Since the Challenger expedition in 1877, when the chemical composition of seawater was first reported, no changes in the composition of seawater have been observed. Thus, it can be supposed that for the time-scales pertinent to climate change, viz., decadal to centennial, salinity behaves as a conservative tracer.
Thus, its time evolution is given by the three-dimensional transport of salinity by advection (as water parcels carry properties) or diffusion (tendency to smooth salinity gradients even in still water). At the surface of the oceans (up to a depth where the turbulent action of the wind is balanced by the laminarity of the stable ocean stratification), salinity concentrations are also modified by the dilution/concentration resulting from mass fluxes through the air-sea interface such as evaporation and precipitation, river runoffs, and the thawing/ freezing of ice caps. In open oceans, the lowest values of salinity (below 30 psu) are found at high latitudes and at the mouth of the largest rivers. The highest salinities are found in the subtropics (over 35 psu), where evaporation dominates. In the tropics, which tend to be regions of strong precipitation, salinity is around 34 psu.
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