1.4 The Ocean in Motion!

Teacher Background

The interior of Antarctica is a lifeless desert, except for human research stations. But the coasts, especially the oceans themselves, are rich with life. To appreciate the research done at Palmer and on board the Polar Duke, it helps to understand some pretty astonishing and sometimes counter-intuitive phenomena.

When we see the beautiful, deep blue colors of a tropical ocean, we assume it's teeming with life. But relatively speaking, blue waters are the deserts of the sea. The farther down light penetrates, the bluer the water seems. This means there's no microscopic life to stop the light. Murky, dark-green oceans are usually nutrient-rich oceans (full of microscopic life). The Arctic and Antarctic Oceans are, at certain seasons, more productive than any others on Earth. Because of its size, the Antarctic is, on average, the richest ocean for life. And, because of the dynamics of hot and cold water, Antarctic waters also function as a gigantic planetary Mixmaster, revitalizing sterile waters which eventually reach out across the world.

At the equator, water is warmed to great depths without any sharp separation from a cold layer underneath. Except for storm action, it doesn't cool much in winter and consequently sink and incorporate nutrients from the bottom waters, into which decaying life and dissolved mineral are mixed. In fact, rich bottom water seems to flow right from the northern to the southern oceans in one continuous, bottom-hugging layer, passing right underneath warm, equatorial regions without much intermixing.

Westerly winds drive the surface waters of the Antarctic Ocean eastward, round and round the continent. The very cold bottom layer is heavy with salt and decaying organisms. (As ice forms near the surface, more and more salt sinks to the bottom, and flows away north.) Antarctic surface water becomes less salty by dilution with heavy rains and melting ice. This lightweight water also drifts northward rather than sinking. With both surface and bottom water moving northward, bottom water from warmer regions is drawn down between the layers. This warmer, less salty, bottom water bumps against the Antarctic continent and rises, creating a continuous upwelling of nutrient-rich water throughout the year. This inflowing water has moved as a huge mass all the way from the North Atlantic in a journey which takes several thousand years.

The density of sea water depends primarily upon how much salt it contains and/or at what temperature it's measured. Cold sea water is more dense than warm sea water of the same salinity. It's important to understand how density differences, mixed layer depths, and heating, cooling and ice formation affect the mix in the water column because these factors are linked to the growth of phytoplankton-the foundation of the food chain-in the Southern Ocean, as we'll see in Activity 1.5.

If the top layer of the water column is shallow, the microscopic plants in that layer will be mixed around and will receive lots of light. If the mixed layer depth is great-600 m for example, as it was in the Ross Sea in early 1996 before the bloom started-the plants will spend part of the day in the lighted part and part of the day in the darker part of the water column and will not receive enough daylight to grow. When the difference in density between two layers of water in the water column is great, the water column is said to be "stable."


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