Sea
A sea is a large body of salt water that is surrounded in whole or in part by land.[1] More broadly, the sea (with the definite article) is the interconnected system of Earth's salty, oceanic waters—considered as one global ocean or as several principal oceanic divisions. The sea moderates Earth's climate and has important roles in the water cycle, carbon cycle, and nitrogen cycle. Although the sea has been travelled and explored since prehistory, the modern scientific study of the sea—oceanography—dates broadly to the British Challenger expedition of the 1870s.[2] The sea is conventionally divided into up to five large oceanic sections—including the IHO's four named oceans[3] (the Atlantic, Pacific, Indian, and Arctic) and the Southern Ocean;[4] smaller, second-order sections, such as theMediterranean, are known as seas.
Owing to the present state of continental drift, the Northern Hemisphere is now fairly equally divided between land and sea (a ratio of about 2:3) but the South is overwhelmingly oceanic (1:4.7).[5] Salinity in the open ocean is generally in a narrow band around 3.5% by mass, although this can vary in more landlocked waters, near the mouths of large rivers, or at great depths. About 85% of the solids in the open sea are sodium chloride. Deep-sea currents are produced by differences in salinity and temperature. Surface currents are formed by the friction of waves produced by the wind and by tides, the changes in local sea levelproduced by the gravity of the Moon and Sun. The direction of all of these is governed by surface and submarine land masses and by the rotation of the Earth (theCoriolis effect).
Former changes in the sea levels have left continental shelves, shallow areas in the sea close to land. These nutrient-rich waters teem with life, which provide humans with substantial supplies of food—mainly fish, but also shellfish, mammals, and seaweed—which are both harvested in the wild and farmed. The most diverse areas surround great tropical coral reefs. Whaling in the deep sea was once common but whales' dwindling numbers prompted international conservation efforts and finally a moratorium on most commercial hunting. Oceanography has established that not all life is restricted to the sunlit surface waters: even underenormous depths and pressures, nutrients streaming from hydrothermal vents support their own unique ecosystem. Life may have started there and aquaticmicrobial mats are generally credited with the oxygenation of Earth's atmosphere; both plants and animals first evolved in the sea.
The sea is an essential aspect of human trade, travel, mineral extraction, and power generation. This has also made it essential to warfare and left major cities exposed to earthquakes and volcanoes from nearby faults; powerful tsunami waves; and hurricanes, typhoons, and cyclones produced in the tropics. This importance and duality has affected human culture, from early sea gods to the epic poetry of Homer to the changes induced by the Columbian Exchange, fromViking funerals to Basho's haikus to hyperrealist marine art, and inspiring music ranging from the shanties in The Complaynt of Scotland to Rimsky-Korsakov's "The Sea and Sinbad's Ship" to A-mei's "Listen to the Sea". It is the scene of leisure activities including swimming, diving, surfing, and sailing. However, population growth, industrialization, and intensive farming have all contributed to present-day marine pollution. Atmospheric carbon dioxide is being absorbed in increasing amounts, lowering its pH in a process known as ocean acidification. The shared nature of the sea has made overfishing an increasing problem.
Definition[edit]
Further information: List of seas
Both senses of sea date to Old English; the larger sense has required a definite article since Early Middle English.[4] As the term has been applied over time, there are no sharp distinctions between seas and oceans, although seas are smaller and are—with the notable exception of the Sargasso Sea created by the North Atlantic Gyre[6](p90)—usually bounded by land on a smaller scale than multiple continents.[7] Seas are generally larger than lakes and contain salt water, but the Sea of Galilee is a freshwater lake.[8] There is no accepted technical definition of "sea" among oceanographers.[a] In international law, the United Nations Convention on the Law of the Sea states that all the ocean is "the sea".[12][b]
Physical science[edit]
Earth is the only known planet with seas of liquid water on its surface,[6](p22) although Mars possesses ice caps and similar planets inother solar systems may have oceans.[14] It is still unclear where Earth's water came from, but, seen from space, our planet appears as a "blue marble" of its various forms: oceans, ice caps, clouds.[15] Earth's 1,335,000,000 cubic kilometers (320,000,000 cu mi)[16] of sea contain about 97.2 percent of its known water[17][c] and cover more than 70 percent of its surface.[6](p7) Another 2.15% of Earth's water is frozen, found in the sea ice covering the Arctic Ocean, the ice cap covering Antarctica and its adjacent seas, and various glaciers and surface deposits around the world. The remainder (about 0.65% of the whole) form underground reservoirs or various stages of the water cycle, containing the freshwaterencountered and used by most terrestrial life: vapor in the air, the clouds it slowly forms, the rain falling from them, and the lakes and rivers spontaneously formed as its waters flow again and again to the sea.[17] The sea's dominance of the planet is such that the British author Arthur C. Clarke once noted that "Earth" would have been better named "Ocean".[6](p7)
The scientific study of water and Earth's water cycle is hydrology; hydrodynamics studies the physics of water in motion. The more recent study of the sea in particular isoceanography. This began as the study of the shape of the ocean's currents[22] but has since expanded into a large and multidisciplinary field:[23] it examines the properties ofseawater; studies waves, tides, and currents; charts coastlines and maps the seabeds; and studies marine life.[24] The subfield dealing with the sea's motion, its forces, and the forces acting upon it is known as physical oceanography.[25] Marine biology (biological oceanography) studies the plants, animals, and other organisms inhabiting marine ecosystems. Both are informed by chemical oceanography, which studies the behavior of elements and molecules within the oceans: particularly, at the moment, the ocean's role in the carbon cycle and carbon dioxide's role in the increasing acidification of seawater. Marine and maritime geography charts the shape and shaping of the sea, whilemarine geology (geological oceanography) has provided evidence of continental drift and the composition and structure of the Earth, clarified the process of sedimentation, and assisted the study of volcanism andearthquakes.[23]
Seawater[edit]
| Solute | ‰ of water (by mass) | % of total solutes | ||
|---|---|---|---|---|
| Chloride | 19 | .3 | 55 | .0 |
| Sodium | 10 | .8 | 30 | .6 |
| Sulfate | 2 | .7 | 7 | .7 |
| Magnesium | 1 | .3 | 3 | .7 |
| Calcium | 0 | .41 | 1 | .2 |
| Potassium | 0 | .40 | 1 | .1 |
| Bicarbonate | 0 | .10 | 0 | .4 |
| Bromides | 0 | .07 | 0 | .2 |
| Carbonate | 0 | .01 | 0 | .05 |
| Strontium | 0 | .01 | 0 | .04 |
| Borate | 0 | .01 | 0 | .01 |
| Fluoride | 0 | .001 | < 0 | .01 |
| All others | < 0 | .001 | < 0 | .01 |
Main article: Seawater
Seawater is invariably salty and, although its degree of saltiness (salinity) can vary, about 90% of the water in the ocean has 34–35 g(1.2 oz.) of dissolved solids per liter, producing a salinity between 3.4 and 3.5%.[27] To easily describe small differences, however, oceanographers usually express salinity as a millage (‰) or part per thousand (ppt) instead of using percents. The surface salinity of waters in the Northern Hemisphere are generally closer to the 34‰ mark, while those in the South are closer to 35‰.[5] The solutes in ocean water come both from inflowing river water and from the ocean floor.[28] The relative composition of the solutes is stable throughout the world's oceans:[26][29] sodium (Na) and chloride (Cl) make up about 85%. Other solutes include metal ions such asmagnesium (Mg) and calcium (Ca) and negative ions such as sulfate (SO₄), carbonate (CO₃), and bromides. In the absence of other pollution, seawater would not be harmful to drink except that it is much too saline;[d] similarly, it cannot be used for irrigating mostplants without being desalinated. For scientific and technical purposes, a standardized form of artificial seawater is often used.
Variations in salinity are caused by many factors: currents flowing between the seas; incoming freshwater from rivers and glaciers; precipitation; the formation and melting ofsea ice; and evaporation, which is in turn affected by temperature, winds, and waves. For example, the upper level of the Baltic Sea has a very low salinity (10 to 15‰) because the low temperatures of the surrounding climate produce minimal evaporation; it has many inflowing rivers; and its small connection to the North Sea tends to create a cold, dense under-layer that hardly mixes with the surface waters.[32] By contrast, the Red Sea lies between the Sahara and Arabian Deserts; it has high evaporation but little precipitation; it has few (and mostly seasonal) inflowing rivers; and its connection to other seas—the Suez Canal in the north and the Bab-el-Mandeb in the south—are both very narrow. Its salinity averages 40‰.[33] The Mediterranean is a little lower, at 37‰, while some landlocked lakes are much higher: The Dead Sea has 300 grams (11 oz) of dissolved solids per liter (300‰).
Sea temperature chiefly depends on the amount of solar radiation it absorbs. In the tropics where sunlight falls more directly, the temperature of the surface layers can rise to over 30 °C (86 °F); near the poles, the temperature is in equilibrium with the sea ice at its freezing point. Its salinity makes this lower than freshwater's, usually about −1.8 °C (28.8 °F). These temperature differences contribute to the continuous circulation of water through the sea. Warm surface currents cool as they move away from the tropics; as the water becomes denser, it sinks. The cold water in the deep sea moves back towards the equator before welling up again to the surface. Deep seawater has a temperature between −2 and 5 °C (28 and 41 °F) in all parts of the globe.[34] In freezing seas,ice crystals begin to form on the surface. These break into small pieces and coalesce into flat discs that form a thick suspension known as frazil. In calm conditions, frazil will freeze into a thin, flat sheet called nilas, which thickens as new ice forms in the sea beneath it. In turbulent waters, frazil instead join together into larger flat discs known as "pancakes". These slide over and under one another to form floes. During these processes, salt water and air are trapped amid the ice. Nilas forms with a salinity around 12–15‰ and is grayish in color but grows fresher over time: after a year, it is bluish and closer to 4–6‰ saline.[30][35]
The amount of light that penetrates the sea depends on the angle of the sun, the local weather, and the sea's turbidity. Of the light that reaches the surface of the sea, much of it is reflected at the surface and its red wavelengths are absorbed in the top few meters. Yellow and green reach greater depths, and the longer blue and violet wavelengths may penetrate as deep as 1,000 m (3,300 ft).
The amount of oxygen present in seawater depends primarily upon its temperature and the photosynthetic organisms living in it, particularly algae, phytoplankton, and plantssuch as seagrass. During the day, their photosynthetic activity produces oxygen, which dissolves into the seawater and is used by marine animals. The water's oxygen saturation is lower during the night and much lower in the deep sea. Below a depth of about 200 m (660 ft), there is insufficient light for photosynthesis[36] and consequentlylittle dissolved oxygen. Below this, anaerobic bacteria break down falling organic material, producing hydrogen sulfide (H₂S).[37] It is projected that global warming will reduce oxygen both in surface and deep waters, due to oxygen's decreased solubility as temperatures increase[38] and increased oceanic stratification.[39]
Waves[edit]
Ocean surface waves are oscillations caused by the friction from air moving across the surface of the water. This friction transfersenergy and forms surface waves in the water perpendicular to the direction of the wind. The top of a wave is known as its crest and its foot as its trough; the distance between two crests is the wavelength. These waves are mechanical: as they approach, the water molecules at a given point rise up and, as they pass, the water molecules go down, tracing a roughly circular path. The energy is passed across the surface and does not represent a horizontal motion of the water itself. The sea state of the ocean is determined by the size of these waves, which—on the open ocean—depends upon the wind speed and the fetch, the distance over which the wind blows upon the water. The smallest waves are called ripples. As strong and prolonged winds push against ripples' raised crests, larger and more irregular waves form, which known as seas. These waves reach their maximum height when the rate at which they are traveling nearly matches the speed of the wind and, over time, they naturally separate[e] into long, powerful waves with a common direction and wavelength. These swells are particularly common in the Roaring Forties of the Southern Hemisphere where the wind blows continuously.[40][41] When the wind dies down, ripples easily disappear owing to water's surface tension, but seas and swells are only slowly reduced by gravity anddestructive interference from other waves.[40] Constructive interference, however, can also cause individual rogue waves much higher than normal.[42] Most waves are less than 3 m (10 ft) high[42] and it is not unusual for strong storms to double or triple that height;[43] offshore construction such as wind farms and oil platforms use these measurements in computing the hundred-year wave they are designed against.[44]Rogue waves, however, have been documented at heights above 25 meters (82 ft).[45][46]
As waves approach land and move into shallow water, they change their behavior. If approaching at an angle, waves may bend or wrap rocks and headlands. When the wave reaches a point where its deepest oscillating molecules contact the seabed, friction begins to slow the wave down. This pulls the crests closer together and increases the waves' height. When the ratio of a wave's height to its wavelength exceeds 1:7, it "breaks", toppling over in a mass of foaming water.[42] This rushes in a sheet up the beach before retreating into the sea under the influence of gravity.[40]
A tsunami is an unusual form of wave caused by a sudden and powerful event such as an underwater earthquake or landslide, ameteorite impact, a volcanic eruption, or a collapse of land into the sea. These events can temporarily lift or lower the surface of the sea in the affected area, usually by a few feet. The potential energy of the displaced seawater is turned into kinetic energy, creating a shallow wave radiating outwards at a velocity proportional to the square root of the depth of the water. Tsunamis, therefore, travel much faster in the open ocean than on a continental shelf.[48] Despite traveling at speeds of over 600 mph (970 km/h),[49] tsunamis in deep seas have wavelengths from 80 to 300 miles (130 to 480 km) and an amplitude of less than three feet.[50] Standard surface waves in the same region may only have wavelengths of a few hundred feet and speeds up to 65 mph (105 km/h) but, when compared to their possible amplitudes of up to 45 ft (14 m), tsunamis at this stage are often able to pass unnoticed.[50] Tsunami warning systems rely on the fact that seismic waves caused by earthquakes travel around the world at around 14,400 kilometers (8,900 mi) per hour, allowing threatened regions to be alerted to the possibility of a tsunami.[51] Measurements from a network of sea-level measuring stations make it possible to confirm or cancel a tsunami warning.[52] A trigger event on the continental shelf may cause a local tsunami on the land side and a distant tsunami that travels out across the ocean. The energy of the wave is dissipated only gradually but is spread out over the wave front. As the wave radiates away from the source, the front gets longer and the average energy reduces, so distant shores will generally be hit by weaker waves. However, as the speed of the wave is controlled by the water depth, it does not travel at the same speed in all directions and this affects the direction of the wave front. This effect, known as refraction, can focus the strength of an advancing tsunami on some areas while weakening it in others, according to the undersea topography along its path.[53][54]
Just as with other waves, moving into shallow water causes the tsunami to slow but grow in height.[50] Either the trough or the crest of the tsunami can arrive at the coast first.[48] In the former case, the sea draws back and leaves subtidal areas unusually exposed.[55] When the crest arrives, it does not usually break but rushes inland, flooding all in its path. Much of the disaster's destruction can be produced by these flood waters, which drain back into the sea while pulling people and debris along. Several tsunamis can be caused by a single geological event. In such cases, it is common for the later waves to arrive between eight minutes and two hours after the first, which may not be the biggest or most destructive.[48] Occasionally, in a shallow bay or estuary, a tsunami may transform into a bore.[49]
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