The secret of the earth's oceans?

We live on the earth. Among the nine planets in the solar system, you may be able to name many features of our "hometown", but there is a very unusual feature that you may not notice, that is, only the temperature of the earth's surface can make water exist in solid, liquid and gas at the same time. Moreover, the earth is the only celestial body in the solar system with a huge ocean. Some statistics of this connected ocean surprise you: the total area is 36.2 billion square kilometers (accounting for 70.8% of the total area of the earth), the average depth is 3795 meters, and the total volume of seawater is 65.438+037 billion cubic kilometers. No wonder some people say that our planet is better called "water polo" than "earth".

This continuous seawater layer covering the earth's surface can be divided into Pacific Ocean, Atlantic Ocean, Indian Ocean and Arctic Ocean according to its distribution characteristics. Less than 20% of the remaining mainland areas have intermittent surface water, especially rivers and lakes. There is still persistent groundwater in the soil and rocks below the surface. Surface water and groundwater are connected, and finally they are connected with seawater. In this way, the earth's surface includes water at a certain depth below the surface, which actually constitutes a complete circle. This is the hydrosphere.

It is almost 100% certain that life on earth originated from the ocean, which is the cradle of life. At present, there are more than 200,000 species of organisms in the ocean, and their annual growth accounts for about 43% of the global biomass.

However, for most of the time after the birth of human beings, human activities have always been confined to land. In their view, the ocean is an insurmountable "kingdom of water" and a mysterious "strange giant".

People's exploration of the ocean has been at a very low level for a long time: observing the ups and downs of waves, recording the ups and downs of tides, tasting seawater and observing the depth of offshore waters ... Until recently, people's understanding of the ocean was still superficial. It can even be said that geologists know far less about the seabed surface than astronomers do about the moon surface.

The founder of modern oceanography is American scholar Murray. He used to be a captain officer in the United States Navy. At the age of 30, an accident crippled his leg, which was really a great misfortune for a young man, but he turned it into a good thing. After being disabled, Murray was sent to take care of charts and instruments. This idle position not only didn't discourage him, but gave him a chance to show his skill. Since then, he devoted himself wholeheartedly to the study of the ocean.

Murray was the first scholar to study the relationship between ocean and meteorology. He collected a large number of observation records about wind, current and water temperature, and compiled the first volume of Marine Meteorological Observation Report in 1846. In the second year, he drew an accurate nautical chart, recording in detail the wind, wind direction, trade winds, equatorial windless zone, surface water temperature, as well as the number of storms and the frequency of rain and fog in different months. Marine Physical Geography, published by 1855, is Murray's representative work, which systematically describes natural geographical phenomena such as ocean current, wind, salinity, temperature and the relationship between ocean and atmosphere for the first time, and becomes the first important work of modern oceanography, so Murray became the founder of modern oceanography.

One of Murray's important contributions is the systematic and detailed record and study of ocean currents. Specifically, he studied and discussed the Gulf Stream in particular. This current was first discovered and studied by American scholar Franklin in 1769. After studying this ocean current, Murray wrote vividly in the book Marine Physical Geography: "There are rivers in the ocean, which will not dry up in the case of maximum drought and will not overflow the banks in the case of maximum flood. The banks and riverbed of this river are composed of cold water, while the flowing water in the river is warm water. Mexico is its source and the Arctic Ocean is its estuary. "

You can't find such a spectacular current on land. Generally, ocean currents are tens of kilometers to hundreds of kilometers in width, hundreds of meters in depth, tens of meters in velocity per minute, and millions and tens of millions of cubic meters in flow per second. The Gulf Stream is not only much larger than any river on land, but also the champion of "rivers" in the ocean. Its current in straits of florida is 26 million cubic meters per second, and when it approaches chesapeake bay, it has reached 890 million cubic meters per second, which is equivalent to the seawater flow of thousands of Mississippi rivers.

How ocean currents are formed. After years of observation and research, scientists have finally basically figured out their causes and some laws. The distribution of Shanghai current on the earth is mainly influenced by prevailing winds, seawater density, earth deflection, seabed topography, coastal contours and islands. According to its genesis, ocean currents can be divided into wind currents, density currents, gradient currents, inclined currents and compensation current. Current can be divided into warm current and cold current according to whether its water temperature is higher or lower than the sea area it flows through.

Ekman, a Swedish oceanographer, made the greatest contribution to the study of ocean currents. He first put forward the theory of wind current (drifting), and in 1905, he designed and manufactured the "ekman current meter", which can simultaneously measure the velocity and direction of seawater. After three improvements, the instrument has become the main tool for measuring current. He also studied the theories of density current, gradient current, deep current and turbidity current. Ekman is famous for his research on ocean current dynamics, and he is a pioneer of physical oceanography.

Some major ocean currents near the surface of the world's oceans belong to wind currents. The wind blew away the water, and the water here went away, and the neighboring water came to replenish it; If the wind always blows in one direction, it will push the sea to make a long voyage in the ocean along the direction of the wind, which is called wind flow. For example, the northeast trade winds prevail in the northern hemisphere and the southeast trade winds prevail in the southern hemisphere. These winds are directional winds. With directional winds, there is directional flow, so general winds and currents flow regularly.

Ocean currents are also influenced by the earth's rotation. Because the earth is constantly rotating from west to east, the moving objects on the earth will be subjected to an inertial force, which was first studied by the French mathematician Coriolis in 1835, also called Coriolis force. Take the earth that rotates from west to east as an example. Under the action of Coriolis force, objects moving in the northern hemisphere will shift to the right, and objects moving in the southern hemisphere will shift to the left. Therefore, in the northern hemisphere, the right side of the rail wears more, and rivers scour the right bank more, while in the southern hemisphere, the opposite is true. Similarly, the surface flow direction of the northern hemisphere ocean is always 45 degrees to the right of the wind, forming a clockwise circulation, and the surface flow direction of the southern hemisphere ocean is always 45 degrees to the left of the wind, forming a counterclockwise circulation.

Affected by wind and the earth's rotation, seawater is generally less than several hundred meters deep and belongs to "surface current". Surface currents can often be felt intuitively, and the velocity and direction can be measured by ordinary current meters. However, it is not so easy to directly understand the deep-sea currents. Some people used to think that since almost all ocean currents in the ocean are blown by the wind, and the depth involved is only a few hundred meters, if there is no wind, there will definitely be no ocean currents, just a stagnant pool. But it is hard to imagine, especially when people know that there are creatures in the depths of the ocean, making people believe that the deep sea water is also flowing. Otherwise, how can we transport oxygen and nutrients to the depths of the ocean to meet the needs of underwater life?

The next question is: how does the sea water flow in the depths of the ocean? One of the most memorable factors is the density of seawater. Because of the different temperature and salinity, the density of seawater is different, and the high-density seawater flows to the low-density place, thus forming a density current corresponding to wind and current.

How the deep water flows. The previous statement is very inconsistent. Due to the lack of actual observation data, it is difficult to determine at the moment, and each has its own statement, and no one can convince anyone. The flow of deep seawater is too weak, and there is no accurate instrument to measure their speed and direction.

From the beginning of the 20th century, scientists began to think of indirect methods, such as analyzing and tracking where the deep seawater flows and how far it flows according to the density of seawater and its horizontal and vertical distribution. With the increase of ocean observation data, especially deep-sea temperature data, great progress has been made in the research of deep-sea currents. According to ocean current dynamics theory, oceanographers Sfey Droop and Huste put forward a more realistic ocean deep circulation model by analyzing the data of seawater temperature, salinity and dissolved oxygen.

In the Antarctic region, people have discovered the Weddell Sea, which goes deep into the Antarctic continent. In winter, the surface water with low temperature and high salinity here is denser than the seawater below, so it sinks to the bottom of the sea, goes north along the bottom of the sea and overflows most of the bottom of the three oceans, maintaining the characteristics of low temperature and high salinity from the Weddell Sea. In the Atlantic Ocean, this Antarctic "bottom water" can go north to Newfoundland Shoal; In the Pacific Ocean, it reached the Aleutian Islands; In the Indian Ocean, it came to the Bay of Bengal and the Arabian Sea.

In the North Atlantic, people also found a high-density seawater area-the Norwegian Sea, but it is separated from the Atlantic Ocean by a sea sill. Therefore, when the low-density water in the Norwegian Sea dives over the sea sill and overflows, it will have a strong mixing effect with the surrounding seawater, so that its density will be reduced, and it will not sink to the bottom of the Atlantic Ocean, but will only be located at the bottom of the Antarctic water 1500 meters to 4,000 meters, and will spread deep water. You can also bypass the Cape of Good Hope and enter the Indian Ocean, then enter the western Pacific from the Indian Ocean to the north, turn clockwise, cross the equator, go south from the eastern Pacific and rise in the Antarctic waters, which also becomes the compensation for the Antarctic bottom water, thus forming a huge deep bottom circulation between the north and the south.

Except for the sinking of the Weddell Sea and the Norwegian Sea, the seawater in other sea areas "converged" and sank, namely Antarctic convergence and subtropical convergence. The Antarctic gathers around the Antarctic continent, and the sea water density here is not very high, so this part of the sea water can only sink to the middle layer of 800 meters to 1500 meters, which is called Antarctic middle water, and it also spreads from south to north like Antarctic bottom water, flooding the deep waters of various oceans. Subtropical convergence sinking seawater has a low density and can only sink to surface water and middle Mizunokami, which is called upper water.

In this way, the whole seawater is divided into five layers: surface layer, upper layer, middle layer, deep layer and bottom layer. Except that the surface current is mainly wind current, other layers are basically density current. Like the surface currents, the currents in each layer are connected end to end and continuous, forming a complete ocean circulation.

1957, British and American oceanographers organized a joint ocean expedition, and obtained many more direct evidences to prove that deep seawater really flows in this way, thus overthrowing the long-standing ocean circulation theory. They mainly used the special float invented by the British oceanographer Swarow, and the float with the right weight can float in the seawater with a predetermined depth; According to its ultrasonic signal, we can know its moving direction and speed. In addition, it can also be used to measure temperature, salinity and depth simultaneously.

All kinds of ocean currents have their own motion patterns. 1973, the first astronauts on the "Sky Lab" space station launched by the United States used remote sensing instruments to find large eddies in coastal waters such as Mexico, and then found large eddies in waters near the east and west coasts of the United States, Australia, New Zealand, Africa and Hawaii Islands. These eddies are channels for seawater exchange between ocean currents. They are large and small, varied, and some people call them "whirlpool grand view garden" The discovery of "Swirl Grand View Garden" is considered as one of the major advances in oceanography since 1950s.

All the sea water is in constant motion. Waves, tides and currents have great influence on the geographical environment and human activities. They are navigation assistants, drought and flood forecasters and climate regulators. The Mexican warm current, like a huge warm current, flows across the Atlantic Ocean to the cold Arctic Ocean, full of spring all the way, bringing a good port all year round to the polar regions and a warm winter to the high latitudes. In addition, the convergence zone is a sea area where cold current and warm current meet, with rich nutrition, mild water temperature and abundant fish and shrimp, which is almost the most famous fishing ground in the world.