Stratification of the ocean water is a natural occurrence and one that is important for the structure, productivity and circulation of the ocean (centerforoceansolutions).
Normal Stratification of Ocean Column:
Natural stratification of the ocean column occurs as pictured below. With natural stratification a vertical water column is formed due to the varying densities of the ocean water- will less dense water towards the surface of the ocean and more dense water towards deeper ocean (centerforoceansolutions) The varying dense layers of the water are able to mix through wind upwelling and down-welling. Upwelling of water from about hundred meters depth to the ocean surface acts to cool the surface of the ocean. (Columbia) Down-welling on the other hand, takes the water surface water and mixes it with the lower colder water. It is through this process that nutrients are able to reach the varying ocean depths. (Columbia, Richardson et al. 09)
|Normal Ocean Stratification.|
Picture: Earth Observatory NASA
Stratified Ocean Stratification:
Stratified Ocean Stratification occurs when water with different properties such as salinity, density and temperature form layers, which act as barrier for water mixing. This barrier prevents water from passively mixing across the thermocline boundary (boundary between warm water and cold water) (Boyce). Usually wind, upwelling, down welling (as shown below in the figure) and storms help move the cool water across the boundary and mix with the warm upper layer of water (centerforoceansolutions, Boyce). With increasing temperatures, decreasing wind current, strong and prolonged stratification is occurring. This is leading to decrease amount of nutrients traveling up to the surface layer of the water and oxygen of deeper parts of the ocean (centerforoceansolutions)
|Picture courtesy of Ole Nielsen (blog)|
As temperatures increase due to climate change, the ocean water becomes more stratified. (Richardson. Et al) The increase in temperature has even a greater effect on the coastal waters, because the thermocline becomes even a more powerful boundary causing potential reduction in upwelling (centerforoceansolutions). This reduction in upwelling and down welling makes it more difficult for nutrient (such as nitrate and phosphate) rich water to reach the surface and for oxygen to reach the lower depths of the ocean (centerforoceansolutions). These kinds of reductions can result in biomass loss and changes in species compositions as certain species abundance goes down (centerforoceansolutions, Boyce)
The figure below shows effects of anthropogenic caused changes in ocean stratification and what it means for other organism in the ocean.