Hello, my name is David Schultz. Welcome to Our Earth, It's Climate, History, and Processes. In this lecture, I want to talk about how the ocean works. Now, most of the sun's energy that's retained by the earth is stored in the oceans. The reasons for this are really quite simple. First, there is a lot more ocean surface than there is land surface on the Earth. Second, the heat capacity of water is four times larger than that of air. So, although air can move very quickly and move energy from the tropics to the poles more quickly than water does, because of the faster wind speeds than the ocean currents, the fact that the water is carrying more energy per unit mass makes this an effective way to transport heat from the tropics to the poles. Now, in fact, in the mid latitudes, the atmosphere transports about twice as much energy as the oceans. So, we got about four petawatts in the atmosphere and compare that to two petawatts in the ocean. As we'll see in the next lecture, what controls the circulation is the density of seawater. Two things control it's density, it's temperature and it's salinity. The salinity is essentially the salt content in the water. Now if we look at the map of January average sea surface temperatures, sometime abbreviated SST, from Build Your Own Earth, the pattern is pretty clear. Green and yellow represent relatively high sea surface temperatures, whereas blue represents relatively low sea surface temperatures. As we would expect, the tropical oceans receive more solar energy so they tend to be warm. The temperature can be as high as 30 degrees Celsius and it drops as you move poleward in each hemisphere to temperatures of 0 degrees Celsius, or freezing. But even in the mid latitudes, you'll see temperatures 10, 15, 20 degrees across much of the oceans. Now let's look at the corresponding map of average salinity in January from Build Your Own Earth. Yellow and orange represent relatively high salinity, or more salty water. The green represents relatively low salinity or less salty water. We have high salinity in tropical regions, particularly in the Atlantic Ocean, where the values can be as high as 37 parts per thousand, or 3.7%. The values are high here because the amount of evaporation from the ocean to the atmosphere exceeds the amount of precipitation of fresh water from the atmosphere to the ocean. This makes the ocean water in this location particularly salty. You can see very high salinity values in the Mediterranean Sea as well for exactly the same reason. Now as we move poleward in each hemisphere, the salinity generally decreases due to fresh water input from polar ice sheets. Also, as you move from the center of the oceans towards the coast line of the continents, the salinity also tends to decrease here due to the increase of the fresh water input from the rivers. Then there are the surface currents, these are largely driven by the winds. Here's a map of the generic ocean surface currents around the world. You can see relatively warm currents in red, moving from the equator to the polar regions, particularly along two enhanced regions of flow in the Northern Hemisphere. There's the Gulf Stream on the East Coast of North America and the Kuroshio Current on the East Coast of Asia. In particular, these two strong ocean currents transport much of the energy from the tropics to the Northern Hemisphere pole. In each ocean basin in the Northern Hemisphere, you see a clockwise gyres circulation of which one component is either the Gulf Stream or the Kuroshio. In the Southern Hemisphere, we see the same thing except moving in the opposite direction. In this case, counter-clockwise gyres in the Southern Hemisphere in each ocean basin. In the next lecture, we'll show how these surface currents are part of the global network of flow in the ocean that we call the global oceanic conveyor belt. Now one more aspect of the structure of the ocean that we need to talk about, this is the vertical structure of the ocean temperature in salinity. To help explain this, I'm joined by Dr. Gregory Lane Surf, a senior lecturer here in the school of Mechanical, Aerospace, and Civil Engineering at the University of Manchester. So Gregory, can you explain to us about this vertical structure of the oceans? >> Yeah, so the oceans are generally stably stratified, that is they get denser as you go down in depth. However, the surface layers tend to be mixed up by wind and wave action, and so on, and also heated somewhat by the sun, giving lower sea surface temperatures and lower densities there. The depth of this upper mixed layer varies during the year. So in the summer when there is strong solar radiation and relatively little wind, you have a shallow relatively warm buoyant mixed layer. And in the winter, as there's some cooling, which results in lower surface temperatures, smaller density contrast, but also much stronger mixing from the wind and the waves, resulting in a mixed layer that can extend to 50 meters or sometimes even 100 meters depth or so. So we can do a simple experiment with a straight mixed layer deepening. We fill a pint glass two thirds full of water and then mix in about 3 teaspoons of salt, and in a separate tumbler, I've got some fresh water with about 1 cm cubed of food coloring mixed in. Now what I'm going to do is carefully spoon or pour some of this water onto the salty water in here. And I'm just going to stick an ice cube there because it just makes it a little bit easier to spoon this on without it going everywhere, so let's just see how again. So if you just start slowly. And just by directing onto the ice cube, we can reduce the amount of mixing and what you should be seeing is I'm gradually building up a layer of fresh water which is colored on top of this. If I get a bit braver I can start to cover it a bit more vigorously. [SOUND] And it won't mix quite so much. Let's see, here we go. Okay, so let's see if I, oops, I'll pour it a bit more slowly to start with. Okay, so I now have a layer of relatively fresh water on top colored green here, and a layer of salty, denser water on the bottom and actually quite a strong interface in between. And if I just move it around a bit, you might be able to see some interesting internal waves generated at the interface between these two. The ice cube, I should say, was, as I say, just there to help me introduce fresh water without mixing it up so much, but there are regions of the earth, for example, in the polar regions, where there's some ice in the system, as well. Now, we can pretend to be a winter storm. So we're going to add some energy to the top by making up this surface layer. So start slowly, just use the spoon and you'll start to see some mixing extending down into the saltier water below, and as I keep on going, You'll see that I've increased the mix layer to a much more substantial depth. And if I stop there you'll see, again, these interesting internal waves going on in the bottom. By mixing, we've actually increased the potential energy of the system. Some of the salt that was originally in the lower two thirds has been raised to occupy part of the upper part of the glass. These mixing places are actually very inefficient, typically only about 3% of the energy put in by the storm actually increases the potential energy of the water column. Most is dissipated to heat via turbulent processes or carried away by those internal waves that I mentioned. >> Well thanks, Gregory. So, to summarize this lecture, we learned that the oceans are an important contributor to moving the excess energy received from the sun poleward. We've also seen the variability in temperature, salinity in the ocean currents that exist around the world. We also have seen a dramatic illustration of how the thermocline forms and how it separates warm, less salty water from the surface, from deeper, colder and more saline water below. Finally, Gregory showed us a dramatic illustration of how storms can lead to mixing in the surface layer, deepening this layer and increasing the depth of the thermocline. In the next lecture, we'll see more on how these characteristics of the ocean change on longtime scales.
