The question of whether ocean salinity increases with depth does not have a simple yes or no answer, as the relationship is highly layered and depends heavily on location. Salinity, the concentration of dissolved salts in seawater, dictates water density alongside temperature. Denser water sinks beneath less dense water, creating a stratified ocean structure. Therefore, the change in salt content from the surface to the seafloor is highly variable, often decreasing sharply in a middle layer before becoming stable or slightly increasing in the deep ocean.
Understanding the Surface Layer and Shallow Depth
The salinity of the ocean’s surface layer, typically extending to about 200 meters, is the most dynamic and variable part of the water column. This upper zone, known as the mixed layer, is constantly influenced by atmospheric conditions that either add or remove fresh water. Evaporation, particularly in the subtropical belts, removes fresh water and leaves the salt behind, causing surface salinity to increase.
Conversely, areas that receive high levels of precipitation, such as the equatorial zone, or regions near the mouths of major rivers, experience a dilution of surface waters. Ice formation and melting in polar regions also play a role; when seawater freezes, the salt is mostly excluded, increasing the salinity of the surrounding water, while melting ice decreases it. Because of these competing forces, the shallow ocean’s salinity profile is often seasonal, with values ranging widely from about 33 to 37 practical salinity units (psu) across the globe.
The Halocline and Vertical Salinity Changes
Below the surface mixed layer lies a transition zone where salinity changes rapidly with increasing depth, a layer known as the halocline. This region is where the vertical stratification of the ocean becomes most apparent. In the middle latitudes, the surface water is usually saltier due to high evaporation, meaning the halocline is characterized by a rapid decrease in salinity down to the deep-water value.
In contrast, high-latitude regions, like the Arctic or parts of the Southern Ocean, often have a low-salinity surface layer due to massive inputs of fresh water from melting ice and river runoff. In these areas, the halocline shows a rapid increase in salinity with depth as it transitions to the saltier water mass below. This vertical change in salinity, along with temperature change (thermocline), contributes to the pycnocline, the layer of sharp density change that prevents easy mixing between the surface and deep layers.
Deep Ocean Stability and Global Circulation
Below the halocline, typically starting at depths of 1,000 meters or more, the ocean water is characterized by stability in its properties. In the deep ocean, salinity remains consistent, generally falling between 34.5 and 35 psu across the major ocean basins. This stability is due to the lack of direct interaction with the atmosphere and the surface processes that cause variability.
The deep and bottom waters that fill these basins originate predominantly in the polar regions. Here, surface water becomes extremely cold and saltier due to the process of brine rejection during sea ice formation. The salt excluded from the forming ice concentrates in the remaining liquid water, creating a dense water mass. This cold, dense, and slightly saltier water sinks to the ocean floor, driving the planet’s global ocean current system. Because this dense water sinks, the deepest parts of the ocean are generally filled with the densest, coldest, and often the most saline water mass overall.