What are the five water masses in the world's oceans?

Following ekman's drift theory, many scholars have simulated different ocean shapes and conducted various experiments to study the global ocean circulation according to the actual wind field characteristics in the ocean, taking into account the fact that Coriolis force varies with latitude and the friction effect of the ocean coast.

As early as 1948, H. Stommel studied the equilibrium relationship between vertical turbulent shear stress and Coriolis force according to the upwind stress of the sea surface. The uniform ocean circulation structure as shown in figure 5- 12 is obtained. In Figure (a), the Coriolis force is zero or constant, while in Figure (b), the Coriolis force increases with latitude. In the experiment, he assumed that the ocean is a rectangle with equal depth, located on the equatorial side, and the wind stress changes with latitude, and calculated the flow field under three different equilibrium conditions: (1) When the Coriolis force is zero, that is, only the current situation when the wind stress and turbulent shear stress are balanced is considered; (2) When the Coriolis force is constant, the situation is similar to (1), that is, the streamlines are all symmetrical, as shown in Figure 5- 12 (a); (3) Considering the variation of Coriolis force with latitude, the obtained streamline is very similar to a main feature of ocean flow field.

2. Hot salt cycle

Wind-driven wind-driven circulation is mainly manifested in the upper ocean. The cycle caused by temperature and salt changes is usually called hot salt cycle. Relatively speaking, it is dominant in the middle and lower oceans. Compared with wind-driven circulation, hot salt circulation flows slowly, but it is the main reason for the distribution characteristics of temperature and salt in the middle and lower ocean and the layered structure of the ocean.

A relatively simple model to describe the hot salt circulation is to regard the North and South ocean basins as a set of superimposed "pots", and each "pot" corresponds to an isodensity surface (strictly speaking, an isodensity surface). The cold water with high polar density sinks the deepest along the isodensity surface, and the mid-latitude seawater can only sink to a medium depth (Figure 5- 14). Of course, the actual situation in the ocean is much more complicated.

According to the analysis of temperature-salt structure on the isodensity surface, the seawater movement caused by heat-salt action can be determined. Because the temperature and salinity characteristics of deep-sea seawater depend on the characteristics of its source and its mixing with the surrounding seawater in the process of movement, the distribution and trend of the main characteristics of its source can be traced, so as to infer the movement and distribution of circulation. This method is called core layer analysis.

A typical example is to analyze the distribution of Mediterranean spillage across the Atlantic. Due to the high salinity of Mediterranean water (close to 38.0), it still has a high density despite its high temperature (close to 13.0℃). Warm and salty Mediterranean water overflowed the bedrock of Gibraltar, flowed into the Atlantic Ocean and began to sink. During the sinking process, it was mixed with seawater with relatively low temperature and salinity in the Northeast Atlantic, and the gravity and buoyancy of the mixed water were roughly balanced at the depth of 1 100 m, and then the high-salt "core" continued to spread in the North Atlantic.

According to the analysis of the nature of seawater, the seawater in the depths of the world's oceans is mainly formed by the sinking of surface seawater, and its main sources are the Greenland Sea in the North Atlantic, the Norwegian Sea and the Weddell Sea on the edge of the Antarctic continent. In the past, people thought that the speed of deep-sea circulation formed by hot salt was very small (a few millimeters per day), but recent observations show that not all deep-sea circulation speed is very slow.

G. Hurst pointed out in 1935 that, based on the calculation of dissolved oxygen content and geostrophic current in the depths of the Atlantic Ocean, there is a current moving northward along the ocean floor in the southern hemisphere and a current moving southward along the Wüst side in the bottom water field in the northern hemisphere. Later observation of neutral floating objects confirmed this conclusion.

Stormer put forward the ocean thermohaline circulation model. He believes that due to the conservation of seawater volume, the sinking seawater at high latitudes will inevitably lead to the upward movement of seawater in the ocean. In addition to the divergence zone of Antarctic sea surface to be mentioned later, according to the fact that the main thermocline of the ocean is actually stable, he proposed that the sinking of seawater is local, but the upward movement covers the whole middle and low latitude sea area. The reason is that the low latitude sea area has net heat income every year. If the cold water below is not compensated, the main thermocline will deepen.

K Wiltke (196 1) discussed a meridional thermohaline circulation by numerical method. Considering the polar current between the upper layer and the surface layer of the ocean, the high-density water in the high-latitude sea area sinks and spreads towards the equator in the deep layer, and the seawater rises through the main thermocline. By studying the thermal balance of the upper ocean, it is inferred that the rising speed is (1 ~ 5) × 655.

Assuming that the average water transport capacity is 45× 106m3/s, the total cycle of ocean thermohaline circulation is about 1000 years, about 500 years in the North Atlantic and more than 2000 years in the North Pacific.

3. World Ocean Circulation and Water Mass Distribution

I. Main horizontal circulation in the upper ocean of the world

The general characteristics of the upper ocean circulation in the world can be explained by the theory of wind-induced circulation. The circulation patterns of the Pacific Ocean and the Atlantic Ocean are similar: there is a huge anticyclone circulation corresponding to the subtropical high in both hemispheres (clockwise in the northern hemisphere and counterclockwise in the southern hemisphere); There is equatorial countercurrent between them; The western boundary current in the northern hemisphere (the Atlantic Ocean is called the Gulf Stream, and the Pacific Ocean is called the Kuroshio) is very strong, while the western boundary current in the southern hemisphere (the Brazil Stream and the East Australian Stream) is very weak; There is a cold current from the north on the west side of the ocean basin in the North Pacific and the North Atlantic. There is a small cyclone circulation in the north of the main vortex.

The difference of ocean circulation patterns is caused by their different geometric shapes. Generally speaking, the circulation pattern of the South Indian Ocean is similar to that of the South Pacific and the South Atlantic, while the circulation pattern of the North Indian Ocean is monsoon, and the circulation direction in winter and summer is opposite. In the high latitudes of the southern hemisphere, there should be a strong polar current from west to east relative to the westerlies. In addition, there is an circumpolar wind-driven current from east to west along the coast near the Antarctic continent (Figure 5- 15).

(1) equatorial flow system

Corresponding to the trade winds in the two hemispheres are the westerly south equatorial current and the north equatorial current, also known as the trade winds. These are two relatively stable wind-driven drifts caused by trade winds, both of which are components of the huge cyclone circulation in the northern and southern hemispheres. Between the north and south trade winds, corresponding to the equatorial windless area, there is an equatorial countercurrent moving eastward, and the flow amplitude is about 300 ~ 500 km. Because the average position of the equatorial windless zone is between 3 and10 n, the air flow between the north and south equator is also asymmetric with the equator. In summer (August), the northern equatorial airflow is between10 n and 20 ~ 25 N, and the southern equatorial airflow is between 3° N and 20 S ... It is slightly southerly in winter.

The equatorial airflow gradually strengthens from east to west. At the edge of the basin, equatorial countercurrent and trade winds become more complicated. The equatorial airflow system is mainly confined to the upper layer from the surface to 100 ~ 300 m, with an average velocity of 0.25 ~ 0.75 m/s and a strong thermocline at the lower part. Above the thermocline is fully mixed warm and salty surface water with high dissolved oxygen content and low nutrient content, which makes it difficult for plankton to reproduce, so it has the characteristics of high seawater transparency and high water color. In a word, equatorial flow is a flow system characterized by high temperature, high salinity, high water color and transparency.

The equatorial flow system in the Indian Ocean is mainly controlled by monsoon. The wind direction in the equatorial region is mainly meridian direction, which changes with the seasons. 165438+1The northeast monsoon prevails from October to March, and the southwest monsoon prevails from May to September. South equatorial current exists all the year round in the south of 5 s, and equatorial countercurrent exists all the year round in the south of the equator. The northern equatorial current flows westward from June 1 1 when the northeast monsoon prevails to March of the following year, and flows eastward at other times due to the influence of the southwest monsoon, which can merge with the equatorial countercurrent and is difficult to distinguish.

The equatorial countercurrent region is rich in precipitation, which has the characteristics of high temperature and low salt compared with the equatorial flow region. There is a divergent upward movement of seawater between it and the north equatorial current, which transports seawater with low temperature and high nutrition upward, resulting in fertile water quality, which is beneficial to the growth of plankton, so the water color and transparency are relatively reduced.

The Pacific Ocean is located in the southern equatorial current zone (in the thermocline below the equator, there is a flow from west to east opposite to the equatorial current, which is called equatorial undercurrent or Cromwell current). It is generally distributed in a strip shape, with a thickness of about 200m and a width of about 300km. The maximum velocity is as high as 1.5 m/s, and the flow axis is often consistent with the thermocline, which is located below 50m in the eastern ocean and above 200m in the western ocean. Obviously, the equatorial undercurrent is not directly caused by the wind. There are different opinions about its formation and maintenance mechanism. Among them, some people think that the southern equatorial current accumulates surface seawater on the west coast of the ocean, which makes the sea surface tilt from west to east, thus producing an eastward pressure gradient force. Because the Coriolis forces on both sides of the equator are in opposite directions, the undercurrent flowing eastward is concentrated on both sides of the equator. This undercurrent has been found in the Atlantic Ocean and Indian Ocean.

(2) Upper boundary current, Gulf current and Kuroshio in the west.

The western boundary Shanghai current refers to the current flowing from low latitude to high latitude along the continental slope on the west side of the ocean, including Kuroshio and Dongao currents in the Pacific Ocean, Gulf currents and Brazil currents in the Atlantic Ocean, and Mozambique currents in the Indian Ocean. They are all part of the main anticyclone circulation in the northern and southern hemispheres, and they are also the continuation of the north-south equatorial current. Therefore, compared with coastal seawater, it has the characteristics of high temperature, high salinity, high water color and high transparency of equatorial current.