“Roll on, roll on, thou deep, dark ocean, roll,” thus opens a famous poem. Ocean waters are never at rest; they are always in motion. Waves, tides and currents are the three kinds of movements affecting waters in the oceans and the seas. You are familiar with waves. Waves are formed when particles of water move up and down in rhythm. The movement is passed on from particle to particle resulting in a rhythm of movement in which waves appear to travel across the surface of the water. It is the movement only which travels, water does not, it only keeps on rising and falling. Waves are, thus, moving ridge-like curves on the surface of the sea. The top part of a wave is called its crest and the lower part between two crests is known as the trough. Waves are caused by winds exercising friction upon the surface of the water When winds are strong, very high waves may be generated. Such waves have great force. They erode the shores and sometimes even sink large ships. Very high and dangerous waves are caused by storms at sea and by under-sea earthquakes and volcanoes. They cause severe damage in coastal areas and may kill thousands. Severe storms hit Bangladesh in July and August and our eastern coastal areas in Tamil Nadu in October, November due to monsoon winds. Tides Tides are the periodic rise and fall in the level of the sea which occurs twice every day Tides are not noticeable in the open seas, but near the shores they are very obvious. The rising waters at high-tide rush up the beaches and flood the land for considerable distance. A little after, at low-tide the waters retreat and leave the land. Tides are caused by the gravitational pull of the sun and the moon. Name the days on which spring and neap tides occur. It is believed that tides are caused by the gravitational pull of the sun and the moon acting on the earth. Water, being fluid, is pulled out noticeably in the direction of the pull. The moon’s pull on the earth is stronger owing to its relative nearness to the earth. When the sun and the moon pull from the same or directly opposite directions, high or spring tides are caused. Such tides occur on full moon and new moon days. When the pulls of the moon and the sun act at right angles to one another, very low or neap tides occur. The pull of the moon. This happens when the moon is just half visible on the first and last quarter days. Tides are useful to us in many was. The increased depth of water at high tide enables ships to enter harbours. Many ships use the retreating waters of low tide to leave harbours. Tides constantly sweep the coasts, carrying away the mud etc. brought by rivers This prevents many harbours from getting choked by sand and silt. Tides are of vital importance to certain types of fishing. High tides can be used as a source of energy. They are also useful in salt industry. Sea water at high tide is impounded between a dam in a natural valley. At low tide, it is allowed to escape gradually. Its movement both ways is used to turn turbines to generate electricity. In the salt industry, high tide is used for filling up salt-pans with sea water for making salt. Tides in many places cause damage too. At places, tides erode away the coastal land. Ocean Currents The surface temperature of sea water is not the same everywhere. It varies from place to place giving rise to movement of waters on a large scale. This is being explained by an experiment as shown in Fig. 5.4. However, other factors are also responsible for the movement of ocean water. Oceans are said to be great distributors and moderators of temperatures. How do cold and warm currents help in this regard? Moves from one place to another in broad streams in the oceans. Such more or less permanent streams of water which flow in a definite direction are known as ocean currents. They are like rivers in the ocean. There are two kinds of ocean currents – the warm currents and the cold currents. Generally, the currents which originate near the equator are warm currents. They carry warm waters from low latitudes and spread them over the cold waters in the high latitudes. The cold currents generally originate in the high latitudes and move towards the low latitudes. Cold currents also from the upwelling of cold ocean water in tropical regions to com-pen sate for warm water carried away from the equatorial region. This results in general circulation of the sea water. Of the various causes of ocean currents, the most important are the prevailing winds and the difference in density of ocean waters due to variation in temperature or salinity. The slow movements of ocean waters, caused by prevailing winds, are known as drifts. Some deflection in the direction of currents is caused by the rotation of the earth. The direction to the ocean currents is also influenced by the shapes of the continental coasts. You can see from the map that (i) Warm currents hug the eastern coasts in lower and middle latitudes bringing warm waters from equatorial regions. (ii) They drift finally to western coasts of continents in higher latitudes on the opposite side of the oceans. (iii) Cold currents flowing along eastern coasts in high latitudes bring cold waters from polar regions. (iv) Another set of cold currents flows along western coasts of continents in tropical regions, resulting from upwelling of cold ocean waters to compensate for the removal of water from equatorial region. You will note from the map that the general pattern of currents in various oceans is the same. Let us now learn about some of the important currents In the Atlantic and the Pacific Oceans. The Gulf Stream criminates in the Gulf of Mexico in the Atlantic Ocean. It is a swift flowing warm current. Flowing northwards along the coast of North America, it meets the cold Labrador Current near Newfoundland. It is now pushed north-eastwards by the westerly towards the coasts of north-western Europe, and is known as the North Atlantic drift. The British Isles, Norway and Western Europe are benefited immensely by the North Atlantic Drift. For their latitudes, these would be cold countries. But the drift makes their climate mild and keeps the coasts free from ice in winters. The Gulf Stream and the Labrador Current meet near New found land. The mixing of warm and cold waters results in formation of thick fog over the area. This creates poor visibility and is a danger to shipping. The rocky material brought by the floating ice in the cold current is also dropped here, making the sea shallow. The cold current brings much plankton or small sea plants and animals with it. The net result is the formation of shallow banks, the Grand Banks near New found land where fish can find food in plenty. This has, thus, become a great fishing ground. The parallel of Gulf Stream and North Atlantic Drift in the Pacific Ocean are the Kuro Shio and the North Pacific Drift. The Kuro Shio makes eastern Japan to have mild winters and keeps the coasts free from ice. Across the ocean, the beneficial warmth is given to the coasts of Alaska and Canada. Note the cold current coming from the Artic Ocean and passing along the western coasts of Japan. It is the Oya Shio. It makes north-eastern Japan and the opposite Asian coast very cold in winters. Note also the cold currents along the western coasts of North America South America and Africa, the Canaries and Beguile currents respectively. These currents are a result of upwelling ocean waters and make the respective coasts cooler. Importance of Ocean Currents You have seen above how currents influence the temperatures of the coastal regions. Winds passing over warm currents get warmed up and pick up moisture to bring rain to western areas of continents. Cold currents make winds cool and dry. Where warm and cold currents meet, shallow banks are built up. Fish are found abundantly in such areas. Currents help in shipping too. Warm currents keep ports, even in cold northern areas of Europe and America, open all the year round. Ships sailing with the current-flow gain in speed and use less fuel. Warm currents also help in melting dangerous icebergs.