In most ocean regions the wind-driven circulation, which was the focus of discussion so far, does not reach below the upper kilometre of the ocean. Water renewal below that depth is achieved by currents which are driven by density differences produced by temperature (thermal) and salinity (haline) effects. The associated circulation is therefore referred to as the thermohaline circulation. Since these movements are mostly very sluggish, it is often impractible to use current meters to measure them directly; they are usually deduced from the distribution of water properties and the application of geostrophy.

The driving force for the thermohaline circulation is water mass formation. Water masses with well-defined temperature and salinity characteristics are created by surface processes in specific locations; they then sink and mix slowly with other water masses as they move along. The two main processes of water mass formation are deep convection and subduction. Both are linked to the dynamics of the mixed layer at the surface of the ocean; so it is necessary to discuss thermohaline aspects of the upper ocean first.

Oceanographers refer to the surface layer with uniform hydrographic properties as the surface mixed layer. This layer is an essential element of heat and freshwater transfer between the atmosphere and the ocean. It usually occupies the uppermost 50 - 150 m or so but can reach much deeper in winter when cooling at the sea surface produces convective overturning of water, releasing heat stored in the ocean to the atmosphere. During spring and summer the mixed layer absorbs heat, moderating the earth's seasonal temperature extremes by storing heat until the following autumn and winter, and the deep mixed layer from the previous winter is covered by a shallow layer of warm, light water. During this time mixing is achieved by the action of wind waves, which cannot reach much deeper than a few tens of meters. Below the layer of active mixing is a zone of rapid transition, where (in most situations) temperature decreases rapidly with depth. This transition layer is called the seasonal thermocline. Being the bottom of the surface mixed layer, it is shallow in spring and summer, deep in autumn, and disappears in winter, when heat loss at the surface produces instability and the resulting convection mixes the water column to greater depth (Figure 7.1). In the tropics winter cooling is not strong enough to destroy the seasonal thermocline, and a shallow feature sometimes called the tropical thermocline is maintained throughout the year.

The depth range from below the seasonal thermocline to about 1000 m is known as the permanent or oceanic thermocline. It is the transition zone from the warm waters of the surface layer to the cold waters of great oceanic depth The temperature at the upper limit of the permanent thermocline depends on latitude, reaching from well above 20 ºC in the tropics to just above 15 ºC in temperate regions; at the lower limit temperatures are rather uniform around 4 - 6 ºC depending on the particular ocean.

Below the surface layer which is in permanent contact with the atmosphere, temperature and salinity are conservative properties, ie they can only be changed by mixing and advection. All other properties of sea water such as oxygen, nutrients etc. are affected by biological and chemical processes and therefore non-conservative. Water masses can therefore be identified by their temperature-salinity (T-S) combinations (Figure 7.2).

Water mass formation by deep convection occurs in regions with little density stratification (ie mostly in polar and subpolar regions). When the water in the mixed layer gets denser than the water below, it sinks to great depth, in some regions to the ocean floor. The density increase can be achieved by cooling or an increase in salinity (either through evaporation or through brine concentration during freezing) or both.

Water mass formation by subduction occurs mainly in the subtropics. Water from the bottom of the mixed layer is pumped downward through a convergence in the Ekman transport and sinks slowly along surfaces of constant density (Figure 7.3).

Figure 7.4 gives a summary of water masses in the world ocean. Antarctic Bottom Water is formed mainly in the Weddell and Ross Seas by deep convection and fills all ocean basins below 4000 m depth; in the Pacific and Indian Oceans it is mixed with North Atlantic Deep Water, the mixture being known as Circumpolar Water. North Atlantic Deep Water is the product of a process that involves deep convection in the Arctic Ocean, the Greenland Sea and the Labrador Sea. Most Antarctic Intermediate Water is formed by deep convection east of southern Chile and west of southern Argentina and spreads into all oceans with the Circumpolar Current. Intermediate Water in the northern hemipshere may be formed by convection or subduction. Central Water, the water of the permanent thermocline, is formed by subduction in the subtropics. Mediterranean and Red Sea waters are intrusions of high temperature, high salinity waters from two mediterranean seas (see the discussion of mediterranean seas below).

It is worth stressing that the picture developed here is of a very schematic nature. The real ocean is a fluid in turbulent motion full of eddies, fronts and other instabilities. It should also be kept in mind that significant zonal (east-west directed) movement occurs in every ocean basin and that the schematic distribution shown in Figure 7.4 cannot depict the variations that occur from the eastern to the western coasts. However, as a summary of the principal features of the water masses in the world ocean it is correct and adequate.

A summary of the TS characteristics of all water masses is given in Figure 7.5. Antarctic Bottom Water is represented by a single TS point (in the white region of the salinity scale). Antarctic Intermediate Water also has its own TS point but is usually only seen as a salinity minimum in the TS-curve; the minimum is slowly eroded by mixing as the Intermediate Water progresses northward (Figure 7.6). The Central Waters are represented by TS curves rather than points (compare Figure 7.3).

A complete description of water mass movement requires horizontal property distributions as well as vertical sections and TS-diagrams. It is then seen that the path of Antarctic Bottom Water in particular is strongly affected by the topography. For example, the deep basins of the eastern Atlantic Ocean are separated from the Southern Ocean by a sill and cannot be reached by Antarctic Bottom Water directly. They are filled through a gap in the Mid-Atlantic Ridge near the equator known as the Romanche Fracture Zone; in other words, flow of Antarctic Bottom Water in the eastern South Atlantic Ocean is southward, from the equator toward the pole. In the Pacific Ocean, input is mainly along 170ºW (east of New Zealand), followed by spreading east and westward in the northern hemisphere; recirculation into the southern hemisphere occurs in the east. Input into the Indian Ocean is from the west, and in smaller quantities from the east.

Circulation in Mediterranean Seas

Mediterranean Seas are large bodies of water characterized by very restricted water exchange with the major ocean basins. This results in different hydodynamics and sets them apart from the remainder of the world ocean. While the circulation in most of the world ocean is dominated by wind-driven currents, the circulation in mediterranean seas is determined by thermohaline processes. Two basic types of circulation can be distinguished, the concentration basin and the dilution basin. Concentration basins occur where evaporation in the region exceeds precipitation; such mediterranean seas are therefore also sometimes called arid mediterranean seas. Examples are the (Eurafrican) Mediterranean Sea, the Red Sea and the Persian Gulf. Dilution basins occur when precipitation and river input exceed evaporation; such basins are therefore also known as humid mediterranean seas. Examples are the Black Sea, the Baltic Sea and the Australasian Mediterranean Sea (the seas of the Indonesian archipelago).

The circulation in mediterranean seas and their water exchange with the remainder of the world ocean differs strikingly between the two types (Figure 7.7). In concentration basins (Figures 7.8, 7.9), evaporation increases the salinity of the surface waters, raising their density and producing convection. Deep water renewal is therefore a nearly continuous process, and the waters of the basin are well ventilated (have relatively high oxygen content) at all depth.

In dilution basins, the freshening of the surface waters resulting from excess rain and freshwater input from rivers reduces the density of the surface layer. This prevents the freshened water to reach the deeper layers. The result is the establishment of a fresh upper layer and a strong halocline. Water below the halocline is renewed only very slowly through mixing across the halocline and inflow of oceanic water through the connecting strait. As discussed in Lecture 5, oxygen at depth is consumed by remineralisation of nutrients. Oxygen content is therefore very low when active ventilation is inhibited through the stable halocline (Figure 7.10). If the basin is large and the exchange with the open ocean very restricted, oxygen levels at depth can fall to zero, preventing the existence of higher marine life. Such conditions are occasionally found in some basins of the Baltic Sea. The Black Sea, which is more than 1500 m deep, is devoid of oxygen below 150 m depth.