3.2.2 Oceanic Structure and Deep Water Formation in the Arctic Ocean
Near the surface of the world's oceans, the water is directly affected by solar radiation, agitation by the wind, evaporation and precipitation. This surface layer, called the mixed layer, is thoroughly churned by the action of waves and convection currents. Below the mixed layer, where temperature and salinity are fairly uniform, the thermocline and halocline are observed, where the temperature and salinity change abruptly. In most oceans, the thermocline and halocline occur at almost the same depth. During the summer, forced convection by the wind restricts the mixed layer to a relatively shallow depth of 10-20m, but in the winter, natural convection causes the mixed layer to extend up to 200m deep or more.
The occurrence of sea ice is closely connected with the underlying oceanic structure. The temperature of maximum density of seawater is below the freezing temperature; thus, in a homogenous ocean the entire water column would have to be cooled down to its freezing temperature before ice could form on the surface. In fact, winter convection is limited by the polar haloclines discussed above.
Vertical distributions of water temperature and salinity and formation of halocline in the Arctic Ocean (Carmack, 1986)
A unique feature of the structure of the Arctic Ocean is that the thermocline and the halocline exist at different depths. The mixed layer at the freezing point includes both a low-salinity surface layer and a halocline whose salinity increases with depth. This ocean structure is supported by a cycle in which a low-salinity water mass spreads over the continental shelf as an enormous volume of fresh water is supplied from the arctic rivers, and brine of high salinity is released in the winter freezing process; when the ice melts in the summer, a huge volume of fresh water is supplied again to the seawater. Only the Arctic Ocean exhibits this structure.
A warm, high-salinity current (with a temperature of 3-4℃ and salinity of 3.5%) called the Spitsbergen Current flows through the Fram Straits from the North Atlantic Ocean into the Arctic Ocean and sinks below the halocline to an intermediate layer. At the same time, the Arctic Ocean's surface low-salinity water (temperature -1.5-+2℃, salinity 3.1%) flows out into the North Atlantic in the form of the East Greenland Current. The volume of this flow is estimated to reach approximately 3.5 million metric tons of water per second. On the Pacific side, about 300,000 metric tons per second flows into the Arctic Ocean through the Bering Strait. An annual average of 100,000 metric tons of lighter water is estimated to flow into the Arctic Ocean from coastal rivers, but seasonal variations are immense.
The water from the North Atlantic Ocean that flows underneath the halocline transfers heat to the low-temperature, low-salinity water on top, causing the incoming flow to become still heavier and sink, forming a low-temperature, high-salinity water in the Arctic Ocean.
Structure of the Arctic Ocean and deep-water formation (Aagaard, 1985)