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Oceanic waters are stratified by density, which is mainly a function of salinity and water temperature. The colder or saltier a parcel of water is, the greater its density. Temperature–salinity plots, or T‐S diagrams, help characterize ocean layering and water‐column characteristics. Figure 1.11, a T‐S curve for an oceanographic station in the Atlantic, is an example. Note that the lines of equal density, or isopycnals, each comprise a variety of temperatures and salinities; the same density can result from many different temperature–salinity combinations.

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.