way to the bottom where it kept spreading.
Is there another way to change the density of water? Yes, by changing its temperature.
Objects take more space when they are hot than when they are cold. This means that an objects has a larger volume and the same mass when it is hot than when it is cold. What does that mean in terms of density? That hot water has less density than cold water. Hot water will be above cold water.
4. DENSITY AND CURRENTS
When we have liquids of different density they will move so the denser liquid is at the bottom and the less dense on top, if not already in that configuration. This means that differences in density in the ocean can produce motion.
Imagine you put cold water on your bathtub and let it sit there for a couple of hours. You will probably not see much motion when you get back. Now make blue ice cubes by adding food coloring on the water and placing them on your freezer. Once fully frozen, place the ice cubes gently on the surface of the water at one end of the tub. What do you think you would observe?
The ice cubs would be floating. They will release blue water as they melt. The blue water is colder than the tub water, which means it is denser. Since the blue water is denser than the clear water, the blue water will begin to sink. If you add a lot of blue ice cubes, the blue water will sink and travel along the bottom of the tub.
You have produced the same type of motion that we saw when we had salty and fresh water together.
5. OCEAN THERMOHALINE CIRCULATION
Let us go back to our original ocean with no motion. Remember that there are salts in this ocean. During winter, in the polar regions, close to where we are in the boat now, the air removes heat from the water and the water cools down. The water can cool down so much that it freezes. When the water freezes it can no longer dissolve the salt, so the salts are left behind in the unfrozen water. This unfrozen water is not only very cold, it is also very salty! This water is very dense and will sink to the bottom of the ocean.
The sinking water cannot leave a hole at the surface of the water, so the surrounding water at the surface move towards the void left by the sinking water. Voila! our ocean is now moving. Water at the surface moves towards the place where water sinks to the bottom. The cold salty water sinks and moves along the bottom of our ocean. And that is the basic principle of the Thermo (temperature) haline (salt) circulation. It is the circulation produced by changes in density produced by differences in temperature and salinity.
Is this circulation important? Most of the heat that arrives from the Sun arrives around the equator. The atmosphere and Oceans distribute this energy towards the poles. The thermohaline circulation moves surface water from the tropics and subtropics towards the poles, transporting some of the energy that arrived in the tropical areas.
An oceanographers view of the thermohaline circulation. A bit more complex than my explanation.
At least one of you is about to object to all this explanation by saying, if colder water is more dense than warmer water, and the denser goes under the less dense, how come ice cubes float? aren't they colder water on top of warmer water? Water is the only substance that gets denser as it cools down until it is close to freezing. It then becomes less dense. There is an explanation in terms of what is called hydrogen bonds, but I am afraid that if there were anyone still reading at this point, that they would stop if I continue with explanations.