The formation and impact of sea ice?

The formation of sea ice:

Pure fresh water freezes at 0℃ and has the highest density at 4℃. But seawater is different. Both the freezing point temperature I (referring to the temperature when seawater begins to freeze) and the temperature at maximum density are related to salinity. Both temperatures decrease linearly with the increase of salinity, and decrease rapidly. When the salinity is 24.69, I and I of seawater take the same value, both are -1.33°C. When S<24.69 seawater, > I, therefore, when the temperature drops, it is reached first. At this time, there is vertical convective mixing. When the water temperature continues to drop and approaches I, the density of the surface water is no longer the maximum and gradually tends to be stable. , so the water freezes quickly when the temperature is slightly lower than the freezing point. Sea water with S>24.69, < I. Therefore, the process in which the water temperature gradually drops to the freezing point temperature is the process in which the density of seawater continues to increase, so it becomes heavier and sinks, and convection occurs. This convection process will continue until the seawater freezes. When seawater freezes, not all the salt is contained in the sea ice, so the salinity of the seawater under the ice increases, enhancing convection of the seawater. Sea ice forms when the water temperature drops below freezing and the seawater reaches a certain degree of supercooling.

Salt water ice formed by freezing in the ocean. Sea ice initially forms in the form of needles or flakes, then gathers and condenses, and under the action of wind, currents, waves and tides, stacks on top of each other to form overlapping ice and accumulation ice; and then forms a paste or sponge; After further freezing, it becomes ice skin or ice cake floating on the sea surface, also called lotus leaf ice. When the sea surface is covered with this ice, it extends in the thickness direction to form gray ice and white ice covering the sea surface.

Sea ice formation can begin in any layer of seawater, even at the bottom of the sea. Ice formed below the water surface is called submerged ice, also known as latent ice, and ice that adheres to the seafloor is called anchor ice. Since the density of deep-seated ice is lower than that of sea water, when they grow to a certain extent, they will float to the sea surface from different depths, causing the ice on the sea surface to continue to thicken.

Impact:

1. Impact on the vertical distribution of ocean hydrological elements. Due to the vertical convective mixing of seawater that exists during the freezing process, it often reaches a considerable depth, which can occur in shallow water areas. Directly to the seafloor, resulting in a relatively uniform vertical distribution of all ocean hydrological elements. This process can transport seawater with high dissolved oxygen in the surface layer downward, and at the same time transport fertile seawater in the bottom layer rich in nutrient salts needed by phytoplankton to the surface layer, which is conducive to the massive reproduction of organisms. Therefore, ice-covered seas, especially polar seas, tend to be rich in fishery resources. For example, Antarctica's lobster and whale fisheries are world-famous, which is directly related to this.

When ice melts, a warm and fresh water layer will form on the surface to cover the high-salt cold water, and a pycnocline will appear, which in turn will affect the vertical distribution of various hydrological elements and the exchange of upper and lower water. .

2. Influence on ocean dynamic phenomena. The existence of sea ice has a great impact on tides and currents. It will dampen the decline of tide levels and the movement of tidal currents, and reduce tidal range and flow speed; similarly, sea ice It will also reduce the wave height and hinder the propagation of waves.

3. Impact on the thermal conditions of seawater

When there is sea ice on the sea surface, the heat exchange between seawater and the atmosphere through evaporation and turbulence is greatly reduced. At the same time, due to Sea ice has extremely poor thermal conductivity and acts as a "skin jacket" for the ocean. Sea ice's high reflectivity of solar radiation energy and its high latent heat of melting can restrict changes in sea water temperature, so the annual variation in water temperature in polar seas is only about 1°C.

4. Ocean bottom water forms in the polar seas. Especially on the Antarctic continental shelf, a large amount of seawater freezes, which makes the subglacial seawater have the characteristics of increased salinity, low temperature and high density. It can slide down along the continental shelf to the bottom, forming The so-called Antarctic bottom water spreads to the three oceans, thus having a very important impact on ocean hydrological conditions.

In short, sea ice will not only have a huge impact on the ocean hydrological conditions itself, but also on atmospheric circulation and climate change, and will also directly affect human social practice activities. For example, it can directly block ports and waterways, block maritime transportation, and destroy marine engineering facilities and ships. Since the 1940s, high-latitude coastal countries have successively carried out sea ice observation and research work, and issued iceberg danger and sea ice forecasts. Observe sea ice and icebergs using shore stations, ships, aircraft, ice drifting stations, radars and satellites, and use mathematical statistics, synoptic and dynamic numerical methods to issue long-, medium- and short-term forecasts of sea ice .

Characteristics:

Sea ice generally floats on the sea surface. The height of regular-shaped sea ice above the water surface is 1/7 to 1/10 of the total thickness, and the peak ice is exposed. The height reaches 1/4~1/3 of the total thickness. The reflectivity is 0.50~0.70, and the compressive strength is about 3/4 of freshwater ice.

1. Density

Because sea ice contains bubbles, the density is generally lower than this value. The density of new ice is roughly 914~915. The density of sea ice increases with the salinity and Increased as air content decreases.

The longer the ice age, the smaller the density due to the leakage of marinade in the ice. Sea ice density can drop to around 860 in late summer. Since sea ice is less dense than sea water, it always floats on the surface of the sea.

2. Salinity

The salinity of sea ice refers to the salinity of seawater after melting, which is generally around 3 to 7‰. When seawater freezes, the water in it freezes and the salt in it is squeezed out. Some of the salt that has no time to flow away is surrounded in the form of marinade in the gaps between the ice crystals to form "salt bubbles". In addition, when seawater freezes, gases that have no time to escape are trapped between ice crystals, forming "bubbles." Therefore, sea ice is actually a mixture of freshwater ice crystals, brine, and air bubbles.

The salinity of sea ice depends on factors such as the salinity of the seawater before freezing, the speed of freezing, and the age of the ice. The higher the salinity of seawater before freezing, the higher the salinity of sea ice is likely to be. The salinity of sea ice measured in the waters near the Antarctic continent is as high as 22 to 23. The lower the temperature when freezing, the faster the freezing speed, and the more marinade that has no time to flow out and is surrounded by ice crystals, so the salinity of sea ice is naturally higher. In ice layers, salinity decreases with depth because the lower layer freezes slower than the upper layer. When the sea ice passes through the summer, the melting of the ice surface will also cause the brine in the ice to flow out, causing the salinity to decrease. In the multi-year old ice in the polar regions, the salinity is almost zero.

3. Specific heat capacity

The specific heat capacity of sea ice is larger than that of pure water ice, and it increases with the increase of salinity. The specific heat capacity of pure water ice is not greatly affected by temperature, while the specific heat capacity of sea ice decreases as the temperature decreases. At low temperatures, because it contains less marinade, it does not change much with temperature and salinity, and is close to the specific heat of pure water ice. However, at high temperatures, especially near the freezing point (-2°C), the brine in sea ice changes phase as the temperature rises and falls, that is, the pure water in the brine freezes and precipitates when the temperature drops, and the ice melts into the brine when the temperature rises. As a result, its specific heat capacity is reduced and increased respectively. Its decrease and increase values ​​vary greatly depending on its salinity. When the salt is low, its specific heat capacity is small, while when the salt is high, its specific heat capacity will be several times or even ten times greater than that of pure water ice.

4. Thermal conductivity

The latent heat of melting sea ice is also greater than that of pure water ice. The thermal conductivity coefficient of sea ice is smaller than that of pure water ice because sea ice contains bubbles and the thermal conductivity coefficient of air is very small. The thermal conductivity coefficient of sea ice is slightly greater than the molecular thermal conductivity coefficient of sea water. Therefore, sea ice limits the heat transfer from the ocean to the atmosphere, and also greatly reduces the evaporation heat loss of the ocean, thus forming a protective layer for the ocean.

Since there are more gaps in the upper part of sea ice than in the lower layer, its thermal conductivity coefficient also increases with depth, that is, the thickness below the ice surface. The thermal conductivity coefficient of sea ice exceeding 1m is the same as that of pure ice. Water ice is not much different, about 1/3 of pure water ice near the surface.

5. Expansion coefficient

The thermal expansion coefficient of sea ice changes with the temperature and salinity of sea ice. For low-salinity sea ice, as the temperature decreases, it begins to expand and then contracts. The critical temperature value for changing from expansion to contraction decreases with increasing sea ice salinity. For high-salt sea ice, it always expands as the temperature decreases, but the expansion coefficient becomes smaller and smaller.

6. Compressive strength

The compressive strength of sea ice mainly depends on the salinity, temperature and age of the sea ice. Generally, new ice has greater compressive strength than old ice. Low-salinity sea ice has greater compressive strength than high-salinity sea ice, so sea ice is not as dense and hard as freshwater ice. Under normal circumstances, sea ice is about the same strength as freshwater ice. 75% of the time, people can walk safely on 5 cm thick river ice, while safe walking on sea ice requires 7 cm thick ice. Of course, the cooler the ice, the greater its compressive strength. During the severe ice freeze in the Bohai Sea in 1969, in order to rescue ships, the Air Force dropped 30 kilograms of explosive packets on the 60 cm thick ice layer, but the ice layer was not broken.