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Outcome: Explain the relationship between fresh water and ocean dynamics.
Grade level: 9-12
Theme: water cycle

Activity 1.6: Convection Under Ice (p. 10). In oceanography, density is used to characterize and follow water masses as a means to study ocean circulation. Many processes are caused by or reflect differences in the densities of adjacent water masses or differences in densities between fluids and solids. Plate tectonics and ocean basin formation, deep-water formation and thermohaline circulation, and carbon transport by particles sinking from surface waters to depth are a few examples of density-driven processes. This activity is designed to highlight links to oceanic processes.

In the left panel in the photo at right, an ice block floats in tap water because the density of ice is lower than that of freshwater. As the ice melts, however, cold, colored meltwater sinks to the bottom because it is denser than the tap water. Warmer water from the bottom is then displaced and upwells, resulting in a convective flow visible in the dye patterns. Ice melting in the center of the tank is analogous to a convection "chimney" formed in the open ocean, while ice melting at the tank's edge is analogous to a chimney on a continental shelf (near a land mass). Such chimneys in the ocean, created and sustained by convective processes, appear as "columns" of mixed water that flow downwards. For a given set of oceanic and meteorological conditions, open-water convection tends to entrain (mix with) more of the surrounding waters than does convection near a land mass. The open-ocean case therefore results in downwelled water that is less dense.

In the right panel, the ice block floats in dense, salty water. As the ice melts, only a small amount of dye sinks because the density of the saltwater is greater than the density of the newly melted, fresh, ice-cold water. Most of the meltwater accumulates in a surface layer on top of the denser salt layer.

Read the Background section (p. 4-5) of Chapter 1 (Density) in preparation for this activity.

Note: Even though this experiment focuses on temperature only, it can be used to discuss how salinity, like temperature, affects the density of ocean water.
Grade level: 9-12
Theme: water cycle

Activity 1.5: Effect of Stratification on Mixing (p. 9-10). In open ocean regions (with the exception of polar seas), the water column is generally characterized by three distinct layers: an upper mixed layer (a layer of warm, less-dense water with temperature constant as a function of depth), the thermocline (a region in which the temperature decreases and density increases rapidly with increasing depth), and a deep zone of dense, colder water in which density increases slowly with depth.

Mixing of stratified layers requires work. Without energetic mixing (e.g., due to wind or breaking waves), the exchanges of gases and nutrients between surface and deep layers will occur by molecular diffusion and local stirring by organisms, which are slow, ineffective modes of transfer. The energy needed for mixing is, at a minimum, the difference in potential energy between the mixed and stratified fluids. Therefore, the more stratified the water column, the higher the energy needed for vertical mixing.

Density is fundamentally important to large-scale ocean circulation. An increase in the density of surface water, through a decrease in temperature (cooling) or an increase in salinity (ice formation and evaporation), results in gravitational instability (i.e., dense water overlying less-dense water) and sinking of surface waters to depth. Once a sinking water mass reaches a depth at which its density matches the ambient density, the mass flows horizontally, along "surfaces" of equal density. This process of dense-water formation and subsequent sinking is the driver of thermohaline circulation in the ocean. It is observed in low latitudes (e.g., the Gulf of Aqaba in the Red Sea, the Gulf of Lions in the Mediterranean Sea) as well as in high latitudes (e.g., deep water formation in the North Atlantic).

This experiment looks at the energy required to mix two layers. Students should read the Background section (p. 4-5) of Chapter 1 (Density) in preparation for this activity.

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