17.3: Convection and Convection Cells

Essential to Know

• Vertical motions in a fluid can be caused by temperature or compositional changes that alter the density of parts of the fluid.
• When density continuously decreases within one layer of a fluid (often at its lower boundary with another fluid or solid) and continuously increases in a higher layer (often the fluid surface), a convection cell is established.
• In a convection cell, a plume of lower-density fluid rises within the fluid until it reaches an equilibrium level where the surrounding fluid is of equal density. If the rising fluid has a lower density than all fluid layers above it, it rises to the surface. At its equilibrium level (or the surface), the fluid spreads out. If it is cooled or otherwise altered so that its density increases, the fluid eventually becomes dense enough to sink back to its original level, where it replaces rising fluid and reenters the cycle.
• Some convection cells have toroidal (doughnut-shaped) circulation, in which a rising column of fluid spreads out laterally in all directions at the top of the convection cell and returns by sinking in a ring-shaped band surrounding the rising plume. This pattern may be reversed with a column of sinking fluid surrounded by a ring of rising fluid.
• In other convection cells, both the rising fluid and the sinking fluid form an elongated curtain. The simplest example of such a convection cell is cylindrical.
• The rising plume is called an “upwelling plume,” and the sinking plume is called a “downwelling plume.”
• At the top or bottom of a convection cell, the fluid flows horizontally away from a location at a divergence. The fluid flows toward a location at a convergence.
• At the top of a convection cell, divergences are regions of upwelling, and convergences are regions of downwelling. At the bottom of the convection cell, divergences are regions of downwelling, and convergences are regions of upwelling.
• Several convection cells may be formed side by side within a fluid. Convergences and divergences always alternate across the top or bottom of the cells.

Understanding the Concept

CC1 explains how density differences cause stratification in fluids, in which successive layers have higher density with depth. If the density of the layers in a vertically stratified fluid does not change, the system is stable and vertical motions do not occur.

Three stratified fluids are important to oceanographers: the Earth’s interior layers, the oceans, and the atmosphere. Heat is introduced into the Earth’s layers by energy released during radioactive decay in the core and mantle (Chap. 4). In the oceans, a small amount of heat is introduced by conduction from the mantle through the seafloor, and a much larger amount by solar heating of surface water. Heat is also exchanged (gained in some areas and lost in others) between oceans and atmosphere by conduction, radiation, evaporation, and precipitation (Chaps. 5, 7). The atmosphere exchanges heat with the oceans and land, gains heat from solar radiation, and loses heat by radiation to space (Chap. 7).

The heat transfer processes between sun, atmosphere, ocean, and land all vary with time and with location on the Earth. Variable heat transfers produce density changes in the mantle, ocean, and atmosphere and cause the stratification to become unstable. Changes in composition can also alter density. For example, changes in the salinity of ocean water (Chap. 5) or in the water vapor pressure of air (Chap. 7) alter the fluid density. When stratification becomes unstable, vertical motions called convection occur.

We can best understand convection using a simple kitchen analogy: the motion of water in a saucepan heated on the stove (Fig. CC3-1). Initially, when we place the saucepan on the burner, all the water within the pan is at room temperature. No vertical motion occurs because the water is uniform in density. When we begin heating the base of the saucepan, the water at the bottom is heated, while the water above remains at room temperature. As bottom water is heated, its density decreases, and it rises through the overlying water to the surface. If you watch a saucepan of water carefully as you start to heat it, you can see the vertical water movements as swirls of motion.

Figure CC3-1. In a saucepan heated at the center of its bottom, water will establish a circulation in which heated water rises to the surface, spreads toward the pan’s sides, cools, and then sinks back to the bottom. This toroid-shaped convection cell is established because the water’s density decreases when it is heated and increases when it is cooled. The idealized toroidal convection cell depicted would, in practice, normally be distorted because saucepans are usually heated across their entire base, not just at the center, and by turbulence in the flow patterns caused by variations in the rate of heating and cooling.

When the water is heated above room temperature, it begins to lose heat through the saucepan’s surface and sides (Fig. CC3-1). Heated water rises to the surface, spreads out, and cools. As more warmed water rises to the surface, it forces the cooler surface water toward the sides of the saucepan. Because heat is lost through the water surface, the surface water that is displaced to the edges of the pan is cooled slightly and has a slightly higher density than the newly warmed water rising to the surface at the center of the pan. Accordingly, warmer water continuously flows toward the edge of the pan, cooling as it flows. As we continue to heat the saucepan, a continuous recycling motion is established. Water heated at the bottom of the water column rises at the center of the pan and is replaced by cooler water flowing in from the sides. The heated water rises to the surface, then spreads out, cools, and sinks down the sides of the pan to be heated again (Fig. CC3-1). The closed cycle of fluid circulation, driven by warming at depth that causes the fluid to rise, cooling at a lower-density level, and sinking back to its original depth to rejoin the cycle, is called a convection cell.

Convection is not as uniform as depicted in Figure CC3-1. The heating of the saucepan bottom and the heat loss through the water surface and the pan sides vary. Therefore, the location of the rising plume of the convection cell fluctuates, and more than one rising plume is often present. In addition, the water flow is turbulent rather than smooth. Turbulence can be seen in the internal swirling, churning motions in fast-running streams.

If we set our stove on low heat, the water in the saucepan does not boil, and convection cell motion continues indefinitely. Heat is added continuously and lost at the same rate that it is added. In nature, most convection cells operate in this way.

Convection cells transport heat vertically within a fluid. Convection also establishes areas on the surface and at the bottom of the cell where warmer fluid (in the center of the saucepan) and cooler fluid (at the edges of the saucepan) are concentrated. These features of convection cells are important in many ocean processes, including plate tectonics (Chap. 4), ocean water circulation (Chap. 8), and atmospheric circulation (Chap. 7). The upper and lower boundaries of convection cells are often the interfaces between two fluids (e.g. water and air) or between a fluid and a solid (e.g., the ocean surface and the seafloor). However, convection cells may develop between any two density surfaces within a vertically stratified fluid.

The convection cell just described is toroidal (doughnut-shaped), with a concentrated rising plume of heated water at its center and a ring of sinking water surrounding this plume (Fig. CC3-1). In another configuration of convection cells, both the rising plume and the sinking plume are extended laterally. In its simplest form, this type of convection cell is cylindrical in shape (Fig. CC3-2). Because fluid cannot both rise and sink at the same location, convection cells must be arranged so that the rising and sinking plumes alternate (Fig. CC3-2). The areas where fluid rises are upwelling zones, and areas where it sinks are downwelling zones. Upwelling and downwelling areas must alternate across the top or bottom of the fluid. We cannot go from one upwelling zone to another upwelling zone without passing through a downwelling zone. The locations of upwelling and downwelling in the atmosphere, ocean, and Earth’s mantle are important to many aspects of Earth’s geology, chemistry, climate, and biology.

Figure CC3-2. Cylindrical convection cells. Note that convergences and divergences alternate between adjacent cells.

At the top of a convection cell, fluid flows horizontally away from upwelling areas as it is displaced by upwelled fluid. Such areas are called divergences (Fig. CC3-3). Similarly, fluid flows into downwelling areas to replace downwelled fluid. Such areas are called convergences (Fig. CC3-3).

This text primarily considers processes that occur at the tops of convection cells (e.g., in the upper mantle and ocean surface waters), where upwelling zones are divergent, and downwelling zones are convergent (Fig. CC3-3). When we consider atmospheric circulation, we are concerned primarily with the bottom of the convection cell at the Earth’s surface, where upwelling zones are convergences and downwelling zones are divergences (Fig. CC3-3).

Figure CC3-3. The mantle, oceans, and atmosphere each have convection cells with alternating divergences and convergences across their upper and lower boundaries. In the atmosphere, we are generally concerned with the lower (sea-level) boundary of the convection processes, where divergences are downwelling zones, and convergences are upwelling zones. In the mantle and oceans, we are generally concerned with the upper (sea surface and upper surface of the asthenosphere) boundaries of the convection processes, where divergences are upwelling zones, and convergences are downwelling zones.