There are two basic circulation systems in our oceans. One is the wind-driven surface circulation, and the other is the deepwater density-driven circulation. It is controlled primarily by differences in temperature and salt content (thermohaline circulation - "thermo" for temperature and "haline" for salinity). This section focuses on density-driven circulation.
The ocean is a single body of water, but it is not homogeneous. There are water property differences (such as temperature, salinity, and density) in various parts of the ocean. Oceanographers define distinct water masses based on their physical and chemical characteristics. Temperature and salinity are the primary components used to recognize a particular water mass. Only about 10% of the ocean volume is involved in wind-driven surface currents. The other 90% circulates due to density differences in water masses (primarily caused by differing temperatures and salinities).
Penguin Note: Why is warmer water less dense? Density is a ratio (mass per unit volume). When water warms up, its molecules vibrate and move around more. As they do this, they take up more volume (but gain no more mass), causing the water density to decrease! So...why is saltier water more dense?
Denser water masses sink below lighter ones. A water mass seeks its appropriate density level and then spreads laterally. When one water mass sinks, another rises elsewhere. When water masses run into each other at a convergence zone, the less dense water mass floats on top of the denser one.
Ultimately, all water masses gain their particular characteristics because of interaction with the surface during their development. Different inputs of freshwater, different patterns of precipitation and evaporation, and different temperature regimes at the surface all combine to create distinct water masses.
Once water masses sink, their temperature and salinity are modified primarily by mixing with other water masses. In the deep open ocean there is little energy to mix up the various layers of water (no wind blows down there!) so this process can be quite slow. Individual water masses can retain distinct properties for a long time, allowing oceanographers to identify specific ones.
Penguin Note: It takes about 1,000 years for newly formed bottom water to transit through the ocean before it reaches the surface again!
Let's meet a few of these water masses! You'll notice their names generally incorporate information about the depth levels they occur at. These levels are:
North Atlantic Deep Water forms in the region around Iceland. It actually is modified from another water mass - North Atlantic Intermediate Water - that has come near the surface and has been cooled by the contact with the air. The cooling increases the density of the water mass and it sinks (salinity of 35 parts per thousand and temperature of 3°C or 37.4°F).
Mediterranean Outflow Water is a deep water mass that results from high salinity, not cooling. The high evaporation rate in the Mediterranean increases salinity. As the water leaves the Mediterranean basin it spreads into the Atlantic. Mediterranean Outflow Water is saltier (38 parts per thousand) than the North Atlantic Deep Water, but much warmer, so it floats above it.
Antarctic Bottom Water is the most distinct of all deep water masses. It is cold (-0.5°C or 31.1°F) and salty (34.65 parts per thousand). It forms at the edge of the Antarctic continent and flows under all other water masses into the deep basins as it moves equatorward, hugging the bottom. Antarctic Bottom Water travels far from its origin, penetrating into the North Atlantic and North Pacific basins. A cold, salty deep water mass also forms in the Arctic, but the Arctic basin keeps most of the water contained.
Entry and distribution of Mediterranean Outflow water in the Atlantic Ocean (modified from Davis, 1991). Because the flow is restricted by the Straits of Gibraltar (and the sill), and the Mediterranean Sea is in a region that has high evaporation rates, the water of the Mediterranean is very salty. It is a distinct water mass in the global ocean.
Penguin Note: The average temperature for all ocean water is 3.51°C and its average salinity is 34.72 parts per thousand. For the ocean surrounding Antarctica (south of 55°), the average temperature is 0.71° and the average salinity is 34.65 parts per thousand. Of the major ocean regions, the North Atlantic is the warmest and saltiest (averages: 5.08°, 35.09 parts per thousand).