The Arctic Ocean is a unique body of water, and its salt content, or salinity, is often misunderstood. Salinity is the measure of dissolved salts, primarily sodium chloride, in a given volume of water. Arctic water is saline, but it is the least salty of the world’s major oceans. This lower salt concentration is a defining feature of the Arctic, stemming from its distinct geography and frigid climate, and creating a highly stratified water column that influences sea ice formation and global ocean circulation patterns.
The Arctic’s Salinity Profile
Oceanographers use the Practical Salinity Unit (PSU) to measure the salt content of seawater, a dimensionless quantity that roughly corresponds to grams of salt per kilogram of water. The global average salinity for the world’s oceans is typically around 35 PSU.
In contrast, the surface waters of the Arctic Ocean are significantly less salty, generally ranging from 28 to 33 PSU. The reduced salinity is most pronounced near coastlines and river mouths, where the reading can drop well below 28 PSU. Even with this lower concentration, the water remains far from freshwater, which has a salinity near zero.
Salinity is a primary factor determining water density, especially in cold water where temperature differences have less effect. The less dense, fresher water stays at the surface, creating a stable barrier that impacts the ocean’s interaction with the atmosphere and the layers below. This stratification is a direct consequence of the massive freshwater inputs the Arctic basin receives.
The Influence of Massive Freshwater Inflow
A primary reason for the Arctic Ocean’s lower surface salinity is the enormous volume of freshwater poured into it by surrounding continents. The Arctic basin receives about 11% of the total global river discharge, despite containing only about 1% of the volume of the world ocean. This disproportionate influx of freshwater constantly dilutes the ocean’s surface layer.
The largest contributors are the great Eurasian and North American rivers, including the Ob, Yenisei, Lena, and Mackenzie. These rivers discharge a massive amount of water that spreads out across the shallow shelf areas before moving into the central basin. This process creates vast plumes of low-salinity water along the coastlines.
Precipitation, in the form of rain and snow, acts as a secondary diluting factor, adding pure water directly to the surface of the ocean. The net result is a persistent surface layer of relatively fresh, less dense water across much of the Arctic Ocean.
How Sea Ice Formation Affects Salinity
The interplay between freezing and melting sea ice profoundly influences the salinity of the Arctic Ocean. When seawater freezes to form sea ice, it undergoes a process known as brine rejection. The crystalline structure of ice is unable to incorporate the salt molecules, effectively excluding them from the newly formed ice.
This rejected salt is concentrated into pockets of extremely saline water, or brine, which is then expelled into the ocean water directly beneath the growing ice sheet. Because this brine is significantly denser and saltier than the surrounding water, it sinks rapidly. This process increases the salinity and density of the deeper water column, contributing to the formation of dense water masses.
Conversely, the melting of sea ice acts as a powerful source of freshwater. Since the ice crystal structure had already expelled most of the salt during freezing, the resulting meltwater is nearly pure. When this meltwater is released back into the ocean’s surface layer during the summer thaw, it dramatically lowers the surface salinity.
The seasonal cycle of freezing and melting creates a dynamic feedback loop. Freezing increases salinity in the water column below, while melting reduces salinity at the surface. This process accentuates the difference in salt content between the surface and the deeper layers.
The Halocline: Layering the Arctic Ocean
The combined effects of massive river runoff and sea ice processes result in a distinct vertical structure within the Arctic Ocean known as the halocline. A halocline is a layer of water where salinity changes rapidly with depth. In the Arctic, this layer acts as a sharp boundary, or stratification, separating two major water masses.
The surface layer, which extends down to approximately 50 to 250 meters, is characterized by low salinity and low temperature, a direct product of freshwater input and ice melt. Beneath this layer lies the much saltier and significantly warmer Atlantic water, which flows into the Arctic basin from the North Atlantic. This Atlantic layer is a major source of heat that would rapidly melt the overlying sea ice if it were to mix upward.
The halocline is tremendously important because it prevents this mixing between the layers. Because the low-salinity surface water is less dense than the warm, salty water below, it floats like a lid, effectively trapping the heat of the Atlantic water at depth. This strong stratification is largely responsible for the continued stability of the Arctic sea ice cover, insulating it from the deep ocean heat. The presence of this salt-driven layering is a defining characteristic of the Arctic Ocean.