Fluctuations in water levels World ocean Phenomenon of tides

2023-04-02

Table of contents

Ocean and sea level
Tidal difference
Tidal amplitude and characteristics on oceans and seas.
BASICS OF PREVENTION OF INCOME FACTORS.

Ocean and sea level

Sea level terms

a. Sea level

– The free level position of the surface of the world ocean at a particular place, at a specific time, i.e., the actual depth of the sea in the specific area, at a specific time specified.

The main force acting on the water particles of the oceans and seas is gravity, which always brings the elements back into balance.

b. Mean sea level is the value obtained after averaging the observed water level values over a long period.

The observation time for determining the average water level over many years depends on the characteristics of the water regime of the sea under study and practical requirements.

Depending on the observation period, the water levels are distinguished: daily averages, monthly averages, annual averages, and multi-annual averages.

c. Zero depth: this is the average water level for many years in waterless seas.

For tidal seas, depth “0” is the lowest possible water level for astronomical reasons.

In practice, the number “0” on the map, which is conventionally called the reference plane, is chosen as the origin to measure the depth of the sea; it is the horizontal plane, which is specified for each sea area using this “0” number.

Sea and ocean surface oscillation

Many causes make sea and ocean levels unstable and unstable in continuous motion. The main force that drastically changes the water level is the moon’s and sun’s cosmic gravity, which causes the water level to fluctuate periodically.
Influence of hydrometeorological processes, causing vertical and horizontal disturbances of water masses. We can cite: the change in pressure, causing the difference in the height of the sea.

When the barometric pressure increases by one mb, the sea level decreases by 10 mm, and conversely, when the pressure drops by 1 mb, the story of the sea increases by 10 mm. Evaporation, precipitation, and river flows are associated with water changes in different parts of the oceans and seas.

Variations in water density are responsible for fluctuations in sea and ocean levels. In addition, significant fluctuations in water levels can be caused by underground earthquakes. This phenomenon does not occur often or everywhere in the oceans and seas.
In nature, the fluctuations in the level of pure water mentioned above are not observed but only receive a combined effect caused by different causes.

The characteristics of water level fluctuations, often the most recognizable, are periodic. The cycle duration can be a day and a night, a month, a season, a year, or several years.
A type of period oscillation caused by random changes in wind direction and speed, precipitation, evaporation, etc.

Instruments for monitoring sea level fluctuations.

The most straightforward and widely used tool is the water locator. Water is a metal ruler marked with horizontal lines divided by white and black squares. The width of a separator is usually 2 cm; there are also separators 1 dm wide.

Thuy Chi is firmly buried in the vertical direction, in a stable place, and the altitude does not change for long. The “0” point of the water solstice should be set so that the lowest water level has not yet reached it.

The waterway’s “0” landmark only forms the “0” story of the monitoring station and is usually linked to the national elevation system. The reading of the sea level by hydrology requires an accuracy of 1 cm; when its divisions are 2 cm, the accuracy is reduced to half a division.

When reading, observers should look directly at the solstice from the water, reading the dividing line near the edge of the water. When there is a wave, the fluctuating water level should read both high and low water levels, then take the average value.
For the continuous recording of water level fluctuations, tidal dynamics are used. There are two types of tide records: one located at the shore station; the other – off.
The principle of operation of the first type is based on the modulation of the vertical displacement of a floating object, proportional to the removal of the needle, drawn on the strip of paper wound around the drum, driven in rotation by the clock structure.
The second type is based on hydrodynamic pressure changes. The hydrodynamic pressure receiver is located on the seabed, away from the shore, or in the water near the beach.

Tidal phenomena

Important tidal definitions and terms.

a. Definition – Tides are periodic fluctuations in sea and ocean levels caused by the cosmic gravity of the moon and sun.

Sea level reaches its maximum position shortly after the moon’s highest elevation on the local meridian, then slowly drops to its lowest level as it nears the horizon. The constant motion of the moon below the horizon leads to a conductive rise in sea level, and a new high water level is reached near the time the moon crosses the lower meridian of the same name.

Then, again, the sea level drops, and near the time the moon rises, the position of the sea level reaches its lowest point. After that, the sea level gradually rose again, depending on the moon’s height, as the sea level rose after the previous moonrise.

Thereby, the tides are repeats of the highest and lowest sea level after about the same time interval, about 12.5 hours. Waves of this type, known as semi-diurnal tides, are most commonly observed in the World Ocean.

Tidal oscillations of ocean water levels, with very long wavelengths, up to 2000 km. The speed of the tidal waves is also very significant – 160 km/h.

b. Some of the most important terms.

– High water – the maximum water level when the water rises. Low water – the minimum water level when the water is low.

– Tidal period (T) is the interval between two consecutive high or low water times. Depending on the quantity of the cycle, people divide the tide types: into diurnal, semi-diurnal, and mixed tides:

+ Diurnal tide: has an average period equal to the moon day (24g50 minutes); on a moon day, there is a time of high water and times of low water;

+ Semi-diurnal tide: has an average period equal to half a day of the moon (12g25 minutes); On a moon day, there are two times high water and two times low water;

+ Mixed tide: this is the most complicated phenomenon; during the half-lunar month, the tidal cycle changes from daily to diurnal and vice versa.

If it is the Japanese cycle, the mixed tide is said to be irregular semi-diurnal; that is to say that the timing of the rising and falling tides is not the same, and when the diurnal-diurnal wave is periodic.

– Tide height – elevation of the water level above the depth figure “0”, also called depth correction, reflects the actual sea level at the time of monitoring.

– Tidal range – the vertical distance between high water and the next low tide. The tidal amplitude is also known as tidal magnitude.

For tidal characteristics over time, the following definitions are used:

+ High water time: the time when the water is highest.

+ Standing tide period during which the tide at a specified height does not change.

+ Lunar interval: the time between when the moon is at the highest of the local meridian and when it reaches the nearest ample water.

The influence of various factors on the quantities and characteristics of the tide.

In addition to astronomical factors, there are other factors affecting the amount and characteristics of tides, such as coastal features, watershed size, sea depth, presence of islands, etc. the depth and width of the ocean, the amount of wave is close to the theoretical and is equal to 1m.

Meanwhile, different amounts are observed on the continent’s coasts, especially in straits and long straits, where the amplitude can reach 12 m. Maximum tidal range observed in the Fandi Strait on the North American coast.

The enormous amplitude is due to Fandi’s waist being too long and narrow, accompanied by a gradual decrease in width and depth.

Figure 1. Mixed tide diagram and its features

The influence of shallow waters is almost permanent on the semidiurnal tide, resulting in an unequal break in the symmetry of the high and high tide times; that is to say that the times rise and low tide are unequal.

The direction of the wind is opposite to the direction of propagation of the tidal wave, reducing its speed of propagation. The opposite direction of the wind is in the direction of propagation of the tidal waves, increasing the rate of propagation and increasing the amplitude of the tides.

Stormy and robust winds can change the timing of high and low water, and the extreme wind is stable in one direction.

Explain the phenomenon of tides

Tides, as mentioned above, occur due to the cosmic gravity of the moon and the sun. These forces are called tidal forces.

For the sake of simplicity, we first assume that the moon exerts only a tidal force on the components of the earth.
Like all other planets, the moon’s gravitational pull attracts particles of terrestrial matter, and the greater the distance to the moon’s center, the less force decreases.

More precisely, the strength of gravity is proportional to the product of the moon’s mass and the water element in question and inversely proportional to the square of the distance from the moon’s center to this element.

Thus, the amount of gravity is not equal for particles at different distances. Also, the directions of gravity are not parallel but are directed toward the center of the moon from different positions of the elements.

Figure 2. The tidal force diagram of the moon. Spherical Earth; Pn – north pole; T- the center of the moon; H – gravity; L – centrifugal force; F – tidal force.

In addition to the moon’s gravitational force, there is a centrifugal force, which acts on every element of matter. The centrifugal force is created by rotating the system of two earth-moon stars around their common center of gravity.

Calculations show that the general center of gravity is in the earth, at a distance of 0.73 times the planet’s radius. The moon makes one revolution around available gravity, with a period of precisely one lunar month.
It has been shown that the centrifugal force acts on every element of the earth of equal magnitude and in the direction of the meridian plane of the observer towards the moon.
Each water element is solicited by two forces at the same time: the gravitational force of the moon and the centrifugal force. The vector sum of these two forces at each point is called the tidal force.

Suppose the world ocean covers the earth with a layer of water of equal thickness. The moon’s position on the diagram is not only in the plane of the observer’s meridian but also in the equatorial plane; that is, its declination is equal to 0°.
From figure 2, we see: in the water near points A and A’, the tidal force is directed vertically toward the center of the earth, lowering the water level; at points G and X – in the direction of the meridian plane, from the center of the world outwards, pulling water on either side of the meridian, causing the water level to rise, that is- that is to say towards great waters; at points, B, B’ and C, C’ are mutually symmetrical by the line – direction tangent to the sphere of water.
Under tidal forces, the entire surface of the world ocean presents a tidal ellipse. Each half of the ellipse resembles a tidal wave with a peak at high water points and a base at low water points.

Due to the earth’s daily rotation, tidal waves constantly cross the bridge from west to east, and high and low levels cross each meridian.

Figure 3. The ellipsoid is the tide when the moon’s declination is zero.

In addition to the moon, the sun exerts tidal forces on the earth. Diagram of the tidal force formation due to the sun, similar to the moon.

However, since the sun is 390 times farther from the planet than the moon, even though its mass is 30 million times that of the moon, the sun’s tidal force is still less than its tidal force of 2, 17 times.
The two tidal force systems are entirely independent of each other, but by nature, they are connective, and composite moon-solar tides are observed. Due to the reciprocal positions of the earth, the moon and the sun constantly change, and their tidal forces also change.

They can combine to make the tide stronger, but in other places, they cancel each other out and weaken the wave. This affects the characteristics and quantities of the observed tides and clearly shows their changes.

Tidal difference

Using tidal observations, deviations from the average values of the tidal quantities and the times of reaching high water and low water, which vary daily, were established at mixed tides – one day at a time. These differences are called tidal differences. Changes in the reciprocal positions of the earth, moon, and sun cause them.

The following types of tidal difference exist: daily, half-day, month tidal difference, month, and long cycle.

The daily tide difference

The daily tide difference is caused by the difference in the height of high water and low water of mixed tides and the time of high tide and low water during the day.

The daily tidal difference depends on astronomical causes – lunar declination and solar declination and geographical conditions of the location.
These disparities are strongest in mixed tides. Indeed, during irregular diurnal tides, when the lunar declination is essential, the daily height difference at high water causes the loss of low tides and net high water, i.e., transition from a semi-diurnal wave to a purely diurnal tide.
The difference depends on the lunar declination characteristic of the daily tides here; the tides reach a very high amplitude when the lunar declination is maximum.

These tides are called tropical tides. When the moon’s declination is zero, the tidal range is minimum, and it is called the equilibrium tide or the religion of the equinox.
The difference depends on the characteristic lunar phase of the semidiurnal tide. They include: the maximum amplitude of the tides observed on the days of the new and full moon, the periods of the moon, the sun, and the earth distributed on the same isoline, called squirrel tide, and The minimum when the moon and sun are distributed at right angles to the earth is called the orthogonal tide.

Figure 4. Diagram of Soc Vong tide and direct tide

In I+III positions – Soc Vong dynasty; at positions II+IV – direct tide;

Because of the moon, the large water, black crescents, and the sun – can be the porch by the white; The arrows are the direction of movement of the earth around the sun and the moon around the earth.

The semi-monthly tidal difference is usually dependent on altitude and time.

The tidal amplitude changes over the course of two weeks, corresponding to the moon’s phase and tilt. In a lunar month, there are twice high water elevation and twice low water; The duration of these variations is about 14.6 days.

The half-moon difference in time is the change of the moon due to the change in the distribution of the positions of the moon and the sun. The maximum value of the lunar interval occurs when the lunar meridian crosses before and after the sun at around 4 and 8 o’clock.

A monthly tidal difference

A monthly tidal difference is a change in the number of tides caused by varying distances between the moon and the sun. When the distance from the earth to the moon is the smallest, the waves are most incredible and the largest – the smallest. In addition to tidal height, it is also detected in lunar changes around it.

The tidal difference of a moon is called the parallax difference because for a quantitative assessment of the distance from the earth to the moon, the slope of the horizon of the moon – parallax is added.

The long-term tidal difference

The long-term tidal difference is primarily due to the variation of the slope and its distance from the earth within a year. The interpretation of solar declination is related to the variation in the amount of tropical and equatorial tides and diurnal variations.

In addition to the half-year, one-year difference, there is a slow difference with a period of 18,61 years due to the inclination time between the lunar orbit and the ecliptic plane.

Tidal amplitude and characteristics on oceans and seas.

The largest tidal amplitude is observed in the Atlantic Ocean.

At its peak, the amplitude can reach 18m, as in the Fandi Strait, which stretches between the North American continent and the New Sotlan peninsula.

This is the largest tidal amplitude for the entire World Ocean. In the port of Halegos on Argentina’s south coast, the measured tide is 14 m.
Near the southwest coast of England, significantly large tidal ranges are observed, up to 11.5 m in the Briston Strait, 8 m in Livecpun, and up to 6.3 m near the mouth of the More River. The island’s coastal area in the tidal range of Raikavik is 4 m.
The tidal range of the islands off the ocean is about 1-2 m; for example, Adop islands are 1.2 – 1.8 m, and Tristan-da-Cunha island – is 1.5 m.
The tides in the Atlantic Ocean are primarily semi-diurnal. The tidal features are particularly pronounced in the region near the west coast of Europe: the most unique.

In the Caribbean Sea and the strait of Mexico, tidal features are very diverse, including semi-diurnal, pure diurnal, and mixed tides.

Pacific

In many parts of this ocean, the tidal amplitude is 9 m. exceeds the Strait of Kuca in Alaska, with amplitude up to 7-8 m, in the Panama Strait and California – over 9 m, near the coast of Chile -8 m. Penrin Sea of Okhot – up to 13 m.

It is observed that the tidal characteristics in the Pacific Ocean are less than the daily and mixed tides. The tides are mainly mixed near the coasts of Australia, Asia, and North America.

In the Indian Ocean, the most excellent tidal range

In the Indian Ocean, the most excellent tidal range is found on the north coast of Australia, in the Bengal and Arabian Straits South Australia 2m.

Tidal amplitude in the Arctic Ocean

Tidal amplitude in the Arctic Ocean is most observed in the White Sea, especially in the Gulf of Meden, which is expected to reach 8.5 m. The largest observed off the Murman coast in the Barents Sea is up to 4 m; Snow Sea – 0.5 – 1 m, Trukot Sea – 1.5 m.

BASICS OF PREVENTION OF INCOME FACTORS.

Calculating the factors of tides is essential for the safety of navigation and seaports.

The natural tidal phenomena are widely used in harmonic analysis.

The essence of harmonic analysis is that it is possible to view the complex curve of sea level change under the influence of tides as the summation form of standard component curves, each of which has the characteristics of simple harmonic oscillation Z as a sine wave:

Z = R cos (qt – ξ )

Inside: R – amplitude of the component wave under real conditions.

q – average angular velocity per hour for each component wave, constant and independent of geographical conditions.

t – mean solar time

ξ – the first stage of the wave.

These component tidal waves, which can be seen more conveniently and simply as a result of the motion of some celestial bodies, rotate around the earth in circular orbits of different radii and at different rotational angular velocities.

Likewise, when the object’s mass, orbital radius, and angular momentum are included, a composite result can be obtained using the true tidal wave.

Therefore, the height of the water level at any time $h_{t}$ is determined by the formula:

$h_{t}$ = Σ R cos(qt – ξ)

In nature, of course, there are no component waves. There is a tidal wave that we observe. Therefore, the whole method is considered a purely artificial interpretation of the wave phenomenon since it is assumed that all phenomena occur as if they were separate waves.

Taking into account the influence of local conditions on the amplitude, the original formula for the shaped tidal wave sin z can be written as:

z = f H cos [ qt + ( $V_{0}$ + U ) – g ]

Inside f is the reduction factor calculated according to the divine laws of motion.

H – average tidal amplitude depending on geographical conditions.

$V_{0}$ + U – astronomical argument taken as the celestial hour angle during the first

$0^{g}$ days of observations or tidal projections;

g is the wave position angle that determines the slow arrival of the high water moment of each component wave relative to a particular tidal wave, or, in other words, the moment that crosses the celestial meridian causing the characteristic deviation.

According to the stationary tide phase, natural wave characteristics depend on local conditions.

The tidal amplitude H and the angle of the g-wave location are determined by the data observed for that location. They are invariant quantities, and hence they are called harmonic constants.
The harmonic analysis problem includes 1/ determining the harmonic constants, 2/ estimating the standard tidal height at a particular point in time, and 3/taking into account the main tidal factors of a given topic.
The complete formula with tide height calculation includes 93 components. However, in practice, it is necessary to determine only 8 main features of waves for deep sea and 11 for shallow water.
Based on the harmonic analysis, a tide table is created. It is possible to pre-calculate the tide factors in different ports – high water heights, low tides, when they are reached, and other characteristics of the waves at any time.
Tide tables or tide calendars are the official way of calculating tide elements on ships. They are published annually.
In developed countries, the tidal calendar consists of several sets that pre-calculate tidal factors for almost all major ports in the seas of the World Ocean.

Most of these calendars usually have two main parts: Part I – high-water and low-water advance tables for large ports; part II – main indication for tide calculations for small ports; and some other auxiliary tables.

Thanks to the tidal calendar, the following problems can be solved:

– Calculate the height and time of high and low water in main ports on a given date.

– Calculate the sea level at the main port at any given time between high and low water,
– Determine the time when the tide reaches the given maximum value

– To calculate the tide in the supplementary tables, you must first become familiar with the headings, have a clear overview of their contents, and at the same time take care to read the descriptions and operating instructions.