Vertical structure in the ocean can be divided into three density zones: an upper mixed layer, a layer of changing density, and the deep layer.
The upper mixed layer is a region of fairly uniform density because of the action of wind mixing, waves, and currents. Depending on place and season, it can vary from being very shallow (<30 m) to depths of greater than 200 m and is the only region of the ocean that interacts with the atmosphere. The upper mixed layer contains about 2% of the volume of the ocean.
Beneath the upper mixed layer is the pycnocline, where density increases rapidly with depth and the increasing density acts as a barrier between the upper and deep layers. If declining temperatures are mainly responsible for the increasing density, the pycnocline is also a thermocline. If salinity is the major cause, it is a halocline. The pycnocline extends from the bottom of the mixed layer to the cold and stable deep zone. In most regions of the world ocean, the pycnocline ends at about 1000 m of depth and contains about 18% of the world ocean volume.
Table 1.2 Boundary currents in the Northern Atlantic and Pacific Oceans. Data from Gross (1990), Schwartzlose and Reid (1972), Sverdrup et al. (1942), Zhou et al. (2000).
| Location | Current | Speed (cm s−1) | Transport (sv a) | Common features | Special features |
|---|---|---|---|---|---|
| Western Atlantic | Gulf Stream | 120–140 | 55 | Narrow (100–150 km) and deep (2 km) | Sharp boundary with coastal circulation system; little or no coastal upwelling; waters tend to be depleted in nutrients, unproductive |
| Western Pacific | Kuroshio Current | 89–180 | 65 | ||
| Eastern Atlantic | Canary Current | 10–15 | 16 | Broad (~1000 km) and shallow (<500 m) | Diffuse boundaries separating from coastal currents; coastal upwelling common |
| Eastern Pacific | California Current | 12.5–25 | 10 |
a sv = sverdrup (1 sv = 1 million cubic meters per second)
Figure 1.10 Upwelling and downwelling. (a) Ekman transport caused by wind blowing from the north moving surface water offshore, results in deeper water upwelling to the surface in the northern hemisphere. (b) Ekman transport due to winds blowing from the south moves surface water onshore and subsequently down slope.
In the cold and relatively stable deep zone, temperature varies very little with depth and density increases only gradually. The deep zone contains the remaining 80% of the global ocean at depths greater than 1000 m, well away from surface influences.
Water Masses
The global ocean has a variety of different water masses, parcels of ocean identifiable by their temperature, salinity, and density characteristics that determine their place within the vertical structure of the oceanic water column. It is important to keep in mind that certain properties of a water mass are determined during its sojourn at the surface, e.g. temperature and salinity. Those are conservative properties and are changed only when a water mass mixes with another. In the deep ocean, water masses can mix only when their densities are roughly equal, otherwise they remain stratified. Therefore vertical movements of water, away from or toward the surface, require a weakly stratified water column, such as is found near the poles. Figure 1.12 is a standard diagram of oceanic temperatures at depth at low, middle, and high latitudes.
Figure 1.11 T‐S diagram. Temperature–salinity plot from an oceanographic station in the Atlantic. The axes represent salinity (X) and temperature (Y). The curved lines represent isopycnals (equal density). AAIW, Antarctic Intermediate Water; NADW, North Atlantic Deep Water; AABW, Antarctic Bottom Water.
Source: Brown et al. (1989), figure 6.26 (p. 191). Reproduced with the permission of Pergamon Press.
Five generic water masses are found at temperate and tropical latitudes. Surface water extends from the surface to about 200 m depth and includes the seasonal thermocline. Central water extends from just below surface water to the bottom of the permanent thermocline, usually at about 1000 m. Intermediate water resides below central water to a depth of about 1500 m, where deep water begins. Deep water is found below intermediate water but is not in contact with the bottom; it is found between 1500 and 4000 m. Deepest of the oceanic layers is bottom water, which is in contact with the seafloor.
Each of the generic water masses has a large number of specific examples that can be identified by their temperature and salinity; they are named for their source region. Upper water masses (surface and central water), shown in Figure 1.13, are numerous and correspond fairly closely with surface currents. Intermediate waters are mapped in Figure 1.14. It is important to note the difference in areal extent between upper and intermediate waters. Antarctic Intermediate Water (AAIW), for example, is widespread at intermediate depths throughout the world ocean. The temperature and salinity characteristics that define AAIW (temperatures of 2–4 °C and salinities of about 34.2) provide a fairly uniform environment for the species that live within it. It is thus easy to understand why deeper‐living oceanic species are typically far more wide‐ranging than those inhabiting surface waters (Briggs 1974).
Figure 1.12 Standard depth profiles of temperature at low, middle, and high latitudes.
Deep and bottom water masses are even less numerous and more widespread than intermediate water masses. North Atlantic Deep Water (NADW) forms in the north Atlantic above 60 °N when cold, saline waters from the Norwegian and Greenland seas spill over the mid‐Atlantic ridge into the depths of