Tidal currents. Tidal currents Examples of tidal currents in the world's oceans

In the World Ocean, currents are caused by the action of wind on the water surface, the action of gravity and tidal forces. Regardless of the cause of its occurrence, the current is influenced by the internal friction of water and the deflecting effect of the Earth's rotation. The first slows down the flow and causes turbulence at the boundary of layers with different densities, the second changes its direction, deflecting it to the right in the northern and left in the southern hemispheres.
Based on their origin, currents are divided into friction(the main reason is the friction of moving air on the surface of the water), gravity-gradient(the reason is the desire of gravity to level the surface and eliminate the uneven distribution of density) and tidal(the reason is a change in level due to tidal forces).
In friction currents, we can distinguish wind currents, caused by temporary winds, and drift currents, caused by constant (or prevailing) winds. In the circulation of the waters of the World Ocean, drift winds are of greatest importance.
Gravity-gradient currents are divided into drainage (runoff) and density currents. Sewage currents occur in the case of a steady rise in the water level, caused by its influx and abundance of precipitation, or, conversely, in the case of a decrease in the level, caused by the outflow of water and its loss through evaporation. An example of a drainage current associated with a rise in level as a result of the influx of water from a neighboring sea (the Caribbean) would be the Florida Current, which provides flow from the Gulf of Mexico to the Atlantic Ocean. The waste current, caused by an increase in level due to river flow, is observed in the Kara and Laptev seas. The waste current can cause wind (surges and surges of water).
Density currents are the result of unequal density of water at the same depth. They arise, for example, in straits connecting seas with different salinities (Strait of Gibraltar, Bosporus, etc.). Differences in water density can be caused by unequal atmospheric pressure on different parts of the Ocean. The resulting density currents are called barogradient currents.
Tidal currents are created by the horizontal component of tidal forces. These currents cover the entire thickness of the water. The speed of tidal currents is directly proportional to the height of the tide. In straits and bays it depends on their cross section. If in the open Ocean the speed of the tidal current is only about 1 km per hour, then in narrow straits it reaches 22 km per hour. With depth, the tidal current very slowly (slower than any other) loses speed. The period of tidal currents depends on the period of tide (semidiurnal, daily). The tidal current maintains a straight direction of movement (back and forth) only in the straits. In the open ocean, the tidal current deviates from rectilinear motion and takes on a rotational character, making full turn(clockwise in the northern hemisphere and counterclockwise in the southern hemisphere) in 12 hours. 25 min. or 24 hours 50 minutes.
Since the causes of currents can act simultaneously, currents are often complex.
Currents can exist as inertial some time after the action of the force that caused it has ceased.
Depending on the location in the ocean water column, currents are distinguished surface, deep, bottom.
According to the duration of existence, currents can be distinguished permanent, periodic and temporary(random). The belonging of currents to one group or another is determined by the nature of the action of the forces causing them. Constant currents maintain their direction and average speed from year to year. They can be caused by constant winds (for example, trade winds). The direction and speed of periodic currents change periodically in accordance with the nature of the change in the causes that caused them (for example, monsoon winds, tides). Temporary flows are caused by random reasons, and there is no pattern in their changes.
Currents may be warm, cold and neutral. The former are warmer than the water in the region of the Ocean through which they pass; the latter, on the contrary, are colder than the water surrounding them; still others do not differ in temperature from the waters through which they flow. The temperature of the cold Peruvian Current in the area of ​​the Galapagos Islands reaches 22°, but it is 5-6° lower than the temperature of surface waters in the equator region. Warm current penetrating at some depth from Atlantic Ocean in the Arctic, has a temperature of only 2° (and even lower), but above and below it there is water with a temperature of 0°.
Usually, currents moving from the equator are warm; Currents going towards the equator are cold.
Cold currents are usually less salty than warm currents. This is explained by the fact that they flow from areas with big amount precipitation and less evaporation or from areas where the water is desalinated by melting ice.

When warm and cold currents interact, the cold currents, unless they are less salty, sink under the warm ones. However, the combination of salinity and temperature can cause cold water appears above the warm one (for example, in the Arctic Ocean).
Studying drift currents made it possible to derive a number of patterns to which these currents obey:
1) the speed of the drift current increases with the strengthening of the wind that caused it and decreases with increasing latitude:

2) the direction of the current does not coincide with the direction of the wind: it deviates to the right in the northern hemisphere and to the left in the southern hemisphere. Provided there is sufficient depth and distance from the shore, the deviation is theoretically equal to 45°. Observations show that in real conditions the deviation at all latitudes is slightly less than 45°;
3) due to friction, the movement of water caused by the wind on the surface is gradually transmitted to the underlying layers. In this case, the speed of the current decreases exponentially, and the direction of the flow (under the influence of the Earth’s rotation) deviates more and more and at some depth turns out to be opposite to the surface one (Fig. 83). The countercurrent speed is 1/23 of the surface speed (4%). The depth at which the flow turns 180° is called the friction depth. At this depth, the influence of the drift current practically ends. Observations show that drift currents cease at all latitudes at a depth of about 200 m.
Transmission of the current into depth takes time. It takes about five months for the current to spread to the depth of friction.
In shallow places, the deviation of the current from the wind direction decreases, and where the depth is less than 1/10 of the friction depth, no deviation occurs at all.
The influence of the bottom topography affects surface currents even at relatively large depths (up to 500 m).
The configuration of the banks greatly influences the direction of the current. The current, heading towards the shore at an angle, bifurcates, with its largest branch going towards the obtuse angle. Where two currents approach the shore, a drainage-compensatory countercurrent arises between them due to the connection of their branches.
General diagram of surface currents of the World Ocean. Since the main cause of surface currents is constant (or prevailing) winds in three oceans - the Atlantic, Pacific and Indian - the general distribution of currents is the same (Fig. 84).
On both sides of the equator, trade winds cause north and south trade winds (equatorial) currents, which deviate from the direction of the wind and move from east to west. Meeting the eastern coast of the mainland on their way, the trade wind currents bifurcate. Their branches heading towards the equator, meeting, form a drainage-compensating inter-trade wind countercurrent, flowing east between the trade wind currents. The branch of the northern trade wind current, deviated to the north, moves along the eastern shores of the continent, gradually moving away from it under the influence of the Earth's rotation. North of 30° N. w. this current is influenced by the prevailing westerly winds here and moves across the Ocean from west to east. At the western coast of the mainland (about 50° N), this current is divided into two currents diverging in opposite directions. One of them goes to the equator, compensating for the loss of water caused by the northern trade wind current, and joins it, closing the subtropical ring with an anticyclonic (clockwise to the center of the region) system of currents. The second current along the coast of the mainland follows north. One part of it penetrates into the Arctic Ocean, and the other joins the flow from the Arctic Ocean, completing another, smaller (and less pronounced) than the subtropical ring with a cyclonic system (counterclockwise from the center of the area) of currents.

In the southern hemisphere, just like in the northern, a subtropical ring (anticyclonic) of currents arises. A second, smaller (cyclonic) ring of currents does not form. In the south, where there is a continuous expanse of water (the Southern Arctic Ocean), there is a powerful drift current of westerly winds connecting the waters of three oceans.
Surface currents of the Atlantic Ocean. In the Atlantic Ocean, as shown in Figure 84, there are northern and southern trade wind currents and countercurrents between them. The southern trade wind current is located at the equator, the northern trade wind current and countercurrent are shifted north of it in the same way as the thermal equator, the equatorial low pressure zone and, consequently, the trade winds over the Ocean are shifted.
The northern trade wind current begins at Cape Verde, crosses the Ocean and approaches the Antilles. Part of it enters the Caribbean Sea (Caribbean Current) and from there penetrates the Gulf of Mexico. Some of the water flows along the Antilles (Antilles Current) and merges with the Florida Current leaving the Gulf of Mexico.
From the confluence of the Florida (more powerful) and Antilles (less powerful) currents, the Gulf Stream is formed, stretching from Cape Hatteras to the Great Newfoundland Bank.
The Gulf Stream is a relatively narrow strip (75-120 km) of water with high speeds (up to 3-10 km/h), separating the warm waters of the Sargasso Sea from the cold waters coming from the north. At a depth of 1350-1800 m the current is very weak, and from a depth of 2800 m there is a water movement opposite to the surface one. The flow trunk consists of a number of multidirectional jets (strips), vortices, and branches. Characterized by constant pulsation and the formation of convolutions. The change in current speed is periodic and is caused by changes in the speed of trade winds and westerly winds. The more intense the trade wind circulation, the lower the speed of the Gulf Stream. The current temperature also depends on the intensity of the trade winds. When they intensify, the water temperature first rises. This occurs 3-6 months after the northeast trade wind strengthens and 6-9 months after the southeast trade wind strengthens, as a result of the surge of warm water into the Gulf of Mexico. 9-11 months after the strengthening of the northeastern trade wind and 10-12 months after the strengthening of the southeastern trade wind, a decrease in temperature is observed. Following the warm water moved by the trade winds from the coast of Africa, the winds drive the colder water that has risen from the depths. The average annual water temperature on the surface of the Gulf Stream is 25-26°, salinity is 36.2-36.4‰.
To the southeast of the Great Newfoundland Bank (slightly north of 40° N and about 40° W), the Gulf Stream ends, breaking up into a series of jets heading south and southeast and joining the general anticyclonic circulation of waters in this part of the Atlantic Ocean.
At the eastern edge of the Great Newfoundland Bank, under the influence of westerly winds, the North Atlantic Current arises, continuing the Gulf Stream to the northeast. About 50° N. w. the current is divided into two branches: northern and southern. The southern branch forms the Portuguese Current. Between the Canary Islands and Cape Green, the waters of this current merge with those that differ from them in physical properties(due to the influence of the cold deep waters rising here) by the waters of the Canary Current. At Cape Verde, the Canary Current joins the northern trade wind, closing the subtropical ring of currents in the northern part of the Atlantic Ocean.
The northern (main) branch of the North Atlantic Current goes to the shores of Europe and, under the name of the Norwegian Current, goes into the Arctic Ocean. Around the 60th parallel, the Irminger Current departs from the North Atlantic Current (under the influence of the bottom topography) to the west. Most of it at Cape Farwell joins the East Greenland Current, forming together with it the West Greenland Current. A smaller part of it, going around the island from the west and north. Iceland, flows into the East Iceland Current (a branch of the East Greenland Current).
The West Greenland Current, following the coast of Greenland, goes into Baffin Bay. Some of it penetrates into the Arctic Ocean. The rest of the water mass of this current turns south and, strengthened by cold waters flowing through the straits from the Arctic, forms the Labrador Current. The latter, meeting the Gulf Stream, is divided into a number of jets. Western jets, merging with the current coming out of the Cabot Strait, move along the coast North America South. There is always cold water between the mainland coast and the warm waters of the Gulf Stream. The temperature of the Labrador Current in January is 0°, in August 12°. Its cold waters gradually go deeper under the warm waters of the Gulf Stream. The Labrador Current brings icebergs of various shapes to the Newfoundland Bank, descending south to 41° N. w. (in exceptional cases to the south).
The southern trade wind current, the most constant of all currents in the world's oceans, crosses the Atlantic Ocean, following along the equator, and off the coast South America is divided into the Guiana and Brazilian currents. The Guiana Current, together with the North Equatorial Current, carries water to the Caribbean Sea and the Gulf of Mexico. The Brazilian goes south and, deviating to the east around the 40th parallel, joins the flow of the Western winds. Only a small branch of the Brazilian Current continues to move south along the coast of the mainland, clinging to it.
Towards the Brazilian Current, penetrating between its two branches (at a distance of 30-50 km from the coast), the cold Falkland Current heads, turning (after connecting with the Brazilian Current at 35° S) to the east. Off the coast of Africa, the Benguela Current departs from the West Winds to the north. It closes the southern subtropical ring of currents in the Atlantic Ocean.
The equatorial countercurrent in the Atlantic Ocean is expressed throughout the summer; from December to March it persists only in the east. The continuation of the countercurrent is the Guinea Current, connecting with the South Equatorial Current.
Surface currents in the Pacific Ocean. The northern trade wind current is always observed north of the equator (between 10 and 22° N). In the western part of the ocean near the Philippine Islands, it is divided into 3 unequal branches: one becomes part of the inter-trade wind countercurrent, the second goes to the Sunda Islands, and the third, the most powerful, forms the warm Kuroshio Current (analogous to the Gulf Stream). Near the island of Kyushu, a western branch departs from Kuroshio, penetrating through the Tsushima Strait into the Sea of ​​Japan - the Tsushima Current.
Kuroshio washes the eastern shores of the Japanese islands and off the island. Honshu (near the 40th parallel) turns east, turning into the transverse Ceeepo-Pacific Current. Near the coast of North America, it is divided into the Californian (more powerful) and Alaskan (less powerful) currents.
The northern subtropical ring of currents in the Pacific Ocean consists of the following currents: North Equatorial - Kuroshio-North Pacific - California.
The Alaska Current, following along the coast of Alaska and the Aleutian Islands, partially penetrates the Bering Sea and the Arctic Ocean, and partially turns to the south and southeast, forming a small ring.
From the Bering Sea along the coast of Kamchatka and the ridge of the Kuril Islands, the waters of the cold Kuril-Kamchatka Current move south. It gradually goes down, turning into a deep current.
The intertrade countercurrent in the Pacific Ocean exists all year, but in the summer in the northern hemisphere it moves north and expands. In the east, off the coast of America, the countercurrent divides into two opposite branches, flowing into the trade wind current. In summer, most of the countercurrent turns north.
The Cromwell countercurrent has been discovered beneath the surface inter-trade current in the Pacific Ocean. It is located at a depth of more than 100 m, its thickness reaches approximately 200 m, and its speed is 1.5 m/sec. It runs from west to east for more than 4.5 thousand km and disappears at the Galapagos Islands. Under the Cromwell Current, the water moves west again. The existence of currents similar to the Cromwell Current is assumed in other oceans.
The southern trade wind current, more stable and stronger than the northern one, goes from east to west near 23° south. w. Near Australia and New Guinea it is divided into two currents.
The main part of it flows into the countercurrent, a smaller part forms the East Australian Current. It causes a circular movement of water on the surface of the Tasman Sea, and then joins the current of the Western Winds. Off the coast of South America, from the current of the Western Winds to the north, the powerful Peruvian Current (Humboldt Current) goes to connect with the South Trade Wind Current. The water temperature is 8-10° lower than the air temperature.
Surface currents of the Indian Ocean. The size and position of the Indian Ocean explain some of the differences in its surface currents from those of the Atlantic and Pacific oceans.
In the northern part of the Indian Ocean, divided by the Hindustan Peninsula, monsoon currents, changing their direction with the seasons, become of primary importance. There is no constant Northern trade wind current here; it is expressed only from November to March in the same way as the inter-trade wind countercurrent.
The southern trade wind current exists constantly, but in comparison with similar currents of the other two oceans, in accordance with the position of the trade winds, it is shifted by 10° to the south.
In the western part of the ocean, first the Madagascar Current and then the Mozambique Current branches south from the Southern Trade Wind Current, but the bulk of its waters turns north. In summer it forms the Somali Current, and in winter it gives rise to the inter-trade wind countercurrent.
In summer, during the southwest monsoon, in the northern part of the Indian Ocean, water generally moves from west to east, while in winter, during the northeast monsoon, from east to west. During this period, a current passes off the coast of Somalia, also called the Somali Current, but opposite in direction to the summer Somali Current.
In the southern part of the Indian Ocean (south of Madagascar), the Madagascar and Mozambique Currents merge to form the stable Agulhas Current, but most of the water goes east and joins the current of the Western Winds. The Needle Current partially enters the Atlantic Ocean, flowing into the Benguela. The Western Wind Current in the south and the Western Australian Wind Current in the east complete the subtropical ring of currents in the Indian Ocean.
The Western Wind Current, covering the southern parts of the three oceans, is the greatest current in the World Ocean. Its width in the Bellingshausen Sea is 1300 km. The speed is low (on the surface - 0.2-0.3 m/sec) and decreases with depth. To go around Antarctica, surface waters need 16 years, deep waters - more than 100 years.
Currents of the Arctic Ocean. The distribution of currents in the Arctic Ocean, compared to other oceans, is very unique, although it also depends on the prevailing winds.
Strong winds blowing from east to west along the northern shores of the Eurasian continent, and from north to south along the eastern shores of Greenland, cause ice and surface water to drift generally towards the Atlantic Ocean. In this case, several interconnected circulations arise: one in the Beaufort Basin is anticyclonic, two in the Nansen Basin - anticyclonic (north of Greenland) and cyclonic (northeast of Novaya Zemlya). The last two circulations contribute to the formation of the East Greenland Current, which carries large amounts of water and ice into the Atlantic Ocean.
The Norwegian Current brings warm Atlantic water (145,000 km3/year). At the North Cape, it is divided into the North Cape (35,000 km3/year), going east along the coast of the mainland, and Spitsbergen (78,000 km3/year), following to the north and gradually sinking (due to relatively high salinity) to a depth of 100-900 m. The warm water of this current, pressing against the continental slope, moves east and creates an intermediate layer of relatively warm (up to 2.0-2.5°) water with a thickness of up to 600 m.
Pacific water, penetrating through the Bering Strait (44,000 km3/year), does not form an independent current in the Arctic Ocean.
Currents in the seas, bays and straits. Currents in the seas are caused by the same reasons as in the oceans, but limited size and shallower depths determine the scale of the phenomenon, and local conditions give them unique features. Many seas (Black, Mediterranean, etc.) are characterized by a circular current caused by the deflecting force of the Earth's rotation. In some seas, tidal currents are very well expressed (for example, the White Sea). Currents in a number of seas (for example, in the North, Caribbean) are a branch of ocean currents.
According to the nature of the currents, straits can be divided (following N.N. Zubov) into flow-through and exchange. In flowing straits, the current, as in a river, is directed in one direction (Strait of Florida). In exchange straits, water moves in two opposite directions, and multidirectional water flows can be located one above the other (vertical water exchange) or next to each other (horizontal water exchange). Examples of straits with vertical exchange can be the Bosporus and Gibraltar, with horizontal exchange - La Perouse and Davis. In narrow and shallow straits, the direction of the current can change to the opposite depending on the direction of the wind (Kerch Strait).
General circulation of the World Ocean. Surface currents are part of the complex and still very little studied general circulation of the waters of the World Ocean.
The main reasons that determine the movement of water - the movement and pressure of the atmosphere, differences in the distribution of temperature and salinity - act primarily on the surface of the Ocean. The movement of surface water caused by wind generally has a latitudinal direction with sharp deviations in either direction. Under the influence of heat, water on the surface of the Ocean moves towards the cold (cold water compacts and sinks, warm water expands and rises), i.e. from the equator to the poles. In the equatorial region, the ascending movement of waters dominates; in the polar regions, on the contrary, it is downward. With thermal circulation in the bottom layers, there should be a general movement of water from the poles to the equator.
In areas of high salinity, water tends to sink; in areas of low salinity, on the contrary, it tends to rise (the effect of density). Accordingly, horizontal movement of water occurs in one direction or another.
The existence of systems of surface currents with a general direction of movement towards the center or from the center of the system leads to the fact that in the first case a downward movement of water occurs, in the second - an upward movement. An example of such areas in the Ocean can be subtropical ring current systems.
The lowering and rising of waters is also caused by the surge and flow of water on the surface (for example, in the area of ​​​​the trade winds).
Zones of convergence of currents (zones of convergence) are areas of lowering water, zones of divergence of currents (zones of divergence) are areas of their rise.
Since the various reasons that determine the movement of ocean waters either coincide or turn out to be in opposite directions, their overall circulation becomes very complicated. The thermal circulation scheme can be taken as a basis. If in the polar and temperate latitudes the subsidence of water sharply predominates, then the equatorial region is characterized by its rise. On the surface of the Ocean, the dominant movement of water is from the equator; at depth, it is towards the equator. The existence of currents throughout the entire water column, including its bottom layers, is currently beyond doubt.
The importance of ocean currents large and varied. The great influence of currents on climate is well known.
Thanks to the continuous movement of water, there is a constant transfer of not only heat and cold, but also nutrients, necessary for organisms.
In zones of convergence of currents and sinking water, deep layers are enriched with oxygen; in zones of divergence of currents and rising water, nutrients (phosphorus and nitrogen salts) are carried from the depths to the surface. These processes are very important for the development of life in the Ocean.
Currents determine the distribution of plankton in the open ocean and in the seas, and transport fish larvae and fry from spawning sites to habitats. An example is the larvae of the European eel, which hatch in the Sargasso Sea and move in a passive drift (taking two to three years) to the shores of Europe. With the help of currents, eggs, larvae and fry of cod and herring move; for example, cod larvae and fry that appear off Newfoundland and the Lofoten Islands are carried by the current to the Norwegian and Barents Seas.
The flow of warm and salty Atlantic waters into the Arctic Ocean plays a big role in the life of its seas and is important for fisheries. It was discovered that changes in temperature, quantity and salt content in Atlantic waters fluctuate with approximately a four-year period, which significantly affects the herring fishery.
A change in the direction of currents off the Far Eastern shores (the departure of warm current jets) led to the cessation of the catch of the Far Eastern sardine - iwasi.
Currents played huge role in the era of the sailing fleet and now have great importance. They compile current maps, descriptions and tables for sailors.

Tidal sand ridges are elongated sand bodies formed by tidal currents. [ ...]

Tidal deltas (tidal flow deltas) form at the mouth of a channel on the landward side of a lagoon, and are best developed in tidal channels dominated by wave processes, where waves amplify the tidal current. The newly formed tidal delta is a series of overlapping cones or curved lobes, such as at Chatham Port. Massachusetts, where two fused lobes are overlapped by rectilinear and sinusoidally curved large tidal current wave marks, but with tidal flow predominant. Over time, the tidal current is concentrated within the channel, and the mature tidal delta is an inclined plane, dissected by tidal channels and dissected into a series of alluviums and nappes formed by the tidal flow. Landward deposits of tidal deltas are dominated by areas with plane-parallel and trough-shaped coarse cross-bedding, interspersed with areas of coarse cross-bedding oriented towards tidal currents, especially at the top of the section. Sedimentation rates in tidal deltas are often high, and these sediments can form a significant portion of the lagoonal facies, particularly if they migrate laterally with tidal channel migration. [ ...]

Intertidal sand beds consist of well-sorted medium- to fine-grained sand with shell fragments. Often the grain size in bedform material is finer than would be expected from the strength of the current associated with the bedform (, p. 49). The sand ridges around the British Isles are typically 50 km long, 1–3 km wide, 10–50 m high, and spaced up to 12 km apart. No simple relationship has been established between bedform size and water depth, although some groups show systematic variations in length and height, as in the Norfolk bedforms in eastern England (Fig. 9.12), which decrease in size with distance from the shore. The oblique orientation of most bedforms to the direction of the tidal current means that transport of sediment to each of the two sides of the bedform was carried out either predominantly by ebb-tide or predominantly by tidal flow (Fig. 9.13). The irregularity typical of such flows (see Fig. 9.35) causes the development of an asymmetrical cross-section of active beams, which is preserved as a series of main gentle (3-7°) internal bedding planes separated by smaller-scale cross-bedding (Fig. 9.14, b ). This latter reflects sand waves formed in the direction of ebb and flow on the surface of modern active ridges. Sand waves are oblique, but towards the crest the ridges become parallel to it, indicating convergence of flow directions along the crest. The progressive change in the orientation of sand waves is explained by their refraction, since the wedge-shaped shape of the ridge progressively impedes the flow. [ ...]

Although tidal currents are bidirectional, linear or circular, they carry out predominantly unidirectional transport of sediment due to the fact that 1) ebb and flow currents are usually not equal in maximum strength and duration (Fig. 7.39, e); 2) ebb and flow currents can follow mutually exclusive transport routes; 3) the retarding effect associated with the circular tide delays the supply of sediment; 4) a unidirectional tidal current can be enhanced by other currents, for example, a drift wind current. The interaction of these processes is well demonstrated by the example of the most studied seas in the world, namely the seas of North-West Europe, the hydrodynamic regime of which is in partial equilibrium with the shapes of the bottom surface and the directions of sediment transport. [ ...]

Linear tidal sand ridges (or sand bars) are common in modern tidal environments in both coastal and offshore areas (Section 9.5.3); morphologically similar forms of the seabed are also widespread in the Mid-Atlantic Gulf with a predominantly storm regime (Section 9.6.2). At present, there are no diagnostic criteria for distinguishing linear sand ridges in the geological record, formed predominantly by tidal currents or predominantly by storm currents, since the features of the internal structure of modern sand ridges are still very poorly known. [ ...]

Seasonal and tidal currents. Depending on the position, shape and structure of the coast, the volume and movement of water masses will change once or twice a day, ranging from very large values ​​to negligible values. Seasonal currents can further promote vertical mixing of water, eroding layers or preventing stratification of water masses, depending on the temperature and density of the water. [ ...]

The long axis of a tidal sand ridge is approximately parallel to the direction of the tidal current (Figure 6.7-2). [ ...]

Key Features facies of the tidal channel are: the presence of basal erosion of the bottom surface with deposits of shell gravel with a mixed faunal complex; the presence of large lateral accretion surfaces inclined towards the channel bed and reflecting the former position of the sedimentary side of the channel, and large-scale areas of large (more than 1 cm) cross-bedding of the tidal current, separated by thin wavy layers of silt and clay. [ ...]

The migration of dunes(?) to the SE was driven by the influence of tidal currents, probably strengthened by storms. [ ...]

Peculiar changes occur in the biology of reproduction in fish of the intertidal zone. Many of the fish in particular; Sculpins move away from the littoral zone during spawning. Some species acquire the ability to give birth viviparously, such as the eelpout, whose eggs undergo an incubation period in the mother's body. The lumpfish usually lays its eggs below the low tide level, and in those cases when its eggs dry out, it pours water on it from its mouth and splashes it with its tail. The most curious adaptation to reproduction in the intertidal zone is observed in American fish? ki Leuresthes tenuis (Ayres), which lays eggs at spring tides in that part of the intertidal zone that is not covered by quadrature tides, so that the eggs develop outside the water in a humid atmosphere. The incubation period lasts until the next syzygy, when the juveniles emerge from the eggs and go into the water. Similar adaptations to reproduction in the littoral zone are also observed in some Galaxiiformes. Tidal currents, as well as vertical circulation, also have an indirect effect on fish, mixing bottom sediments and thus causing better development of their organic matter, and thereby increasing the productivity of the reservoir. [ ...]

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Seasonally varying semi-permanent ocean currents, the California and Davidson currents, also have a strong influence on the shelf during lateral migration towards the shelf, especially in winter when the bottom current is directed north. In summer the opposite happens. Currents are too weak to erode the seafloor, but can transport suspended sediment and enhance northward wind drift currents during the winter. Mixed and semidiurnal tides with a height of 2-3 m cause circular tidal currents, which strengthen other bottom currents, but are themselves relatively weak. Tidal currents on the middle and outer shelves have an average speed of only 10 m/s. However, on the inner shelf the average current speed can reach 30 cm/s and is often intensified by wave swells. [ ...]

Most of the criteria widely used to distinguish ancient intertidal deposits are derived from observations primarily of modern intertidal deposits. Many of these criteria are not applicable either to the subtidal zone in general or to the shelf environment in particular. In coastal settings, tidal currents are usually the only significant source of energy, whereas in offshore settings, winds, waves and storms generate indefinitely variable and therefore less predictable processes and products. Nevertheless, some combinations of sedimentological features are indicative of tidal deposits located far from the shore. [ ...]

Sand waves are much smaller and oriented normal to the direction of tidal currents. The waves have a height of 1 to 10 m, they are asymmetrical and the distance between them is several hundred meters. [ ...]

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Sedimentation processes are divided into 1) calm weather processes (including tidal currents, ocean currents, and surge waves) and 2) storm processes (storm-related surge currents and high-energy oscillatory waves). In accordance with this, in the corners of the triangular diagram (Fig. 9.33, B) there are three facies types associated with three groups of processes: 1) predominantly tidal, 2) predominantly wave, 3) predominantly storm. This classification of processes is related to a similar classification of modern shelf environments (Fig. 9.4). This preliminary framework provides a basis for considering the three main groups of shallow marine terrigenous facies mentioned above, based on the combination of sand and silt content and on the dominant processes of sedimentation. [ ...]

Sand ribbons are elongated bodies parallel to the direction of the strongest tidal current. They are up to 15 km long, 200 m wide and no more than 1 m thick. [ ...]

They are common in tidal, partially enclosed epicontinental seas and straits. They are usually composed of large-scale diverse cross-beds (the thickness of cross-bedded series is about 1-10 m, rarely up to 20 m). Internal structures vary from simple avalanche-sloping fore-layers to complex areas consisting of large, gently dipping bedding surfaces that are separated by areas of small-scale cross-bedding, dipping generally downslope but in some cases upslope. [ ...]

Characteristics: Finger-shaped channel sands grading offshore into elongated sandy ridges of tidal currents. [ ...]

Relatively thick, elongated sand bodies towards the tide, formed by sand ridges and shoals of tidal currents, which form a complex of secondary channels and sands with megaripple marks (Fig. 6.6-32), represent the main geometric elements observed in this range of the delta . [ ...]

A depositional sub-environment with a high energy level, where sedimentary material is constantly reworked by tidal currents, marine longshore currents and waves (water depth no more than 10 m). It includes delta front cover sands, delta arm mouth bar, river mouth tidal deposits, nearshore and shore bar deposits, and stream mouth bar deposits. The delta front is a relatively large-scale sequence characterized by an increase in grain size up the section. It records a change in facies from fine-grained distal or prodeltaic to a shoreline facies typically dominated by sandstone. These sequences are the result of lateral accretion of the delta front, and may be truncated by deltaic arm or braided tidal drainage channel sequences as accretion continues. [ ...]

The physical processes imprinted in ancient lake sediments are similar to those associated with marine environments. However, there are no tidal currents in the lakes, wave activity here is reduced, but the bottom outcrops above the water surface are characteristic, reflecting frequent, even annual fluctuations in the water level in the lakes and the position of their coastline. [ ...]

The unevenness of the coastline divides the shelf into a large number of areas of sedimentation (Fig. 9.22, b). The characteristics of the coastline and the driving force of the Agulhas Current maintain the coastal position of these areas and do not allow them to spread far from the coastline. When in a leeward position, whirlpools develop in them, moving clockwise, for example, near Maputo and Durban (Fig. 9.22, b). On the seafloor, north-facing lee slopes of sand waves and ripple marks indicate countercurrents that may transport sediment to the shelf break. Where the southward Agulhas Current turns toward the shelf, the boundary between the southern end of the eddy system and the main current becomes a discharge zone (Fig. 9.22b), which can migrate along the shelf up to 10 km in any direction. As a result of this migration, these areas may contain sedimentary structures that are similar to those produced in other areas by tidal return current systems (Section 9.5.2). [ ...]

In the seas and oceans the situation is completely different. Marine habitats are vast and interconnected; they are more or less accessible to pelagic larvae, since the latter are quickly carried away by constant and tidal currents. In marine invertebrates, the dispersal stage is usually the short-lived pelagic larva, and the sessile adult usually corresponds to the phase of the life cycle in which feeding and growth are mainly carried out. All this is the exact opposite of freshwater insects (Fig. 5.9). [ ...]

Visher Navajo sandstones as shallow marine sediments. They provide structural data for these rocks and compare the cross-bedding in them with that which is supposedly observed in modern bottom forms formed by tidal currents. However, the structural data cannot be considered unambiguous, and when comparing cross-bedding, these authors for some reason did not take into account that echograms recorded over shallow marine bottom forms are characterized by exaggerated vertical contrast. [ ...]

Conditions suitable for the regular deposition of silt plumes existed in the Early Cretaceous channel of southern England. The muddy layers lie in the central part of the quartz silts and sands and are diagnostic of two periods of tidal calm separated by tidal current deposits (stages B, C and O in Fig. 7.39). The fore-layer sands reflect the migration of a sand wave or mega-ripple during the current-dominated stage (Fig. [ ...]

The biological productivity of the biosphere, of all living matter on the Earth, is 1.7x1015 MJ/year. In its absolute value, it is comparable, within the same order of magnitude, with such global geological processes as the energy of tidal currents (2.3x1015 MJ/year), the energy of movement of atmospheric air masses (1.3x1015 MJ/year) and the magnitude of thermal flow from the bowels of the Earth, equal to 1.3x1015 MJ/year; an order of magnitude higher than the energy of Earth earthquakes and two orders of magnitude higher than the energy of river runoff and volcanic eruptions. [ ...]

Open shelves (Ginsburg, James) are inclined towards the shelf edge, located at a depth of 140-230 m, and since there are no physical barriers, the bottom of the shelf is strongly affected by wave processes, oceanic and tidal currents are also active. On such shelves, high-energy environments can occur and coarse-grained detritus is abundant on them. Coarse-grained detritus includes “pure” calcarenites. The presence of finer-grained carbonate is mainly confined to the deeper (low energy) outer shelf edges where pelagic sedimentation becomes significant. The absence of significant bottom slopes is reflected in the presence of wide, irregularly shaped facies belts and the absence of redeposition due to gravity flow. [ ...]

According to most researchers, the active development of modern canyons was associated with periods of falling sea levels in the Pleistocene. During glaciations, the coastline moved significantly closer to the shelf edge, so material carried by rivers and tidal currents flowed directly onto the slope and eroded its surface, resulting in the formation of gullies and canyons. The Holocene transgression of the sea led to the fact that the canyons on the passive margins lost their direct connection with the sources that fed them and gradually lost activity. However, in the late Pleistocene they represented an effective system of transport arteries, through which most of the sedimentary material carried to the shelf was ultimately dumped into deep-sea areas. outskirts. [ ...]

Submarine denudation and sedimentation intensified after the sediment balance in the lagoon was disrupted due to the diversion of river mouths. The erosion of the bridge separating the lagoon from the sea has increased and in some places its condition is considered critical. In the passages connecting the lagoon with the sea, currents cause erosion of sediments on one side of the bridge and their accumulation on the other. Due to the construction of the piers, the speed of the currents increased. Thanks to this, the processes of deepening and expanding shipping channels self-develop. In some places in the Malamocco Passage, for example, underwater denudation has extended to a depth of 20 m. Increased tidal currents have improved water quality in the lagoon, but at the same time, abrasion activity has intensified, posing a threat to some structures. In particular, the destruction of Fort San Andrea had already occurred in the Lido Passage. [ ...]

The shelf and its sandy cover exhibit certain variations in the operating processes and sedimentation responses associated with geographical location. The features of the northern Georges Bank are partly inherited from the substrate (they predominate), and partly due to the action of the tidal current, which processes relatively coarse Pleistocene glacial sediments. On the southern North Atlantic Shelf, erosion predominates, sediment is not deposited, and biogenic carbonates are formed in situ. In the middle part of the North Atlantic shelf, signs of ancient river sediments are combined with the products of modern predominantly wave processes and currents. [ ...]

The predominant feature is well-graded sand (Figure 6.7-1), with a moderate to high grain-matrix ratio. The grain size distribution across the beds is relatively uniform. Grain size may increase upsection within the ridge and regionally in the direction of transport by tidal currents. [ ...]

There are relatively few examples of facies of ancient estuarine associations. Several Pleistocene sections in Holland, interpreted as subtidal estuarine channel deposits, are represented by a basal erosional surface overlain by thin intraformational conglomerate that grades into sands with trough-like cross-bedding and evidence of bimodal paleocurrents. The frontal slopes of the cross-beds have clay laminations and alternating clays and silts, indicating that the migration of the bed topography occurred in accordance with fluctuations in the tidal current. The channel sands are followed by finer-grained facies with lenticular and laser bedding and also show the presence of bimodal paleocurrents. In this example, the interpretation of the facies as etaurian rather than tidal channel facies is supported by their close proximity to fluvial facies. [ ...]

When considering the general concept of energy “subsidies,” one more point needs to be made. A factor that increases productivity in some conditions may contribute to energy leakage in other conditions, reducing productivity. Thus, increased evapotranspiration in a dry climate leads to excessive energy consumption, and in a humid climate, for example, it provides additional energy (G. Odum and Pidgin, 1970). Flowing water ecosystems, such as those included in Table. 7 streams in Florida are generally more productive than standing water ecosystems, but too fast (and therefore destructive) or irregular water flow reduces productivity. The smooth ebb and flow of tides on salt marshes, mangrove estuaries, or coral reefs contributes to the high productivity of these communities, but on northern rocky coasts, which suffer from ice in winter and heat in summer, tidal currents can drain the energy of the community. Even in agriculture human attempts to help nature often lead to undesirable consequences. For example, plowing the soil in the north is beneficial, but in the south it leads to rapid leaching of nutrients and loss of organic matter, which can greatly harm future crops. It is symptomatic that agronomists are now seriously discussing the possibility of "no-till" farming - an encouraging shift towards the concept of "mind helping nature, not fighting it." Finally, some types of pollutants, such as treated wastewater, can, depending on the volume and frequency of discharge, be a beneficial factor that increases productivity or serve as a source of stress (see Fig. 216). If treated wastewater enters an ecosystem at a constant, moderate rate, it can contribute to increased productivity, but massive discharge at irregular intervals can almost completely destroy the system as a biological entity. [ ...]

Sedimentation in lakes depends on three main factors: water chemistry, shoreline fluctuations, and the relative amount of debris carried by rivers. Open lakes are characterized by a fairly stable coastline, since the influx of water plus precipitation is in equilibrium with the amount of outflow plus evaporation. The outflow of water acts as a buffer, preventing particularly large fluctuations in lake level (such as in the Great Lakes of North America), but despite this, lake level fluctuations can be significant (as in Lake Nyasa in East Africa). Shoreline fluctuations can also be caused by phenomena such as isostatic arching, which occurs after glaciation. Thus, the northern shore of Lake Superior rises relative to the southern one by 0.46 m per 100 years, and the channel of Lake Ontario rises by 0.37 m over the same time. From a geological point of view, the periods of time during which these movements occur are instantaneous. Other lakes (Maracaibo in Venezuela) are directly connected to the sea, which also determines the water level in the lake. An unusual situation occurs in Pitt Lake in British Columbia, where water levels are controlled by tidal currents in the Fraser River estuary. The sedimentation of open lakes is usually dominated by the supply of clastic material by rivers, but where its supply is small (for example, in Lakes Tanganyika - Kivu, Fig. 14.8), chemical and biochemical sedimentation may dominate.

TIDAL CURRENTS tidal currents, movements of water caused by the tidal forces of the Moon and the Sun. see also Tides are quadrature And Spring tides.

• - see Air currents...

  Dictionary of winds

• - air currents, atmospheric currents - wind systems over a large area and in a significant thickness of the atmosphere, possessing a certain stability in time and space...

  Dictionary of winds

• - wind currents, temporary, periodic or permanent, arising on the surface of the water under the influence of wind. They deviate from the wind direction in the northern hemisphere to the right at an angle of 30-45°...

  Dictionary of winds

• - part of the seabed that drains during low tides...

  Ecological dictionary

• - deep currents is a generalized name for currents developing in the ocean below a layer of water under the direct influence of the wind...

  Geographical encyclopedia

• - see Currents...

  Marine dictionary

• - currents arising as a result of tidal phenomena, periodically changing direction and speed and reaching the highest speeds in coastal areas and in narrow areas...

  Marine dictionary

• - currents of surface waters of oceans and seas resulting from the action of wind on the water surface...
• - currents that arise in the seas and oceans as a result of the formation of a pressure difference in the water column. The pressure difference is created under the influence of wind surges and surges of water, uneven distribution...

  Great Soviet Encyclopedia

• - tidal adj. the same as...

  Dictionary Efremova

• - ...

  Spelling dictionary-reference book

• - adj. "excellent"...

  Russian spelling dictionary

• - influences of time, prevailing views Wed. Completing legislative work does not sometimes mean implementing it in practice, especially if unfavorable for it occurs...

  Mikhelson Explanatory and Phraseological Dictionary

• - Current influences of time, prevailing views. Wed. To carry out legislative work does not sometimes mean to carry it out in practice, especially if unfavorable trends are encountered. A. Ѳ...

  Michelson Explanatory and Phraseological Dictionary (orig. orf.)

• - adverb, number of synonyms: 1 tide...

  Synonym dictionary

• - adj., number of synonyms: 1 tidal...

  Synonym dictionary

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The difference between zero depth and the high water level is called the high water height hPV. The difference between zero depth and low water level is called low water height hMV. The difference between the heights of high water and the following low water is called the magnitude of the tide B = hPV hMV. The time between two adjacent moments of high or low water is called the high tide period.

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rocks of classical navigation. Pilot.

18. Tidal phenomena

and their accounting in navigation.

The surface of the oceans is not at rest, but periodically changes its position and fluctuates. This occurs under the influence of various processes and forces, which can be combined into the following main groups:

Geodynamic and geothermal phenomena in the earth's crust - earthquakes and seaquakes, volcanic eruptions (tsunamis), rise and fall of land (tectonics), heat flow through the ocean floor.

Mechanical and physico-chemical effects on the ocean surface solar radiation, changes in atmospheric pressure, wind, which causes surge fluctuations, precipitation, coastal runoff, etc.

Cosmic (astronomical) tidal forces, which are the main ones in tidal phenomena.

The concept of tides and terminology

Tidal phenomena are complex wave movements of ocean water masses. The consequence of these movements is periodic changes in level and currents.

Tidal phenomena occur due to the action of tidal forces between the Earth, Moon and Sun. The tidal force of the Moon is 2.17 times greater than the tidal force of the Sun (due to its distance), therefore the main features of tidal phenomena are determined mainly by the relative positions of the Earth and the Moon.

Physiographic conditions have a significant influence on the magnitude and nature of tidal phenomena in each specific place: depths, coastline, the presence of islands, and others. Due to the influence of physical and geographical conditions, the nature of the tides can vary within very wide limits. Thus, in the Baltic Sea they are practically absent; in the Bay of Fundy, located at approximately the same latitude, level fluctuations reach 18 meters.

Tidal phenomena are characterized by two main factors:

Level changes;

Tidal currents.

Both sides of this process are interconnected, however, due to the lack of a unified theory, tidal level fluctuations and tidal currents are studied separately.

Tidal phenomena have a great impact on navigation and navigation safety, therefore information about them is regularly published in special manuals. In order to use them correctly to solve various navigation problems, navigators must have a good understanding of the nature of this phenomenon.

Tidal fluctuations can be represented graphically.

On the daily tide graph, the x-axis is time, t , and along the ordinate axis is the height of the tide, h , above the conventionally accepted level zero depth, 0 gl.

The process of sea level rise is called high tide, low tide low tide.

The highest level position at high tide is called full water PV, low tide low water MV.

The difference between zero depth and full water level is calledhigh water height h PV.

The difference between zero depth and low water level is calledlow water height h MV.

The difference between the heights of high water and the following low water is calledthe magnitude of the tide

B = h PV - h MV.

Beyond zero depths on Russian nautical charts on tidal seas, the lowest theoretical level (LTU) is adopted the lowest level possible under astronomical conditions, that is, according to the relative positions of the Earth, Moon and Sun.

The time between two adjacent moments of high or low water is calledhigh tide period.

Depending on the size of the period, tides are divided into daily, semidiurnal, mixed, irregular semidiurnal, irregular diurnal and anomalous.

Daily allowance tides (C) those whose average period is equal to the lunar day (24 hours 50 minutes). Diurnal tides occur most often in the Pacific Ocean.

Half daily allowance tides (T) are those whose period is equal to half a lunar day (12 hours 25 minutes). Semi-diurnal tides are observed along the Murmansk coast of the Barents Sea, throughout most of the White Sea and almost throughout the entire Atlantic Ocean.

At semidiurnal tides, high water occurs twice a day, high water, and low water, low water, twice a day. Since both PV and both MV have different heights, they are designated as follows:

ERW high full water;

NPV low full water;

WWII high low water;

NMV low low water.

The heights of PV and MV of semidiurnal tides above zero depth are designated as follows:

h ERW height of high full water;

h IVC height of low high water;

h WWII height of high low water;

h NMV height of low low water.

Mixed tides are those whose period changes from semidiurnal to daily during the lunar month. Mixed tides are divided into irregular diurnal (ID), in which the diurnal period predominates, and irregular semidiurnal (SI), in which the semidiurnal period predominates.

Abnormal tides those in which the nature of the rise and fall of water is complicated by shallow water, these are daily shallow tides (SM) and semidiurnal shallow tides (SM). Abnormal tides are observed in some ports of the English Channel and in the White Sea.

The magnitude of tide B varies throughout the month, and on some days it reaches its maximum value, and on others it reaches its minimum. The magnitude of the tide varies according to the phase of the Moon, that is, it depends on the relative positions of the Earth, Moon and Sun.

The highest high water and the lowest low water, that is, the maximum tide (B) is observed after full moons and new moons, that is, when the Earth, Moon and Sun are approximately in the same straight line, and the tidal forces of the Moon and Sun add up. Such periods are called syzygy (gr. sizigia connection).

The lowest high water and the highest low water, that is, the minimum tide, are observed after I and after IV quarters in the phases of the moon. At this time, the Moon and the Sun are located approximately at right angles to the Earth, and the tidal forces of the Sun weaken the tidal forces of the Moon. Such periods are called quadrature (lat. quadrature fourth part, quarter).

The tides are also influenced by the declination of the Moon. At high declinations of the Moon, tides are called tropical , and when the Moon passes through the equatorequatorial.

The time interval between the moment of the upper or lower climax of the Moon and the moment of the onset of full water on a given meridian is calledlunar interval Tl.

The average of the lunar intervals on syzygy days, calculated from a large number of observations, is calledport application hour IF.

The following terms are used to characterize tides over time:

t PV moment of full water;

t MV moment of low water;

T r time of level rise time from the moment of low water to the moment of high water:

T r = t PV t MV;

T p time of level drop time from the moment of high water to the moment of low water:

T p = t MV t PV;

T st level standing time time during which the level, having reached a certain height, remains unchanged.

Russian tide tables

Tidal phenomena in different areas of the world's oceans have been studied differently. Depending on the degree of study, all points are divided into three groups:

Main points (ports) for which detailed tide data is available.

Additional points linked to the main ones, for which the tides are calculated through the main point.

Points for which applied clocks are given, from which it is possible to calculate the moments of PT and MV and their heights, based on the moments of the culmination of the Moon.

The Oceanographic Institute annually publishes Tables from which it is possible to pre-calculate the moments and heights of tides. Tide tables are published in four volumes:

Volume I . Waters of the European part of Russia.

Volume II . Waters of the Asian part of Russia.

Volume III . Foreign waters. Atlantic, Indian and Arctic oceans.

Volume IV . Foreign waters. Pacific Ocean.

Volume I and Volume II each consist of three parts:

Part I - Tides at main points.

Part II - Amendments for additional points.

Part III - Tidal currents.

Volume III and Volume IV Each consists of two parts:

Part I - Main points.

Part II Additional items.

At the beginning of each volume general information about tides is given, and at the end there are auxiliary tables and alphabetical index points.

The General Information section provides the following information:

The influence of hydrometeorological conditions on tides;

Basic terms and designations;

Information about tidal inequality;

Criteria that determine the nature of the tides;

Examples of using tide tables.

There may be differences in tide tables from different years of publication. general information, so you need to get acquainted with them every time you use new tables.

B I Part “Tides at the main points” shows the moments and heights of high and low waters for every day of a given calendar year for the main points, a list of which is given in alphabetical order on the back cover of the table.

In II Part “Corrections for Additional Items” contains corrections for moments and heights, introducing which into selected ones from the part I information on tides in the main port; you can obtain data on the moments and heights of PV and MV at additional points.

The “Auxiliary Tables” show:

Interpolation table for calculating the level at moments intermediate between MV and PV;

Average heights of syzygy and quadrature PV and MV and mean sea level (MSL) for some points;

Tables of mean sea level corrections for seasonal changes and atmospheric pressure;

Tables for converting standard time to local time;

Feet to meters conversion tables;

Astronomical data (phases, declination, perigee and apogee of the Moon).

Problems solved using tables

Determination of the moment and height of high and low waters at the main point.

Determination of the height of the tide level at any intermediate moment between MF and SW at the main point.

Determination of the moment and height of high and low waters in an additional paragraph.

Determination of the height of the tide level at any intermediate moment between MF and MF at an additional point.

Teacher of the highest category Kisenkov Vladimir Ilyich

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