The Arctic Ocean is a unique body of water that is salty, but its surface layer is notably less saline than the rest of the world’s oceans. Salinity, the measure of dissolved salt content, is expressed in practical salinity units (psu). The global ocean average hovers around 35 psu, reflecting the concentration of dissolved minerals like sodium chloride. In contrast, the surface waters of the Arctic Ocean frequently range between 28 and 33 psu, dropping lower near coastlines and river mouths. This relatively low surface salinity is a defining characteristic of the Arctic marine environment.
Major Sources of Freshwater Dilution
The primary reason for the Arctic Ocean’s reduced salinity is the massive and consistent influx of freshwater from surrounding landmasses. The Arctic Ocean basin receives approximately 11% of the total global river discharge, despite making up only about 1% of the world’s ocean volume. This disproportionate input acts as a constant diluting agent for the surface waters in this semi-enclosed basin.
Continental runoff is delivered by major river systems across Eurasia and North America. Four of the largest contributors are the Yenisei, Ob, Lena, and Mackenzie Rivers, which collectively discharge approximately 4,300 cubic kilometers of freshwater annually. This freshwater, being less dense than saltwater, tends to spread out across the surface of the ocean, lowering the average salinity.
A second major source of dilution comes from the melting of land-based ice, primarily the Greenland Ice Sheet. Unlike sea ice, land ice is composed of non-saline water accumulated from snowfall. As glaciers and ice sheets melt, this pure meltwater flows directly into the Arctic and surrounding seas. This glacial runoff represents a net addition of freshwater to the ocean system, contributing significantly to surface dilution.
The Salinity Impact of Freezing and Melting Sea Ice
The seasonal cycle of sea ice formation and decay plays a dynamic role in regulating Arctic Ocean salinity. When seawater begins to freeze, the crystalline structure of the ice lattice naturally excludes salt ions, a process known as brine rejection. The resulting sea ice is largely composed of nearly pure freshwater.
As salt is expelled from the forming ice, it concentrates in the remaining unfrozen water immediately beneath the ice sheet. This localized concentration of salt creates dense, highly saline water masses that sink toward the deeper layers of the ocean. This process increases the salinity of the deep water locally, contributing to the formation of dense water that can drive ocean circulation patterns.
Conversely, when the sea ice melts during the summer months, it releases the freshwater trapped within its structure back into the surface layer. This meltwater acts as an input, reducing the surface salinity during the warmest season. The freezing and melting cycle continuously redistributes salt throughout the water column, lowering surface salinity while increasing the salinity of the water below.
Physical Barriers and Water Layering
The persistence of the low-salinity surface layer is maintained by the unique physical geography and hydrodynamics of the Arctic basin. The basin is relatively isolated, with restricted connections to the global ocean that limit the inflow of highly saline water from the Atlantic and Pacific. For example, the exchange with the Atlantic Ocean is primarily funneled through the narrow Fram Strait and the shallow Barents Sea.
The Pacific Ocean inflow is similarly restricted, passing only through the extremely shallow and narrow Bering Strait. This geographical confinement prevents rapid, large-scale mixing with the saltier global ocean water masses. The limited exchange allows the massive freshwater inputs from rivers and ice melt to accumulate and remain within the Arctic basin.
Strong density stratification, particularly the formation of the halocline, is the most important mechanism for maintaining low surface salinity. The halocline is a distinct layer where salinity increases rapidly with depth, creating a density barrier. Colder, less-dense, low-salinity surface water floats atop the warmer, denser, and saltier Atlantic water masses flowing into the deep Arctic basin. This density difference is dominated by salinity, giving the Arctic Ocean a high degree of stability. The halocline acts like a lid, preventing the fresh surface layer from mixing with the warmer water below, effectively trapping freshwater inputs near the surface.