![]() So if you take a kilogram of seawater, about 965 grams of that is water, H 2O, and about 34.4 of that is in weight, in mass, grams, is the salt. Let’s take a look at the salt itself, and this will connect a little bit with the lab that you’re currently writing up. So the water near the surface would be salty there, but not at the bottom. Whereas in the tropics, for example, in the belt of deserts, the descending branch of the Hadley cell, you’d have a lot of evaporation, but very little precipitation. ![]() The water might be a little less saline at the surface than it is down deep. The surface waters, notice the salinity scale is reversed on this diagram. For example, the high latitudes where you might have a lot of precipitation and because it’s cold, not very much evaporation. These profiles vary from place to place in the world ocean. So here it’s the halocline referring to the salt, there the thermocline referring to the temperature, and the pycnocline refers to that combined quantity which is the density of the water. The word pycnocline is used when you’re referring to this gradient region as it applies to density. For example, if you wanted to apply the hydrostatic relation to find out how fast pressure increases with depth, you’d want to use the density derived from the salinity and the temperature. And since density is what gravity acts on, we’re particularly interested in this. So the greater the salt content the greater is the density, and the fresher the water is the less is the water density. And salt, when you add salt to water, the salt goes a little bit into the pores between the water molecules, which increases the density. The colder it is, the more it contracts, the density is greater. The warmer the water is, the more it expands a little bit and its density is less. And that is a generally a function of temperature and salinity. Then the dynamical quantity we’re interested in is the seawater density with units that you’re familiar with, kilograms per cubic meter, for example. But always staying in this remarkably narrow range between 34 and 35 and 1/2 parts per thousand. In this case, it’s saltier, then gets a bit fresher, than gets very slowly saltier below. Salinity is somewhat similar in that you can have strong gradients near the air surface, but more uniform conditions below. So this point I was emphasizing last time about how the surface temperature does not represent the deep ocean temperature is shown nicely here. Then you get down to temperatures below that that are four, five, six degrees Celsius and colder, and that fills most of the interior of the ocean basin. Maybe in this case, just a couple hundred meters below the surface you go from a mixed layer to a strong gradient region called the thermocline, “cline” referring to change, and “thermo,” of course, referring to temperature. The temperature is shown here with a distinct cooling as you go down that starts pretty quickly below the surface. Zero refers to sea level, and then depth is in meters below that, and this one going down to 4,000 meters. So let me just start here, and I showed you this last time. And I wanted to pursue that just a little bit further, and then get into some quantitative methods for estimating how the atmosphere forces various things that go on in the ocean. Some places are deeper, some places are shallower, but there is that reference level from which we work.Īny questions on that? Then we moved into a discussion of how to measure salinity and temperature in the oceans, and I talked about some of the methods to do that. It’s an oversimplification to say, of course, that the ocean basins have flat bottoms like a bathtub or something.īut there is a little bit of a tendency that way because they do have-they do have ocean crust beneath them which floats at a certain level and gives you the depth of about 5 kilometers in many, many places around the world ocean. And that’s why there are these vast areas of the ocean that are at about the same depth below sea level, these abyssal planes. The continental crust which sits a bit higher, and the ocean crust which sits a bit lower. We connected that to plate tectonics, both to make the point that those ocean basins are changing through geologic time, but also to get at this curious issue of how the oceans are not just kind of a random roughness on the Earth, but they really represent two basic levels of Earth crust. ![]() And last time I gave an overview of the nature of the ocean basins, basically the geometry of the basins in which the water sits. Professor Ron Smith: So now we are into this new section of the course, oceanography. The Atmosphere, the Ocean, and Environmental Change GG 140 - Lecture 20 - Ocean Water Density and Atmospheric ForcingĬhapter 1: Ocean Depth Profiles
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