The formation and influence of sea ice?

Pure fresh water freezes at 0℃, and its density is the highest at 4℃. However, the sea water is different. The freezing point temperature I (the temperature at which seawater begins to freeze) and the temperature at the maximum density are all related to salinity. These two temperatures decrease linearly with the increase of salinity, and the decline rate is very fast. When the salinity is 24.69, I of seawater takes the same value as-1.33℃. When S & lt24.69 seawater, > I Therefore, when the temperature drops, it reaches it first, and there is convection and mixing in the vertical direction at this time. When the water temperature continues to drop close to I, the density of surface water is not the maximum and tends to be stable gradually, so it freezes quickly when the water temperature is slightly lower than freezing point. S & gt24.69 seawater,

Sea water freezes in the ocean. At first, the sea ice was needle-like or flaky, then it gathered and condensed, and under the action of wind, ocean current, waves and tides, it overlapped with each other to form overlapping ice and accumulated ice. Then forming paste or sponge; After further freezing, it becomes ice skin or ice cake floating on the sea, also called lotus leaf ice; After the sea surface is covered by this ice, it extends to the thickness direction, forming gray ice and Bai Bing covering the sea surface.

The formation of sea ice can start at any seawater layer, even at the bottom of the sea. The ice formed below the water surface is called underwater ice, also called diving ice, and the ice attached to the seabed is called anchor ice. Because the density of deep 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, making the ice on the sea surface thicken.

Impact:

1. Influence on the vertical distribution of marine hydrological elements Because the vertical convection and mixing of seawater often reach a considerable depth during freezing, it can directly reach the seabed in shallow water, resulting in a more uniform vertical distribution of all marine hydrological elements. This process can transport seawater with high dissolved oxygen in the surface layer downward, and at the same time transport fertile seawater rich in nutrients needed by phytoplankton to the surface layer, which is beneficial to the mass reproduction of organisms. Therefore, frozen seas, especially polar seas, are often rich in fishery resources. For example, Antarctic krill and whale fishing grounds are world-famous, which is directly related to this.

When the ice melts, a warm and light water layer will be formed on the surface to cover the cold water with high salt, and a density thermocline will appear, which will affect the vertical distribution of various hydrological elements and the exchange between water and water.

2. Influence on ocean dynamic phenomena The existence of sea ice has a great influence on tides and tidal currents, which will damp the decline of tidal level and the movement of tidal currents, and reduce tidal range and velocity; Similarly, sea ice will also reduce the wave height and hinder the wave propagation.

3. Influence on seawater thermal condition

When there is sea ice on the sea surface, the heat exchange between sea water and atmosphere is greatly reduced through evaporation and turbulence, and at the same time, because of the poor thermal conductivity of sea ice, it plays the role of "fur coat" for the ocean. The high reflectivity of sea ice to solar radiation energy and its high latent heat of melting can restrict the change of seawater temperature, so the annual change of water temperature in polar waters is only about 65438 0℃.

4. The sea bottom water formed in polar seas, especially the sea water on the Antarctic continental shelf, is frozen in large quantities, which makes the sea water under the ice have the characteristics of salt increase, low temperature and high density. It sinks along the continental shelf and reaches the bottom, forming the so-called Antarctic bottom water, which spreads to the three oceans, thus having a very important impact on marine hydrological conditions.

In a word, sea ice will not only have a great impact on marine hydrology, atmospheric circulation and climate change, but also directly affect human social practice. For example, it can directly block ports and waterways, block maritime transportation, and destroy offshore engineering facilities and ships. Since the 1940s, high-latitude coastal countries have successively carried out sea ice observation and research, and issued iceberg hazards and sea ice forecasts. Sea ice and icebergs are observed by shore stations, ships, planes, floating ice stations, radars and satellites, and long-term, medium-term and short-term forecasts of sea ice are issued by using mathematical statistics, meteorology and dynamic numerical methods.

Features:

Generally speaking, sea ice floats on the sea surface. The height of regular sea ice above the water surface is1/7 ~110 of the total thickness, and the height of spire ice is 1/4 ~ 1/3 of the total thickness. The reflectivity is 0.50 ~ 0.70, and the compressive strength is about 3/4 of that of fresh water ice.

1, density

Because there are bubbles in sea ice, the density is generally lower than this value. The density of new ice is about 9 14 ~ 9 15, and the density of sea ice increases with the increase of salinity and the decrease of air content.

The longer the ice age, the smaller the density will be due to the leakage of marinade in ice. At the end of summer, the sea ice density can be reduced to about 860. Because sea ice is less dense than sea water, it always floats on the sea surface.

2. Salt

The salinity of sea ice refers to the salinity of sea water after melting, which is generally around 3 ~ 7 ‰. When seawater freezes, it is the water inside that freezes, and the salt inside is squeezed out, and some of the salt that cannot flow away is surrounded in the cracks of ice crystals in the form of marinade, forming "salt bubbles". In addition, when seawater freezes, the gas that cannot escape in time is surrounded by ice crystals, forming "bubbles". Therefore, sea ice is actually a mixture of fresh water ice crystals, marinade and bubbles.

The salinity of sea ice depends on the salinity of seawater before freezing, freezing speed and ice age. The higher the salinity of seawater before freezing, the higher the salinity of sea ice may be. The sea ice salinity measured in the waters near the Antarctic continent is as high as 22 ~ 23. When freezing, the lower the temperature, the faster the freezing speed, and the more marinade wrapped in ice crystals can flow out, so the salinity of sea ice is naturally higher. In the ice layer, the lower layer freezes slower than the upper layer, so the salinity decreases with the increase of depth. When the sea ice passes through summer, the melting of the ice surface will also make the marinade in the ice flow out, which will lead to the decrease of salinity. In the polar old ice, the salinity is almost zero.

3. Specific heat

The specific heat capacity of sea ice is greater than that of pure water ice, and it increases with the increase of salinity. The specific heat capacity of pure water ice is not affected by temperature, while sea ice decreases with the decrease of temperature. At low temperature, 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 temperature, especially near the freezing point (-2℃), the salt water in sea ice changes phase with the rise and fall of temperature, that is, the pure water in salt water freezes and precipitates when the temperature drops, and the ice melts into salt water when the temperature rises, so that its specific heat capacity decreases and increases respectively. Because of its salt content, the decrease value and increase value are quite different. At low salinity, its specific heat capacity is small, while at high salinity, its specific heat capacity will be several times or even ten times larger than that of pure water ice.

4. Thermal conductivity

The latent heat of sea ice melting is also greater than that of pure water ice. The thermal conductivity of sea ice is smaller than that of pure water ice, because there are bubbles in sea ice and the thermal conductivity of air is very small. The thermal conductivity of sea ice is slightly larger than the molecular thermal conductivity of seawater, which limits the heat transfer from the ocean to the atmosphere and greatly reduces the heat of vaporization loss of the ocean, thus forming a protective layer of the ocean.

Because there are more gaps in the upper part of sea ice than in the lower part, its thermal conductivity increases with the depth, that is, the thickness from the ice surface down. The thermal conductivity of sea ice above 1m is similar to that of pure water ice, and it is about 1/3 of pure water ice near the surface.

5. Expansion rate

The coefficient of thermal expansion of sea ice varies with the temperature and salinity of sea ice. For low-salt sea ice, with the decrease of temperature, it expands first and then contracts. The critical temperature from expansion to contraction decreases with the increase of sea ice salinity. For high salt sea ice, it always expands with the decrease of temperature, but the expansion coefficient is smaller and smaller.

6, compressive strength

The compressive strength of sea ice mainly depends on its salinity, temperature and ice age. Generally, the compressive strength of new ice is greater than that of old ice, and the compressive strength of low salinity sea ice is greater than that of high salinity sea ice, so the density of sea ice is not as hard as that of fresh water ice. Generally speaking, the firmness of sea ice is about 75% of that of fresh water ice. People can walk safely on 5 cm thick river ice, while walking safely on sea ice requires 7 cm thick ice. Of course, the lower the temperature of ice, the greater the compressive strength. 1969, during the extremely cold period in Bohai Sea, the air force dropped 30 kilograms of explosives on the 60 cm thick accumulated ice layer in order to rescue the ship, but the ice layer was not blown up.