11.4: The Ocean Layer Cake

Now, to get the most out of this next section, you’ll want to place a multilayered cake in front of you. The concepts here will make more sense if you place a slice of your favorite, delicious, hard-to-resist layer cake in front of you. Even if you don’t like to eat cake, the presence of a slice will help. Avoid eating it for now. We’ll refer to cake throughout the rest of this chapter. And when you finish the chapter, you’ll have a nice reward!

The three climate zones mentioned above—the tropical, temperate, and polar zones—receive varying amounts of sunlight due to Earth’s orbit around the Sun. This means that the parts of the ocean within these zones receive seasonally varying amounts of sunlight. Tropical oceans heat the most; polar oceans heat the least. This differential heating of the planet drives a number of important processes. Here we take a look at how seasonal changes in heating affect the physical structure of the ocean; that is, how heating and cooling cause changes in the temperature structure of the water column. Fear not, if you like swimming in lakes in the summer, this section will be a piece of cake.

Seawater Density (Revisited)

In earlier chapters, you learned that changes in seawater temperature and salinity can cause changes in seawater density, the mass of molecules occupying a given volume of seawater (i.e., mass per unit volume). Here we examine those changes in greater detail.

The exchange of heat across the air–sea interface may bring heating or cooling of the surface of the ocean. The amount of heating or cooling depends on the solar intensity and the temperature of the atmosphere. When sunlight is intense—or the atmosphere is warmer—the surface waters will warm. When sunlight is less intense—or the atmosphere is cooler—the surface waters will cool. Of course, a number of additional factors can affect heating and cooling of the ocean’s surface. Clouds, airborne or waterborne particles, winds, waves, and even the color of the water may affect temperature changes. But in general, sunlight and atmospheric heat are considered the major factors governing ocean heat content (e.g., Lindsey and Dahlman 2020).

Temperature affects seawater density because it affects the spacing between the molecules of water and its dissolved salts. As we heat seawater, its molecules move faster, and the space between the molecules gets larger. As we cool seawater, the molecules move slower, and the molecules get closer together. In the first case—heating—a rise in temperature causes the density of seawater to decrease. We say the seawater becomes less dense. In the second case—cooling—the lowering of temperature causes the seawater to become more dense. Temperature and density have an inverse relationship: As one quantity goes up, the other goes down, and vice versa.

The concentration of salts in seawater—its salinity—also affects its density. As the salinity—the concentration of salts—increases, the density increases. That’s because there are now more molecules packed into the same volume of seawater. If we add freshwater and lower the salinity, we decrease the density. Salinity and density have a positive relationship (or correlation). As one increases or decreases, so does the other.

And we shouldn’t forget that the solid form of water—ice—is less dense than the liquid form of water. That’s because water molecules repel each other and the structure of water changes at temperatures below 39.2°F (4°C). Because ice is less dense than liquid water, it floats.

Finally, as noted in Chapter 11, freezing of seawater and formation of sea ice cause brine rejection, extrusion of a syrup of salts into the surrounding ocean. The sea ice is fresh, but the brine increases local seawater salinity as it dissolves.

The Concept of Buoyancy

Heating and cooling and increases and decreases in salinity change the density of seawater. As a result of changes in density, a seawater parcel—an oceanographer’s informal term for an unspecified volume of seawater—may sink or rise. The position of a water parcel in the ocean depends on the buoyant force, the upward force exerted on a fluid or object immersed within it. The buoyant force counteracts the gravitational force—the pull of gravity on the water parcel or objects within it. It’s these two forces that determine buoyancy—the tendency of a fluid or object to rise or sink in a fluid. Buoyancy underlies the different density layers in the ocean—that is, the ocean layer cake.

A brief explanation provides some context for the physics. The buoyant force arises because water pressure—exerted in all directions—increases with depth (as the weight of water above an object increases). The deeper the depth, the greater the buoyant force. The buoyant force also depends on the volume of the parcel or object. The larger the volume, the greater the buoyant force on that object. Objects of equal volumes at the same depth will experience the same buoyant force. However, if their masses differ, the gravitational force on them will differ. That’s because the gravitational force depends on the mass of the object (i.e., the gravitational force equals the mass of the object times the gravitational constant). The mass of a given volume depends on its density: the higher the density, the greater its mass. In short, water parcels with a higher density will experience a greater gravitational force. And this will influence where the water parcel comes to rest in the water column.

Here are the key concepts related to the importance of buoyancy for understanding ocean layering. If the buoyant force is less than the gravitational force, then the fluid or object sinks, which is known as negative buoyancy. If the buoyant force exceeds the gravitational force, then the fluid or object rises, a condition known as positive buoyancy. If the buoyant force and the gravitational force are equal, then the fluid or object remains stationary—it has achieved neutral buoyancy. If you are on a sinking ship, you will want to grab something that will help you stay afloat, something that has positive buoyancy in water. If you want to hide a dead body, then you will want to attach it to something that will sink, something with negative buoyancy. All kidding aside, the principles of buoyancy have important practical applications, such as scuba diving, shipbuilding, and operating a submarine, among others.

The principles of buoyancy are more obvious with a familiar example, such as a hot air balloon—invented in 1783, making it the oldest form of human flight (Kotar and Gessler 2011). By heating air inside a flexible container—the balloon—it becomes positively buoyant. The air becomes less dense, the balloon gains buoyancy, and it rises. When the hot air cools, either by conduction through the fabric of the balloon or by letting cool air into the balloon, the balloon loses buoyancy and it sinks. But how does a balloon remain stationary at an ideal altitude above the ground? There comes a point in its upward transit where the density (and pressure) of the surrounding air matches the density (and pressure) of the air inside the balloon. At that point, the hot air balloon stops its ascent. The air inside and outside the balloon has the same density, and the balloon achieves neutral buoyancy.

Scuba divers maintain their buoyancy using a combination of weights (negative buoyancy) and a special air-holding vest called a buoyancy compensator, or BC, for short (positive buoyancy). When the diver wishes to descend, they need negative buoyancy, so they release air from their BC. When they wish to maintain a particular depth—to hover over a shipwreck or beautiful reef, for example—they inflate their BC just enough to balance the gravitational force of their weights and so achieve neutral buoyancy. When they want to ascend and return to the surface, they add a little more air to their BC, which causes them to acheive positive buoyancy and rise slowly.

Water parcels also sink and rise in the water column, albeit without weights and BCs. If the water parcel loses heat or gains salts, it may become negatively buoyant and sink. Gaining heat or freshwater, the parcel may become positively buoyant. It rises—unless it’s already at the surface. If the water parcel is neutrally buoyant, it remains in place. Just as they do with hot air balloons and scuba divers, the principles of buoyancy govern the movements of water masses and play a role in the productivity of the ocean.

A Stable or Unstable Water Column

In a given region of the ocean or a lake, the water parcels will naturally arrange themselves according to their density and buoyancy. The least dense and most buoyant water parcels will be at the surface. The most dense and least buoyant water parcels will be at the bottom. We can view these water parcels as density layers—like a layer cake—where the water parcels are arranged according to their density. When the layers are arranged in order of increasing density—least dense on the top and most dense on the bottom—we have a stable water column. If one or more of the layers are out of order—if they are not arranged in order of increasing density from the surface to the bottom—then you have an unstable water column.

Looking at the water column as a stack of water parcels with different densities helps explain many of the phenomena we observe in the ocean or even a lake. Ever swim in a lake during summer and noticed that it’s very cold when you dive deeper? That’s because the lake consists of different layers of water, each with its own density. The top layer is warm, relatively light (i.e., less dense), and positively buoyant. The bottom layer is cold, relatively heavy (i.e., more dense), and negatively buoyant. What seems like a nice, warm summer swim turns into a shockingly cold experience when you take a dive. In a nutshell, the same thing happens in the ocean.