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Captain’s Diary: Mechanical and Convective Ocean Mixers | Outside

In my previous column, we looked at the different ocean layers and “…clines” including thermocline, halocline, pycnocline. These change constantly, some quickly and some at a (very fast) snail’s pace. To end this discussion (if ever there can be an end), let’s look at the forces that mix the ocean vertically.

Water mixes at varying speeds depending on the depth and the causal force of mixing like agitated water. There are two basic types of mixing…mechanical and convective, two very distinct types of action.

The mechanical mixing process is caused by wave action, surface storms, etc. Wave action agitates warmer surface water which is carried downward, where it mixes with cooler groundwater. Eventually, a layer of water with a fairly constant temperature is produced.

This process is more important in summer than in winter because surface water is much warmer and less dense than groundwater, thus producing a stable water column. The mechanical mixing process is faster and irregular than the convective mixing process.

The convective mixing process occurs as a result of changes in water stability. When surface water becomes denser than groundwater, an unstable condition exists. Examples are an increase in surface salinity due to evaporation, ice formation or a drop in surface water temperature.

A decrease in temperature of 0.01°C or an increase in salinity of 0.01% is sufficient to restart the convective mixing process.

For example, a mass of cold polar or arctic air moving over warm water cools surface water before it can cool groundwater. As surface water cools and becomes colder than groundwater, it becomes denser and sinks. As colder surface water flows, warmer, less dense groundwater 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 the winds and resulting wave action are generally stronger in winter and spring, convective mixing – caused by colder winter temperatures – produces a deeper mixed layer than can be reached by a mechanical mixing. Thus, convective mixing is considered the more important of the two, and the predominant process in winter and spring.

The convective process is strongest in northern waters where the vertical gradients of temperature and salinity are not extreme and where surface waters experience a high degree of cooling. Convective mixing attributed to changes in salinity is most visible wherever evaporation far exceeds precipitation.

Both processes can and often do occur simultaneously. When this occurs, the mixed layer normally reaches a depth greater than that which would be reached by either process individually.

– Captain David Bacon operates WaveWalker Charters and is President of SOFTIN Inc., a non-profit organization providing boating opportunities to 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.

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