1.a. Accuracy and precision. Units.
1.b. Pressure and depth
Figure. Depth versus pressure
calculated from a CTD profile near Japan.
1.c. Temperature and potential temperature
1.d. Salinity and conductivity
Figure. Gulf of Alaska salinity, conductivity, temperature.
1.e. Density, potential density and neutral density
Figure. Density and freezing point of seawater and freshwater
Figure. Potential density relative to 0 dbar as a function
of temperature and salinity.
Figure. Potential density relative to 4000 dbar as a function
of temperature and salinity.
(a) Density as a function of temperature
at pressure = 0, salinity = 35.0
(b) Density as a function of pressure at temperature = 0.,
salinity = 35.0,
(c) Density as a function of temperature
of freshwater at pressure = 0, and
(d) Freezing point temperature as a fucntion of salinity.
Figure. Vertical section of potential density relative to 0 dbar along approximately 25W in the Atlantic Ocean. Note the deep inversion of sigma theta with depth in the South Atlantic due to use of surface pressure for referencing the density.
Figure. Vertical section of potential density relative to 4000 dbar along approximately 25W in the Atlantic Ocean. Note that there is no deep inversion in this sigma 4 since an appropriate local reference pressure is used.
1.f. Sound speed
North Pacific profile of sound speed, Brunt-Vaisala frequency,
potential temperature, salinity.
1.g. Tracers
Oxygen - non-conservative tracer. Source is primarily air-sea interaction, some subsurface source in outgassing by plankton. Oxygen is consumed in situ. Oxygen content decrases with age, so it can be used in a rough way to date the water. It is not a good age tracer because the consumption rate is not a constant (what does it depend on?), and since waters of different oxygen content mix, the age is not simply related to content.
Per cent saturation of oxygen depends strongly on temperature (show figure). Cold water holds more oxygen.
Nutrients: nitrate and phosphate. Also non-conservative. Nitrate and phosphate are completely depleted in surface waters in the subtropical regions where there is net downwelling from the surface and hence no subsurface source of nutrients. In upwelling regions there is measurable nitrate/phosphate in the surface waters due to the subsurface source (figure from Hayward and McGowan; other figures based on woce data). Nitrogen is present in sea water in dissolved N2 gas, nitrite, ammonia, and nitrate, as well as in organic forms. As water leaves the sea surface, particularly the euphotic zone, productivity is limited by sunlight and nutrients are "regenerated". That is, the marine snow is dissolved by what and produces nitrate and phosphate. Nitrate and phosphate thus increase with the age of the water. Vertical sections and maps of nitrate and phosphate appear nearly as mirror images of oxygen, but there are important differences in their patterns, particularly in the upper 1000 meters; vertical extrema are not always co-located and sometime large multiple extrema appear on one parameter and not in the others (e.g. in oxygen but not in nitrate/phosphate).
Nitrate/oxygen and phosphate/oxygen combinations - nearly conservative tracers. Nitrate/oxygen and phosphate/oxygen are present in seawater in nearly constant proportions, given by the Redfield ratio. There are small variations in this ratio, with particularly large deviations near the sea surface. Because of the near constancy of this ratio, a combination of nitrate and oxygen and of phosphate and oxygen is a nearly conservative tracer (Broecker).
Silica - non-conservative. In seawater it is present as H2SiO4 (silicic acid) rather than silicate (SiO3), but many people use the term silicate. This nutrient is also depleted in surface waters similarly to nitrate and phosphate - completely depleted in downwelling areas and small but measurable quantities in upwelling areas. Subsurface distributions of silicate look something like nitrate and phosphate and mirror oxygen since silicate is also regenerated in situ below the euphotic zone. However, silica in marine organisms is associated with skeletons rather than fleshy parts and so dissolves more slowly in the water. Much of the silica thus falls to the bottom of the ocean and accumulates in the sediments (map of types of sediments). Dissolution from the bottom sediments constitutes a source of silicate for the water column which is not available for nitrate, phosphate or oxygen. Another independent source of silicate are the hydrothermal vents which spew water of extremely high temperature, silica content, and helium content, as well as many other minerals, into the ocean. The three named quantities are used commonly to trace hydrothermal water.
Other tracers used commonly for ventilation and deep water circulation include chlorofluorocarbons, tritium, helium-3 and carbon-14. CFC's and tritium are strictly anthropogenic. Their source functions have been well described and they are used to trace recently ventilated waters into the ocean, and various combinations of CFC's, tritium/helium3 are used to attach ages to water parcels, although not without approximation.
Vertical profiles of potential temperature and salinity:
Western subtropical gyre
Eastern subtropical gyre
Western subpolar gyre
Eastern subpolar gyre
3.b. Volume, mass, heat and freshwater transport
4.b. Heat gain/loss, evaporation/precipitation
