Joseph J. Torres

Life in the Open Ocean


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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.

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)

Schematic illustration of upwelling and downwelling.

      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

Schematic illustration of t-S diagram. Temperature–salinity plot from an oceanographic station in the Atlantic.

      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.

Schematic illustration of standard depth profiles of temperature at low, middle, and high latitudes.