Ocean Tides: Causes, Patterns, and Tidal Energy

Ocean tides are the regular rises and falls of sea levels. Tides are primarily the result of the gravitational effects of the moon and sun. The characteristics of tides vary depending on the time of day and year and the geographic region. Two daily high tides alternate with two daily low tides in most coastal regions. Tide height is influenced by the varying distance of the moon and the alignment of the moon with the sun. Tide patterns are influenced by the shape of the coastline, the depth of the ocean basin, the strength of the winds, barometric pressure, and other factors. People have made use of tides for practical purposes, ranging from sailing ships to harnessing tidal energy for electricity generation.

LUNAR, SOLAR, AND OTHER INFLUENCES ON TIDES. As tides rise and fall, there are associated horizontal currents of ocean water. Flood currents are tidal currents that move toward the coastline, and ebb currents are the tidal currents that move away from the coastline. Flood and ebb currents become strongest near the times of the high and low tides, respectively. The forces of the tides and currents depend on complex factors that influence the gravitational effects of the moon and sun. Because the sun has a much greater mass than the moon, its gravitational attraction to Earth is almost 180 times greater than that of the moon. However, because the sun is much farther away from Earth than the moon, its gravitational influence on tides is only about half that of the moon. Thus, the moon’s gravity is the dominant influence on tides.

The pull of the moon’s gravity on ocean water is strongest on the side of Earth that faces the moon, causing a high tidal bulge in the ocean on that side. On the opposite side of the planet, another tidal bulge forms as a result of inertia, a force that acts to counterbalance the force of gravity. As the moon orbits Earth and as Earth rotates, the positions of these bulges move to stay in alignment with the moon. The alignment of the sun also influences the positions and heights of the tidal bulges, but to a much lesser extent than the moon. High tides occur in association with tidal bulges. In most coastal areas, there are two tidal bulges, two high tides, and two low tides every lunar day—the twenty-four-hour, fifty- minute period during which a site on Earth’s surface rotates from a point beneath the moon to the same point again. There are twelve hours and twenty-five minutes between one high tide and the next, and six hours and twelve and a half minutes between a high tide and a low tide. Extreme high tides and extreme low tides (both known as “spring tides”) happen when Earth, the moon, and the sun are in direct alignment, coinciding with full and new moons. “Neap tides” occur when the sun and moon are positioned at right angles to each other, resulting in a moderating influence on tide height. The gravitational effects of the moon are further enhanced once a month when the varying orbital path of the moon brings it closest to Earth (a position called perigee).

There are different types of tidal patterns, which vary depending on the way the continental shapes affect water movement within each ocean basin. In a semidiurnal tidal pattern (such as in many areas along the US East Coast), the two daily high tides are about the same height, as are the two daily low tides. In a mixed semidiurnal tidal pattern (such as in many areas along the US West Coast), the two daily high tides vary in height, as do the two daily low tides. In a diurnal tidal pattern (such as in the Gulf of Mexico), which is the rarest type of pattern, there is only one high tide and one low tide once a day. Tide heights tend to be magnified in areas with wide continental margins and in funnel-shaped bays, while they are minimized around mid-oceanic islands and in estuaries with strong river flows. Strong offshore winds cause low tides to last longer as water is pushed away from coastlines, whereas strong onshore winds may prevent the development of low tides by continually piling water onto coastlines. High-pressure weather systems make low tides lower than normal as sea levels are depressed. Conversely, low-pressure systems make high tides higher than normal.

MONITORING, STUDYING, AND USING TIDES. People have studied and monitored tides and tidal currents for centuries for both practical reasons and basic scientific understanding. For early civilizations, understanding tides was important to maximize catches of desired fish species, because certain species are more abundant during flood tides and others are more numerous during ebb tides. Ship navigators have long studied the heights, directions, and speeds of tides and currents in order to help them maneuver their vessels. Although most important for sailing ships centuries ago, such knowledge remains important for modern powered vessels. Engineers designing docks, bridges, and other construction projects at sea must possess an understanding of tides and currents to make sure their projects can withstand the pressures of sea water.

Scientists use a variety of instruments to measure tides. Tidal stations—such as those operated by the National Oceanic and Atmospheric Administration (NOAA)—consist of electronic measuring equipment mounted on platforms at different coastal locations. Tidal height is measured by devices that transmit audio signals down through a tube and measure the time it takes for the reflected signals to return from the water surface. Other devices at tidal stations measure water current speed and direction, wind speed and direction, water and air temperature, and barometric pressure. These data are typically collected once every several minutes throughout the day and night, with the timing controlled by Geostationary Operational Environmental Satellites (GOES). As the data are collected, the information is relayed to the satellites, which then transmit it to NOAA headquarters for collection and analysis.

Tidal turbines are similar to underwater windmills that are used to generate electricity. The UK is generally considered the world leader in tidal turbine development.

FURTHER READING: Marine Current Turbines: An Atlantic Company. “Tidal Energy.” http://www.marineturbines.com/Tidal-Energy. Accessed November 15, 2016.

McCully, James Greig. 2006. Beyond the Moon: A Conversational, Common Sense Guide to Understanding the Tides. Hackensack, NJ: World Scientific Publishing Company.

National Oceanic and Atmospheric Administration, National Ocean Service Education Discovery Kits. 2005. “Welcome to Tides and Water Levels.” http://oceanservice.noaa.gov/education/kits/ tides/lessons/tides_tutorial.pdf. Accessed November 15, 2016.

Smithsonian National Museum of Natural History, Ocean Portal. 2016. “Currents, Waves, and Tides: The Ocean in Motion.” http://ocean.si.edu/ocean-news/currents-waves-and-tides-ocean -motion. Accessed November 15, 2016.