Ocean Tides: Introduction, Origin and Types | Ocean Tides | Geography

In this article we will discuss about:- 1. Introduction to Ocean Tides 2. Origin of Ocean Tides 3. Types 4. Theories 5. Water Currents.

Introduction to Ocean Tides:

Waves, currents and tides are of vital signifi­cance among various types of oceanic movements. Tides are, in fact, the most important of all the oceanic movements because tidal currents affect the whole water mass from the sea surface to the bottom. The rise and fall of seawater due to gravitational forces (cen­tripetal) of the sun and the moon are called tides. The sea waves generated by tides are called tidal waves.

The rise of seawater and its movement towards the coast is called tide and the resultant high water level is known as high tide water (H.T.W.). The fall of seawater and its movement towards the sea is called ebb and the resultant low water level is called low tide water (L.T.W.).

The difference between high tide water and low tide water is called tidal range. There is much variation in the height of high and low tides at different places in different oceans because of varying characteristics of the depth of ocean water, configuration of sea coasts and coastlines and openness or closeness of the seas.

Origin of Ocean Tides:

The origin of tides in the oceans is primarily concerned with the gravitational forces of the sun and the moon. It may be pointed out that the earth rotates from west to east and revolves around the sun follow­ing an elliptical orbit.

Similarly, the moon rotates from west to east and revolves around the earth (fig. 28.1) along an elliptical orbit so that the distance between the moon and the earth changes (fig. 28.2) during different times in every month. The period of the farthest dis­tance between the moon and the earth (407,000 km) is called apogee while the period of the nearest distance (356,000 km) is called perigee (fig. 28.2).

The surface of the earth with its diameter of 12,800 km (8,000 miles) is 6,400 km nearer to the moon than its centre. The centre of the moon is 3,84,800 km (2,40,00 miles) away from the centre of the earth. The earth’s outer surface is 3,77,000 km (2,36,000 miles) away from the outer surface of the moon. It is evident that the earth’ s outer surface, which is opposite to that surface of the earth which faces the moon (fig. 28.3T) is 3,90,400 km away from the moon’s surface.

The gravitational force of the moon will be maximum at the earth’s surface facing the moon (at T in fig. 28.3) while it will be minimum at the opposite side of the earth (at A in fig. 28.3). Consequently, the water of the earth’s surface facing the moon is attracted and pulled and high tide occurs (fig. 28.3), High tide is also formed at the opposite side of the earth (A in fig. 28.3) simultaneously because of the reactionary force (cen­trifugal) of the gravitational (centripetal) force of the moon causing outward bulge of water.

Thus, two tides and ebbs are experienced twice at every place on the earth’s water surface within 24 hours. When the sun, the earth and the moon are in the same line (at the time of full moon and new moon) their gravitational forces work together and high tides are formed (fig. 28.4).

On the other hand, when the sun and the moon are at the position of right angle with refer­ence to the earth (fig. 28.5), the gravitational forces of the sun and the moon work against each other and hence low tides are formed. This situation occurs during the 8th day of each fortnight of a month.

On an average, every place experiences tides twice a day. Since the earth completes its rotation in roughly 24 hours, every place should experience tide after 12 hours but this never happens. Each day tide is delayed by 26 minutes because the moon also rotates on its axis (west to east) while revolving around the earth.

Since the earth rotates from west to east and hence the tide centre shifts westward. When the tide centre completes one round, the moon’s position is ahead of the tide centre by that time because the moon also revolves around the earth, with the result the tide centre takes another 52 minutes to come under the moon.

Thus, a particular tide centre takes 24 hours 52 minutes to come under the moon but by that time there is another tide at the opposite side of the referred tide centre and this happens after 12 hours 26 minutes.

Let us understand this process with the help of a diagram (fig. 28.6). Suppose if P experiences first tide at 4 P.M., the second tide will occur at 4.26 A.M. and the next tide will be experienced at 4.52 P.M. The moon is at ‘K’ location (fig. 28.6) and the place ‘P’ on the earth’s water surface under the moon (K) will experience tide at 4 p.m. The place ‘P’ after completing its full rotation in 24 hours comes to its original place but by that time the moon moves to ‘L’ position which is above ‘F’ place on the earth’s surface.

Now the place ‘P’ has to cover extra distance of P-F so that it may come under L position of the moon and ‘P’ may experience next tide. The earth has to spend 52 minutes to cover P-F distance. The moon completes its one revolution around the earth in 27 days, 7 hours, 43 minutes and 17.5 seconds (average 27.5 days). Thus, the P-F distance is 2/55th part of the moon’s orbit. The place ‘P’ will take 24 x 60 x 2/55=52 minutes to cover the distance of 2/55 (P-F) part of the moon’s orbit, therefore, the place ‘P’ will experience next tide at 4.26 A.M. when it is at O place and subsequent tide occurs at 4.52 P.M.

It is evident that at each place every day tide occurs after 12 hours and 26 minutes and after the tide, ebb occurs after 6 hours 13 minutes. It may be pointed out that each place experiences tide twice a day i.e. when the place is under the moon and when the place is at the opposite side of the moon and thus each tide at particular place is delayed by 26 minutes daily.

Types of Ocean Tides:

The oceanic tides are caused due to tide produc­ing forces of the sun and the moon. There is a lot of temporal and spatial variation in the tide producing forces because of different positions of the sun and the moon with the earth. Because of variations in the intensity of tide producing forces several types of tides are caused.

A few important types of tides are given below:

(1) Spring Tide:

Very high tide is caused when the sun, the moon and the earth are almost in the same line. Such high tides are called spring tides. The position of the sun, the moon and the earth in a straight line is called syzygy. When the sun, the moon and the earth are in sequential order in a straight line, in other words when the sun and the moon are in one side of the earth, the position is called conjunction (the situation of solar eclipse).

When the position of the earth is in between the sun and moon, this is called opposition (fig. 28.7). On the other hand, when sun, the earth and the moon are in a position of a right angle (fig. 28.5), this position is called quadrature. The positions of conjunction and opposition take place during new moon and full moon respectively.

In these situations the gravitational forces of the sun and the moon work together with combined force and thus high tide is caused. The height of such spring tides is 20 per cent more than the normal tides. Such tides occur twice every month (during full moon and new moon) and their timing is fixed.

(2) Neap Tides:

The sun, the earth and the moon come in the position of quadrature (i.e., form right angle) on seventh or eighth day of every fortnight of a month and thus the tide producing forces of the sun and the moon work in opposite direction, with the result low tide is caused. Such tide, which is lower in height than the normal tide, is called neap tide. The height of neap tides is generally 20 per cent lower than the normal tides.

(3) Tropical and Equatorial Tides:

Like the sun there is also northward and southward position of the moon in relation to the equator of the earth. If the sun completes its northward and southward position in one year i.e., in roughly 365 days, the moon completes it in 27.5 days or say in one synodic month. When there is maximum declination of the moon to the north of equator, the moon’s rays fall vertically on the tide centres (near the Tropic of Cancer) and hence spring tides are caused. Such tropical tides move westward along the Tropic of Cancer.

Spring tides are also caused along the Tropic of Capricorn which is opposite to the Tropic of Cancer. Thus, successive high and low water occurring along the tropics of Cancer and Cap­ricorn are of unequal heights. Such tides and ebbs are of higher and lower heights than the normal tides and ebbs respectively. Such tides recur twice every month when the moon’s rays fall vertically on the tropics of Cancer (during northward position of the moon) and Capricorn (during southward position of the moon).

Thus, the tides occurring along the tropics of Cancer and Capricorn are called tropical tides. There is no diurnal inequality of tides in terms of heights of two neap tides and two spring tides because the moon is vertical on the equator every month. Such tides are called equatorial tides.

(4) Apogean and Perigean Tides:

The nearest position of the moon with the earth is called perigee when the distance between them is 3,56,000 km. The tidal force of the moon is most powerful during this position and hence high tides are caused. Such tides, called as perigean tides, are 15 to 20 per cent higher than the normal tides. On the other hand, the tidal force of the moon is minimum during the position of apogee when the moon is at the farthest distance (4,07,000 km) from the earth and hence low tides are caused.

Such low tides, called as apogean tides, are 20 per cent lower than the normal tides. When the spring tide and perigean high tide occur at the same time, the resultant tide becomes abnormal. Similarly, when neap tide and apogean tide occur at the same time, the water level becomes significantly low.

(5) Daily and Semi-diurnal Tides:

The tides re­curring at the interval of 24 hours 52 minutes daily are called diurnal or daily tides while the tides recurring at the interval of 12 hours 26 minutes are called semi­diurnal tides.

(6) Equinoctical Spring Tides:

The tides recur­ring at an interval of 6 months due to the revolution of the earth around the sun and sun’s varying declinations are called equinoctical tides.

Theories of Ocean Tides:

Numerous theories have been put forth from time to time to explain the origin of ocean tides. These theories include equilibrium theory by Issac Newton (1687), dynamical theory by Laplace (1755), progres­sive was theory by William Whewell (1833), canal theory by G.B. Airy (1842), stationary wave theory by R.A. Harris etc.

(1) Equilibrium theory:

Sir Issac Newton propounded his theory of gravitation in his Principia in the year 1687 wherein he stated that every celestial body of the universe pos­sesses gravitational force. The celestial bodies attract each other through their gravitational force in such a way that they remain in equilibrium.

Thus, the sun, the earth and the moon are also in equilibrium due to their respective pull towards each other. Though the gravi­tational force of the sun is far greater than that of the moon but the lunar gravitational force has more effect on the earth than the sun because of its nearness to the earth.

The earth and the moon while attracting each other revolve around their common centre of gravity and thus two types of force are produced e.g.:

(i) Centrifugal forces which work outward from the centre, and

(ii) Centripetal force which works towards the centre. Centrifugal force is similar at all points on the earth’s surface.

The earth’s surface under the moon is 6,400 km (4,000 miles) nearer to the moon’s surface than its (earth’s) centre so that the centripetal or gravi­tational force (attractive force) is greater than the centrifugal force at the earth’s surface. Consequently, the water of the earth’s surface under the moon is attracted and pulled and high tide is caused.

The opposite side of the earth’s surface also experiences tide because the reactionary force (centrifugal force) of the gravitational force (centripetal force) of the moon is more effective. In other words, the centrifugal force of the moon is greater than the attractive force at the opposite side of the surface of the earth and thus a bulge of water is caused.

Gravitational and centrifugal forces balance each other along the line joining both the poles resulting in a resultant force which is directed towards the centre of the earth. This force causes lowering of sea level and depression i.e., low tide. It is apparent that each place experiences two high tides every day.

Similarly, there are two tides on the earth’s water surface at the same time, one is caused due to gravitational (centripetal) force being greater than the centrifugal force while the second one is caused due to centrifugal (outward) force being greater than the gravitational force so that both may balance each other.

It may be stated that the highest points of rise of water or say high tides lie nearest to and farthest away from the moon while the lowest points of water surface or say lowest tides lie at places perpendicular to the above places. These four phenomena viz. two high tides and two low tides occur simultaneously at one time on the earth’s surface.

Limitation of the Theory:

Since the earth’s sur­face is comprised of land and water and hence the gravitational force of the moon will not be so effective as it would have been if the earth’s surface would have been composed of only water. Secondly, the bulge of water may not be possible unless some sort of horizon­tal movement of tide is involved.

In other words, changes in the position of water masses in the form of horizontal movement are essential for the bulging of sea water outward. In order to overcome this shortcoming the theory was subsequently amended. The theory envisages that waves are generated during the occur­rence of tides and these waves move westward with their crests directly under the moon.

Thirdly, the time of high tide should be the same at all places along each meridian but this never hap­pens. For example, Liverpool and Leith, both are situated on 8°W longitude (meridian) but the time difference of high tides of these two places is of three hours. Fourthly, the proposed theoretical time by this theory for a tidal wave to move round the earth would be slightly more because there is vast variation in the configuration of the coasts of different oceans and their depths and thus the tidal waves have to move under internal as well as external frictions.

In fact, the tidal waves are not free but are forced waves which are very often obstructed by continental and oceanic barriers (bottom reliefs). G.B. Airy regarded this equilibrium theory as an erroneous approach to explain ocean tides. According to him it is erroneous to explain the origin of tides on the basis of gravitational force.

(2) Progressive Wave Theory:

The ‘progressive wave theory’ of William Whewell propounded in the year 1883 and ‘the canal theory’ of G.B. Airy postulated in the year 1942 to explain the origin of ocean tides are based on the following facts:

(i) The earth is a heterogenous body and not a perfect fluid,

(ii) Tide occures at different times at different places on same longitude,

(iii) There is a lagging of time of tides away from the source,

(iv) There is variation in the magnitude and amplitude of tides at different places,

(v) Tide is in the form of tidal wave which travels from east to west.

The crests and troughs of such tidal waves become tides and ebbs respectively. These waves are originated in the oceans under the influence of tidal force of the moon. The length and velocity of tidal waves depend on the depth of seas and oceans. In a globe completely surrounded by water the tidal waves would travel freely from east to west but the position of land and water hinders the velocity and direction of these waves.

Since the continents roughly stretch from north to south and hence they hamper the free movement of tidal waves. These waves are least hampered in the oceans surrounding the Antarctic continent.

Thus, tidal waves are generated in the southern ocean in the southern hemisphere under the influence of tide-producing force of the moon. These waves are called primary waves which move from east to west in the form of forced waves. These waves are obstructed by the continents and are consequently refracted north­ward.

Secondary waves are generated when the west ward movement of primary waves is obstructed by land masses. These northward moving waves are called secondary waves or derived waves which also move from east to west. Further minor waves are generated from these secondary waves. These secondary and minor waves progressively move northward though there is gradual decrease in their magnitude and ampli­tude but these waves generate tides every-where.

It may be pointed out that the primary waves are influ­enced by the moon but the minor waves move freely. It is, thus, apparent that the tidal waves after being originated in the southern ocean progressively move northward with continuous lag of time and dissipation of wave energy. In other words, the arrival of these progressive waves at successive places northward along the same longitude is also progressively delayed.

This is why there is difference of time of tide at different places on the same longitude. In other words, the time of tides is progressively delayed northward along the longitude. These progressive waves become ineffec­tive after reaching North Pole. The crests and troughs of these waves after reaching the coasts cause tides and ebbs respectively. Fig. 28.8 depicts the co-tidal lines (the lines joining the points of high waters occurring at the same hour are called co-tidal lines) of the Atlantic Ocean.

Evaluation of the Theory:

According to the pro­gressive wave theory the age of tides increases north­ward. In other words, if tide is generated in the south on a particular longitude it reaches quite late at the points located further north on the same longitude. On the other hand, the data available so far about the time of tides denote that the time of spring tides is almost the same from Cape Horn to Greenland in the Atlantic Ocean.

Normally, the tides are local or regional phe­nomena rather than phenomena originating in the south­ern ocean and moving progressively northward. At some latitudes daily and semi-diurnal, both types of tides are observed. Further, there is spatial variation in the irregularity of tides in different oceans. These variations cannot be explained on the basis of progres­sive wave theory.

(3) Stationary Wave Theory:

R.A. Harris of the U.S. Coast and Geodetic Survey propounded the concept of stationary waves as opposed to the progressive waves. This theory offers almost satisfactory explanation for local differences in tides, their types and their age. According to Harris tide phenomena are not due to progressive waves which originate in the southern oceans as claimed by William Whewell but are due to stationary waves which origi­nate independently in each ocean.

In other words, tide phenomena are regional phenomena. The stationary wave theory can be explained with the help of an experiment. If a rectangular tank or ‘developing tray’ containing water is rocked from one side to the other or is simply tilted, the water level rises along one side of the tray but falls along the other side.

This generates oscillation in the water contained in the tray. Such oscillations in the water are called stationary waves. There is such a centre in the middle of the tray where there is no change in the level of water. This point is called nodal point (fig. 28.9). The water level moves rhythmically from one end of the tray to the other end along a line which is called nodal line.

The period of oscillaition of water in the tray depends on the length and depth of the tray and the force of shocks applied to the tray. The aforesaid example is the case of uninodal system (fig. 28.9A) but there may also be binodal oscillation system (fig. 28.9B).

Based on above analogy, different oceans of the earth are like giant water containing trays. The tidal forces of the sun and the moon cause oscillations in the oceanic waters but the oscillations do not occur along straight limes as in the case of the tray rather they occur around a central point because of the rotational force of the earth, with the result several amphidromic points are generated.

The oceanic water remains calm and stationary at these points whereas water level changes around them. This mechanism results in the formation of waves which move in anti-clockwise direction around these amphidromic points. Such oscillatory mecha­nism of water occurs in every ocean and is collectively called as oscillation system. Numerous stationary waves are generated from these amphidromic points.

Every stationary wave has a definite time of its oscillation. The osciallation system and mechanism are affected by the depth, configuration and length of the ocean basins and the rotational speed of the earth. The sta­tionary waves after being originated from the amphidromic centres move towards the coasts.

The forward movement of these waves is hampered by the continental peninsulas, islands, bays etc. When these waves reach the coasts, their crests and troughs cause tides and ebbs respectively. There is positive correla­tion between the depth of the oceans and the height of tides. In other words, if the depth of the ocean becomes greater, higher stationary waves are generated and high waves generate high or spring tides. Low tides are caused in shallow seas because of lower height of stationary waves.

Water Currents Generated by Ocean Tides:

Water currents are generated by ocean tides due to upward and downward movement of sea level in the open and near shore regions. The coastward movement of tides causes flood currents which pile up seawater against the sea coast. The currents caused by returning tides are called ebb currents. Thus, flood currents move coastward while ebb currents move away from the coasts.

The currents associated with tides in the open ocean are called rotary currents which turn in counter clock wise direction in the northern hemisphere and clock wise in the southern hemisphere. When the rotary currents enter the shallow water of near shore areas they suffer from friction and ultimately they change to alternating or reversing currents.

The velocity of rotary currents in the open sea is very slow, around one kilometer per hour, but the reversing currents move very fast with the velocity of 44 kilometers per hour. The reversing tidal currents assume greater velocities in the region of irregular coastlines, in narrow and constricted bays, and in narrow coastal rivers.

The rise and fall of tides, and flood currents and ebb currents have greater potentials for the generation of electricity wherever there are bays with narrow openings and constricted tidal inlets or narrow estuaries.