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Tides, Waves, and Currents

• Tides are caused by the force of gravity from the moon. They result in diurnal (daily) changes in water levels.
  
  
• Ordinary ocean waves get their energy from the wind. Higher waves need more energy. 'Sea state' (the typical height of wind waves) is determined by the wind speed, the length of time the wind has been blowing steadily (the 'duration'), and the distance over the water that the wind blows in a single direction (the 'fetch').
  
  
• On May 27, 1990, a severe storm washed 39,466 pairs of Nike athletic shoes overboard from a ship in the North Pacific about 2,700km west of the North American coast. Curt Ebbesmeyer and James Ingraham tested computer models of ocean currents by using the time and location of arrival of the shoes that drifted ashore.
  
  
• In July 1870, British scientists aboard the Porcupine made the first observations of a subsurface current in the Strait of Gibraltar that flows westward against the surface current, and that carries water from the Atlantic into the Mediterranean. William Carpenter, one of the scientists on board, used this information to develop an explanation of the vertical circulation of the world ocean.
  
  
• Ocean surface currents are driven by the wind. In the surface layer the force exerted by the wind is balanced by friction with the underlying water and the apparent force due to the Earth's rotation. The overall transport of water in the surface layer is at an angle to the direction towards which the wind blows, to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
  
  
• Since 1972 scientists have been able to use satellites to track transmitters on small buoys drifting with the surface currents. The buoys are usually attached to large sea anchors, or drogues, to reduce the errors due to the effect of the wind on the buoy itself.
  
  
• Wind-generated ocean waves continue to travel after the wind stops. Longer waves travel faster than shorter ones, and go farther before friction causes them to disappear. Groups of waves from storms in the Pacific near Antarctica have been detected in Hawaii and Alaska, more than 10,000km away.
  
  
• In the night of Feb. 6-7, 1933, officers aboard the USS Ramapo measured the highest officially recorded sea waves by observing the height of the wave crests in reference to the ship's foremast when the ship was in the trough of the wave. They were 34.2m from crest to trough, and the interval between crests was 14.8 seconds.
  
  
• A wave generated by an underwater landslide struck the island of Lanai in the Hawaiian chain about 105,000 years ago. It deposited sediment to an altitude of 380m.
  
  
• Waves and/or currents generate ripples on beaches. Those formed by waves will be symmetrical, with rounded crests and troughs, while those formed by currents will be asymmetrical with a steeper downstream face and a gentler upstream face.
  
  
• The ocean tide is a regular oscillation in water levels and in speed and direction of ocean currents. Tides originate with the gravitational forces of the moon and sun.
  
  
• The 'range' of a tide is the vertical distance between high water and the succeeding low water. 'Flood' is horizontal flow in the landward (upstream) direction. 'Ebb' is flow in the seaward (downstream) direction. Slack water is the short period of rest as the flow reverses from flood to ebb, or vice versa.
  
  
• The greatest difference between high and low tide occurs at Burntcoat Head in the Minas Basin of the Bay of Fundy, between Nova Scotia and New Brunswick, where the average range for spring tides is 16m. Very large tides also occur in the Leal Basin of Ungava Bay, Quebec.
  
  
• The fastest ocean currents are tidal currents in Nakwakto Rapids, on the Pacific Coast of Canada, which reach a speed of 16 knots (29 km/hr).
  
  
• A tide is 'semidiurnal' when it exhibits two high waters and two low waters in a lunar day (about 25 solar hours), 'diurnal' when it exhibits only one of each. Most places have two daily tides differing from each other in height and range.
  
  
• The Greek philosopher Posidonius (135-50 BC) traveled to the Atlantic coast of Europe to observe ocean tides. He described the spring/neap cycle, with the greatest tides at the full moon and the dark of the moon, smaller tides at the quarters, and annual variation, with the greatest tides at the equinoxes.
  
  
• In the year 730 AD the British scholar the Venerable Bee recognized many of the major features of the tides. He wrote that high and low tide occur approximately 50 minutes later each day, do not occur at the same time everywhere, go through cycles of larger and smaller tides (springs and neaps) which are linked to the phases of the moon, and repeat themselves on a 19 year cycle, again like the moon.
  
  
• In his Principia (1687), Sir Isaac Newton was the first to explain the major features of the tides as a combination of the effects of two separate movements of the water, one produced by the moon and a smaller due to the sun. This required an understanding that gravitation is proportional to mass and that the force of attraction decreases as the square of the distance between the bodies.
  
  
• In 1872 British scientist Lord Kelvin invented a machine that combined tidal theory with astronomical observations to provide the predictions needed for tide tables. Results from this machine and later versions were the basis for tide tables around the world until the late 1960s when digital computers became powerful enough to take over the job.
  
  
• Tides can be predicted more accurately than almost any other natural phenomenon on Earth. 95 per cent of the time tidal height can be predicted within an accuracy of one per cent a month ahead, and with an accuracy of five per cent 19 years ahead. Most of the variation in tides is linked to the position in space of the Earth, Sun and Moon, which can be predicted very accurately for long periods.
  
  
• An undulating bore is not an obnoxious partygoer but a series of waves propagating up a river as the tide rises. Under certain conditions the influx of the tide is held back by the river flow until a steep wave is formed which then proceeds upstream. The best known examples are in the Seine (France), the Severn (England), the Ch'ien-tang (China) where the bore can be 4.6 m high, and the Petitcodiac (Canada). A bore with a steep foaming crest is called a 'crashing bore'.
  
  
• The strength and location of ocean currents can be calculated from a map of sea surface elevation in the same way that winds can be calculated from a map of barometric pressure. The height of the ocean surface varies to create pressure gradients balancing the Coriolis force on water moving in ocean currents.
  
  
• Special satellites carry radar altimeters that can map the height of the sea surface with an accuracy of about four centimeters, to map changes in ocean currents. The first such satellite was Seasat, launched in 1978.
  
  
• Automatic internally recording current meters anchored to the seafloor have been successfully used to measure ocean currents since about 1967. Moorings that can operate unattended for a year are now common, but required development of technology such as acoustically commanded anchor releases, special mooring cable, and reliable deep flotation.
  
  
• In both the Atlantic and Pacific oceans north of the equator, the combined effects of the strong westerly winds (between 30 and 60° N) and the Northeast Trade Winds (between 10 and 20° N) cause the near-surface waters to circulate in a large, clockwise gyre between about 10° N and 40 to 60° N. Wind drag on the water is the primary driving force.
  
  
• Currents on the western sides of the oceans, such as the Gulf Stream and the Kuroshio, are narrow (100km) and strong (up to 9km/hr). Currents running west to east or east to west and those on the eastern sides of the oceans are broad (500 to 1,000 km) and slow (0.4km/hr). The intensification of currents on the western side of the oceans results from an increase in the Coriolis effect between the equator and the North Pole, caused by the Earth's spherical shape.
  
  
• If a parcel of ocean water is given a short strong 'push' by a quickly moving wind system it will move in circles for several days. This is similar to what happens when a person sitting on a swing is given a push, except that the water goes in circles rather than back and forth. The length of time to go around the circle depends on the latitude. It takes 12 hours at the poles, 24 hours at 30oN or S and takes infinitely long at the Equator.
  
  
• The largest current in the ocean is the Antarctic Circumpolar Current, which circles Antarctica. It is 24,000km long, up to 200km wide, extends from the surface to depths of 4,000m, and is estimated to carry about three million cubic kilometers of water per year.
  
  
• Since 1974 oceanographers have been able to measure water motion at mid-depths by using shore-based or anchored sound receivers to track floats drifting at depths of 500m to 2,000m. Each float makes a distinctive sound at frequencies near 'middle C' on the piano.
  
  
• Eddies 50km to 200km across, up to 4km deep, and taking from one month to a few months to rotate are common in the ocean, particularly near strong currents or boundaries between water masses. They travel a few kilometers a day, and tend to move westwards and/or towards the equator. Eddies can last for two or three years
  
  
• Ocean eddies can transport bodies of water with particular temperature and salinity characteristics from one part of the ocean to another. Eddies about 100km in diameter containing 'Mediterranean Sea Water' (13oC, 37.3 grams of salt/kg) from the Mediterranean outflow at Gibraltar have been found as far away as the Bahamas, and appear to persist for two or three years.
  
  
• Strong currents at boundaries between two water masses, like the Gulf Stream and Kuroshio, can develop large meanders that close off into loops. These loops or 'rings' then drift away from the main current as large eddies. 'Cold core rings' carry water from the cooler side of the current into the warm ocean on the other side, while 'warm core rings' do the opposite.
  
  
• Turbidity currents are underwater avalanches of mud, sand and water that flow down steep slopes. The November 1929 Gulf of St. Lawrence earthquake caused a turbidity current which ran for 700km at speeds of 40 to 55km/h, breaking transatlantic telephone and telegraph cables.
  
  
• On Dec. 30, 1972 the British ship Weather Reporter, in the North Atlantic at 50oN, 19oW, recorded a wave height of 26.2m. This is the highest instrumentally recorded wave.
  
  
• On July 9, 1958, a landslip from the land into the ocean caused a wave to wash 524m high along fjord-like Lituay Bay, Alaska.