The Arctic Ocean is salty, like other oceans, but generally has lower salinity than the global average. While average ocean salinity ranges from 32 to 37 practical salinity units (PSU), the Arctic Ocean’s surface layer is often between 28 and 33 PSU. Even lower values occur in coastal areas influenced by river plumes.
Factors Influencing Arctic Ocean Salinity
The Arctic Ocean’s lower salinity stems from geographical and climatic factors that introduce significant freshwater. A primary contributor is substantial inflow from large rivers, particularly those in Siberia and North America. Rivers like the Ob, Yenisei, Lena, and Mackenzie collectively deliver immense freshwater volumes, totaling about 1900 cubic kilometers per year. This accounts for approximately 60% of the annual river flow into the Arctic Ocean, diluting seawater near coastlines.
Seasonal melting of extensive ice also reduces Arctic salinity. As sea ice and glaciers melt, they release freshwater into the ocean, particularly in the surface layer. This less dense meltwater tends to remain near the surface, freshening the upper ocean. Over recent decades, increased freshwater volume from enhanced river discharge and ice melt has measurably decreased the Arctic’s overall salinity.
Limited evaporation also contributes to the Arctic’s lower salinity. Extremely cold temperatures and extensive ice cover significantly suppress water evaporation compared to warmer oceans. Elsewhere, evaporation concentrates salts, increasing salinity. Reduced evaporation in the Arctic prevents this, contributing to its fresher state.
The Arctic Ocean’s semi-enclosed geography also influences its salinity. It connects to the Pacific Ocean via the shallow Bering Strait and to the Atlantic Ocean through the Fram Strait, Davis Strait, and Barents Sea Opening. While exchange occurs with saltier Atlantic and Pacific waters, these pathways are restricted. The shallow Bering Strait, for instance, primarily allows low-salinity Pacific water to enter. This limited exchange helps trap freshwater within the Arctic Basin.
Ecological and Climatic Significance of Arctic Salinity
Lower Arctic salinity significantly influences sea ice formation. Unlike freshwater, which freezes at 0°C (32°F), seawater freezes at a lower temperature, typically around -1.8°C (28.8°F). Lower salinity further depresses this freezing point, allowing less saline Arctic water to freeze more readily than saltier ocean water. This impacts the timing and extent of sea ice formation, important for the Arctic ecosystem and global climate regulation.
Salinity also influences the stratification, or layering, of water masses in the Arctic Ocean. Less dense, fresher surface water forms a distinct layer above denser, saltier water below. This strong layering, known as a halocline, acts as a barrier, limiting vertical water mixing. This restricted mixing affects nutrient and oxygen distribution throughout the water column, preventing nutrient-rich deeper waters from reaching the sunlit surface where marine life, like phytoplankton, would utilize them.
The Arctic’s salinity and temperature conditions support specialized marine ecosystems. Many organisms, from microscopic algae and zooplankton to fish and marine mammals, have adapted to thrive here. For example, some Arctic fish possess antifreeze proteins in their blood, enabling survival in sub-zero temperatures. The halocline’s stability also creates distinct habitats, influencing species distribution at different depths.
The outflow of cold, less saline Arctic water influences global ocean circulation patterns. This freshwater export, particularly into the North Atlantic via straits like the Fram and Davis Straits, can impact North Atlantic water density. Changes in water density, driven by temperature and salinity, are key to the thermohaline circulation, often called the “global conveyor belt.” An increase in Arctic freshwater could potentially slow this circulation, which distributes heat globally and influences regional climates.