Stratification (water)

Stratification in water is the formation in a body of water of relatively distinct and stable layers by density. It occurs in all water bodies where there is stable density variation with depth. Stratification is a barrier to the vertical mixing of water, which affects the exchange of heat, carbon, oxygen and nutrients. Wind-driven upwelling and downwelling of open water can induce mixing of different layers through the stratification, and force the rise of denser cold, nutrient-rich, or saline water and the sinking of lighter warm or fresher water, respectively. Layers are based on water density: denser water remains below less dense water in stable stratification in the absence of forced mixing. Stratification occurs in several kinds of water bodies, such as oceans, lakes, estuaries, flooded caves, aquifers and some rivers.

Mechanism

The driving force in stratification is gravity, which sorts adjacent arbitrary volumes of water by local density, operating on them by buoyancy and weight. A volume of water of lower density than the surroundings will have a resultant buoyant force lifting it upwards, and a volume with higher density will be pulled down by the weight which will be greater than the resultant buoyant forces, following Archimedes' principle. Each volume will rise or sink until it has either mixed with its surroundings through turbulence and diffusion to match the density of the surroundings, reaches a depth where it has the same density as the surroundings, or reaches the top or bottom boundary of the body of water, and spreads out until the forces are balanced and the body of water reaches its lowest potential energy. The density of water, which is defined as mass per unit of volume, is a function of temperature ( T {\displaystyle T} ), salinity ( S {\displaystyle S} ) and pressure ( p {\displaystyle p} ), which is a function of depth and the density distribution of the overlaying water column, and is denoted as ρ ( S , T , p ) {\displaystyle \rho (S,T,p)} . The dependence on pressure is not significant, since water is almost perfectly incompressible. Since water is typically densest at 4 °C, an increase in the temperature of the water above that temperature causes expansion and the density will decrease. A decrease in temperature below 4 °C also causes slight expansion and water expands the most when it freezes, resulting in a decrease in density. An increase in salinity, the mass of dissolved solids, will also increase the density. Density is the decisive factor in stratification. It is possible for a combination of temperature and salinity to result in a density that is less or more than the effect of either one in isolation, so it can happen that a layer of warmer saline water is layered between a colder fresher surface layer and a colder more saline deeper layer. A pycnocline is a layer in a body of water where the change in density is relatively large compared to that of other layers. The thickness of the pycnocline is not constant everywhere and depends on a variety of variables. Just like a pycnocline is a layer with a large change in density with depth, similar layers can be defined for a large change in temperature, a thermocline, and salinity, a halocline. Since the density depends on both the temperature and the salinity, the pycno-, thermo-, and haloclines have a similar shape.

Mixing Mixing is the breakdown of stratification. Once a body of water has reached a stable state of stratification, and no external forces or energy are applied, it will slowly mix by diffusion until homogeneous in density, temperature and composition, varying only due to minor effects of compressibility. This does not usually occur in nature, where there are a variety of external influences to maintain or disturb the equilibrium. Among these are heat input from the sun, which warms the upper volume, making it expand slightly and decreasing the density, so this tends to increase or stabilise stratification. Heat input from below, as occurs from tectonic plate spreading and vulcanism is a disturbing influence, causing heated water to rise, but these are usually local effects and small compared to the effects of wind, heat loss and evaporation from the free surface, and changes of direction of currents. Wind has the effects of generating wind waves and wind currents, and increasing evaporation at the surface, which has a cooling effect and a concentrating effect on solutes, increasing salinity, both of which increase density. The movement of waves creates some shear in the water, which increases mixing in the surface water, as does the development of currents. Mass movement of water between latitudes is affected by coriolis forces, which impart motion across the current direction, and movement towards or away from a land mass or other topographic obstruction may leave a deficit or excess which lowers or raises the sea level locally, driving upwelling and downwelling to compensate. The major upwellings in the ocean are associated with the divergence of currents that bring deeper waters to the surface. There are at least five types of upwelling: coastal upwelling, large-scale wind-driven upwelling in the ocean interior, upwelling associated with eddies, topographically associated upwelling, and broad-diffusive upwelling in the ocean interior. Downwelling also occurs in anti-cyclonic regions of the ocean where warm rings spin clockwise, causing surface convergence. When these surface waters converge, the surface water is pushed downwards. These mixing effects destabilise and reduce stratification.

By water body type

Oceans

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