Waves, Currents, Tides, and Sea Level Changes
Waves, Currents, Tides, and Sea Level Changes
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Waves transfer energy from one place to another. Waves in the oceans (or large lakes) are caused by wind blowing over the surface of the water.
The size of a wave is determined by how far, how fast and how long the wind blows. A gentle breeze forms patches of tiny ripples on the surface of the water. Strong, steady winds over long distances create large waves. But even if you feel no wind at all, you may seer large waves caused by distant storms.
All waves have certain characteristics in common.
Or click here to see a diagram showing parts of a wave.
Things that float In the open ocean bob up and down when a wave passes, instead of moving along with the wave. This is because the waves are caused by energy travelling through water. Waves do not move the water along with them. As a wave arrives it lifts water particles. The water particles travel forward, then down and back. Each water particle completes a circle. Circling movements of water particles near the surface set off smaller circling movements below them.
The water particle moves in a vertical circle as the wave passes.
The particle moves forward with the wave crest, and backward with the wave trough.
Image courtesy of Office of Naval Research.
Click here to see diagram of circular motion of water particles decreasing with depth, and waves approaching a shore. From Water on the Move, Museum of Science.
Human activity along the coastline sometimes involves building structures to trap sand or to stop coastal erosion. Structures built perpendicular to the beach (i.e., sticking out into the water) will tend to trap sand transported by longshore drift on the up-current side. But unfortunately, once the flow of sand is interrupted, erosion will occur on the down-current side of the man-made structure, accelerating coastal erosion. This occurs because the waves have energy, but the sand is trapped. So the wave energy erodes more sand. In many cases, man-made structures along the coastline actulaly enhance coastal erosion (seawalls for example, or down-current from groins and jetties).
Sea Arch, Kilauea Volcano, Big Island of Hawaii
Video: Portrait of a Coast discusses the differences between winter and summer waves, and the problems of coastal erosion from winter storms, and attempts to prevent coastal erosion.
Video: The Beach - A River of Sand - discusses longshore drift, and how man building along the coast interferes with natural processes. A classic paper in Scientific American has aerial photos of many of the sites mentioned in the movie, and you may be able to locate it.I located this paper in a Scientific American book called Ocean Science, which is a compilation of articles.
Currents are the horizontal, unidirectional flow of water. Ocean currents influence the weather in coastal areas. In order to map and predict currents,scientists release floating buoys and track their positions.
The horizontal movement of water is caused by a number factors. These include:
There is a system of large-scale global ocean currents that form because of uneven heating of the Earth's surface by the Sun, (and the resulting wind patterns), and the Coriolis Effect. There is a circular pattern of currents known as gyres, centered in the north Atlantic Ocean, south Atlantic Ocean, north Pacific Ocean, south Pacific Ocean, and Indian Ocean. They are centered roughly around 30 degrees latitude. The currents in the gyres flow clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. (See image on this web page and here .) The Gulf Stream in the Atlantic Ocean, and its counterpart, the Kuroshio Current, in the Pacific Ocean are strong currents that are part of this system. These currents carry heat northward from the tropics to the higher latitudes.
Ocean currents carry heat from the tropics (near the equator) to higher latitudes (near the poles). This helps to maintain Earth's temperature. Sunlight warms sea surface in the tropics. Wind-driven surface currents carry the heat toward the poles. In the North Atlantic Ocean, the warm tropical currents feed the North Atlantic Current (see diagram). As the North Atlantic Current flows northward toward Norway and Greenland, it loses heat to the atmosphere and begins to cool. In winter, the water near Norway and Greenland is cold and dense that it sinks to the bottom of the ocean. The cold bottom water feeds bottom currents shown in blue and green. Eventually, mixing brings the bottom water back to the surface in other parts of the ocean. When the water reaches the surface, sunlight warms the water, and the cyclebegins again.
Deep circulation is important for two reasons: Cold water carries dissolved gases, including carbon dioxide, deep into the ocean and away from the atmosphere. Also, surface currents that sink and feed deep currents carry much more heat toward Europe than do surface currents. Because of this, northern European countries (such as Norway at 60o north latitude) are far warmer than southern Greenland and northern Labrador, which are at the same (or similar) latitudes.
Click here to see color brochure from NOAA with safety information about rip currents.
The elements of a rip current.
Image from NOAA.
There is a vertical rise and fall in sea level of approximately a meter or more, once or twice per day.
Cause of tides?
The Moon's gravitational pull, and to a lesser extent, the Sun's gravitational pull.
But gravity is only one of the major forces responsible for creating tides. Another is inertia, which is the force that acts to counterbalance gravity. It is the tendency of moving objects to continue moving in a straight line (see Newton's Laws in Gravity course notes). Together, gravity and inertia are responsible for the creating the two major tidal bulges on the Earth.
Click here to see a diagram.On the side of the Earth facing the moon, a tidal bulge occurs. On the opposite side of the Earth from the moon, the gravitational attraction of the moon is less because it is farther away. Here, the inertial force exceeds the gravitational force. Water tries to keep going in a straight line, moving away from the Earth, also forming a bulge.
A bulge in the water on the Earth's surface occurs on both the side facing the moon and the side away from the moon.
The combination of gravity and inertia create two tidal bulges of water. One forms where the Earth and moon are closest, and the other forms where they are furthest apart. Over the rest of the globe the two forces are in relative balance. Because water is fluid, the two bulges stay aligned with the Moon as the Earth rotates.
Tidal Cycle: A high and low tide that occur in succession.
Semi-diurnal Tide: The most common tidal pattern, featuring two high tides and two low tides each day. Successive high or low tides are approximately the same height.
Diurnal Tide: There is only one high tide and one low tide during each day. Successive high and low tides are approximately the same height. Diurnal tides occur in the Gulf of Mexico.
Mixed Tide: Wide variation in heights of successive high and low waters, and longer tide cycles than those of the semidiurnal cycle. Mixed tides occur along the west coast of the U.S. and on many Pacific islands.
See map of locations of semi-diurnal, diurnal, and mixed semidiurnal tides.
Tidal Range: The vertical difference in water level between the high tide and low tide, during one tidal cycle.
The shape of the shoreline can be influence tidal range.
When tidal bulges hit areas with wide continental shelves, the height of the tides can be magnified. Conversely, when tidal bulges hit mid-oceanic islands, tides are low, commonly 1 meter or less.
The shape of bays and estuaries along a coastline can also magnify the intensity of tides. Funnel-shaped bays dramatically alter tidal magnitude. The Bay of Fundy in Nova Scotia is the classic example of this effect.
The highest tides in the world occur in the Bay of Fundy in Nova Scotia, Canada. The tidal range is 15-16 meters (approximately 45-50 feet). Because there are 2 high tides and 2 low tides in each day (roughly a 24 hour period) - i.e. semidiurnal tides - then the tide must come in within about a 6 hour period. As a rough estimate, the tide rises about 8 feet (or 96 inches) an hour (48 feet divided by 6 hours). This translates to a tide which rises at more than one inch per minute. If you have walked down a beach with a steep cliff along side (which is common there), be sure to watch the tides. If you walk for about an hour and then notice that the tide is coming in, the water will be over your head before you get back to where you started!
High tide and low tide at Five Islands Provincial Park, Bay of Fundy, Nova Scotia, Canada
High tide and low tide at Five Islands Provincial Park, Bay of Fundy, Nova Scotia, Canada
Areas that are alternately submerged and exposed by rising and falling tides are called tidal flats.
Tides at certain times of the month are unusually lower or higher than at others. The reason for this has to do with the position of both the sun and the moon relative to the Earth. If all three are lined up in a straight line, the tides will have a higher tidal range. They are called spring tides. Spring tides occur twice a month, at full moon and at new moon.
But if the sun and moon are at right angles to one another, the tidal range will be lower. These tides are called neap tides. Neap tides occur twice a month, at first quarter and last quarter.
In addition, once a month when the Moon's orbit brings it closest to the Earth (which we call perigee), unusually high and low tides occur, and the tidal range is greater than normal.
About two weeks later, when the Moon's orbit takes it farthest from Earth (which we call apogee), the moon's gravitational force is smaller (because it is farther away), and the tidal ranges are less than average.
When the Earth is closest to the sun (which we call perihelion), around January 2 each year, tidal ranges are also much greater, with unusually high and unusually low tides.
When the Earth is farthest from the sun (which we call aphelion), around July 2 each year, tidal ranges are much less than average.
Tides are used in Nova Scotia (at Annapolis Royal) to generate electrical power (and also in France, Russia, and China).
Morer than 75 percent of the world's population lives within 60 km of a coast. Because of this, it is important to study the effects of any changes in sea level.
Sea level is caused by a combination of factors,the most important of which is climate change.
Climate changes toward global cooling lead to the onset of glaciation. Glaciation causes sea level to drop.
Climate changes toward global warming lead to the melting of the glaciers, which causes sea level to rise.
Sea level has risen on the order of 100 meters over the last 18,000 years, as a result of the end of the last Ice Age and the melting of the glaciers.
Mankind has contributed to sea level rise through the burning of fossil fuels and release of greenhouse gases like carbon dioxide. The greenhouse effect can cause the oceans to gather thermal inertia that will heat the continents and slowly melt the polar ice caps. This increases sea level worldwide.
Another factor causing global sea level rise is increased sea floor spreading rates and increase in undersea volcanism. Extrusion of lava on the sea floor along the mid-ocean ridges changes the volume of the ocean basins, and causes the displacement of sea water onto the land.
Sea level may also change locally due to localized subsidence or uplift of the coastline.
Subsidence of a coastline can also occur in an area where the water table is dropping as a result of humans interacting with the environment. Venice, Italy has experienced ground subsidence and a simultaneous rise in sea level. This has resulted in an average rise in relative sea level of 13 centimeters per century, going back to the second century A.D. Venice sunk at a faster rate during the 20th century (about 23 cm or 9 inches), in part due to an ill-planned industrial complex that pumped water from beneath the city from 1930 to 1970. The drop in the water table accelerated ground subsidence.
Sea level may also change in a localized area as a result of the deposition and compaction of sediment. This is most likely the cause of the subsidence in the Mississippi delta region.
Because of sea level increasing or decreasing, shorelines may be described as emergent coasts (coastal land is rising relative to sea level) or submergent coasts (coastal land is sinking relative to sea level; sea level is rising).
Global warming seems to be causing sea level to rise.
Sea level has been rising at a rate of 10-15 cm over the past 100 years. (That is equal to 1 to 1 1/2 cm of sea level rise in 10 years.) Recent research indicates that sea level is rising approximately 2mm/yr. That seems almost too slow to notice, but the rate appears to be accelerating.
Sea level rises has serious impacts on our beaches. A 1 cm rise in sea level erodes beaches about 1 m horizontally. In addition, rising sea levels lead to larger storm surges that quicken the rate of beach erosion. At the present rates of sea level rise, 70% of the world's beaches are eroding and retreating. If the rate of sea level rise continues to increase, the loss of beaches due to coastal erosion will increase.
Rising sea level also allows saltwater to penetrate farther inland and upstream, raising the salinity of both surface water and groundwater supplies. Fresh water is limited in supply and essential for life and also for most industrial processes. Rivers and groundwater can be contaminated with saltwater, becoming unavailable or unsuitable for life.
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Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA
Page created March 31 - April 1, 2005
