Ebbs and flows. Non-periodic currents Thus, one would not want to get into a giant ocean whirlpool

The phrase in the title is a literal translation of the Japanese word “tsunami” and refers to a unique natural phenomenon: several successive long ocean waves generated by sharp displacements of large areas of the ocean floor caused by earthquakes.

Tsunamis formed at great depths are a transverse long wave (100-300 kilometers long) of low height (no more than 2 meters), propagating at a speed of about 0.2 kilometers per second (700 kilometers per hour), their period is 15-60 minutes . But when they reach shallow water, these waves sharply increase in height, their length decreases, the crests begin to collapse and, in essence, huge waves of movement are formed, to which the name “tsunami” actually refers. In some cases, the wave height reaches 30-40 meters.

The arrival of a tsunami on the coast is usually preceded by a drop in sea level and the arrival of relatively small waves. Then there may be a secondary drop in level, and after that a tsunami comes. After the first wave, as a rule, several more waves of larger magnitude come at intervals from 15 minutes to 1-2 hours. Usually the third or fourth wave is the maximum.

The waves penetrate deep into the land, depending on its topography, sometimes 10-15 kilometers and, having high speed, cause enormous destruction. After receiving a tsunami warning, it is necessary to take the ship out to the open sea to meet the wave.

In coastal areas, there are frequent cases of the formation of another natural phenomenon - large standing waves - suloya, which means a whirlpool, a crush. Small suloi are observed in the Black Sea (in the Kerch Strait), stronger ones - in the narrows off the Pacific coast of Canada and the skerries of Scandinavia. But suloi reach their largest sizes in shallow water areas with strong reverse currents - in the Kuril Straits, Singapore Straits, Portland Firth, etc. (up to 4 meters). The formation of ripples is usually associated with the interaction of two counter flows of water (Fig. 4.36a.). In this case, vortices are formed in the frontal zone, emerging to the surface in the form of random waves, and the higher the flow speed, the greater the energy of these waves.

Suloi can also appear as a result of a flow entering shallow water. In this case, large velocity gradients are formed in the water stream, flow discontinuities, vortices and, as a consequence, waves on the surface (Fig. 4.36b).

The ripples reach their greatest size during the maximum speeds of tidal currents. This dependence of suloi on the nature of the tide allows them to be predicted very reliably.

Suloi is very dangerous for navigation. Vessels passing through the swell experience unpleasant, disorderly rolling, go off course, and a high wave can tear off mechanisms and life-saving equipment from their fastenings. Crossing such areas by small vessels threatens them with death.

When water in the sea has a jump in density at any depth, waves called internal waves can arise at the boundary between the upper less dense layer and the lower layer with a sharply increased density.

Internal waves can have a height several times greater than surface waves (up to 90 m, period up to 8 minutes).

When internal waves are excited, a phenomenon known as “dead water” is observed.

A ship in dead water loses speed and can remain almost in place when the machinery is fully operational.

When following “dead water” in a calm state, the surface of the sea takes on an unusual appearance. Transverse waves increase greatly behind the stern, and a huge wave appears in front of the ship, which the ship is forced to push. On “dead water”, almost the same wave movements occur as when a ship moves through shallow water. If the speed of the ship coincides with the speed of propagation of free internal waves, then during its movement the ship creates not only ordinary ship waves on the surface of the water, but also generates waves at the interface of two layers - the “light” upper and “heavy” lower ones. The wave occurs when the interface layer is located approximately at the depth of the keel. In this case, the water masses of the upper layer, with a thickness equal to the ship’s draft, move in the opposite direction and cause a loss of speed of the ship; wave resistance increases greatly, since the ship has to “drag along” the suddenly arisen wave. This phenomenon explains “dead water”.

The phenomenon of “dead water” occurs everywhere near the mouths of large rivers - the Amazon, Orinoco, Mississippi, Lena, Yenisei, etc. But it is especially often observed in Norwegian fiords and in the Arctic seas in calm spring weather during ice melting, when there is a relatively thin layer of almost fresh water located above highly saline and dense sea water.

Internal waves pose a serious threat to underwater navigation. This is manifested both in the direct, physical impact of internal waves, internal surf on submarines, and indirectly - the complication of the conditions for the passage of sound in water.

An in-depth study of the structure of large ocean currents has revealed that these flows are far from being a “river with liquid banks,” as previously thought. It turned out that the currents consist of a number of alternating jets moving at different speeds. Moreover, a speed of 2.7 m/s (5.2 knots) was measured in the Gulf Stream. In addition, it was discovered that there are narrow countercurrents on both sides of the main flow (can reach 2 knots).

Another interesting feature of currents was revealed: streams bend in space, forming bends - like river meanders. Meanders, increasing in size, move with the current, and sometimes break away from it and move independently. The separated meanders form vortices of various sizes. To the left of the general flow, the vortices rotate clockwise, to the right - counterclockwise. The current speed in these eddies is up to 2.0 knots.

Observations have shown that, for example, in the Gulf Stream field, 5-8 pairs of cyclones and anticyclones are formed per year. The most developed Gulf Stream cyclones have a diameter of up to 200 km and capture a layer of water masses almost to the ocean floor (2500-3000 m). Gulf Stream cyclones drift generally to the southwest at speeds of up to 3 miles per day.

The discovery of vortices is of great importance for navigation in the open ocean. The vortex circulation system is the real field of currents that affects a ship located in the ocean. When passing through areas with constant currents marked on hydrometeorological maps and atlases, navigators should be aware that the real variability of current directions and speeds, and therefore the actual drift of the vessel, can differ greatly from the directional direction of the current.

Many navigators have noted that often, especially in tropical latitudes, at night, the glow of water flowing onto the bow of the ship is clearly visible; The seething water at the sides glows, flowing around the hull; a swirling, gradually narrowing and fading light strip forms behind the stern. The glow of the water highlights the shore, rocks, reefs, shallows, buoys, ships and jetties against the general background of the sea.

As hydrobiologists have found out, the glow of the sea is caused mainly by the bioluminescence of marine organisms. The most common is the sparkling or flickering glow of various unicellular and multicellular plankton creatures ranging in size from tens of microns to several millimeters. When there are many such luminous beings, individual points of light merge into an uneven glow. This glow occurs when organisms are mechanically irritated, for example, when animals and fish move, when an oar hits the water, and also when exposed to chemicals.

For a long time, sailors returning from the tropical seas of Southeast Asia spoke of encountering gigantic luminous wheels, several miles in diameter, rotating at high speed on the surface of the sea. Western European sailors dubbed them the “devil’s carousel”; in the East they are called “Buddha wheels”.

The formation of small-scale vortices can be considered an explanation for these phenomena. Such vortices and whirlpools arise at the edges of currents, at the junction of differently directed flows of any origin, where the depth is shallow, tidal currents are strong and internal waves arise.

Falling winds

The general name “falling winds” includes coastal winds observed in the foothills of some seas; These winds are called differently in different areas: foen, bora, mistral, sarma. They are united by such qualities as surprise, great force and the nature of the impact on ships. Many ships suffered accidents during bora near the Novaya Zemlya coast, off the coast of Greenland, and in the roadsteads of such large ports as Trieste, Marseille, and Novorossiysk.

The speed of falling winds reaches 40 meters per second at the sea surface, and with gusts 50-60. Naturally, they pose a great danger to coastal navigation, to the mooring of ships in roadsteads and at berths, and to the operation of ports.

When studying this phenomenon, researchers noticed that bora usually occurs in winter, and in those areas where coastal mountains border a fairly high plain, which becomes very cold in winter. A high pressure area often forms over the plain, while a cyclonic area persists over the sea. This creates large horizontal gradients that move huge masses of cold air. Due to the action of gravity, the speed of air movement increases sharply as it passes over the ridge.

The rapid fall of cold air onto the surface of the bays creates strong waves in the coastal zone; at subzero temperatures, water splashes cause icing of ships and port facilities. Ice armor reaches up to 4 meters, which often causes catastrophic consequences. Vertically, the bora extends to 200-300 meters, and horizontally - only a few miles from the coast.

The mechanism of hair dryer formation is slightly different. The proper name of the wind “fen” (warm) gives the key to understanding the nature of the phenomenon. It has been established that the foehn is formed due to a significant difference between atmospheric pressure inland and over the sea. When a cyclone passes over the sea near the coast, when a high-pressure core remains inland, the pressure field forms flows of air masses directed from the land to the sea. And if there are mountains on the path of these flows, then masses of air, accumulating behind the ridge, begin to slowly rise. As the air rises, the air temperature drops, and the humidity gradually increases and reaches a maximum at a certain point.

At the top of the ridge, where the air is supersaturated with water vapor, it begins to condense, forming a cloud bank that covers the entire mountain range - a characteristic “foehn wall” appears. From this height, the air rushes to the sea, heating up, so it arrives at the coast with a higher temperature and low humidity.

Sometimes, under appropriate weather conditions, small-scale atmospheric vortices are formed - tornadoes (or as they are sometimes called - tornadoes, blood clots, typhons).

An ordinary tornado is formed as follows: as a result of intense ascending air currents, the edge of a formidable cloud begins to rise, twisting horizontally around an axis parallel to the cloud boundary - a small rotor is formed. The rotor, rotating rapidly, lowers one end (usually the left one according to the movement of the cloud) to the ground in the form of a funnel. This funnel - the main component of a tornado - is a spiral vortex consisting of extremely rapidly rotating air.

The internal cavity of the funnel, with a diameter ranging from several meters to several hundred meters, is a space limited by walls; it is almost clear, cloudless, sometimes small lightning flashes from wall to wall; the air movement in it weakens. The pressure here drops sharply - sometimes by 180-200 mb. Such a catastrophically rapid drop in pressure causes a peculiar effect; Hollow objects, in particular houses, other buildings, car tires, explode when they come into contact with a tornado funnel.

There are no direct measurements of wind speed in tornadoes: not a single device can withstand enormous accelerations. However, experts in the strength of materials calculated these speeds based on the nature of destruction and accidents: up to 170-200 m/s, and sometimes even 350-360 m/s - more than the speed of sound.

The lifetime of a tornado varies and ranges from several minutes to several hours.

The speed at which tornadoes move is also different. Sometimes the cloud moves very slowly, almost stands still, sometimes it rushes at high speed. Meteorologists determine the average speed of tornadoes to be 40-60 km/h, but sometimes this speed reaches 200 km/h. During its movement, a tornado travels an average distance of 20-30 km. However, cases of tornadoes passing 100-120 km are not uncommon.

Marine waterspouts usually originate in groups from a single parent cloud. They most often form and reach their greatest strength near thunderstorm cumulonimbus clouds. Sometimes they accompany tropical cyclones.

Tornadoes are visible from a fairly large distance and are easily detected on the radar screen, and therefore, when they see the approach of this natural formation, navigators must take measures to avoid meeting it.

Rare but very dangerous phenomena have long been noticed at sea: - loss of buoyancy during the eruption of underwater volcanoes, of which there are many in the oceans (this creates a water-air mixture) or due to gas breakthrough from the bottom of the sea.

CONCLUSION

In conclusion, we should recall the basic rule of a sailor - there is nothing secondary at sea . At a given specific moment in time, in a given place, the effect of any natural factor can be most strongly manifested, resulting in consequences - even a catastrophe.

Therefore, the skipper must always "consider your place closer to danger" not only in the literal navigational sense of this, but also taking into account all other navigation conditions. Even simple knowledge of the very factor of the influence of these phenomena on navigation, and even more so a qualitative assessment of the effect, allows us to minimize possible negative consequences.

Ebbs and flows, periodic fluctuations in water levels (rises and falls) in water areas on Earth, which are caused by the gravitational attraction of the Moon and the Sun acting on the rotating Earth. All large water areas, including oceans, seas and lakes, are subject to tides to one degree or another, although in lakes they are small.

The highest water level observed in a day or half a day during high tide is called high water, the lowest level during low tide is called low water, and the moment of reaching these maximum level marks is called the standing (or stage) of high tide or low tide, respectively. Average sea level is a conditional value, above which the level marks are located during high tides, and below which during low tides. This is the result of averaging large series of urgent observations. The average high tide (or low tide) is an average value calculated from a large series of data on high or low water levels. Both of these middle levels are tied to the local foot rod.

Vertical fluctuations in water level during high and low tides are associated with horizontal movements of water masses in relation to the shore. These processes are complicated by wind surge, river runoff and other factors. Horizontal movements of water masses in the coastal zone are called tidal (or tidal) currents, while vertical fluctuations in water levels are called ebbs and flows. All phenomena associated with ebbs and flows are characterized by periodicity. Tidal currents periodically reverse direction, while ocean currents, moving continuously and unidirectionally, are driven by the general circulation of the atmosphere and cover large areas of open ocean (see also OCEAN).

During transition intervals from high tide to low tide and vice versa, it is difficult to establish the trend of the tidal current. At this time (which does not always coincide with the high or low tide), the water is said to “stagnate.”

High and low tides alternate cyclically in accordance with changing astronomical, hydrological and meteorological conditions. The sequence of tidal phases is determined by two maxima and two minima in the daily cycle.

Explanation of the origin of tidal forces.

Although the Sun plays a significant role in tidal processes, the decisive factor in their development is the gravitational pull of the Moon. The degree of influence of tidal forces on each particle of water, regardless of its location on the earth's surface, is determined by Newton's law of universal gravitation. This law states that two material particles attract each other with a force directly proportional to the product of the masses of both particles and inversely proportional to the square of the distance between them. It is understood that the greater the mass of the bodies, the greater the force of mutual attraction that arises between them (with the same density, a smaller body will create less attraction than a larger one). The law also means that the greater the distance between two bodies, the less attraction between them. Since this force is inversely proportional to the square of the distance between two bodies, the distance factor plays a much larger role in determining the magnitude of the tidal force than the masses of the bodies.

The gravitational attraction of the Earth, acting on the Moon and keeping it in near-Earth orbit, is opposite to the force of attraction of the Earth by the Moon, which tends to move the Earth towards the Moon and “lifts” all objects located on the Earth in the direction of the Moon. The point on the earth's surface located directly below the Moon is only 6,400 km from the center of the Earth and on average 386,063 km from the center of the Moon. In addition, the mass of the Earth is 81.3 times the mass of the Moon. Thus, at this point on the earth’s surface, the Earth’s gravity acting on any object is approximately 300 thousand times greater than the Moon’s gravity. It is a common idea that water on Earth directly below the Moon rises in the direction of the Moon, causing water to flow away from other places on the Earth's surface, but since the Moon's gravity is so small compared to the Earth's, it would not be enough to lift so much water. huge weight.

However, the oceans, seas and large lakes on Earth, being large liquid bodies, are free to move under the influence of lateral displacement forces, and any slight tendency to move horizontally sets them in motion. All waters that are not directly under the Moon are subject to the action of the component of the Moon's gravitational force directed tangentially (tangentially) to the earth's surface, as well as its component directed outward, and are subject to horizontal displacement relative to the solid earth's crust. As a result, water flows from adjacent areas of the earth's surface towards a place located under the Moon. The resulting accumulation of water at a point under the Moon forms a tide there. The tidal wave itself in the open ocean has a height of only 30-60 cm, but it increases significantly when approaching the shores of continents or islands.
Due to the movement of water from neighboring areas towards a point under the Moon, corresponding ebbs of water occur at two other points removed from it at a distance equal to a quarter of the Earth’s circumference. It is interesting to note that the decrease in sea level at these two points is accompanied by a rise in sea level not only on the side of the Earth facing the Moon, but also on the opposite side. This fact is also explained by Newton's law. Two or more objects located at different distances from the same source of gravity and, therefore, subjected to the acceleration of gravity of different magnitudes, move relative to each other, since the object closest to the center of gravity is most strongly attracted to it. Water at the sublunar point experiences a stronger pull towards the Moon than the Earth below it, but the Earth in turn has a stronger pull towards the Moon than water on the opposite side of the planet. Thus, a tidal wave arises, which on the side of the Earth facing the Moon is called direct, and on the opposite side - reverse. The first of them is only 5% higher than the second.

Due to the rotation of the Moon in its orbit around the Earth, approximately 12 hours and 25 minutes pass between two successive high tides or two low tides in a given place. The interval between the climaxes of successive high and low tides is approx. 6 hours 12 minutes The period of 24 hours 50 minutes between two successive tides is called a tidal (or lunar) day.
Tide inequalities.

Tidal processes are very complex and many factors must be taken into account to understand them. In any case, the main features will be determined:

1) the stage of development of the tide relative to the passage of the Moon;

2) tidal amplitude

3) the type of tidal fluctuations, or the shape of the water level curve. Numerous variations in the direction and magnitude of tidal forces give rise to differences in the magnitude of morning and evening tides in a given port, as well as between the same tides in different ports. These differences are called tide inequalities.

Semi-diurnal effect.

Usually within a day, due to the main tidal force - the rotation of the Earth around its axis - two complete tidal cycles are formed. When viewed from the North Pole of the ecliptic, it is obvious that the Moon rotates around the Earth in the same direction in which the Earth rotates around its axis - counterclockwise. With each subsequent revolution, a given point on the earth's surface again takes a position directly under the Moon somewhat later than during the previous revolution. For this reason, both the ebb and flow of the tides are delayed by approximately 50 minutes every day. This value is called lunar delay.

Half-month inequality.

This main type of variation is characterized by a periodicity of approximately 143/4 days, which is associated with the rotation of the Moon around the Earth and its passage through successive phases, in particular syzygies (new moons and full moons), i.e. moments when the Sun, Earth and Moon are located on the same straight line. So far we have touched only on the tidal influence of the Moon. The gravitational field of the Sun also affects the tides, however, although the mass of the Sun is much greater than the mass of the Moon, the distance from the Earth to the Sun is so greater than the distance to the Moon that the tidal force of the Sun is less than half that of the Moon. However, when the Sun and Moon are on the same straight line, either on the same side of the Earth or on opposite sides (during the new moon or full moon), their gravitational forces add up, acting along the same axis, and the solar tide overlaps with the lunar tide. Likewise, the attraction of the Sun increases the ebb caused by the influence of the Moon. As a result, the tides become higher and the tides lower than if they were caused only by the Moon's gravity. Such tides are called spring tides.

When the gravitational force vectors of the Sun and the Moon are mutually perpendicular (during quadratures, i.e. when the Moon is in the first or last quarter), their tidal forces oppose, since the tide caused by the attraction of the Sun is superimposed on the ebb caused by the Moon. Under such conditions, the tides are not as high and the tides are not as low as if they were due only to the gravitational force of the Moon. Such intermediate ebbs and flows are called quadrature. The range of high and low water marks in this case is reduced by approximately three times compared to the spring tide. In the Atlantic Ocean, both spring and quadrature tides are usually delayed by a day compared to the corresponding phase of the Moon. In the Pacific Ocean, such a delay is only 5 hours. In the ports of New York and San Francisco and in the Gulf of Mexico, spring tides are 40% higher than quadrature ones.

Lunar parallactic inequality.

The period of fluctuations in tidal heights, which occurs due to lunar parallax, is 271/2 days. The reason for this inequality is the change in the distance of the Moon from the Earth during the latter’s rotation. Due to the elliptical shape of the lunar orbit, the tidal force of the Moon at perigee is 40% higher than at apogee. This calculation is valid for the Port of New York, where the effect of the Moon at apogee or perigee is usually delayed by about 11/2 days relative to the corresponding phase of the Moon. For the port of San Francisco, the difference in tidal heights due to the Moon being at perigee or apogee is only 32%, and they follow the corresponding phases of the Moon with a delay of two days.

Daily inequality.

The period of this inequality is 24 hours 50 minutes. The reasons for its occurrence are the rotation of the Earth around its axis and a change in the declination of the Moon. When the Moon is near the celestial equator, the two high tides on a given day (as well as the two low tides) differ slightly, and the heights of morning and evening high and low waters are very close. However, as the Moon's north or south declination increases, morning and evening tides of the same type differ in height, and when the Moon reaches its greatest north or south declination, this difference is greatest. Tropical tides are also known, so called because the Moon is almost above the Northern or Southern tropics.

The diurnal inequality does not significantly affect the heights of two successive low tides in the Atlantic Ocean, and even its effect on the heights of the tides is small compared to the overall amplitude of the fluctuations. However, in the Pacific Ocean, diurnal variability is three times greater in low tide levels than in high tide levels.

Semiannual inequality.

Its cause is the revolution of the Earth around the Sun and the corresponding change in the declination of the Sun. Twice a year for several days during the equinoxes, the Sun is near the celestial equator, i.e. its declination is close to 0°. The Moon is also located near the celestial equator for approximately 24 hours every half month. Thus, during the equinoxes there are periods when the declinations of both the Sun and the Moon are approximately 0°. The total tidal-generating effect of the attraction of these two bodies at such moments is most noticeably manifested in areas located near the earth's equator. If at the same time the Moon is in the new moon or full moon phase, the so-called. equinoctial spring tides.
Solar parallax inequality.

The period of manifestation of this inequality is one year. Its cause is the change in the distance from the Earth to the Sun during the orbital movement of the Earth. Once for each revolution around the Earth, the Moon is at its shortest distance from it at perigee. Once a year, around January 2, the Earth, moving in its orbit, also reaches the point of closest approach to the Sun (perihelion). When these two moments of closest approach coincide, causing the greatest net tidal force, higher tidal levels and lower tidal levels can be expected. Likewise, if the passage of aphelion coincides with apogee, lower tides and shallower tides occur.

Observation methods and forecast of tide heights.

Tidal levels are measured using various types of devices.

Footstock- this is an ordinary strip with a scale in centimeters printed on it, attached vertically to a pier or to a support immersed in water so that the zero mark is below the lowest low tide level. Level changes are read directly from this scale.

Float rod.

Such foot rods are used where constant waves or shallow swell make it difficult to determine the level on a fixed scale. Inside a containment well (a hollow chamber or pipe) mounted vertically on the seabed is a float, which is connected to a pointer mounted on a fixed scale or to a recorder stylus. Water enters the well through a small hole located well below the minimum sea level. Its tidal changes are transmitted through the float to measuring instruments.
Hydrostatic sea level recorder.

A block of rubber bags is placed at a certain depth. As the height of the tide (layer of water) changes, the hydrostatic pressure changes, which is recorded by measuring instruments. Automatic recording devices (tide gauges) can also be used to obtain a continuous record of tidal fluctuations at any point.

Tide tables.

There are two main methods used in compiling tide tables: harmonic and non-harmonic. The non-harmonic method is entirely based on observational results. In addition, the characteristics of port waters and some basic astronomical data are involved (the hour angle of the Moon, the time of its passage through the celestial meridian, phases, declination and parallax). After making adjustments for the listed factors, calculating the moment of onset and level of tide for any port is a purely mathematical procedure.

The harmonic method is partly analytical and partly based on observations of tidal heights carried out over at least one lunar month. To confirm this type of forecast for each port, long series of observations are required, since distortions arise due to physical phenomena such as inertia and friction, as well as the complex configuration of the shores of the water area and the features of the bottom topography. Since tidal processes are characterized by periodicity, harmonic vibration analysis is applied to them. The observed tide is considered to be the result of the addition of a series of simple component tidal waves, each of which is caused by one of the tidal forces or one of the factors. For a complete solution, 37 such simple components are used, although in some cases additional components beyond the basic 20 are negligible. Simultaneous substitution of 37 constants into the equation and its actual solution is carried out on a computer.

River tides and currents.

The interaction of tides and river currents is clearly visible where large rivers flow into the ocean. Tidal heights in bays, estuaries and estuaries can increase significantly as a result of increased flows in marginal streams, especially during floods. At the same time, ocean tides penetrate far up rivers in the form of tidal currents. For example, on the Hudson River a tidal wave reaches a distance of 210 km from the mouth. Tidal currents usually travel upriver to intractable waterfalls or rapids. During high tides, river currents are faster than during low tides. Maximum speeds of tidal currents reach 22 km/h.

Bor.

When water, set in motion under the influence of a high tide, is limited in its movement by a narrow channel, a rather steep wave is formed, which moves upstream in a single front. This phenomenon is called a tidal wave, or bore. Such waves are observed on rivers much higher than their mouths, where the combination of friction and river current most impedes the spread of the tide. The phenomenon of boron formation in the Bay of Fundy in Canada is known. Near Moncton (New Brunswick), the Pticodiac River flows into the Bay of Fundy, forming a marginal stream. At low water its width is 150 m, and it crosses the drying strip. At high tide, a wall of water 750 m long and 60-90 cm high rushes up the river in a hissing and seething vortex. The largest known pine forest, 4.5 m high, is formed on the Fuchunjiang River, which flows into Hanzhou Bay.

Reversible waterfall

(reversing direction) is another phenomenon associated with tides in rivers. A typical example is the waterfall on the Saint John River (New Brunswick, Canada). Here, through a narrow gorge, water during high tide penetrates into a basin located above the low water level, but slightly below the high water level in the same gorge. Thus, a barrier arises, flowing through which water forms a waterfall. During low tide, the water flows downstream through a narrowed passage and, overcoming an underwater ledge, forms an ordinary waterfall. During high tide, a steep wave that penetrates the gorge falls like a waterfall into the overlying basin. The backward flow continues until the water levels on both sides of the threshold are equal and the tide begins to ebb. Then the waterfall facing downstream is restored again. The average water level difference in the gorge is approx. 2.7 m, however, at the highest tides, the height of the direct waterfall can exceed 4.8 m, and the reverse one - 3.7 m.
Greatest tidal amplitudes.

The world's highest tide is generated by strong currents in Minas Bay in the Bay of Fundy. Tidal fluctuations here are characterized by a normal course with a semi-diurnal period. The water level at high tide often rises by more than 12 m in six hours, and then drops by the same amount over the next six hours. When the effect of spring tide, the position of the Moon at perigee and the maximum declination of the Moon occur on the same day, the tide level can reach 15 m. This exceptionally large amplitude of tidal fluctuations is partly due to the funnel-shaped shape of the Bay of Fundy, where the depths decrease and the shores move closer together towards top of the bay.

Wind and weather.

Wind has a significant influence on tidal phenomena. The wind from the sea pushes the water towards the coast, the height of the tide increases above normal, and at low tide the water level also exceeds the average. On the contrary, when the wind blows from land, water is driven away from the coast, and sea level drops.

» article « Giant ocean whirlpool ring". Where we will tell you that there are not only whirlpools in the bathtub or on the river, behind the ship. We will talk about whirlpools with a diameter of hundreds of kilometers and stability of years.

Such giant oceanic whirlpools are called rings. From English ring = ring. That is, if translated literally, we get giant oceanic rings. However, in shape they still resemble the familiar whirlpools in bathrooms. But first things first. Let's start from the beginning.

The Pacific Ocean region adjacent to the Japanese Ogasawara Islands has been notorious among sailors for a long time. However, no wonder - according to researchers of anomalous phenomena, it is located on the periphery of the so-called “Devil's Sea” - a sea not indicated on nautical charts, and in the relevant literature its location is interpreted very arbitrarily. In any case, reports came from this area quite regularly about ships that had disappeared without a trace.

In the mid-70s, this area attracted the attention of scientists from Kyoto University. Since ships are avoiding it, it was worth exploring the possibility of sinking radioactive waste in this deep-sea (depths of over 5000 meters) ocean region. And then, 400 kilometers from Ogasawara, they discovered a giant whirlpool - its radius was about 100 kilometers. Research has shown that the whirlpool rises from a depth of 5000 meters to the surface of the ocean.

In the center of this giant funnel there is a depression, the water level in which is several tens of meters below ocean level. According to oceanologists, the energy of this whirlpool is 10 times greater than the energy of a normal current. And one more oddity that has not yet found any explanation: approximately once every 100 days this whirlpool changes the direction of its rotation.

So, the waters of the World Ocean are rarely calm. In addition to storms, storms and waves of gigantic destructive force - tsunamis, there are powerful horizontal currents in the ocean, both surface and underwater. The Gulf Stream, for example, carries enormous amounts of warm water, heating the western and northern coasts of Europe.

But now we are interested vertical currents, leading to the emergence of those very huge whirlpools in the ocean. As in the ocean of air, they appear as a result of vertical movements of water masses caused by differences in water densities arising from differences in temperatures of water layers or their different salinities (warm water is lighter than cold water, salty water is heavier than less salty water).

Such vertical movements of water cause the appearance of giant whirlpools called rings. Moreover, these whirlpools have all the features that distinguish air whirlpools, namely, in the Northern Hemisphere, in the center of cyclonic whirlpools rotating counterclockwise, deep waters rise and fall at the periphery of the whirlpool. In the Southern Hemisphere, the same vertical movement of water leads to the emergence of a whirlpool that rotates clockwise. In the case of lowering of water masses in the center of the whirlpool in the Northern Hemisphere, water movement occurs clockwise, and in the Southern Hemisphere - counterclockwise.

Similar giant whirlpools have been found in the Bermuda Triangle area, near Sri Lanka and even off the coast of Antarctica. In the center of such whirlpools there is a rather deep depression: for example, near Sri Lanka its depth exceeds 100 meters. Depths of depressions of up to 200 meters have been recorded from satellites.

Although legends about such whirlpools have been known for several centuries, the first instrumental measurements of eddies in the open ocean were carried out in 1970 in the tropical Atlantic at the Polygon-70 sea test site by an expedition of the USSR Academy of Sciences. Sea water vortices live much longer than air vortices, but, in general, have the same properties: temporary nature, cyclic origin, movement and destruction within larger circulations.

So, rings were discovered relatively recently, in the seventies of the last century. As studies have shown, ocean eddies can exist for quite a long time, calculated in months and, according to some scientists, years. Their diameters can be tens and even hundreds of kilometers. Regardless of which direction, clockwise or counterclockwise, the water vortex rotates, its surface due to centrifugal force will not be horizontal; the center of the vortex may lie tens of meters below ocean level, as noted by equipment installed on artificial satellites Earth.

The mechanism for the formation of rings is completely identical to the mechanism for the formation of air vortices. The main operating objects of this mechanism are Earth's magnetic field and those moving in it water molecules(having partial positive and negative charges) and positively and negatively charged salt particles, which, when moving in the Earth’s magnetic field, acquire rotational motion. Naturally, the already mentioned differences in the density of warm, cold, salty and less salty water play a significant role.

Direct observation of the entire giant oceanic formation - the ring - is possible only from the orbit of an artificial Earth satellite. Ocean eddies are monitored during expeditions using instruments that measure the speed of sea currents at depths of interest to scientists. For example, the Polygon-70 expedition placed about two hundred meters in the southern part of the northern trade wind current of the Atlantic Ocean, the data from which was recorded for six months. Subsequently, all this information was brought together and processed on a computer. The processing results convincingly proved the presence of a giant water vortex with an anticyclonic rotation pattern.

Then, in the North Atlantic alone, about 10 such rings were discovered. Their occurrence is associated with the Gulf Stream, which, having passed Cape Hatteras, departs from the coast of North America and begins to form loop-shaped meanders. Some of the meanders break away from the main flow and become amateur vortices, the current speed of which can reach 4 or more kilometers per hour. A yacht or raft, having found itself during a long calm in such a whirlpool with a diameter of 150-300 kilometers, after a few days, having traveled quite a long distance, may end up in almost the same place. The drift of such a whirlpool itself is very insignificant and rarely exceeds 3 kilometers per day.

During the study of the rings, it was found that the eddies that separate from the Gulf Stream on its southern side differ from the surrounding warm waters of the Sargasso Sea in that their center has a lower temperature. The same eddies that separate from the north side of the Gulf Stream have a warmer center.

Rings with a warm center usually move at a speed of up to 5 kilometers per day. Such a ring exists for about a year, then, once again in the Cape Hatteras area, it joins the Gulf Stream. The drift of rings with a cold center is mainly southwestern. Place of extinction: off the eastern coast of the Florida Peninsula; lifespan: 2-3 times longer. It was possible to track rings that live up to 4-5 years.

In the centers of cold rings, fogs often occur and are extremely long lasting: after all, here the ocean whirlpool lifts water with a very low temperature from depths of 2.65-3.5 kilometers to the surface. When warm air comes into contact with a cold water surface, the process of condensation of water vapor occurs, an increase in the concentration of which is the cause of deterioration in visibility.

Thus, one would not want to get caught in a giant ocean whirlpool.

Just look from above. For this reason, we invite you to watch the following video:

This, of course, is not a whirlpool with a diameter of 100 kilometers, but it is still impressive.

Sources: P. MANTASHYAN, “Science and Life” No. 5, 2008. Tatiana SAMOILOVA, Columbus magazine No. 15 (2005)

Tidal fluctuations in ocean level are accompanied by horizontal movement of water masses, which is called the tidal current. Therefore, the navigator must take into account not only changes in depths, but also the tidal current, which can reach significant speeds. In areas where there are high tides, the boatmaster must always be aware of the height of the tide and the elements of the tidal current.

Tides allow deep-draft ships to enter some ports located in shallow bays and estuaries.

In some places, the tides are intensified by surge phenomena, which leads to a significant increase or decrease in the level, and this in turn can lead to accidents of ships under cargo operations at berths or in the roadstead.

The nature and magnitude of tides in the World Ocean are very diverse and complex. The magnitude of the tide in the ocean does not exceed 1 m. In coastal areas, due to the decrease in depth and the complexity of the bottom topography, the nature of the tides changes significantly compared to tides in the open ocean. Along straight shores and capes protruding into the ocean, the tide fluctuates within 2-3 m; in the coastal part of the bays and with a heavily indented coastline, it reaches 16 m or more.

For example, in Penzhinskaya Bay (Sea of ​​Okhotsk) the tide reaches 13 m. On the Soviet shores of the Sea of ​​Japan its height does not exceed 2.5 m.

In the seas, the height of the tide depends on what kind of connection a given sea has with the ocean. If the sea extends far into the land and has a narrow and shallow strait with the ocean, then the tides in it are usually small.

In the Baltic Sea, tides are so small that they are measured in centimeters. The tide height in Calais is 7 cm, in the Gulf of Finland and Bothnia about 14 cm, and in Leningrad about 5 cm.

In the Black and Caspian Seas, the tides are almost imperceptible.

In the Barents Sea, tides are semi-diurnal.

In the Kola Bay they reach 4 m, and near the Iokan Islands - up to 6 m.

In the White Sea the tides are semi-diurnal. The highest tide height is observed on the Tersky coast in the throat of the sea, where at the Oryol lighthouse it reaches 8.5 m, and in the Mezen Bay - up to 12 m. In other areas of this sea, the tides are much lower; Thus, in Arkhangelsk it is about 1 m, in Kemi - 1.5 m, and in Kandalaksha - 2.3 m.

A tidal wave, penetrating into the mouths of rivers, contributes to fluctuations in their levels, and also significantly affects the speed of water flow in the mouths. Thus, often the speed of the tidal current, dominating the speed of the river, changes the flow of the river to the opposite direction.

Winds have a significant influence on tidal phenomena.

A comprehensive study and accounting of tidal phenomena is of great importance for the safety of navigation.

The current that is directed in the direction of the movement of the tidal wave is called tidal, the opposite is called ebb.

The speed of tidal currents is directly proportional to the magnitude of the tide. Consequently, for a certain point, the speed of tidal currents at syzygy will be significantly greater than the speed at quadrature.

With increasing declination of the Moon, as well as as the Moon moves from apogee to perigee, the speed of tidal currents increases.

Tidal currents differ from all other currents in that they capture the entire thickness of water masses from the surface to the bottom, only slightly reducing their speed in the near-bottom layers.

In straits, narrow bays and near the coast, tidal currents have the opposite (reversible) character, that is, the tidal current is constantly directed in one direction, and the ebb current has a direction directly opposite to the tidal one.

In the open sea, far from the coast, and in the middle parts of fairly wide bays, there is no sharp change in the direction of the tidal current to the opposite direction, i.e., the so-called change of currents.

In these places, a continuous change in current directions is most often observed, and a 360° change in current occurs with a semi-diurnal tide in 12 hours and 25 minutes and with a diurnal tide in 24 hours and 50 minutes. Such flows are called rotating flows. Changes in the directions of rotating currents in the northern hemisphere, as a rule, occur clockwise, and in the southern hemisphere, counterclockwise.

The change from tidal current to ebb current and vice versa occurs both at the moment of high and low waters, and at the moment of average level standing. Often, a change in currents occurs in the period of time between high and low water. When the tidal current changes to ebb and flow, the current speed is zero.

The general pattern of tidal currents is often disrupted by local conditions. Taking into account the tidal current, as mentioned above, is of great importance for the safety of navigation.

Data on the elements of tidal currents are selected from the Atlas of Tidal Currents, and for some areas of the seas - from tables located on navigation charts. General instructions about currents are also given in sea directions.

Relatively constant currents are shown on maps with arrows. The direction of each arrow corresponds to the direction of the current operating at a given location, and the numbers above the arrow indicate the speed of the current in knots.

The direction and speed of tidal currents are variable quantities, and in order to reflect them on the map with sufficient completeness, you need not one arrow, but a system of arrows - a vector diagram.

Despite the clarity of vector diagrams, they overload the map and make it difficult to read. To avoid this, elements of tidal currents are usually shown on the map in the form of tables placed in free spaces on the map. A complete table is a table that contains the following data:

Watch relative high water at the nearest tidal point; the inscription “Full water”, corresponding to zero hours, is placed on

In the middle of the column, from it up, in ascending order, are the digits of the hours until full water, and downwards, also in ascending order, are the digits of the hours after full water;

Geographic coordinates of points, usually designated by the letters A; B; IN; G, etc. ; the same letters are placed in the corresponding places on the map;

Elements of currents: direction in degrees and speed in syzygy and quadrature in knots (with an accuracy of 0.1 knots).

The determination of the speed and direction of the current at a given moment in a given place according to the Atlas is found as follows.

First, the main port for a given place is determined using the Atlas, after which, using the Tide Table (Part I), the time of high water closest to the given one is found, and the time interval (in hours) before or after the moment of high water in the main port relative to the given moment is calculated. Then, for the calculated period of time before or after the moment of high water, the direction of the current (in degrees) and speed (in knots) are found in the Atlas.

When sailing, the elements of tidal currents must be determined in advance; It is recommended to compile a table of currents for pre-calculated moments (after 1 hour) corresponding to the ship’s countable positions.

Below is an example of a table of tidal currents (Table 7).

Currents arising from the southwest wind cause a significant surge of water in the Taganrog Bay. After the wind stops, strong compensating currents with speeds of up to 1.5 knots or more are established in the bay for some time. (Location of the Azov Sea)

On all tidal maps, atlases and tables of tidal currents, periodic tidal currents are specially marked or directly shown. In practice, tidal currents are the only type of periodic movement of water, the nature of which is known, and its calculation and forecast do not cause difficulties.

But, as a rule, despite the exact indication of the speed and direction of the tidal current on a map or in a table, the values ​​of these quantities do not always coincide with the real ones. The fact is that tidal currents are calculated by filtering and excluding the non-periodic component, but the latter can be tens of times higher than the speed of the periodic current and change its direction even to the opposite. It is excluded from the calculation only because the value of this component is difficult to calculate in advance.

The main reason for the occurrence of non-periodic currents is wind. All changes in wind speed and direction at each point of the sea, spatial and temporal heterogeneity of the wind field over the water area are instantly reflected in the field of currents in the entire basin. Therefore, wind currents are the most difficult to calculate.

In the chapter "Non-periodic sea level fluctuations" we dwelt a little on Ekman's theory of drift currents. In 1905, while solving the problem of wind currents in the open sea, Ekman made a number of important assumptions. He accepted that: a) water is incompressible, its density is constant; b) surge and surge, no water occurs and the sea surface is horizontal; c) the depth of the sea is infinitely great. Having solved the initial equations of water movement, Ekman came to the conclusions we have already discussed regarding wind currents, which in general agree well with the data of numerous observations in the open ocean.

However, near the coast, i.e., where navigation is most difficult, the basic assumptions of Ekman’s theory are not met, that is, this theory is not applicable to phenomena occurring in the coastal zone of the sea. The ideal picture painted by the mathematician begins to change.

As a result of the transfer of water to the coastline, sea level rises (or falls when water outflows). This creates a tilt of the level surface, which causes a flow called gradient. From the theory of drift currents, it follows that the direction of water flow relative to the direction of the wind strongly depends on the depth of the water in that place. At a sufficiently large depth near the shore, a surge or surge, and therefore a gradient current, occurs only if the wind blows at a certain angle to the shore, since in the deep sea the total flow in a drift current is directed to the right relative to the wind (see Fig. 1 ). Obviously, in conditions of great depth, surge or drift does not occur near the shore if the wind blows perpendicular to the coastline. Conversely, the surge reaches its maximum value when the wind blows along the coast located to the right (when looking in the direction of the wind).

In accordance with this, the speed of the gradient flow also changes. This current in the coastal zone covers the entire thickness of water from the surface to the bottom, superimposed on the drift current. As a result, the so-called total coastal current arises, the speed of which is determined as the geometric sum of the velocities of the gradient and wind currents.

Near the deep steep bank there is a current pattern shown in Fig. 3. In a layer of water of thickness D, a surface current develops, which is the sum of currents: a wind current that varies with depth and a constant gradient one. Below depth D, the speed of the drift current is practically zero, and up to depth D, the flows of the deep current are determined only by the level gradient: here a purely gradient current directed along the coast is observed.

In the bottom layer from depth D" to the bottom, the current speed begins to decrease, and the flow deviates to the left from the direction of the general water transfer. In this case, the bottom topography has a significant effect on the water speed. Due to friction between the bottom and water, its flow is slowed down.

In natural conditions, as a rule, there is no wall-shaped shore, especially one with great depth nearby. Therefore, the real picture of wind currents near the coast, according to the observations of oceanologists, is different.

Rice. 3.

1 -- surface current; 2 -- deep current; 3 -- bottom current

Firstly, the angle of deviation of the wind current from the wind direction does not remain constant, but depends on the depth of the sea and the strength of the wind. With decreasing depth (at a constant wind force), the angle a of deviation of the direction of the current from the direction of the wind decreases, the direction of the current approaches the direction of the wind. At a constant sea depth, angle a decreases with increasing wind strength.

Rice. 4.

Rice. 5. Change in the angle a of the deviation of the direction of surface currents (a) and the wind coefficient K (b) depending on the direction of the wind relative to the coast and the distance from it (deep zone)

Secondly, the speed of the current at the same wind force increases with decreasing water depth in a given place. For the convenience of practical calculations, oceanologists introduced the concept of wind coefficient K, which is the ratio of the speed v t of the surface current to the speed v wind of the wind that caused it. The above observations showed that the values ​​of K and a also strongly depend on the wind azimuth, i.e., on what direction the wind has relative to the coastline, if counted clockwise from the normal to the coast (when viewed from the sea), and on whether the shore is deep or shallow in the area. At depths of 35 - 40 m the sea can already be considered deep; at shallower depths it is shallow.

In Fig. 4 and 5 give the values ​​of the angle a of the deviation of the direction of surface currents from the wind direction and the wind coefficient K at various wind azimuths, respectively, for the shallow-water zone and the deep shore. It is interesting that with winds blowing along the coast or in a direction close to it, the wind coefficient reaches its maximum values. The opposite picture is observed with winds blowing normally to the shore or from the shore. In this case, the wind coefficient has minimal values. Studies have shown that the width of the zone of influence of the coast on wind currents in rare cases exceeds 35 miles. It should be noted that when calculating the values ​​of the wind coefficient shown in Fig. 4, 5, the wind speed is expressed in meters per second, and the current speed in centimeters per second.

The presented results were obtained mainly for winds of medium strength (4 - 7 points), however, it was found that the values ​​of the wind coefficient are practically independent of wind strength, and the angle a only slightly decreases with increasing wind. Consequently, these graphs can be used at any wind speed - even storm speeds. Only with very weak winds (1 - 2 points) can one expect some error in determining the values ​​of K and a from the graphs, but with such winds the currents are not of practical interest due to their low speeds.

The changes in the values ​​of the wind coefficient K and angle a for different durations of wind action deserve more attention. Numerous observations of the development of currents in the coastal zone of the sea led to the conclusion that in shallow-water areas the time for establishing speed is much longer than in deep-water areas: the time interval required for the full development of current speed in the deep-water zone is 3-4 hours, while in shallow water it reaches 16-18 hours. In Fig. 6 coefficient T characterizes the ratio of the instantaneous flow speed to the steady flow speed. Surprisingly, the time it takes for the current speed to reach its maximum value does not depend on the wind speed.

Rice. 6.

Rice. 7.

and wave „ - the speed of wave propagation; v -- speed of portable movement

Data in Fig. 4 - 6, the values ​​of K, a, T were obtained for the Baltic Sea, therefore, in relation to other sea basins, they must be used with some caution, but the general patterns of the phenomenon are characteristic of all shallow seas. These patterns can be formulated as follows: on the surface, water flows are directed along the wind and are determined by the wind current itself, and in the bottom layer - against the wind and are determined by the gradient current. For the deep shore, the main surge or surge is created by the wind blowing along the coastline. For a shallow coastline, the wind blowing parallel to the coastline does not create a level slope and gradient currents. The maximum surge and the gradient currents caused by it are observed when the wind blows perpendicular to the coast.

A certain portion of the total coastal current is also contributed by the wave flow - the portable movement of the water mass in the surface layer caused by wind waves. The wave flow is directed along the direction of propagation of wind waves. The reason for its occurrence is the loop-like nature of the trajectories of water particles in a real wind wave (Fig. 7). The speed of transport of water is the same for all particles lying at the same depth; it depends on the height and period of the waves and decays very quickly with increasing depth. Therefore, currents in the surface layers of water near the coast are a complex composition of many factors.

The relief of the coastal zone, the presence of islands and depressions are of no small importance. Thus, sailors more than once had to deal with one, at first glance, surprising factor. When the wind blows from the sea near the islands, the water level drops not only on the leeward side, but also on the windward side. This seemingly paradoxical phenomenon is explained quite simply: the wind drives all the water from the area of ​​the sea where these islands are located to other windward shores, that is, the water is redistributed not only near the islands in question, but throughout the entire reservoir.

It is clear that when sailing near islands it is very important to know the directions and speeds of the currents. In shallow water areas, with the general transfer of water by the wind, the islands flow around from all sides, like a normal obstacle. The speeds and directions of water flows near the shore of the island depend on the depth of the sea, the size and configuration of the island and its location relative to the flow. Changes in currents occur directly near the island.

In stormy weather, navigators do not risk sailing near islands in shallow water. Sailing in the ocean, where large islands can serve as natural shelter from storm waves, is a different matter. Indeed, on the leeward side of the island you can reliably shelter from a strong storm.

But it must be taken into account that the oceanographic observations carried out indicate the existence of a closed anomalous circulation around the oceanic islands. For example, the direction of currents around the islands of Taiwan, Iceland, and the Kuril Islands is opposite to the direction of the general circulation of water in the adjacent area of ​​the ocean. One of the reasons leading to the occurrence of such an anomalous circulation is the vorticity of the wind field over a large oceanic area. In most cases, the anomalous circulation of currents around an island in the northern hemisphere is directed clockwise, i.e., it is anticyclonic in nature, while the general circulation in the ocean area that includes the island is directed counterclockwise.

The vorticity and heterogeneity of the wind field in space and changes in the intensity and direction of the wind according to the seasons of the year lead to the appearance in certain areas of the sea of ​​local circulation formations that differ in direction from currents throughout the sea. These are the currents formed as a result of the influence of breezes and monsoon winds. The time of their action and the direction of the flows are determined by the period and speed of the wind. These same periodic winds can cause more interesting phenomena.

An example is the anomalous circulation in the southeastern part of the Black Sea. Surface currents in the Black Sea, as in all seas of the northern hemisphere, are most often directed counterclockwise and, pressing against the shores, cover a coastal zone approximately 20 miles wide. The main reason for the occurrence of such currents is the wind system over the sea and the intense flow of river waters.

In the southeastern part of the Black Sea in 1937, a circular current in the opposite direction, that is, clockwise, was discovered. Its center is located approximately 40-50 miles from Batumi, and it is in close contact with the coastal current. A detailed study of it showed that the flow has interesting properties. First of all, this is a system of currents in which in summer the temperature of the surface layer of water is much higher, and the intermediate layer is lower, than the average water temperature along the section from Batumi to Yalta. The salinity of the water here is below average.

The intensification of storm activity over the Black Sea contributes to the strengthening of the coastal current, on the one hand, and causes a weakening of currents in the anticyclonic region, on the other. In winter, during the period of maximum intensity of atmospheric activity, northeast winds cause an intensification of the cyclonic coastal current.

If waters with low temperatures and salinity rise to the surface, the anticyclonic circulation may disappear, and a cyclonic circulation appears in this place. Thus, the direction of flow here becomes opposite. However, the anticyclonic region in summer is expressed in this area much more sharply (current speed reaches 1.5 knots) than the cyclonic region in winter (current speed does not exceed 0.4 knots).

Drift currents that arise in the sea under the influence of atmospheric circulation are an extremely difficult phenomenon to study. A change in the pattern of currents even in a very small body of water occurs under the influence of the heterogeneity of the wind field, different depths, configuration of the banks, the presence of islands and banks, etc., therefore, for research it is necessary to simultaneously carry out a large number of observations at different points in the basin. Such research requires a huge number of vessels, instruments, and people.

Given these difficulties in conducting scientific observations, oceanologists have taken the path of using mathematical models to calculate wind currents. Water flows in the sea are described by a system of hydrodynamic equations, which are solved for a large number of nodes of a regular grid, “inscribed” in the geographical contour of the sea. This system allows you to set and take into account the wind speed at each point of the sea, depth, flows at liquid boundaries (in straits) and the level at solid boundaries (near the coast).

Calculations are carried out on modern computers with a time step of 5 - 10 minutes. The distance between adjacent grid nodes is several kilometers, that is, it densely covers the entire sea area. This makes it possible to accurately capture changes in sea currents and water levels near the shore.

However, the complexity of the equations and the large number of specified initial and boundary parameters lead to the fact that the calculation time is long even on modern high-speed computers with large amounts of memory. It is 5-6 hours for one wind situation in, for example, a basin such as the Sea of ​​Azov. It is clear that such calculation schemes are not used for current forecasting purposes. In addition, the calculation must be based on a wind forecast, which has its own error. Therefore, calculation schemes are widely used in determining the regime characteristics of currents: for this, more reasonable averaged characteristics of the wind flow are used as wind fields. Calculated current patterns are published in atlases, reference books, and hydrometeorological maps.

But let's return to the coastal circulation. As we have already established, as a result of the action of wind and wave transport, the resulting currents can cause an increase in the water level near the coast. As the water level increases, so-called compensation currents begin to develop, directed from the shore, the speed of which increases with increasing water level. These compensatory currents are like a link that closes the cycle of movement of water masses. Ultimately, a steady state occurs in which the amount of water flowing to the shore is equal to the amount of water leaving the sea.

Compensation for surge in nature can occur in two ways: in the form of countercurrents and rip currents. Hypothetically, a countercurrent can be thought of like this: a surface current formed by wind blowing towards the shore creates a rise in water near the coastline. The pressure difference resulting from this rise in water level forces the water in the bottom horizon to move from the shore towards the open sea.

Rice. 8.

a - near natural obstacles; b -- with multidirectional flows

In real conditions in a shallow sea, countercurrents are understood not as reverse flow in its pure form, but as the tendency for reverse transfer of water particles that is created by the slope of the level, i.e., the pressure difference creates an obstacle to the forward movement of water during surge: it slows down and can completely stop. If we consider the coastal zone as a whole, then this idea is quite acceptable, but in the near-shore zone it is violated by the effect of rip currents.

Rip currents, unlike compensatory countercurrents, are pronounced, narrowly localized flows that can cover the entire water column from the surface to the bottom. In nature, they are observed in the form of narrow jets, fading as they move away from the shore.

The main reason for the occurrence of rip currents is the tortuosity of the coastline and the unevenness of the water surge along the coast. In this case, during the surge process, a strong along-shore flow is created: water accumulates in uneven bottom topography, near capes and spits, which are natural obstacles to its movement. In these zones, a section of increased level is formed, and at the moment when the force caused by the difference in levels near the coast and in the sea exceeds the force of the flow, a rip current occurs (Fig. 8, a). Indeed, in nature, rip currents are in most cases observed at protruding points of the coast. At the same time, near shallow shores the pattern of occurrence of countercurrents may be different: the complexity of the topography of the underwater coastal slope, even near a shore with a regularly indented coastline, leads to the fact that the direction of alongshore currents is not the same on adjacent sections of the coast. Multidirectional flows arise, which, when they meet, create rip currents (Fig. 8.6).

Rip currents are relatively easily detected by turbulence at the boundaries of their powerful jets, breaks in the line of coastal breakers and sharply visible turbidity of the main part. At shallow depths, rip currents capture the entire thickness of water from the surface to the bottom. At great depths, like all waste currents, they pass into the surface layers. The maximum speeds of rip currents on the surface are approximately 1 meter per second.

The intensity of the rip current is strongly influenced by the concavity indicator of the bay or bay (the ratio of its length to the width of the entrance section). The higher this indicator, the greater the wind surge, which means that the rip current jet is more powerful and therefore penetrates further into the sea.

Due to their locality and high speeds, these currents pose a serious danger to mariners in the coastal zone. A ship that finds itself in the zone of rip currents can be blown off course, and when moving along the coast along a shipping canal, it can be thrown onto the edge. These factors must be taken into account when sailing in areas that are dangerous from the point of view of the conditions for the formation of rip currents.

And another danger is posed by rip currents: in some areas these currents are observed in the form of strong jets of bottom currents, their speed reaches 10 meters per second. At the same time, the bottom flow smoothes out uneven terrain even in strong bedrock, and over time it produces trenches extending from the coast for several miles, causes ruptures in the body of underwater along the coastal levees, and destroys the walls of shipping channels. Such abrupt post-storm changes in the morphology of coastal areas interfere with the established pattern of sediment movement and lead to the formation of shoals and banks in the most unexpected places.

Finally, in the seas and oceans, in addition to wind currents, there may be currents caused by the processes of water penetration through the water-air interface. These currents, called surface currents, are determined mainly by precipitation, evaporation, and condensation. The own speed of these currents, as a rule, does not exceed 1-2 centimeters per second, that is, it is not an obstacle to swimming, but such currents serve as a kind of trigger for other phenomena.

In particular, in calm weather, these currents contribute to intensive mixing of waters and the formation of water masses with different densities. After this, the most powerful force of water movement in the ocean—the force of the density gradient—comes into play, and large-scale circulation arises, which involves large and small masses of water.

When the mass of water increases or decreases in a body of water connected to another narrow strait, strong currents arise in this narrowness. For example, in real conditions of precipitation and evaporation in the Sea of ​​Azov, due to changes in the difference in water levels between the Azov and Black Seas in the Kerch Strait, currents can arise at speeds of 20 - 30 centimeters per second, which poses a danger to navigation. In the recent past, up to 5 billion cubic meters evaporated annually in the Kara-Bogaz-Gol Bay, and the compensating flow of water in the strait of the same name reached a speed of 2.5 meters per second.

Consequently, such processes cannot be discounted when following along the coast near the narrow arms of large bays and estuaries.