The Dynamic Ocean Study Guide

How the Deep Ocean Mixes

The ocean also has a second, very different kind of circulation that has nothing to do with the winds and would exist even in the absence of wind. The technical name for this circulation is the thermohaline circulation. The word is a combination of "thermo" for temperature and "haline" for salt, because these factors determine the density of water.

When water gets cold, say in winter at high latitudes, it becomes denser and tends to sink. When sea ice forms, also in winter at high latitudes, the freezing of fresh water into ice leaves the remaining ocean water saltier. At the same temperature, saltier water is heavier water and therefore tends to sink.

In summary, the driving factor behind the thermohaline circulation is the fact that denser water will tend to sink. Both of the processes of cooling water and increasing its salinity cause it to have increased density, and both processes occur at the high latitudes during winter.

These two factors create the densest water at certain high latitude regions, which are called sites for deep water formation or bottom water formation. One site is in the far North Atlantic, around Greenland and Labrador. The especially salty water in the North Atlantic comes not only from the freezing out of fresher water into sea ice in winter, but also from the fact that water from the relatively salty Mediterranean Sea also manages to contribute (it mixes into the north Atlantic). You see the situation is complex, and oceanographers are always working on refining their understanding of the intricacies of ocean circulation. But certain facts, which we will now review, have become well established.

Water of the far North Atlantic, during winter, becomes very dense and sinks to tremendous depths, almost all the way to the bottom of the ocean (but not quite, for reasons that will become clear in a moment). This is called North Atlantic Deep Water. The water travels south, deep under the ocean's surface, pushed by new deep water behind it, and unable to move upward because its heavy density holds it at a certain depth in the ocean. It even crosses the equator at a depth of 3 kilometers under the surface and enters the Southern Hemisphere and passes into the South Atlantic Ocean.

Then the North Atlantic Deep Water gets mixed into the Antarctic Circumpolar Ocean water. Around the continent of Antarctica is the second major place on Earth where deep or bottom water forms. This region is bitter cold in winter, and the waters around Antarctica become the coldest waters in the ocean. The Antarctic Circumpolar Current (recall this region also goes by that name) is so well mixed that virtually the entire water column for many hundreds of meters could be considered a mixed layer; and the vertical mixing is quite vigorous almost all the way to the bottom.

Some of the North Atlantic Deep Water that enters the Antarctic Circumpolar Ocean is spun up to the surface where it cools during winter to become not only the coldest water on Earth, but also the most dense water on Earth. It sinks downward and then is known as Antarctic Bottom Water. Because it is the densest water on Earth, it dominates the sinking process and reaches deeper than does the North Atlantic Deep Water. In fact, some of the Antarctic Bottom Water travels north into the Atlantic Ocean underneath the North Atlantic Deep Water.

The Antarctic Bottom Water also travels into the Pacific and Indian Oceans. It goes all the way along the bottom in a wide swath a kilometer or so in vertical extent, reaching as far as the North Pacific.

The northern part of the Indian Ocean is still in the tropics (near southern India), so its surface waters never get cold enough to become deep water. But what about the North Pacific? It also experiences intense winters (think Alaska). Well, the deep and bottom waters of the world are, in a sense, in competition with each other. The densest waters make it to the bottom and become the bottom water. The next densest waters make it not quite as far and become the deep water. The waters of the North Pacific get not quite as dense as those that become either the North Atlantic Deep Water or the Antarctic Bottom Water. It is possible that, at other times in Earth's history, when conditions of climate, rivers, and continental positions were different, the North Pacific did produce deep or bottom water.

Consider the geometry of the ocean. Its average depth is about 4 kilometers, or about 2.5 miles. And the typical width of an ocean is many thousands of miles. The distance from the places that form the North Atlantic Deep Water to the equator is about 4,000 miles or more. Consider now the ratio between the ocean's depth and its length, using . That's a ratio of 0.000625, or . The ocean is not deep at all, compared to its length! And yet the ocean's layers of density cause it to be so stratified that the deep and bottom waters can travel thousands of kilometers and stay at their depths, confined within those layers of density. Like the solid earth, the ocean is sratified by its density.

A dramatic demonstration of this stratification can be seen by oceanographers who pull up samples of deep water from high-tech buckets that can be lowered down into the ocean deep, then closed to trap the water they encounter. When these insulated buckets are hoisted to the surface and opened up, the water inside is very cold, just a couple of degrees above freezing. This happens even at the equator, where some of the warmest water on Earth is at the surface!

We've now discussed the deep and bottom waters of the world, traveling around way under the ocean. But that situation can't go on. New deep and bottom water has to come from the surface and that requires taking as well as replacing surface water. How do the deep and bottom waters of the world circulate back to the surface? The answer is little by little, gradually here and there. The amount of bottom water that reaches the North Pacific, for example, is less than the bottom water that entered the South Pacific from the Antarctic Circumpolar Ocean. As the bottom water travels northward into the Pacific, it sheds some of its mass upward toward the surface. This happens because of mixing processes, similar to the way that the Gulf Stream loses some of its flow as it mixes during its travels.

So the return flow to the surface of the deep and bottom waters occurs everywhere. This is different from the situations that form the deep and bottom waters, which happen only in special local places in the North Atlantic and around Antarctica. The return flow to the surface is distributed throughout the world's oceans and is given the term upwelling.

Recall that the levels of nutrientions phosphate and nitrate are almost zero at the ocean's surface but are high down deep in the ocean. How do the nutrients get back to the surface from which they were removed by photosynthesis? The answer is via the ocean's upwelling. The upwelling of the deep waters, which happens everywhere, carries the phosphate and nitrate nutrients back to the surface to be used again by the phytoplankton.

The thermohaline circulation is very powerful, but the ocean is huge. It takes, on average, about a thousand years for the thermohaline circulation to make a complete cycle around the ocean. The stirring time of the world ocean is thus about 1,000 years. In that time period, the entire ocean, from its surface to its deep abyss, is mixed. That seems like a long time to us, but it's short compared to the time scales of geology. In fact, that time is almost instantaneous.

Practice problems of this concept can be found at: The Dynamic Ocean Practice Questions