12.3: Current and Tides
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Water in the ocean, when moving, can move via waves, currents, and tides. Waves have been discussed in chapter 12.1, and this section will focus on the other two. Currents in the ocean are driven by persistent global winds blowing over the surface of the water and water density. They are part of the Earth’s heat engine in which solar energy is absorbed by the ocean water (remember the specific heat of water). The absorbed energy is distributed by ocean currents.
In the above figure, notice the large sub-circular currents in the northern and southern hemispheres in the Atlantic, Pacific, and Indian Oceans. These are driven by prevailing atmospheric circulation and are called gyres  and rotate clockwise in the northern hemisphere and counterclockwise in the southern hemisphere because of the Coriolis Effect (see Chapter 13). Currents flowing from the equator toward the poles tend to be narrow as a result of the Earth’s rotation and carry warm water poleward along the east coasts of adjacent continents. These are called western boundary currents, and they are key contributors to the local climate. The Gulf Stream and the Kuroshio currents in the northern hemisphere and the Brazil, Mozambique, and Australian currents in the southern hemisphere are such western boundary currents. Currents returning cold water toward the equator tend to be broad and diffuse along the western coasts of adjacent landmasses. These warm and cold currents affect nearby lands making them warmer or colder than other areas at equivalent latitudes. For example, the warm Gulf Stream makes Northern Europe much milder than similar latitudes in Northeastern Canada and Greenland. Another example is the cool Humboldt Current flowing north along the west coast of South America. This cold current limits evaporation in the ocean and contributes to the arid condition of the Atacama Desert .
Whether an ocean current moves horizontally or vertically depends on its density. The density of seawater is determined by factors such as temperature and salinity. Evaporation and the influx of freshwater from rivers also affect salinity and therefore the density of seawater. As the western boundary currents cool, some of the cool, denser water sinks to become the deep water of the oceans. Movement of this deep water is called the thermohaline circulation (thermo refers to temperature, haline refers to salinity) and connects the deep waters of all the world’s oceans. This can be best illustrated by the Gulf Stream. After the warm water within the current promotes much evaporation and the heat dissipates, the resulting water is much colder and saltier. As the denser water reaches the North Atlantic and Greenland, it begins to descend and becomes a deepwater current. This worldwide (connected) shallow and deep ocean circulation is sometimes referred to as the global conveyor belt .
The gravitational effects of the sun and moon on the oceans create tides, the rising and lowering of sea level during the day . The earth rotates daily within the gravity fields of the moon and sun. Although the sun is much larger and exerts a more powerful gravity, its great distance from earth means that the gravitational influence of the moon on tides is dominant. The magnitude of the tide at a given location, the difference between high and low tide (the tidal range), depends primarily on the configuration of the moon and sun with respect to the earth. When the sun, moon, and earth line up with each other at full moon or new moon, the tidal range is at a maximum. This is called spring tide. Approximately two weeks later when the moon and sun are at right angles with the earth, the tidal range is lowest. This is called a neap tide.
The earth rotates within the tidal envelope so we experience the rising and ebbing of the tide on a daily basis. Tides are measured at coastal locations and these measurements and tidal predictions based on them are published for those who depend on this information (e.g. this NOAA website) . The rising and falling of the tides (tidal patterns) as experienced at a given shore location are of three types, diurnal, semidiurnal, and mixed.
Diurnal tides go through a complete cycle once in each tidal day. Keeping in mind that the moon orbits around the Earth, a tidal day is the amount of time the Earth rotates to the same location of the Moon above the Earth, which (considering the movement of the bodies) consists of slightly longer than 24 hours. Semidiurnal tides go through the complete cycle twice in each tidal day with the tidal range typically showing some inequality in each cycle. Mixed tides are a combination of diurnal and semidiurnal patterns and show two tidal cycles per tidal day, but the relative amplitudes of each cycle and their highs and lows vary during the tidal month with a diurnal overprint. The pattern at a given shore location and the times of arrival of tidal phases are complicated and determined by the bathymetry (depth) of the ocean basins and continental obstacles in the way of the tidal envelope within which the earth rotates. Local tidal experts use tidal charts (indicated in the example above) based on daily observations to make forecasts for expected tides for the next few days.
Typical tidal ranges are on the order of 3 feet. Extreme tidal ranges occur where the tidal wave enters a restrictive zone. An example is the English Channel between Great Britain and the European continent where tidal ranges of 25 to 32 feet have been observed. The earth’s highest tidal ranges occur at the Bay of Fundy, the funnel-like bay between Nova Scotia and New Brunswick, Canada, where the average range is nearly 40 feet and extremes of around 60 feet have been observed. At these locations of extreme tidal range, a person who ventures out onto the seafloor exposed during ebb tide may not be able to outrun the advancing water during flood tide. Additional information on tides can be found at this NOAA website.
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