In my never-ending quest to catch the next pesky fish, I dig up an awful lot of information, which is one of the really great things about trying to be a good fisherman. I figure the better I understand the ocean and its dynamic forces, the better chance I have of figuring out the important stuff like where the fish are and what they will bite.
So here are a few of the oceanic layers I’ve been wrapping my brain cells around.
Ocean layers
The ocean is divvied up into three horizontal layers, according to scientists who write papers about such things: The mixed layer at the surface; the thermocline; the deep water layer.
The definitions of these layers are based on temperature and salinity. The mixed layer and the deep water layer are relatively uniform in temperature, while the thermocline represents the transition zone between the two. That is the short version of how it all works. The longer version gets interesting and adds additional layers.
The mixed layer is near the surface where the temperature is roughly that of surface water. Mixing in this zone is easy to understand because we see and experience the driving forces, such as long-fetch, trans-oceanic swells, steep wind waves with whitecaps, and oceanic currents.
These forces take the sun-warmed surface water as far down the water column as their mechanical power has influence, which is where the thermocline then begins.
The thermocline is the transition layer between the mixed layer at the surface and the cold deep water layer. In the thermocline, temperature decreases rapidly from the mixed layer temperature to the much colder deep water temperature below. It is the layer of most rapid temperature change per foot of water because it functions as the buffer between the warmer mixed water above and the much colder deeper water below.
The thermocline is where active mixing loses its strength. Where this boundary water is located in the water column changes with depth and varying conditions. Where many of us fish, in relatively shallow water, it may be roughly a third of the way down, more or less, as natural forces, temperature gradients and salinity move it up or down.
Everything below the thermocline is pretty much the deep layer where water slowly moves as extremely cold water with high salinity sinks from the polar regions and mixes at a snail’s pace (make that a hardy snail in a big hurry).
One explanation is that in many ocean regions, temperature and salinity both decrease with depth; however, at very great depth, salinity increases again because the water near the ocean bottom originates from polar regions where it sinks during the winter.
Roughly 90 percent of the total volume of ocean is found below the thermocline in the deep ocean. Much of this deep ocean water is between 0-3 degrees Celsius (32-37.5 degrees Fahrenheit).
The “…clines”
Ready for yet more complexities? There are a couple of “clines” that override the three basic levels in order to describe gradients of change. One is the halocline, which is a depth range where salinity changes most rapidly.
And to package up the “clines” in a not-so-tidy package, the pycnocline encompasses both the halocline (salinity gradients) and the thermocline (temperature gradients). It refers to the rapid change in density with depth. Because density is a function of temperature and salinity, the pycnocline is a function of the thermocline and halocline.
Now, can we use all of this to go find more hungry fish? I’m working on it!
— 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.
