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Oceanography: Bottom topography of the Atlantic, Indian and Pacific Oceans; Temperature and salinity of the oceans; Heat and salt budgets, Ocean deposits; Waves, currents and tides; Marine resources: biotic, mineral and energy resources; Coral reefs, coral bleaching; sea-level changes; law of the sea and marine pollution.
Oceanography also known as oceanology, is the study of the physical and the biological aspects of the ocean. It is an Earth science covering a wide range of topics, including ecosystem dynamics; ocean currents, waves, and geophysical fluid dynamics; plate tectonics and the geology of the sea floor; and fluxes of various chemical substances and physical properties within the ocean and across its boundaries.
The study of oceanography is divided into these four branches
Biological oceanography, or marine biology, investigates the ecology of marine organisms in the context of the physical, chemical, and geological characteristics of their ocean environment and the biology of individual marine organisms.
Chemical oceanography and ocean chemistry, are the study of the chemistry of the ocean. Whereas chemical oceanography is primarily occupied with the study and understanding of seawater properties and its changes, focuses ocean chemistry primarily on the geochemical cycles.
Geological oceanography, or marine geology, is the study of the geology of the ocean floor including plate tectonics and paleoceanography.
Physical oceanography , or marine physics, studies the ocean's physical attributes including temperature-salinity structure, mixing, surface waves, internal waves, surface tides, internal tides, and currents.
Why we should study oceanography?
Oceanography is important to study because the worlds oceans are critically important for sustaining life on Earth. Maintaining ecological balance in the words ocean helps insure we are keeping the worlds carbon levels in check. A large percentage of the worlds oxygen comes from the oceans. If ecological balance should get tipped in the wrong direction it could be detrimental to sustaining life on Earth. Economic Importanceresources, fish, oil and gas. Future medical compounds.
Environmental Reasons
Oceans lay major role in controlling climate71% of earths surfaces is covered by...Ocean
Purpose of ocean
First, it is a large reservoir of chemicals that can contribute to the greenhouse effect in the atmosphere and energy absorbing 90% of the solar radiation
which hits the surface. This reservoir changes very slowly limiting how fast the climate can change.
Second, it works with the atmosphere to redistribute the energy received from the sun such that the heat in the topics, where a lot of energy is received from the sun, is transferred toward the poles, where heat is generally lost to space.
Hydrosphere
A hydrosphere is the total amount of water on a planet. The hydrosphere includes water that is on the surface of the planet, underground, and in the air. A planet's hydrosphere can be liquid, vapor, or ice.
It includes the oceans, seas, lakes, ponds, rivers and streams. The hydrosphere covers about 70% of the surface of the Earth and is the home for many plants and animals.
The hydrosphere, like the atmosphere, is always in motion. The motion of rivers and streams can be easily seen, while the motion of the water within lakes and ponds is less obvious. Some of the motion of the oceans and seas can be easily seen while the large-scale motions that move water great distances such as between the tropics and poles or between continents are more difficult to see.
These types of motions are in the form of currents that move the warm waters in the tropics toward the poles, and colder water from the polar regions toward the tropics. These currents exist on the surface of the ocean and at great depths in the ocean (up to about 4km).
The characteristics of the ocean which affects its motion
Temperature Salinity
Movement of water
Warm water is less dense or lighter and therefore tends to move up toward the surface, while colder water is more dense or heavier and therefore tends to sink toward the bottom.
Salty water is also more dense or heavier and thus tends to sink, while fresh or less salty water is less dense or lighter and thus tends to rise toward the surface.
The combination of the water's temperature and salinity determines whether it rises to the surface, sinks to the bottom or stays at some intermediate depth.
The oceans currents are also affected by the motion of the atmosphere, or winds, above it. The energy in the wind gets transferred to the ocean at the ocean surface affecting the motion of the water there. The effect of wind is largest at the ocean surface.
Hydrological cycle
Distribution of Earth's Water
In the first bar, notice how only 2.5% of Earth's water is freshwater - the amount needed for life to survive.
The middle bar shows the breakdown of freshwater. Almost all of it is locked up in ice and in the ground. Only a little more than 1.2% of all freshwater is surface water, which serves most of life's needs.
The right bar shows the breakdown of surface freshwater. Most of this water is locked up in ice, and another 20.9% is found in lakes. Rivers make up 0.49% of surface freshwater. Although rivers account for only a small amount of freshwater, this is where humans get a large portion of their water from.
Look at the globe to the right. Notice the tiny bubble hovering over Georgia, USA. This ball of water represents all of the freshwater in lakes and rivers. The water in that bubble has the huge responsibility of meeting most of the needs of humans and animals.
The water cycle, also known as the hydrologic cycle, is the process by which water moves from place to place above, on, and below the Earth's surface. This is the process by which water moves around the Earth to different places.
Evaporation is the process by which water is converted from its liquid state to the gaseous state, also known as water vapor. In other words, water leaves the Earth's surface and enters the atmosphere as a gas.
Condensation is the process by which water vapor is changed back into liquid water. Condensation is very important to our weather and climate because it is what is responsible for cloud formation.
Without clouds, we would not get to the third phase, called precipitation, which we will talk about in a minute. Clouds form when water vapor condenses around small particles, like bits of dust or smoke in the air. Depending on the size of the drops, these particles may or may not be visible. Even on a clear, cloudless day, water vapor is always present in the atmosphere, but it does vary in amounts. We know it is present on a very humid day; it often feels like we need to swim through the air! Fog is condensation near the ground
Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. It is the primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth. Most precipitation falls as rain.
Relief of ocean
Ocean bottoms are not plain as believed earlier; they reveal many complex and varied features which rival the relief features on land. The oceans are confined to the great depressions of the earth’s outer layer. The oceans,
unlike the continents, merge so naturally into one another that it is hard to distinguish them. The geographers have divided the oceanic part of the earth into four oceans, namely the Pacific, the Atlantic, the Indian and the Arctic.
Divisions of the Ocean Floors
The ocean floors can be divided into four major divisions(i) the Continental Shelf(ii) the Continental Slope(iii) the Deep-Sea Plain(iv) the Oceanic Deeps.
Besides, these divisions there are also major and minor relief features in the ocean floors like ridges, hills, seamounts, guyots, trenches, canyons, etc.
The Continental Shelf A continental shelf is the edge of a continent that lies under the ocean. This is, in fact, the seaward extension of the continent from the shoreline to the continental edge marked, approximately, by the 100 fathom (600 feet) isobaths.
It is the shallowest part of the ocean showing an average gradient of 1° or even less. The shelf typically ends at a very steep slope, called the shelf break. The width of the continental shelves vary from one ocean to another.
The average width of continental shelves is about 80 km. The shelves are almost absent or very narrow along some of the
margins like the coasts of Chile, the west coast of Sumatra, etc. On the contrary, the Siberian shelf in the Arctic Ocean, the largest in
the world, stretches to 1,500 km in width. The depth of the shelves also varies. It may be as shallow as 30 m in some areas while in some areas it is as deep as 600 m.
The continental shelves are covered with variable thicknesses of sediments brought down by rivers, glaciers, wind, from the land and distributed by waves and currents.
Massive sedimentary deposits received over a long time by the continental shelves, become the sourceof fossil fuelsh
There are various types of shelves based on different sediments of terrestrial origin —
1. glaciated shelf (Surrounding Greenland),2. coral reef shelf (Queensland, Australia),3. shelf of a large river (Around Nile Delta),
4. shelf with dendritic valleys (At the Mouth of Hudson River)5. shelf along young mountain ranges (Shelves between Hawaiian
Islands).
Importance of continent shelves
1. Marine food comes almost entirely from continental shelves;2. They provide the richest fishing grounds;3. They are potential sites for economic minerals [20% of the world
production of petroleum and gas comes from shelves. Polymetallic nodules (manganese nodules; concentric layers of iron and manganese hydroxides) etc. are good sources of various mineral ores like manganese, iron copper, gold etc..]
Continental Slope
The continental slope connects the continental shelf and the ocean basins. It begins where the bottom of the continental shelf sharply drops off into a
steep slope. The gradient of the slope region varies between 2-5°. The depth of the slope region varies between 200 and 3,000 m. The seaward edge of the continental slope loses gradient at this depth and
gives rise to continental rise. The continental slope boundary indicates the end of the continents. Canyons and trenches are observed in this region.
Deep Sea Plain Deep sea plains are gently sloping areas of the ocean basins. These
are the flattest and smoothest regions of the world. The depths vary between 3,000 and 6,000m. These plains
are covered with fine-grained sediments like clay and silt.
The Deep-Sea Abyssal Plain.
⇒ This is the undulating plain lying two to three miles below sea level, and covering two-thirds of the ocean floor, generally termed the abyssal plain.
⇒ It was once thought to be featureless, but modern sounding devices reveal that the abyssal plain is far from being level.
⇒ These flat abyssal plains occur at depths of over 6,500 ft (1,980 m) below sea level. They are underlain by the oceanic crust, which is predominantly basalt—a dark, fine-grained volcanic rock. Typically, the basalt is covered by layers of sediments, much of which is deposited by deep ocean turbidity currents (caused by the greater density of sediment-laden water), or biological materials, such as minute shells of marine plants and animals, that have "rained" down from the ocean's upper levels, or a mixture of both.
⇒ It has extensive submarine plateaux, ridges, trenches, basins, and oceanic islands that rise above sea level in the midst of oceans, e.g. the Azores, Ascension Island.
⇒ Although these are more common and widespread in the Atlantic and Indian ocean basins than in the Pacific, abyssal plains are found in all major ocean basins. Approximately 40% of our planet's ocean floor is covered by abyssal plains. The remainder of the ocean floor topography consists of hills, cone-shaped or flat-topped mountains, deep trenches, and mountain chains such as the mid-oceanic ridge systems.
Oceanic Deeps or TrenchesThese areas are the deepest parts of the oceans. The trenches are relatively steep sided, narrow basins. They are some 3-5 km deeper than the surrounding ocean floor. They occur at the bases of continental slopes and along island arcs and are associated with active volcanoes and strong earthquakes. That is why they are very significant in the study of platemovements. As many as 57 deeps have been explored so far; of which 32 are in the Pacific Ocean; 19 in the Atlantic Ocean and 6 in the Indian Ocean.
Trench Location Depth Notes
Mariana Trench
Western Pacific Ocean(near Philippines and Japan)
10,911 m
Deepest known part of the ocean Lowest elevation on the surface of the earth’s crust Maximum depth is recordedat Challenger Deep, a small valley at its southern end Formed by the subduction of Pacific plate under Mariana plate
Tonga TrenchSouthern Pacific(near New Zealand)
10,882 m
Formed by subduction of Pacific plate under Tonga plate and Indo-Australian plateFastest plate velocity recorded on earth (24 cm per year)
Kuril-Kamchatka Trench Northern Pacific 10,542
m
Philippine Trench Philippines(Pacific Ocean)
10,540 m
Kermadec Trench New Zealand(Pacific Ocean)
10,047 m
Minor Relief Features
Apart from the above mentioned major relief features of the ocean floor, some minor but significant features predominate in different parts of the oceans.
Mid-Oceanic Ridges
A mid-oceanic ridge is composed of two chains of mountains separated by a large depression. The mountain ranges can have peaks as high as 2,500 m and some even reach above the ocean’s surface. Iceland, a part of the mid- Atlantic Ridge, is an example.
Seamount
It is a mountain with pointed summits, rising from the seafloor that does not reach the surface of the ocean. Seamounts are volcanic in origin. These can be 3,000-4,500 m tall. The Emperor seamount, an extension of the Hawaiian Islands in the Pacific Ocean, is a good example.
Submarine Canyons
These are deep valleys, some comparable to the Grand Canyon of the Colorado river. They aresometimes found cutting across the continental shelves and slopes, often extending from the mouths of large rivers. The Hudson Canyon is the best known submarine canyon in the world.
Guyots
It is a flat topped seamount. They show evidences of gradual subsidence through stages to become flat topped submerged
mountains. It is estimated that more than 10,000 seamounts and guyots exist in the Pacific Ocean alone.
Atoll
These are low islands found in the tropical oceans consisting of coral reefs surrounding a central depression. It may be a part of the sea (lagoon), or sometimes form enclosing a body of fresh, brackish, or highly saline water.
Significance of Study of Oceanic Relief
Ocean relief controls the motion of sea water. The oceanic movement in the form of currents, in turn, causes many
variations in both oceans and in atmosphere. The bottom relief of oceans also influences navigation and fishing.
Temperature of Ocean Water
Parts of the oceanThe top part of the ocean is called the surface layer. Then there is a boundary layer called the thermocline. The thermocline separates the surface layers and the deep water of the ocean. The deep ocean is the third part of the ocean.
The Sun hits the surface layer of the ocean, heating the water up. Wind and waves mix this layer up from top to bottom, so the heat gets mixed downward too. The temperature of the surface waters varies mainly with latitude. The polar seas (high latitude) can be as cold as -2 degrees Celsius (28.4 degrees Fahrenheit) while the Persian Gulf (low latitude) can be as warm as 36 degrees Celsius (96.8 degrees Fahrenheit). Ocean water, with an average salinity of 35 psu, freezes at -1.94 degrees Celsius (28.5 degrees Fahrenheit). That means at high latitudes sea ice can form. The average temperature of the ocean surface waters is about 17 degrees Celsius (62.6 degrees Fahrenheit).
90 % of the total volume of ocean is found below the thermocline in the deep ocean. The deep ocean is not well mixed. The deep ocean is made up of horizontal layers of equal density. Much of this deep ocean water is between 0-3 degrees Celsius (32-37.5 degrees Fahrenheit)! It's really, really cold down there!
There is a neat program that is measuring the temperature and salinity of ocean surface waters around the world. The Argo program deploys floats that measure salinity and temperature throughout the surface layer of the ocean. Over 3,000 free-drifting floats have been deployed all over the ocean and each float is programmed to sink 2,000 meters down, drifting at that depth for about 10 days. The float then makes its way to the surface measuring temperature and salinity the whole time. Data is transmitted to a satellite once the float reaches the surface, so that scientists and the public have access to the state of the ocean within hours of the data collection. At a greater depth in the water, measurements are often made with a CTD instrument (CTD = conductivity, temperature, depth), where the instrument is placed in the ocean water from a ship or a platform. These instruments are used by the Bermuda Institute of Ocean Sciences (BIOS), where they have been tracking ocean measurements like temperature, salinity and oxygen concentrations for over 55 years.
The Argos and BIOS program have both published results that confirm that the ocean is warming. Surface water temperatures obviously change from season to season and year to year, but the whole ocean has
warmed about 0.1 degree Fahrenheit (0.055 degree Celsius) in the past 30-50 years. This may not seem like much of a temperature change, but it is significant. Think about a pot of water heating on a stove. A small pot of water will heat quickly, while a large pot of water at the same heat setting will heat very slowly. This is due to a difference in heat capacity. The ocean has an enormous heat capacity because of its large size. So it is like an enormous pot of water, and it takes a great amount of heat to warm the ocean. The fact that the ocean has warmed significantly in 30 to 50 years is remarkable and concerning.
Salinity of ocean water
Salinity is defined as the ratio between the weight of the dissolved materials and the weight of the sample sea water. Generally, salinity is defined as ‘the total amount of solid material in grams contained in one kilogram of sea water and is expressed as part per thousand (%o) e.g., 30%o (means 30 grams of salt in 1000 grams of sea water).
Role of Ocean Salinity
Salinity determines compressibility, thermal expansion, temperature, density, absorption of insolation, evaporation and humidity.
It also influences the composition and movement of the sea: water and the distribution of fish and other marine resources.
Share of different salts is as shown below—
sodium chloride — 77.7% magnesium chloride—10.9% magnesium sulphate —.4.7% calcium sulphate — 3.6% potassium sulphate — 2.5%
Factors affecting salinity
Evaporation Higher the rate of evaporation, higher is salinity. The Highest evaporation has been recorded along the tropic of Cancer and that is one of the reasons that region of Red Sea and Persian Gulf has one of the highest salinity. Another reason is that enclosed seas tend to have more salinity in their water.
Temperature There is a direct relationship between ocean temperature and salinity. So the warmer parts are more saline and frigid parts are less saline.
Precipitation Precipitation is inversely related to salinity. Higher is the precipitation, lower is the proportion of salinity. The equatorial region records highest rainfall and that it is why it has low salinity in comparison to those which are near to tropics.
Influx of Freshwater Low salinity will be found at the mouth of rivers. This salinity is minimum in the raining season.
Atmospheric Pressure High pressure areas have high salinity and vice versa. Circulation of Ocean water Ocean currents play a major role in distribution of salinity
Distribution of Salinity:The average salinity in the oceans and the seas is 35%o but it spatially and temporally varies in different oceans, seas, and lakes. The variation in
salinity is both horizontal and vertical (with depth). Salinity also varies from enclosed seas through partially closed seas to open seas.
Thus, the spatial distribution of salinity is studied in two ways e.g.:(1) Horizontal distribution and
(2) Vertical distribution.
1. Horizontal Distribution:Horizontal distribution of oceanic salinity is studied in relation to latitudes but regional distribution is also considered wherein each ocean is separately described. Similarly, the pattern of spatial distribution of salinity in enclosed seas, partially enclosed seas and open seas is also considered.
(i) Latitudinal distribution:On an average, salinity decreases from equator towards the poles. It may be mentioned that the highest salinity is seldom recorded near the equator though this zone records high temperature and evaporation but high rainfall reduces the relative proportion of salt. Thus, the equator accounts for only 350/00 salinity.The highest salinity is observed between 200-400N (360/00) because this zone is characterized by hi0gh temperature, high evaporation but significantly low rainfall. The average salinity of 350/00 is recorded between 100-300 latitudes in the southern hemisphere. The zone between 400-600 latitudes in both the hemispheres records low salinity where it is 310/00 and 330/00 in the northern and the southern hemispheres respectively.Salinity further decreases in the polar zones because of influx of melt-water. On an average, the northern and the southern hemispheres record average salinity of 340/00 and 350/00 respectively.On the basis of latitudinal distribution of salinity four zones of oceanic salinity may be identified e.g.:(i) Equatorial zones of relatively low salinity (due to excessive rainfall),
(ii) Tropical zone (200-300) of maximum salinity (due to low rainfall and high evaporation),(iii) Temperate zone of low salinity, and
(iv) Sub-polar and polar zone of minimum salinity.
It may be pointed out that the marginal areas of the oceans bordering the continents have lower salinity than their central parts because freshwater is added to the marginal areas through the rivers. The salinity varies in the open seas according to the latitudes though it depends on the ocean currents but there is no control of latitudes on the distribution of salinity in the inland seas. Salinity of partially enclosed seas in the higher latitudes is seldom controlled by latitudes rather it depends on influx of melt-water.
This is why the Baltic Sea records comparatively lower salinity than the North Sea though the latitudinal extent of both the seas is the same. Table 26.2 presents latitude- wise distribution of oceanic salinity in both the hemi-spheres.
(ii) Regional distribution of surface salinity of the oceans and the seas is described in two ways viz:.(a) distribution of salinity in individual oceans and
(b) salinity zones of all the oceans together.
Jenkins has divided the oceans on the basis of salinity variations into three categories as follows:(A) Seas having salinity above normal:(a) Red Sea (34-410/00),(b) Persian Gulf (37-380/00), and(c) Mediterranean Sea (37-390/00).(B) Seas having normal salinity:(a) Caribbean Sea and Gulf of Mexico 35-360/00,(b) Bass Strait (350/00), and(c) Gulf of California (25-35.50/00).
(C) Seas having salinity below normal:(a) Slightly less:(i) Arctic Ocean (20-350/00),(ii) North Australian Sea(33-340/00),(iii) Bering Sea (28-330/00),(iv) Okhotsk Sea (30-320/00),(v) Japan Sea (30-340/00),(vi) China Sea (25-350/00),(vii) Andaman Sea (30-320/00),(viii) North Sea (31-350/00),(ix) English Channel (32-350/00), and(x) Gulf of St. Lawrence (30-320/00);(b) Much below:(i) Baltic Sea (3-150/00), and(ii) Hudson Bay (3-150/00).Pacific Ocean:There is wide range of salinity difference in the Pacific Ocean because of its shape and larger areal extent (fig. 26.1). Salinity remains 34.850/00near the equator. It increases to 350/00between 150-200 latitudes in the northern hemisphere but it becomes still higher (360/00) in the southern Pacific Ocean between the same latitudes.Salinity again decreases further northward in the western parts of the Pacific where it becomes 310/00in the Okhotsk Sea and 340/00 near Manchuria because of influx of melt water brought by the Oyashio current coming from the Bering Strait and due to weakening of Kuroshio warm current.Salinity also decreases along the Californian, Middle American and Peruvian Coasts due to transfer of water and upwelling of cold water from below. Just to the south of high salinity zone (between 150-200S) in the southern Pacific as referred to above (360/00) it becomes low along the Peruvian and Chilean coasts (330/00). Low salinity is noted infront of river mouths (Yellow River = 300/00, and Yangtzekiang = 330/00).
Atlantic Ocean:The average salinity of the Atlantic Ocean is 35.670/00. The highest salinity is not observed at the equator rather it is recorded between 150-200 latitudes. Salinity recorded at 50N, 150N and 150S as 34.980/00,360/00 and 37.770/00 respectively indicates increasing trend of salinity from equator towards the tropics of Cancer and Capricorn. The central zone of the North Atlantic Ocean located between 200N and 300N and 200W-600W records maximum salinity (370/00) and it gradually decreases further northward but with varying trends.The eastern marginal areas of the North Atlantic beyond 400 latitude record comparatively higher salinity than the western margin (east American coast ) because the Gulf Stream carries saline water from the American coast !o the north-western European coast. Maximum salinity of 370/00 in the southern Atlantic is found in a region demarcated by 120S-200S latitudes and 400W-150W longitudes.Salinity, thereafter, gradually decreases southward. It is apparent from fig. 26.2 that salinity is higher along the western margin than the eastern margin between 100-300 in the South Atlantic because of upwelling of water along the African coast. Comparatively low salinity is found in front of river mouths, St. Lawrence 310/00. Amazon 150/00,Congo 340/00 Niger 0/00 Senegal 340/00, Rhine 320/00 etc.
The pattern of spatial distribution of salinity is quite different in the partially enclosed seas of the Atlantic Oceans. The North Sea in spite of its location in higher latitudes records 34%o salinity due to more saline water brought by the North Atlantic Drift. Baltic Sea, on the other hand, records low salinity due to influx of river water. Further northward salinity con-tinues to decrease as it becomes 7 to 80/00 around Rugen Island. It becomes as low as 20/00 in the Gulf of Bothnia due to influx of freshwater.Salinity of 8 to 110/00 is recorded to the south of Sweden (around Bornholm in Baltic Sea). The Mediterranean Sea records high salinity due to evaporation and little mixture of Atlantic water. Salinity increases from the western part of the Mediterranean Sea (36.50/00) to the eastern part (390/00) but it is remarkably reduced to 17-180/00 in the Black Sea due to enormous volume of freshwater brought by the Dneiper, the Danube etc. There is high salinity in the Gulf of Mexico (360/00) and the Caribbean Sea due to more saline water brought by the north equatorial current.Indian Ocean:
The spatial distribution of salinity in the Indian Ocean is more variable and complex than the Pacific and Atlantic oceans. An average salinity of 350/00 is found between 00-100N but it gradually decreases northward in the Bay of Bengal (33.50/00, at 100N lat to 300/00 at the mouth of the Ganga) because of influx of immense volume of freshwater brought by the Ganga river.On the other hand, the Arabian Sea records higher salinity (360/00) than the Bay of Bengal because there is higher rate of evaporation due to relatively less humid conditions and low influx of freshwater as compared to the Bay of Bengal. The western coast of Australia records higher salinity due to dry weather.The partially enclosed seas have higher salinity e.g. it is 370/00 at the head and 400/00 in the interior of the Persian Gulf. The Red Sea records the highest salinity (varying between 360/00 and 410/00 in its different parts) because of low precipitation and very high evaporation.It may be mentioned that spatial distribution of surface salinity of the oceans and the seas is represented by isohalines which are the lines that join the places of equal salinity at the sea surface (on the map).
Inland Seas and Lakes:The amount of salt in the inland seas and lakes is controlled by the rate of evaporation, temperature, influx of river water and the presence or absence of outlets. Wherever a river comes out of a lake or inland sea, salinity is reduced because salt is taken out of the water bodies by the river. The influx of fresh water brought by the river into the lakes and inland seas also lowers down the salinity.
For example, low salinity of the northern part of Caspian Sea (140/00) is because of addition of enormous volume of water brought by the rivers like Volga, Ural etc. but it becomes as high as 1700/00 in the southern part i.e. the Gulf of Karabugas. Very high salinity is found in Great Salt lake0/00, Utah, USA), Red Sea (2400/00), Lake Van (3300/00, Turkey), Dead Sea (2380/00) etc.2. Vertical Distribution:No definite trend of distribution of salinity with depth can be spelt out because both the trends of increase and decrease of salinity with increasing depths have been observed. For example, salinity at the southern boundary
of the Atlantic is 330/00 at the surface but it increases to 34.50/00 at the depth of 200 fathoms (1200 feet).It further increases to 34.75% at the depth of 600 fathoms. On the other hand, surface salinity is 370/00 at 20°S latitude but it decreases to 350/00 at greater depth.The following characteristics of vertical distribution of salinity may be stated:1. Salinity increases with increasing depth in high latitudes i.e. there is positive relationship between the amount of salinity and depth because of denser water below.
2. The trend of increase of salinity with increasing depths is confined to 200 fathoms from the surface in middle latitudes beyond which it decreases with increasing depths. Salinity is low at the surface at the equator due to high rainfall and transfer of water through equatorial currents but higher salinity is noted below the water surface. It again becomes low at the bottom. More studies and data of salinity distribution at regular depths in different oceans and seas are required so that definite characteristic features of vertical distribution of salinity may be determined.
3. Maximum salinity is found in the upper layer of the oceanic water. Salinity decreases with increasing depth. Thus, the upper zone of maximum salinity and the lower zone of minimum salinity is separated by a transition zone which is called as thermocline zone, on an average above which high salinity is found while low salinity is found below this zone. It may be remembered that this should not be taken as a general rule because the vertical distribution of salinity is very complicated.
4. It may be mentioned that the depth zone of oceans between 300m and 1000m is characterized by varying trends of vertical distribution of temperature (fig. 25.7), density of seawater (fig. 25.8), and salinity of ocean water (fig. 25.10).
This zone is characterized by rapid change of seawater density (increase in density with increasing depth in low latitudes, but constant high density in high latitudes) and is known as pycnocline, while this zone represents rapid decrease of temperature with increasing depth upto 1000m in low latitudes (fig. 25.7), and is called as thermocline.
On the other hand, this zone, representing rapid change of salinity (de-crease in sea water salinity with increasing depth in low latitudes, and increase in sea water salinity with increasing depth in high latitudes) is known as halocline (fig. 25.10). It is apparent from fig. 25.9 that thermocline and pycnocline reveal opposite trends of vertical distribution of temperature and density of seawater.
Significance of Salinity:The ocean salinity has significant effects on physical property of seawater and other aspects of the oceans as follows:1. The freezing and boiling points are greatly affected and controlled by addition or subtraction of salts in seawater. The saline water freezes slowly in comparison to fresh water. It is known to all that pure water freezes at the temperature of 00C freezing point. If the salinity of seawater becomes 35% then it would freeze at the temperature of – 1.910C. On the other hand, the boiling point of saline water (seawater) is higher than fresh water.2. Salinity and density of seawater are positively correlated i.e. the salinity of seawater increases its density because solutes (here salts) in water have greater atomic weight than the molecules of fresh water. This is why man is seldom drowned in the seawater with very high salinity.
3. Evaporation is controlled by salinity of the oceans. In fact, solutes (salts) in water lowers the rate of evaporation in the oceans. Thus more saline
water is less evaporated than less saline water. It may be mentioned that evaporation also controls salinity of seawater. More evaporation reduces the volume of seawater and hence the concentration of salts increases (i.e., seawater salinity increases).
4. Spatial variation in seawater salinity becomes potent factor in the origin of ocean currents.
5. The ocean salinity affects the marine organisms and plant community.
Density of ocean water
The density of an object is the ratio of its mass to its volume. Density can also be expressed as specific gravity, which is the ratio of the density of a material to the density of water. Where the density of water in SI units is 1000 kilogram per cubic meter.
The density of seawater varies with temperature and salinity of the water. As temperature increases, density decreases. As salinity of the water increasaes, density also increases. Although the density of seawater varies at different points in the ocean, a good estimate of its density at the ocean's surface is 1025 kilogram per cubic meter. Its specific gravity is therefore 1.025.The density of seawater plays a vital role in causing ocean currents and circulating heat because of the fact that dense water sinks below less dense. Salinitytemperature and depth all affect the density of seawater
Seawater density varies from place to place because it is affected by salinity and temperature. This means that ships float higher or lower in the water, depending on the density of the ocean. If you look near the waterline of a cargo ship, you should
find the International Load Line, once called the Plimsoll Line. This shows the limit of where the fully loaded ship should sit in waters of different densities.
High salinity makes water denser. This is because there is more salt packed into the water.
High temperature makes water less dense. As water gets warmer, its molecules spread out, so it becomes less dense. As it gets colder, it becomes denser. Most chemicals get denser when they turn from a liquid to a solid, but water is different. When liquid water freezes into solid ice, it becomes less dense. When ice forms, water molecules arrange themselves into a rigid but open pattern. This structure is less dense than the liquid water, so ice floats.
Deep water is denser than shallow water. The water molecules are packed together more tightly because of the weight of water above pushing down.
What happens when density changes
Tides
Tides are the rising and falling of ocean levels that are caused by the sun and moon's gravitational pull combined with the earth's rotation. High tide occurs when the wave's highest part reaches a specific location and low tide occurs when the wave's lowest part reaches that specific location. A tide cycle can occur once or twice a day depending on the area and its proximity to the moon. A tide cycle consists of the sea level rising until high tide is reached, then the seal level falling until low tide is reached. Then the cycle begins again.
What Causes Tides?
Gravity is one major force that creates tides
2. Changes in position of the sun and moon in relation to the earth.
3. Uneven distribution of water over the globe.
4. Irregularities in the configuration of the oceans.
Coriolis Effect
Another influence on waves, tides, and ocean currents is the Coriolis effect. Earth is constantly rotating towards the east and the speed of Earth's rotation varies greatly at different latitudes. It travels fastest on the equator and slows in speed at the farther latitudes. Because of this, the Coriolis effect forces moving objects on Earth to follow curved paths. In the Northern Hemisphere, an object will follow a path to the right of its intended course, and in the Southern Hemisphere, an object will follow a path to the left of its intended course.
The Coriolis effect acts on all objects moving freely over the surface of Earth, and has a dramatic effect on atmospheric circulation and ocean currents and tides. Most ocean basins in the Northern Hemisphere will exhibit a circular current flow, known as a gyre, that rotates in a clockwise direction. The opposite is true in the Southern
Hemisphere where ocean gyres predominately rotate counterclockwise.
The Mechanism of Tide FormationInertia -- the tendency of a moving object to keep moving -- affects the earth's oceans too. As the moon circles the earth, the earth moves in a very slight circle too, and this movement is enough to cause a centrifugal force on the oceans. (It's centrifugal force that holds water in a bucket when you swing the bucket in an overhead arc.)
This inertia, or centrifugal force, causes the oceans to bulge on the opposite side facing the moon. While the moon's gravitational pull is strong enough to attract oceans into a bulge on the side of the earth facing the moon, it is not strong enough to overcome the inertia on the opposite side of the earth.
According to the height of the tide
High tide: when the sea water reaches its greatest height within the tide cycle. They are shown in blue on the tide tables.
Low tide: when the sea water reaches its lowest height within the tide cycle. They are shown in red on the tide tables.
According to the lunar phase
In accordance with the lunar phase, we can distinguish two types of tides:
Spring tides
During the full moon and new moon phases, the Moon and the Sun are aligned and their effects combined, producing the spring tides. On the tide tables we can see the high tidal coefficientof the tides when both celestial bodies are aligned.
There is a proven increase in the activity of fish during spring tides, above all if these coincide with sunrise or sunset, and these are the most propitious days for fishing.
Neap tides
During waxing and waning moons, on the contrary, the effects are detracted, thereby obtaining tides of less amplitude (lower tidal coefficient), called neap tides.
The movement on the seabed tends to be less and this normally results in days that are less propitious for fishing than days with spring tides.
Significance of Tides:1. Tides generally help in making some of the rivers navigable for ocean-going vessels. London and Calcutta have become important ports owing to the tidal nature of the mouths of the Thames and Hooghly respectively.
2. Tides also clear away the sediments brought by the rivers and, thus, retard the process of delta formation.
3. The tidal force may also be used as a source for generating electricity. France and Japan, for example, have power stations which convert tidal energy into electricity.
Ocean currents are regular movement of water flowing in the Earth's oceans. The currents are generated from the forces acting upon the water - like the earth's rotation, the wind, the solar radiation, salinity differences in the water, and the gravitation of the moon. The depth contours, the shoreline and other currents influence the current's direction and strength.
There are two type of Ocean Currents:
1. Surface Currents, making up about 10% of all the water in the oceans and staying in the upper 400 meters of the ocean.
2. Deep Water Currents or Thermohaline Circulation that make up the other 90% of the water in the oceans. These deep-ocean currents are driven by differences in the water's density, which in turn is controlled by temperature (thermo) and salinity (haline).
Causes of origin
Ocean Currents are influenced by two types of forces.
1. Primary Forces that start the water moving.
(a) Solar Heating
(b) Winds
(c) Gravity
(d) Coriolis.
Secondary Forces are those that gives direction to the flow of the currents.
Solar heating causes water to expand. Near the equator the water is about 8 centimeters higher than in middle latitudes. This causes a very slight slope and water tends to flow down the slope. Winds blowing on the surface of the ocean push the water. Friction is the coupling between the wind and the water's surface.
When wind blows, water piles up in the direction the wind is blowing. Gravity tends to pull the water down the slope or pile of water against the pressure gradient.
But the Coriolis Force intervenes and causes the water to move to the right (in the northern hemisphere) around the mound of water. (Once air has been set in motion by the pressure gradient force, it undergoes an apparent deflection from its path, as seen by an observer on the earth. This apparent deflection is called the "Coriolis force" and is a result of the earth's rotation.)
Wind blowing on the surface of the ocean has the greatest effect on the surface. However, for the lower layers of the ocean to move they must be pushed by the friction between the layers of water above.
Consequently, the lower layer moves slower than the layer above. With each successive layer down in the water column the speed is reduced.
The net movement of water in the upper levels of the water is 90 degrees to the right of the wind direction (in the northern hemisphere).
Ocean currents flow for thousands of kilometers. They are very important in determining the climates of the continents, especially those regions bordering on the ocean. Perhaps the most striking example is the Gulf Stream, which makes northwest Europe much more temperate than any other region at the same latitude.
Deep ocean currents are driven by density and temperature gradients. Thermohaline circulation refers to the deep ocean density-driven ocean basin currents. These currents, which flow under the surface of the ocean and are thus hidden from immediate detection, are called submarine rivers.
Ocean currents transport enormous amounts of heat around the world. This makes them one of the most important driving forces of climate. The effects of global warming will gradually become noticeable over a period of centuries, because they respond extremely slowly to changes, whereas climate changes associated with wind and sea ice could become recognizable more quickly.
Water plays a central role in the climate system. Its density varies depending on salinity and temperature. Cold, salty water is heavy and sinks to great depths. This causes the circulation of millions of cubic meters of water in the ocean. This powerful phenomenon, which primarily occurs in polar regions of the ocean, is known as convection.
Low temperature and high salinity are the primary driving forces of convection. They pull the dense water of the polar regions downward, which drives a worldwide convection engine called thermohaline circulation. The cold, salty water submerges primarily in the Labrador and Greenland Seas, and then flows southward toward the equator and beyond. Although convection only occurs locally in the polar regions, it propels thermohaline circulation, which spans the globe like a giant conveyor belt. Even the Gulf Stream and its branches are driven by convection and thermohaline circulation. Although wind also influences the transport of water masses, its contribution is significantly less.
Climate
Results in
Warm and rainy climates in tropical and subtropical latitudes [Florida, Natal etc.],
Cold and dry climates on the western margins in the sub-tropics due to desiccating effect,
Foggy weather and drizzle in the mixing zones,
Moderate clime along the western costs in the sub-tropics.
Classification of ocean current based on temperature
Warm ocean currents
Cold ocean current
Cold currents bring cold water into warm water areas [from high latitudes to low latitudes]. These
currents are usually found on the west coast of the continents (currents flow in clockwise
direction in northern hemisphere and in anti-clockwise direction in southern hemisphere) in the low
and middle latitudes (true in both hemispheres) and on the east coast in the higher latitudes in the
Northern Hemisphere;
2. Warm currents bring warm water into cold water areas[low to high latitudes] and are usually
observed on the east coast of continents in the low and middle latitudes (true in both
hemispheres). In the northern hemisphere they are found on the west coasts of continents in high
latitudes.
Currents of Pacific Ocean
The primary forces that influence the currents are:
1. heating by solar energy;2. wind;3. gravity;4. Coriolis force.
The secondary forces that influence the currents are:
1. Temperature difference;2. Salinity difference
Effects of Ocean Currents
Ocean currents have a number of direct and indirect influences on human activities.
Desert formation
Cold ocean currents have a direct effect on desert formation in west coast regions of the tropical and subtropical continents.
There is fog and most of the areas are arid due to desiccating effect (loss of moisture).
Rains
Warm ocean currents bring rain to coastal areas and even interiors. Example: Summer Rainfall in British Type climate.
Warm currents flow parallel to the east coasts of the continents in tropical and subtropical latitudes. This results in warm and rainy climates. These areas lie in the western margins of the subtropical anti-cyclones.
Moderating effect
They are responsible for moderate temperatures at coasts. [North Atlantic Drift brings warmness to England. Canary cold current brings cooling effect to Spain, Portugal etc.]
Fishing
Mixing of cold and warm ocean currents bear richest fishing grounds in the world.
Example: Grand Banks around Newfoundland, Canada and North-Eastern Coast of Japan.
The mixing of warm and cold currents help to replenish the oxygen and favor the growth of planktons, the primary food for fish population. The best fishing grounds of the world exist mainly in these mixing zones.
Drizzle
Mixing of cold and warm ocean currents create foggy weather where precipitation occurs in the form of drizzle [Newfoundland].
ClimateResults in
Warm and rainy climates in tropical and subtropical latitudes [Florida, Natal etc.],
Cold and dry climates on the western margins in the sub-tropics due to desiccating effect,
Foggy weather and drizzle in the mixing zones, Moderate clime along the western costs in the sub-tropics.
Tropical cyclones
They pile up warm waters in tropics and this warm water is the major force behind tropical cyclones.
Navigation
Currents are referred to by their “drift”. Usually, the currents are strongest near the surface and may attain speeds over five knots (1 knot = ~1.8 km). [At depths, currents are generally slow with speeds less than 0.5 knots].
Ships usually follow routes which are aided by ocean currents and winds. Example: If a ship wants to travel from Mexico to Philippines, it can use the
route along the North Equatorial Drift which flows from east to west. When it wants to travel from Philippines to Mexico, it can follow the route
along the doldrums when there is counter equatorial current [we will study this in next post] flowing from west to east.
Explain the factors responsible for the origin of ocean currents. How do they influence regional climates, fishing and navigation? [Mains 2015]
Desert Formation and Ocean CurrentsMajor hot deserts are located between 20-30 degree latitudes and on the western side of the continents. Why?
The aridity of the hot deserts is mainly due to the effects of off-shore Trade Winds, hence they are also called Trade Wind Deserts.
The major hot deserts of the world are located on the western coasts of continents between latitudes 15° and 30°N. and S (Question asked in Previous Mains Exam).
They include the biggest Sahara Desert (3.5 million square miles). The next biggest desert is the Great Australian Desert. The other hot deserts are the Arabian Desert, Iranian Desert, Thar Desert, Kalahari and Namib Deserts.
The hot deserts lie along the Horse Latitudes or the Sub-Tropical High Pressure Belts where the air is descending, a condition least favorable for precipitation of any kind to take place.
The rain-bearing Trade Winds blow off-shore and the Westerlies that are on-shore blow outside the desert limits.
Whatever winds reach the deserts blow from cooler to warmer regions, and their relative humidity is lowered, making condensation almost impossible.
There is scarcely any cloud in the continuous blue sky. The relative humidity is extremely low, decreasing from 60 per cent in coastal districts to less than 30 per cent in the desert interiors. Under such conditions, every bit of moisture is evaporated and the deserts are thus regions of permanent drought. Precipitation is both scarce and most unreliable.
On the western coasts, the presence of cold currents gives rise to mists and fogs by chilling the on-coming air. This air is later warmed by contact with the hot land, and little rain falls.
The desiccating effect of the cold Peruvian Current along the Chilean coast is so pronounced that the mean annual rainfall for the Atacama Desert is not more than 1.3 cm.
Pacific Ocean
The Pacific Ocean is the largest and deepest of Earth's oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south and is bounded by Asia and Australia in the west and the Americas in the east.
It is largest division of the World Ocean—and, in turn, the hydrosphere—covers about 46% of Earth's water surface and about one-third of its total surface area, making it larger than all of Earth's land area combined
Nearly 20,000 islands dot this vast ocean. verage depth is generally around 7,300 metres. Its shape is roughly triangular with its apex in the north at
the Bering Strait.
