The simple answer to whether the Arctic Ocean is saltwater is yes, but it is unique among the planet’s major oceans because its surface is the least saline. This body of water, positioned around the North Pole, is the smallest and shallowest of the world’s five oceanic divisions. The average salinity of the world’s oceans is approximately 35 Practical Salinity Units (PSU). In contrast, the surface layer of the Arctic Ocean often maintains a much lower average salinity, frequently falling below 30 PSU. This significant dilution is a defining characteristic of the Arctic, influencing its physical structure and the persistence of its sea ice cover.
Defining Salinity and the Arctic Standard
Salinity is the measure of dissolved salts in water, with the primary components being sodium and chloride ions. In modern oceanography, this salt content is measured using the Practical Salinity Unit (PSU), a dimensionless unit based on the electrical conductivity of the water sample. PSU is roughly equivalent to parts per thousand, meaning 35 PSU indicates approximately 35 grams of salt compounds dissolved in every kilogram of seawater. The established global ocean average sits at about 35 PSU, providing a necessary benchmark for comparison.
Oceanographers still classify the Arctic as saltwater because even its diluted surface layer contains a substantial concentration of dissolved salts. The Arctic Ocean’s surface typically registers between 28 and 33 PSU, confirming its status as the lowest-salinity ocean basin. The water remains saline enough to maintain the characteristics of an ocean, but this lower concentration makes it particularly susceptible to the effects of freshwater input. Near the mouths of large rivers along the coastline, the surface salinity can drop even lower, creating brackish conditions.
The Primary Drivers of Low Arctic Salinity
The dramatically lower salinity of the Arctic Ocean is a direct result of two major geographic and climatic factors that consistently deliver vast amounts of freshwater.
Continental Runoff
The first driver is the massive continental runoff from surrounding landmasses. The Arctic Ocean receives over 10% of the world’s total river discharge, yet it holds only about 1% of the global ocean volume. This freshwater is funneled into the basin by several of the world’s largest river systems, primarily from Eurasia and North America. The four largest Arctic-draining rivers—the Ob, Yenisei, Lena, and Mackenzie—collectively contribute approximately 1900 cubic kilometers of freshwater annually. This immense volume of river water flows across the surface of the denser saltwater, acting as a continuous dilution agent.
Sea Ice Dynamics
The second driver is the cycle of sea ice dynamics, which involves both the formation and melting of ice. When sea ice forms, the freezing process expels salt from the water molecules, a process called brine rejection, locally increasing salinity beneath the ice. Conversely, when the sea ice melts, it releases nearly pure freshwater back into the ocean. The net effect on the surface layer is dominated by the seasonal influx of meltwater, which significantly lowers the overall salinity of the upper ocean.
The Impact of Salinity on Ocean Structure and Ice
The unique freshwater balance in the Arctic creates a highly stable, layered ocean structure known as stratification, which is fundamentally determined by density differences. Since colder, less saline water is less dense than warmer, saltier water, the low-salinity surface water floats atop the denser layers below. This forms the Polar Mixed Layer, which is typically a cold, fresh layer extending down about 50 meters.
Beneath this surface layer lies a sharp transition zone called the halocline, where the salinity increases rapidly with depth, typically extending to a depth of around 250 meters. The dense, saltier water beneath the halocline includes the warmer Atlantic Water, which enters the Arctic basin from the North Atlantic Current.
This stratification is a physical mechanism because it insulates the surface layer and the sea ice from the heat of the underlying Atlantic Water. The Atlantic Water can be up to 3°C, yet it remains submerged because its higher salinity makes it denser. The halocline effectively prevents the warmer water from mixing upward and melting the sea ice from below, thus allowing the ice cover to persist. This layered structure also impacts the biological environment by limiting the vertical mixing of water, which restricts the cycling of nutrients from the deep ocean to the surface.