ocean water Across the Earth's surface the sea water in each ocean basin has different gross physical and chemical properties. The most important properties for defining a particular element of sea water are its potential temperature and salinity. Potential temperature is the temperature that a volume of water would have at a standard reference level (usually the surface), and it is used instead of the temperature
in situ (i.e. what is actually measured) to calculate density; this takes into account the increase of temperature
in situ with pressure through adiabatic heating (i.e. without exchanging heat with the surroundings). The two properties are conservative: that is, they are altered only at the boundaries of the oceans (e.g. the surface). Away from the surface, the potential temperature and salinity of a volume of water can be significantly altered only by mixing with a volume of water with different potential temperature (
T) and salinity (
S) characteristics.
The differences in
T and
S across the surface of an ocean are due principally to the variation in climatic conditions around the world. The water in the Mediterranean Sea provides a good example. The Mediterranean is an enclosed basin and the ambient climate is relatively dry and hot. The warm atmospheric temperatures result in an excess of evaporation of the surface waters over precipitation, which in turn increases the salinity of the Mediterranean surface waters. Therefore, compared to the North Atlantic waters, the Mediterranean waters are high in both
T and
S. The excess of
T and
S increases the density of the Mediterranean water, and at the Straits of Gibraltar the water sinks below Atlantic water as it enters the North Atlantic. Once away from the surface, the water can be tracked throughout the North Atlantic by its
T and
S values because they are conservative.
There are many areas of the world where uniform climatic conditions create a large volume of water with uniform
T and
S characteristics, this is called a water mass. The variation in climatic conditions around the world enables us to map the geographical distribution and subsequent movements of water masses. Temperature and salinity are not the only properties we can use to track a water mass. We can also use the oxygen and the varying amounts of nutrients such as phosphates and nitrates dissolved in the water. We must be careful, however, for the nutrient concentration and oxygen are non-conservative properties and are depleted by biological activity. Modern techniques have been developed using man-made tracers such as chlorofluorocarbons (CFCs) which are used in refrigeration and as propellants in aerosols. The chlorofluorocarbons have received widespread news coverage as they are thought to be responsible for the destruction of the Earth's ozone layer, but they are also a useful oceanographic tool. The concentration of CFCs in the atmosphere has increased almost exponentially since the 1930s but different chlorofluorocarbons have been released at known different rates. As different CFCs are absorbed at different rates by sea water, it is possible to ‘age’ the ocean water by calculating the ratio of different chlorofluorocarbons: that is, to calculate roughly when the water was last in contact with the atmosphere during the past fifty years or so. Using CFC ratios, oceanographers have determined that there has been a decrease in deep water formation in the Greenland Sea. Using other tracers, such as carbon-14, some deepest water can be ‘aged’ at thousand of years.
To make a point measurement of the water characteristics, oceanographers use an instrument called a CTD. The acronym CTD denotes conductivity, temperature, and depth, and the instrument measures these three parameters. From the conductivity of the sea water we can calculate the salinity. The CTD is lowered from a ship, usually down to full ocean depth, which in the case of water 4000 m deep (roughly the average ocean depth) can take as long as 4 hours. The resulting temperature and salinity profiles can be difficult to interpret, but by far the clearest way to look at the vertical temperature and salinity variation is by plotting the potential temperature against salinity.
Temperature–salinity plots are a common way of interpreting CTD profiles. Because density is a function of temperature and salinity it, too, appears on the plot. It was realized in the 1930s that mixing in the ocean will preferentially take place along lines of equal density across the ocean, rather than along lines of equal depth. Once formed under particular climatic conditions in a particular region, sea water can descend into the deep ocean along density surfaces. As sea water from the high latitudes is cold, relatively fresh, and more dense than the warm water at tropical latitudes, the water from high latitudes descends beneath the more temperate waters. This is one way in which the deep ocean can communicate with (‘ventilate’) the atmosphere.
Water types of different densities from layers throughout the depth of an ocean.
A CTD measurement from the central Atlantic Ocean plotting potential temperature against the salinity shows that the water is structured in such a way that there is a layer of water from the central Atlantic, a layer of water influenced by Mediterranean water, a water type called Antarctic intermediate water (AAIW), then North Atlantic Deep Water (NADW), and finally Antarctic Deep Water (AADW). With such a vertical distribution it is easy to see the value of CFC and other tracer measurements for ‘ageing’ the water types. Because of the mode of formation of a water mass, each type with its own uniform characteristics will by defnition appear as a point in what is called
T–
S space. When two water types of differing
T and
S are mixed together, the resulting water will lie on a straight line between the two source masses (Fig. 1). The distance along the mixing line from each source indicates the relative proportions of each water type in the water sampled. The same principle applies for a more complex mixture of three or more water masses.
Another conservative tracer for a water mass is potential vorticity. This is an oceanic expression of the conservation of angular momentum; it is equal to the absolute vorticity of a water parcel plus the planetary vorticity, all divided by the thickness of the layer. As the relative vorticity is much smaller than the planetary vorticity in most situations, the potential vorticity becomes the Coriolis parameter divided by the thickness of the layer. Geographical maps of potential vorticity can be used to trace water masses. For example, Labrador Sea Water is a well-known North Atlantic potential vorticity minimum. This is not, however, as common a technique as temperature and salinity mapping.
Throughout the oceans, temperature and salinity can typically range from –1.9 to 30 °C and from 30 to 38 practical salinity units (PSU). There are specific areas where water falls outside this range, for example, the Red Sea has ocean temperatures of up to 45 °C and a salinity that is occasionally greater than 40 PSU, whereas near rivers the salinity can fall to almost 0 PSU. These areas are exceptions; 90 per cent of the world's ocean waters falls into the temperature range –1.9 to 10 °C and salinity range 34 to 35 PSU.
By dividing the temperature and salinity ranges into small increments it is possible to calculate the volume of ocean water of a particular character. An accurate volumetric cen-sus of the oceanic water types was calculated in 1981 by Worthington using this method and setting the temperature increment as 0.1 °C and the salinity increment as 0.01 PSU.
Mark A. Brandon
Bibliography
Pickard, G. L. and and Emergy, W. J. (1982) Descriptive physical oceanography. Pergamon Press, Oxford.
Warren, B. A. and Wunsch, C. (eds) (1981) Evolution of physical oceanography. MIT Press, Harvard, Mass.
