The distribution and variation of oceanic salinity are attracting increasing scientific attention due to their relationship to the global water cycle and its influence on circulation, mixing, and climate processes. Over the past 50 years, high-latitude oceans have become fresher while the subtropical oceans have become saltier. This change is slowly spreading into the subsurface ocean layers and may be affecting the strength of the ocean's thermohaline overturning circulation.
As a state variable that partly governs sea water density, salinity is directly linked to the ocean dynamics through the distribution of mass, and provides an important window onto the global water cycle. Regional differences in evaporation (E), precipitation (P), and land runoff (R) affect the freshwater content and hence salinity of surface water. Direct observations of P-E over the ocean remain prone to large uncertainty, yet the oceans harbor 97% of the planet's free water Proand experience 86% of the global evaporation and 78% of global precipitation. Increasing evidence suggests that terrestrial flood and drought cycles are closely linked to variations in the much larger oceanic water cycle, which, in turn, are linked to ocean processes for sequestering and transporting heat and influence climate variability in ways that are not yet well understood.