Generally, water density increases with depth in the ocean or a large body of water. This is primarily due to two factors: the compressibility of water and changes in temperature and salinity.
Factors Influencing Water Density with Depth
1. Compressibility
Water, like any fluid, is compressible, meaning its volume decreases under pressure. As depth increases, the pressure exerted by the water above increases significantly. This higher pressure compresses the water molecules, packing them closer together and increasing the density. Although water is considered relatively incompressible, this effect does contribute to density changes, especially at great depths.
2. Temperature
Temperature plays a significant role in water density. Cold water is denser than warm water. Near the surface, solar radiation warms the water. As you descend, the water temperature generally decreases. This temperature decrease contributes to the increased density at greater depths. There is often a thermocline, a layer characterized by a rapid change in temperature with depth.
3. Salinity
Salinity, the amount of dissolved salts in water, also affects density. Saltier water is denser than less salty water. While salinity variations are less pronounced than temperature variations in many areas, they can still influence density, especially in areas where freshwater rivers meet the ocean or where evaporation rates are high. An increase in salinity contributes to increased density with depth.
Stability Considerations
The principle reason water density normally increases with depth is for stability. If less dense water were below more dense water, it would be an unstable situation. The less dense water would rise, and the more dense water would sink, leading to mixing until a stable density stratification is achieved.
Exceptions
While the general trend is for density to increase with depth, there can be exceptions. For instance, in polar regions, very cold, relatively fresh water can sit on top of warmer, saltier water, creating an inversion. Also, localized mixing events caused by currents, storms, or upwelling can disrupt the typical density profile.
