Ocean tides, types and theories

This Ppt helps you to know very well about ocean tides and its theories

1. OCEAN TIDES Prepared and Presented by, P.C. Jeba Preethi Jansi

2. INTRODUCTION An ocean  tide  refers to the cyclic rise and fall of seawater . Tides are caused by slight variations in  gravitational  attraction between the  Earth and the  moon  and the  Sun  in geometric relationship with locations on the Earth's surface. Tides are periodic primarily because of the cyclical influence of the  Earth's rotation .

3. TIDAL BULGE The moon is the primary factor controlling the temporal rhythm and height of tides . The moon produces two tidal bulges somewhere on the Earth through the effects of gravitational attraction. The height of these tidal bulges is controlled by the moon's gravitational force and the Earth's gravity pulling the water back toward the Earth. At the location on the Earth closest to the moon, seawater is drawn toward the moon because of the greater strength of gravitational attraction . On the opposite side of the Earth, another tidal bulge is produced away from the moon. However , this bulge is due to the fact that at this point on the Earth the force of the moon's gravity is at its weakest. Considering this information, any given point on the Earth's surface should experience two tidal crests and two tidal troughs during each tidal period.

4. TIDAL PERIOD The timing of tidal events is related to the Earth's rotation and the revolution of the moon around the Earth. If the moon was stationary in space, the tidal cycle would be 24 hours long. However, the moon is in motion revolving around the Earth. One revolution takes about 27 days and adds about 50 minutes to the tidal cycle. As a result, the  tidal period  is 24 hours and 50 minutes in length. The second factor controlling tides on the Earth's surface is the Sun's gravity. The height of the average solar tide is about 50% the average lunar tide. At certain times during the moon's revolution around the Earth, the direction of its gravitational attraction is aligned with the Sun's . During these times the two tide producing bodies act together to create the highest and lowest tides of the year. These  springtides   occur every 14-15 days during full and new moons. When the gravitational pull of the moon and Sun are at right angles to each other, the daily tidal variations on the Earth are at their least. These events are called  neap tides  and they occur during the first and last quarter of the moon. 

5. HIGH TIDE AND LOW TIDE When the gravitational pull is at its highest point, the result is  high tide , which is the highest level of the tide. When the pull is at its lowest point, we see  low tide , or the lowest level of the tide. The earth itself is also pulled toward the moon but with less strength. This pulls the earth away from the water on the opposite side of the earth, making the water on that side bulge as well. Therefore, high tide occurs on both sides of the planet at the same time. Meanwhile the earth is rotating. So, we experience tides throughout the day. LOW TIDE HIGH TIDE

6. TYPES OF TIDES Based on the number of high and low tides and their relative heights each tidal day, tides are described as semi-diurnal, mixed, or diurnal . SEMIDIURNAL AND DIURNAL TIDES Now , if the earth were perfectly round with no big land masses, all bodies of water in the world would experience two nearly equal high tides and two low tides each day. This tidal pattern is known as  semidiurnal tides . However, the continents of earth disrupt water bodies, and so this can produce different tidal patterns. For example, some bodies of water, such as the Gulf of Mexico, have  diurnal tides , which means only one high tide and one low tide each day.

7. Mixed tides Many parts of the world experience  mixed tides  where successive high-water and low-water stands differ appreciably In these tides, we have a higher high water and lower high water as well as higher low water and lower low water

8. SPRING TIDES AND NEAP TIDES The earth and moon are constantly in motion around the sun, and all have their own gravitational pull. So, when the alignment between the three bodies changes, it changes the strength of the overall gravitational pull and therefore the size of the tides . SPRING TIDES   Spring Tides are tides that occur when the earth, moon and sun are aligned, and the tidal range between high and low tide is at its maximum. This happens basically twice a month, during the full and new moon phases. At these times, the three bodies are in line and their gravitational pulls reinforce each other. When the spring tide is happening, we see higher than average high tides and lower than average low tides .

9. SPRING TIDE

10. NEAP TIDES A few weeks after the spring tides, we see the  neap tides . These are tides that occur when the moon and sun are at right angles to the earth's orbit, and the tidal range between high and low tide is at its minimum. The neap tides occur when the moon is in its first and last quarter phase. Because of the position of the moon and sun, their gravitational pulls on the waters of earth partially cancel each other out, resulting in smaller differences between the high and low tides.

11. NEAP TIDE

12. TIDAL BORE In shallow steep funnel shapes rivers the tides enhances as a single wall of water. This is called tidal bore. These tides move with a speed of 25km. They move up to height of 8 m. These tidal bores are commonly seen in Quintang river, Amazon, Bay of Fundy and many English and French rivers.

13. The Tide-Generating Forces As the earth revolves around the gravitational centre of the sun/earth system, the orientation of the earth´s axis in space remains the same. This is called revolution without rotation  . The tide generating force is the sum of gravitational and centrifugal forces. In revolution without rotation the centrifugal force is the same for every point on the earth´s surface, but the gravitational force varies . It follows that the tide generating force varies in intensity and direction over the earth's surface. Its vertical component is negligibly small against gravity; its effect on the ocean can be disregarded. Its horizontal component produces the tidal currents, which result in sea level variations. The gravitational force exerted by a celestial body (moon, sun or star) is proportional to its mass but inversely proportional to the square of the distance . The Sun's mass is equivalent to some 332,000 Earth masses, while the mass of the Moon corresponds to only 1.2 percent of the mass of the Earth .

14. The mean distance Sun -Earth is 149.5 million km, the mean distance Earth - Moon only 384,000 km. If the gravitational force of the Sun and Moon are compared, it is found that the Sun's enormous mass easily makes up for its larger distance to Earth, to the extent that the gravitational force of the Sun felt on Earth is about 178 times that of the Moon. As a result the Earth's orbit around the Sun is not seriously distorted by the Moon's movement around the Earth. However, as is evident, tides are not produced by the absolute pull of gravity exerted by the Sun and the Moon but by the differences in the gravitional fields produced by the two bodies across the Earth's surface . Because the Moon is so much closer to the Earth than the Sun, its gravitational force field varies much more strongly over the surface of the Earth than the gravitational force field of the Sun . Quantitative analysis shows that the  differences  of the gravitational forces across the Earth's surface are proportional to the  cube  of the distances Sun - Earth and Earth - Moon. As a result the Sun's tide-generating force is only about 46% of that from the Moon. Other celestial bodies do not exert a significant tidal force.  

15. Tidal theories EQUILIBRIUM   THEORY The first  theory  which attempted to explain the  tides  in the ocean was the  equilibrium   theory  developed by Newton (1687). Newton showed that the attractive forces of the moon on the side of the  earth  nearest to the moon would be greater than the average forces on the opposite side of the  earth  and therefore the water would pulled be less than the average forces and therefore the water would move away from the moon. In an ocean completely covering the  earth  and of equal depth; the horizontal component of the tide generating forces pulls the water towards the points nearest and farthest away from the moon, thus causing elevation of the water at these points, with a corresponding depression in the water level halfway between these two points on the  earth ’s great circle. The sun also would tend to give similar effects related to itself. According to Newton, this process can continue only till the resultant horizontal pressure differences in the ocean tend to return the water to its former position, so that the free surface of the water would be in equilibrium. Equilibrium  theory  can explain certain characteristics of  ocean tides , especially the occurrence of semidiurnal  tides .

16. Thus this  theory  can explain the occurrence of semi-diurnal  tides . This   theory  can also explain the formation of spring and neap  tides . When the sun and the moon are in line with respect to the  earth (full moon and new moon) their tide generating forces are added, producing highest high  tides  known as spring  tides . When the sun and the moon are perpendicular to one another with respect to the earth , their effects tend to cancel one another, producing the lowest low  tides  known as  neap   tides . Dynamic  Theory  of  Tides Nearly a century after Newton put forward his equilibrium  theory . Laplace(1775) formulated the dynamic  theory  of  tides . According to this  theory , the  tide producing forces  produce tide  waves  in the ocean whose period corresponds to that of the generating forces.  The dynamical  theory  attempted to understand  tides  by considering the effect of the depth and width of the o The Coriosis force (deflection due to  earth ’s rotation) is zero at the equator and increases with latitude towards the poles. It varies with the speed of the tidal current

17. Thus , the  earth ’s rotation has a major role in the formation of  tidal currents  and on the general nature of ocean tides . Friction also influence the  tides , since the force of friction reduces the speed of the  tidal currents . In the simplest case so far, we have considered the moon to be directly above the equator . But actually the moon has a declination which changes up to a maximum of 28.5° north and south. Due to this reason the tidal forces will not be symmetrical with  reference  to the equator. Therefore, the problem of wave motion in latitudinal channels would become more complicated . In spite of the complexities of the dynamic  theory , it has one important advantage, in that it enables to predict the course of the  tides  for any given location along a coast .

18. Modern Tidal   Theory Our knowledge on  tides  has not yet reached the final stage, so as to explain the  tides  satisfactorily and quantitatively without referring to observed values, attempts are being made. The following three fundamental ideas have provided the basis from which the investigation is proceeding successfully (Macmillan, 1966). 1. The  theory  of standing oscillations 2. The  theory  of resonance between the natural oscillation periods of water masses in bays, gulfs, seas and oceans and cyclic rhythms of the astronomical disturbing forces which can be predicted accurately. 3. The application of gyroscopic principles to determine the effect of the  earth ’s rotation upon these water masses, which are set in motion horizontally by hydraulic factors, due to the differences in level of the sea surface induced by the atractive forces.

19. Tides Affect Coastal Regions Tides affect coastal regions in different ways. High tides push large amounts of water far up onto beaches and leave the sand and sediment mixed with the water behind when the tide goes out. Therefore, tides transport sand and sediment and shape shorelines. Tides feed estuaries. Estuaries are coastal areas where freshwater mixes with ocean water that is delivered by the tides . High tides bring nourishing sediment and sea life into estuaries. Estuaries are home to biologically diverse and unique plant and animal communities because their waters contain a mix of freshwater and salty ocean water. Tides can present challenges to coastal regions, especially during storms. Storms that form out in the ocean intensify waves. If the storm waves come ashore at high tide, tides can worsen damage and erosion caused by storm waves.

20. TIDES AFFECT MARINE LIFE Organisms living in intertidal zones must be able to live both above and below water, depending on the tide. Other organisms that depend on tides for survival include coral reefs, which depend on tides to deliver food to them. The distances and positions of the sun, moon and Earth all affect the size and strength of the Earth’s tides. The magnitude of tides are also strongly influenced by the shape of the shoreline. The height of tides is greatest when tides hit a wide portion of the continent. On the other hand, mid-oceanic islands usually experience very small tides because they are positioned further away from the continent. Tides carry nutrients, moderate temperatures and influence other conditions . High tide refers to water at its highest level, and low tide is water at its lowest level. Ebb, or falling, tide occurs when the water seems to flow back out between a high tide and a low tide . When the water flows back in between low tide and high tide, this period of time is known as flow, flood or rising tide. Each of the Earth's two tidal bulges travel at a speed of approximately 24 hours, which means there are two high tides and two low tides each day, with high tides occurring 12 hours and 24 minutes apart.