11.7: The Seasons and the Thermoclines

We now come to my honest-to-goodness favorite topic in oceanography: the layering of the ocean due to seasonal heating by the Sun. For me, it’s one of the best examples of how the ocean works as a system. The physics affects the biology, which in turn affects the chemistry of the ocean. Physics drives ocean life, which alters the ocean’s chemistry. But that’s getting ahead of our story. In this chapter, we’ll focus mostly on the physics. And we’ll look just at temperate oceans for the time being. But be prepared. We’ll return to these topics later in the book and explore the marvelous symphony of processes conducted by the Sun.

Winter and the Permanent Thermocline

The different layers that we observe in the ocean result largely from seasonal differences in heat exchange. Salinity plays an important role too—especially in the polar oceans—but we’ll concentrate on heating first. The temperate zone ocean provides the best example of how changes in heat exchange at the surface affect the layering. And the best place to start is winter.

In the winter, the ocean generally loses heat to the atmosphere because the overlying atmosphere is colder. Surface heating is low because daylengths are short and sun angles are low. The loss of heat from the ocean to the atmosphere cools the ocean’s surface. A cold, dense, negatively buoyant layer of surface water forms. Because this cold surface layer rests on top of a warmer, less dense, and more positively buoyant layer, the water column becomes unstable. The surface layer begins to sink.

Sinking of surface waters continues throughout the winter, as long as the top layer gets colder than the layer beneath it. At some point—when the coldest seasonal air temperatures have been reached—the coldest surface layer forms. This layer represents the most negatively buoyant layer formed during the season. As a result, this layer will sink the deepest.

The sinking of surface waters in winter represents an important process by which the upper ocean is mixed. Mixing is just what it sounds like (minus the DJ): the ocean is blended, combined, homogenized, and stirred. Any water parcels with slightly different temperatures (or salinities) may be mixed by the sinking action of denser and thus less buoyant water parcels. This is called buoyancy-driven mixing. It’s important for distributing heat, gases, salts, biologically important nutrients, particles, plankton, larvae, and all sorts of other things in the upper ocean. This wintertime mixing often results in an isothermal water column (iso = “same”; thermal = “temperature”), one in which temperature does not change with depth. This layer of mixed waters in the upper ocean is called—wait for it—the surface mixed layer. The distance from the surface to the bottom of the mixed layer is called the mixed layer depth. The mixed layer depth gives an estimate of the depth of mixing of the water column.

How deep is the mixed layer? Negatively buoyant surface waters sink until they reach a depth where the surrounding water has the same density—they sink until they become neutrally buoyant. In tropical oceans, the surface layer may sink a few tens of meters in winter (e.g., Longhurst 1993). In temperate oceans, the surface layer may sink from 15 to 75 meters on average (e.g., Bathen 1972). In polar environments, where the coldest air temperatures occur, the surface layer may sink thousands of meters, even all the way to the seafloor (e.g., Johnson 2008). The densest waters in the world ocean are formed in polar oceans. These abyssal waters—as they are collectively called—spread along the seafloor from the poles to the equator and beyond. Abyssal waters make up the deepest layers in the world ocean, even in temperate and tropical zones.

Now, if you were to descend in a submarine to the depth where the mixed layer meets the abyssal waters and hold a thermometer out an open porthole, the submarine would fill with water, sink, and kill everyone aboard. (Words to live by: never open a porthole on a submerged submarine.) But if you attached a thermometer to the hull and connected it to a computer so that you could read the external water temperature as you descended, you would notice no change in temperature in the isothermal water column. But in a temperate ocean, you would reach a boundary between the winter-formed deep mixed layer and the polar-formed abyssal waters. At this boundary, you would notice a rapid change in temperature as you crossed from the bottom of the mixed layer to the top of the abyssal waters. This boundary—the region where temperature changes rapidly—is called a thermocline. The suffix -cline means “slope,” just like “incline” means up slope and “decline” means down slope. Where upper ocean waters—mixed ones—meet abyssal waters—ones formed in polar oceans—a permanent thermocline may be formed. Temporary thermoclines—seasonal ones—may be formed, too, as we’ll see in the next section.

The thermocline is a classic structure in descriptions of the physical properties of the ocean. It represents a boundary between different water parcels. The presence of a thermocline indicates that these water parcels are distinct and separate from each other. In graphs of the water column—XZ graphs with the z-axis representing depth and an x-axis representing temperature—the thermocline appears as a slope in the temperature profile. Thus, you may think of the thermocline as a temperature slope.

Spring and the Seasonal Thermocline

As the days get longer and the sun gets higher in the sky in late winter or early spring (depending on the latitude), solar heating is more direct. The sea surface begins to warm. Air temperatures and water temperatures may be highly variable, subject to changing weather conditions. But at some point, the atmosphere in contact with the ocean becomes warmer than its surface. The net exchange of heat now is from the atmosphere to the ocean. When the sea surface warms, the seawater becomes less dense and more positively buoyant. But it doesn’t rise into the sky. Ocean water is about 800 times denser than even the most dense air, so the surface water remains at the surface.

The warming of the surface waters creates a temperature difference between the uppermost layer and the layer beneath it. The boundary between the now-warmer surface layer and the still-cold layer beneath it represents a different kind of thermocline, the seasonal thermocline. It forms as a result of surface warming and will disappear the following fall when the water cools again. It occurs seasonally—hence the name.

Previous to the formation of the seasonal thermocline, the entire mixed layer is isothermal. Now the top of what used to be the mixed layer is warmer. The mixed layer has been split in two, so to speak. As a result, the mixed layer depth becomes shallower. The surface mixed layer is now confined to the region from the surface to the top of the seasonal thermocline. As the surface of the ocean continues to warm, multiple thermoclines may be present. It can get pretty complex, and we’re not going to dwell on all of the possibilities, but just be aware that the seasonal thermocline represents a temporary boundary between water parcels with different densities. The thermocline originates as the surface of the ocean warms. In a graph of the entire water column, from the surface to the seafloor, both the shallower seasonal thermocline and deeper permanent thermocline may be present.

The layering of the ocean that occurs as a result of warming of surface waters and formation of a seasonal thermocline is called stratification. The word stratum means “layer,” and it’s used to describe clouds (e.g., stratus clouds), rocks (e.g., strata of sedimentary rock), or an archaeological dig (e.g., strata of ruins or refuse piles). Stratification of the water column signals a major event in the physics, chemistry, and biology of the upper ocean. It means, in effect, that the upper layers of the ocean are practically cut off from the lower layers. The upper and lower layers can’t mix, or mixing is severely restricted. Exchanges of heat, gases, salts, and biologically important nutrients between the surface mixed layer and lower layers can no longer occur or occur much more slowly. It won’t be until fall or even winter before these waters mix again. Keep this process in the back of your mind. When we get to chapters on productivity and food webs, the isolation of the upper ocean will help you appreciate how physical processes drive the chemistry and biology of the ocean.

Summer and the Multiple Thermoclines

The surface layer continues to warm well into summer as high sun angles permit solar radiation to penetrate deeper into the ocean. The upper ocean warms to its highest temperatures of the year. Double or even multiple thermoclines—more than a single thermocline visible in vertical profiles of the water column—may be observed (e.g., Fan et al. 2014). With layers of positively buoyant water parcels stacked on top of one another, the summer water column represents one of the most stable water columns of the year. The stability of the water column may lead to the formation of phytoplankton thin layers, bands of highly concentrated phytoplankton confined to a narrow depth interval. Having a vertical thickness of less than a few feet, they may extend horizontally for miles. Formation of thin layers requires a very stable water column, and in some places and under the right conditions, they may persist for weeks or longer (e.g., Durham and Stocker 2012).

Though plant life on land tends to be highly productive in summer, phytoplankton concentrations in temperate oceans tend to be low. That’s because stratification of surface waters limits mixing of biologically important nutrients. Starved of nutrients, phytoplankton grow more slowly. Species able to subsist under low nutrient concentrations replace those with high nutrient requirements. That’s just a snippet of the interaction of the ocean’s biology during the seasons of the sea.

Fall and the Disappearing Thermocline

In the temperate zone, all stable water columns must come to an end. The shortening of the days and the lower sun angles in fall reduce solar heating. The atmosphere cools quickly, much quicker than the ocean. At some point during fall, the air becomes cooler than the ocean, and the trend of heat exchange is reversed. The ocean begins to lose heat to the atmosphere. As the surface waters cool, they lose buoyancy and sink. Initially, they sink only a little. They remain plenty warm compared to the temperature minimum of winter, but it just takes a little cooling at the very surface of the ocean to make the water column unstable. As the cooling continues, the surface waters sink deeper. As the surface waters sink, they mix the upper ocean. Deeper waters begin to mix upward. Differences in heat, gases, salts, chemicals, biologically important nutrients, particles, plankton, and larvae disappear as the surface layer becomes colder and mixes deeper. The seasonal thermocline disappears too. The multiple layers of different density that were stacked up in the stable water column of summer become unstable and begin to blend and mix. The disappearance of the seasonal thermocline and the blending of the different layers characterize a process called destratification, the unmaking of the layers of the ocean. Of course, this process will continue throughout winter until the coldest days once again set the minimum temperature for the upper ocean.