MOVEMENTS OF OCEAN WATER 

Horizontal and Vertical Movement of Water  

1). The horizontal motion refers to the ocean currents and waves. The vertical motion refers to tides.   

2). Ocean currents are the continuous flow of huge amount of water in a definite direction while the waves are the horizontal motion of water.  

3). The vertical motion refers to the rise and fall of water in the oceans and seas. Due to the attraction of the sun and the moon, the ocean water is raised up and falls down twice a day. The upwelling of cold water from subsurface and the sinking of surface water are also forms of vertical motion of ocean water.  

  

Waves  

1). Waves are actually the energy, not the water which moves across the ocean surface.  

2). Water particles only travel in a small circle as a wave passes. Wind provides energy to the waves. Wind causes waves to travel in the ocean and the energy is released on shorelines. 

3.). As a wave approaches the beach, it slows down, due to the friction occurring between the dynamic water and the sea floor. And, when the depth of water is less than half the wavelength of the wave, the wave breaks. The largest waves are found in the open oceans. Waves continue to grow larger as they move and absorb energy from the wind. 

4). When a breeze of two knots or less blows over calm water, small ripples form and grow as the wind speed increases until white caps appear in the breaking waves. 

5). Steep waves are fairly young ones and are probably formed by local wind. 

6). The maximum wave height is determined by the strength of the wind, i.e. how long it blows and the area over which it blows in a single direction. 

7). Waves travel because wind pushes the water body in its course while gravity pulls the crests of the waves downward. The falling water pushes the former troughs upward, the actual motion of the water beneath the waves is circular.   

  

Characteristics of Waves  

i). Wave crest and trough : The highest and lowest points of a wave are called the crest and trough respectively.  

ii). Wave height : It is the vertical distance from the bottom of a trough to the top of a crest of a wave.  

iii). Wave amplitude : It is one-half of the wave height.  

iv). Wave period : It is merely the time interval between two successive wave crests or troughs as they pass a fixed point. 

v). Wavelength : It is the horizontal distance between two successive crests.  

vi). Wave speed : It is the rate at which the wave moves through the water, and is measured in knots.  

vii). Wave frequency : It is the number of waves passing a given point during a one second time interval.  

  

Tides  

1). The periodical rise and fall of the sea level, once or twice a day, mainly due to the attraction of the sun and the moon, is called a tide.  

2). Movement of water caused by meteorological effects (winds and atmospheric pressure changes) are called surges. 

3). The moon’s gravitational pull to a great extent and to a lesser extent the sun’s gravitational pull, are the major causes for the occurrence of tides. 

4). Another factor is centrifugal force, which is the force that acts to counter the balance the gravity.  

5). Together, the gravitational pull and the centrifugal force are responsible for creating the two major tidal bulges on the earth. On the side of the earth facing the moon, a tidal bulge occurs while on the opposite side though the gravitational attraction of the moon is less as it is farther away, the centrifugal force causes tidal bulge on the other side.  

6). The ‘tide-generating’ force is the difference between these two forces; i.e. the gravitational attraction of the moon and the centrifugal force.  

7). The tidal bulges on wide continental shelves, have greater height. When tidal bulges hit the mid-oceanic islands they become low. 

8). The shape of bays and estuaries along a coastline can also magnify the intensity of tides. 

9). Funnel-shaped bays greatly change tidal magnitudes. When the tide is channeled between islands or into bays and estuaries they are called tidal currents. 

10). The highest tides in the world occur in the Bay of Fundy in Nova Scotia, Canada. The tidal bulge is 15 - 16 m. Because there are two high tides and two low tides every day.  

  

Types Of Tides  

1. Tides based on frequency are:  

i). Semi-diurnal tide : The most common tidal pattern, featuring two high tides and two low tides each day. The successive high or low tides are approximately of the same height. 

ii). Diurnal tide : There is only one high tide and one low tide during each day. The successive high and low tides are approximately of the same height. 

iii). Mixed tide : Tides having variations in height are known as mixed tides. These tides generally occur along the west coast of North America and on many islands of the Pacific Ocean.  

2. Tides based on the Sun, Moon and the Earth Positions:   

i). Spring tides : The position of both the sun and the moon in relation to the earth has direct bearing on tide height. When the sun, the moon and the earth are in                                             a straight line, the height of the tide will be higher. These are called spring tides and they occur twice a month, one on full moon period and another during new moon period.  

ii). Neap tide : Normally, there is a seven day interval between the spring tides                                   and neap tides. At this time the sun and moon are at right angles to each other and the forces of the sun and moon tend to counteract one another. The Moon’s attraction, though more than twice as strong as the sun’s, is diminished by the counteracting force of the sun’s gravitational pull.  

3. Once in a month, when the moon’s orbit is closest to the earth (perigee), unusually high and low tides occur. 

4. During this time the tidal range is greater than normal. Two weeks later, when the moon is farthest from earth (apogee), the moon’s gravitational force is limited and the tidal ranges are less than their average height.  

5. The time between the high tide and low tide, when the water level is falling, is called the ebb. The time between the low tide and high tide, when the tide is rising, is called the flow or flood.  

  

Importance of Tides  

1. It can be predicted well in advance which helps navigators and fishermen.  

2. Tidal heights are very important, especially harbors near rivers and within estuaries having shallow bars at entrance which prevent boats from entering into harbor. 

3. Help to desilt sediment, remove polluted water from estuaries and generate electrical power. 

4. A 3MW tidal power project at Durgaduaniin Sundarbans.  

  

Ocean Currents  

1). Ocean currents are like river flow in oceans. They represent a regular volume of water in a definite path and direction. 

2). Ocean currents are influenced by two types of forces namely : 

(i). Primary forces that initiate the movement of water;   

(ii). Secondary forces that influence the currents to flow.  

3). The primary forces that influence the currents are: Heating by solar energy, wind, Gravity, Coriolis force. 

4). Heating by solar energy causes the water to expand, near the equator the ocean water is about 8 cm higher in level than in the middle latitudes. This causes a very slight gradient and water tends to flow down the slope.   

5). Large accumulations of water and the flow around them are called Gyres. These produce large circular currents in all the ocean basins.  

6). Water with high salinity is denser than water with low salinity and in the same way cold water is denser than warm water. Denser water tends to sink, while relatively lighter water tends to rise. Cold-water ocean currents occur when the cold water at the poles sinks and slowly moves towards the equator.   

7). Warm-water currents travel out from the equator along the surface, flowing towards the poles to replace the sinking cold water. 

8). Characteristics of ocean currents: The currents are strongest near the surface and may attain speeds over five knots. At depths, currents are generally slow with speeds less than 0.5 knots. The strength of a current refers to the speed of the current. A fast current is considered strong. A current is usually strongest at the surface and decreases in strength (speed) with depth.  

  

Types of Ocean Currents  

1). Based on Depth:  

(i). Surface – 10% of all; found in upper 400 m. 

(ii). Deep water – 90% water; move around ocean basins due to variations in density and gravity.  

2). Based on Temperature: 

(i). Cold Current – cold water to warm areas on west of continents on low and mid latitude & to east in higher latitudes in the North Hemisphere. 

(ii)Warm Current – warm water to cold areas on east of continents on low and mid latitude & to west in higher latitudes in the North Hemisphere.  

  

Major Ocean Currents  

1. Major ocean currents are greatly influenced by the stresses exerted by the prevailing winds and coriolis force. 

2. The oceanic circulation pattern roughly corresponds to the earth’s atmospheric circulation pattern. 

3. The air circulation over the oceans in the middle latitudes is mainly anticyclonic (more pronounced in the southern hemisphere than in the northern hemisphere). 

4. The oceanic circulation transports heat from one latitude belt to another in a manner similar to the heat transported by the general circulation of the atmosphere.  

Effects of Ocean Currents  

1). West coasts of the continents in tropical and subtropical latitudes (except close to the equator) are bordered by cool waters. Their average temperatures are relatively low with a narrow diurnal and annual ranges. There is fog, but generally the areas are arid. 

2). West coasts of the continents in the middle and higher latitudes are bordered by warm waters which cause a distinct marine climate. They are characterized by cool summers and relatively mild winters with a narrow annual range of temperatures. 

3). Warm currents flow parallel to the east coasts of the continents in tropical and subtropical latitudes. This results in warm and rainy climates. These areas lie in the western margins of the subtropical anticyclones.   

4). The mixing of warm and cold currents help to replenish the oxygen and favour the growth of planktons, the primary food for fish population (The best fishing grounds of the world exist mainly in these mixing zones). 

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