GG 140: The Atmosphere, the Ocean, and Environmental Change

Lecture 20.?Ocean Water Density and Atmospheric Forcing

https://oyc.yale.edu/geology-and-geophysics/gg-140/lecture-20

Ocean Depth Profiles [00:00:00]

So now we are into this new section of the course, oceanography. 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. 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.

Today is?a discussion of how to measure salinity and temperature in the oceans, and I talked about some of the methods to do that. And I wanted to get into some quantitative methods for estimating how the atmosphere forces various things that go on in the ocean.?This is a typical ocean sounding. Zero refers to sea level, and then depth is in meters below that, and this one going down to 4,000 meters.

The temperature is shown here with a distinct cooling as you go down that starts pretty quickly below the surface. 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. 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.The surface temperature does not represent the deep ocean temperature is shown nicely here.?

Salinity is somewhat similar in that you can have strong gradients near the air surface, but more uniform conditions below. In this case, it's saltier, then gets a bit fresher, than gets very slowly saltier below. But always staying in this remarkably narrow range between 34 and 35 and 1/2 parts per thousand.

Then the dynamical quantity we're interested in is the seawater density with units that you're familiar with, kilograms per cubic meter.?And that is a generally a function of temperature and salinity. The warmer the water is, the more it expands a little bit and its density is less. The colder it is, the more it contracts, the density is greater. 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.

So the greater the salt content the greater is the density, and the fresher the water is the less is the water density. And since density is what gravity acts on, we're particularly interested in this. 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.

The word pycnocline is used when you're referring to this gradient region as it applies to density. 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.

These profiles vary from place to place in the world ocean. For example, the high latitudes where you might have a lot of precipitation and because it's cold, not very much evaporation. The surface waters, notice the salinity scale is reversed on this diagram. The water might be a little less saline at the surface than it is down deep. Whereas in the tropics or?in the belt of deserts, the descending branch of the Hadley cell, you'd have a lot of evaporation, but very little precipitation. So the water near the surface would be salty there, but not at the bottom. The bottom is more homogeneous.

Salinity [00:06:05]

If you take a kilogram of seawater, about 965 grams of that is water, H2O, and about 34.4 of that is in weight, in mass, grams, is the salt. Then if you break up that little salt wedge into its chemical compositions you get this little pie chart here. It shows the most abundant ions. So assuming that this quantity, these chemicals break apart when they dissolve in the water into their positive and negative ions. By far the largest contribution is the chlorine, 18.96 grams of the 34, a little more than half of that salt mass is due to the chlorine.

And let's look at the other negative ions here. There's a sulfate radical, SO4, and there's a bicarbonate, HCO3?with minus signs for the charge. Then we come to the cations, which is what you're studying in the lab this week. The dominant ones are sodium, which is by far and away the most abundant. But also we have magnesium, calcium, and potassium.?

Even when the salinity varies a little bit, maybe down to 34 or up to 36. These all increase in proportion. So that these relative numbers are quite stable when you go from place to place in the world ocean. And our simple theory of this is that this represents an accumulation over geologic time of small amounts of these chemicals, these salts, that have come into the ocean from rivers.?

Stability in the Ocean [00:09:36]

We spent a good deal of time talking about static stability and?the role of the atmospheric lapse rate--how the temperature changes in the vertical--to whether parcels can rise and fall easily or not. Or whether that atmosphere might even be unstable and break down to convection. So we defined an unstable lapse rate, a stable lapse rate, the inversion, which was an example of a very stable lapse rate. We're interested in the same kind of analysis in the ocean. But there's a couple of essential differences.

One is that the ocean density depends not just on temperature, but on salinity too. So we'll have to take into account both of those quantities. We'll do that simply by computing the density. We'll base our stability analysis on water density, rather than on temperature or salinity alone. The other big difference is that while air is compressible, so as a parcel lifts in the atmosphere and moves to lower pressure, it expands--does work on its environment and its temperature changes. That affect is very, very small in the oceans.

It is so small that for many quick calculations we ignore that volume change and that so-called adiabatic cooling or adiabatic warming as air parcels rise up and down in the atmosphere. We ignore that in the ocean. So because of these two differences, the idea of what determines stability in the atmosphere and the ocean are significantly different.

The basic principle is that while temperature and salinity can either increase or decrease with depth, the density derived from them must always increase with depth. We saw that in the earlier diagram here that the salinity, for example, decreased with depth, then increased with depth. But when I computed the density it increased smoothly. If there was ever a layer where the density got less with depth, that would be an unstable layer, and it would immediately cause convection and would mix.

Density [00:13:24]

This diagram combines together hundreds of CTD and Nansen bottle data sets from three of the major world ocean basins--the Pacific Ocean, the Indian Ocean, and the Atlantic Ocean.?These balloons of data are superimposed on a diagram that has lines of constant density on it.?So on this axis is the salinity in parts per thousand that we're used to working with, PPT. On this axis is temperature in degrees Celsius. And then these lines are lines of constant seawater density.

This unit is the density of seawater with the 1 and the 0 dropped off. So you would read this as 1,028.5 kilograms per cubic meter. And?you can see that the density is greater down here, 29, 28.5, and 28. And that corresponds to salty water and low temperatures. And the seawater density is less up here, 25, 24.4--24.5, and 24 for high temperatures and low salinity.?

Up in the warmer temperature region, up near the top of the diagram, because of the way these lines are tilted, the density is most sensitive to temperature changes. Whereas down in the colder regions where the lines are tilted more like this, the density is more sensitive to salinity changes.?Both are always playing a role, but in warm areas it's mostly the temperature, in cold areas it's mostly the salinity that's controlling the density.

So when you're talking to a tropical oceanographer, he is usually most interested in talking to you about the temperature profiles in the ocean. He hasn't forgotten that salinity plays a role in this. But in the tropical—in the warm parts of the ocean, the temperature is the primary control on the density.?If you're talking to an Arctic oceanographer, she's usually most interested in understanding the salinity field because the salinity field hacking down here,?is more in control of the density field in the colder regions of the ocean.

The Pacific Ocean is generally a little fresher, and maybe even a little bit warmer than the Atlantic Ocean. There is a little part down here which is common to both oceans, they come together down there, called the Antarctic Bottom Water. It's a mass of water that's produced near Antarctica. It flows Northward along the bottom of the ocean, and we find it in both the Atlantic and the Pacific.

Other than that, there is a general systematic difference between the Pacific and the Atlantic Ocean. And that, of course, has to do with atmospheric controls, which I'm just about to get to. If you do a cross-section, this happens to be a North-South cross-section through the Atlantic Ocean from high latitudes to low. 10 degrees North, 20, on up to 60 degrees North, and on down to minus 70 degrees latitude, 70 South.

Here's a depth scale going from 0 down to 6 kilometers. And then the horizontal scale is what I indicated. And contoured here is temperature. Temperature actually gets a little bit below 0 here. Minus 0.2. But by the time you're up in here it's 1.2 degrees, 2.4. And then you've got some strong gradients near the top, well that's the thermocline, and then near the surface of the ocean in the mid-latitudes and low latitudes, you have this little lens of warm water floating on the surface.

There's salinity involved, but mostly it's a temperature control, and you've got a warm layer of water kind of floating in a broad, deep, cold layer of ocean. And you'll find this as you go North to South in all of the world's oceans, you'll find this thermocline that separates this warm sphere from the deep cold sphere. That's kind of a common element in both the Atlantic, the Pacific, and the Indian ocean. This blue water here, that's that Antarctic Bottom Water that I was talking about.?

I would point you to the Great Salt Lake as kind of an analogy to the world ocean. The Great Salt Lake unlike most lakes doesn't have an outflow. When water flows into the Great Salt Lake in Utah, it doesn't spend a few weeks there and then flow in some river into the ocean. No. It basically stays there until it evaporates.?This is a no outflow lake. And for that reason, you get the same kind of concentration of salts?as you do in the ocean. So it's a little mini ocean?in the sense that it has to evaporate its water, get rid of it, and therefore, it concentrates the salts. It concentrates the salts even much more than the ocean does.?

Atmospheric Forcing of the Ocean [00:22:08]

The subject I was going to get?is atmospheric forcing of the ocean. For the most part, any motions you have in the ocean?is driven from above by either sunlight hitting the ocean, or some other interaction with the Earth's atmosphere. There is a little bit of geothermal heat coming out of the bottom of the ocean basins, but that's quite small. So except for that, it's pretty much the oceans are driven by the atmosphere from above.?

You can add or remove heat at the top of the ocean, either by the?Sun's radiation, or long-wave radiation emitted from the ocean surface. Or by contact with the atmosphere. if you have cold air, that'll suck heat out of the ocean. If you have warm air that'll conduct heat into the ocean. Or you can add fresh water by precipitation, or remove fresh water by evaporation, leaving the salt behind. These two things will change the salinity of the surface, and therefore, the density. Or you've got the wind stress. You've got the wind blowing over the ocean surface transferring some momentum to the ocean, and getting the ocean moving in that way.

Here is our best estimate of the heat flux in and out of the ocean. Basically, where you get, for example, cold water and warm air, the heat is going into the ocean, and that's shown as positive on this diagram. Where heat is coming out you've got the blue or the red color shown as negative. But it varies from place to place. It depends on the air temperature. It depends on how much sunlight is being received. It depends on the water temperature, whether heat is going into the ocean or whether heat is coming out.?

Then the other one is E minus P or P minus E. This one is evaporation minus precipitation. The sign is derived in that way. So whenever water is evaporating, you get a yellow signature here, and the units on this are in millimeters per day. How much water is being peeled off and evaporated in terms of a layer depth per day.?Whereas in the blue, and you notice it coincides with the ITCZ?where you have heavy precipitation, precipitation exceeds evaporation. So the sign of this quantity of negative shows up as blue. Then you get up into mid-latitudes again where it's colder, there's less evaporation, but you have the frontal storms, and once again, you're getting an excess of precipitation over evaporation. So this is driving motions in the ocean as well.

Then the final one is the winds over the ocean, which change a bit with the season. There's January, there's July. But generally you've got Westerlies in the mid-latitudes and Easterlies in the low latitudes in both seasons. And that is generating motions in the ocean itself.?So here I've repeated those three ways in which the atmosphere forces the ocean. Adding or removing heat, precipitation and/or evaporation, and the wind stress.

Now, I want to make a further distinction from this.?Here's the cloud that's precipitating, adding fresh water to the surface of the ocean. There's some evaporation, that's this one. Here's some heat being added or removed, either by radiation or by the effect of the atmosphere itself. And here's the wind stress--I'm going to use Greek letter Tau (τ) to represent that.

But I've indicated that there's a layer which feels these inputs directly, and that's called the depth of that capital D. That will vary from time to time and place to place, but typically that's a very small fraction of the ocean depth. When I do examples, I often set D equal to 100 meters. Roughly only the first few tens or couple of hundred meters feels these inputs directly.?

Chapter 6: Atmospheric Forcing of the Ocean: Adding and Removing Heat [00:27:50]

Atmospheric Forcing of the Ocean: Wind Stress [00:43:23]

What happens when the wind blows over the ocean? When the wind blows over the ocean it produces a wind stress. Frictionally, just like when you move your hand across the table pushing down, there's a stress being applied on that table. That table starts to move sometimes in the direction that you're pushing. And that's my question. If I put the wind in that direction, from left to right in this diagram, does the ocean water begin to move in the same direction?

Well, you're probably on your guard because you know that the Coriolis force exists so that some things are not quite as obvious cause and effect-wise when you have the Coriolis force. In fact, here's what happens. So I'm going to draw a plan view here now, North, South, East and West. I'm going to imagine I've got a Westerly wind, that is a wind from West to East. So the wind is blowing in that direction. The wind stress will also be in that same direction.

When I push the water in that direction it goes to the right. It doesn't go in the direction I'm pushing. This will involve a new kind of force balance called the Ekman force balance. It has something in common with geostrophic force balance, but it's also quite different.

So let's say that I start to put this wind stress, it’s going to--in the first few minutes it is going to do the obvious thing. It's going to start the water moving towards East. But then as soon as it gets moving, if feels the Coriolis force and it will begin to bend. After a few hours, I will argue now, that the flow will be towards the South--that'll be the Ekman flow. To the right angles of that will be the Coriolis force acting on that Ekman flow. And that's going to be equal and opposite to the wind stress itself.