In my previous column we looked at the various ocean layers and “… clines,” including thermocline, Halocline, Pycnocline. These are ever changing, some rapidly and some at a (very fast) snail’s pace. To finish that discussion (if there can ever be a finish), let’s look at the forces that mix the ocean vertically.
Water mixes out at varying speeds depending on depth and the causal force of mixing like rough water. There are two basic kinds of mixing … mechanical and convective, two very distinct kinds of action.
The mechanical mixing process is caused by wave action, surface storms, etc. The wave action stirs up the warmer surface water which is driven downward, where it mixes with colder subsurface water. Eventually, a layer of water with a fairly constant temperature is produced.
This process is more important in summer than in winter because surface waters are much warmer and less dense than sub-surface waters, thereby producing a stable water column. The mechanical mixing process is more rapid and irregular than the convective mixing process.
The convective mixing process occurs as a result of changes in water stability. When surface waters become more dense than subsurface waters, an unstable condition exists. Examples are an increase in surface salinity owing to evaporation, the formation of ice, or by lower surface water temperature.
A temperature decrease of .01°C or a salinity increase of 0.01 percent, is enough to jump-start the convective mixing process.
For example, a cold polar or arctic air mass moving over warm water cools the surface water before it can cool the subsurface water. As the surface waters cool and become colder than the subsurface waters, they gain density and sink. As the colder surface water sinks, the warmer and less dense subsurface water rises to the surface to replace it.
This process continues until the water is thoroughly mixed, the density difference eliminated, and the water column stabilized.
Even though winds and the resultant wave action are generally stronger during winter and spring, convective mixing — caused by colder winter air temperatures — produces a deeper mixed layer than can be attained by mechanical mixing. So convective mixing is considered the more important of the two, and the predominant process of winter and spring.
The convection process is strongest in northern waters where vertical temperature and salinity gradients are not extreme and surface waters undergo a high degree of cooling. Convective mixing attributed to salinity changes is most noticeable anywhere evaporation far exceeds precipitation.
The two processes can and often do take place simultaneously. When this occurs, the mixed layer normally attains a greater depth than would be attained by either process individually.
— Capt. David Bacon operates WaveWalker Charters and is president of SOFTIN Inc., a nonprofit organization providing seafaring opportunities for those in need. Visit softininc.blogspot.com to learn more about the organization and how you can help. Click here to read previous columns. The opinions expressed are his own.
