Chapter 74 A Brief Introduction to Oceanography
The ocean may be defined as the vast body of saline water that occupies the depressions of the earth’s surface. More than 97% of the water on or near the earth’s surface is contained in the ocean; less than 3% is held in land ice, groundwater, and all the freshwater lakes and rivers.
Traditionally, we have divided the ocean into artificial compartments called oceans and seas by using the boundaries of continents and imaginary lines such as the equator. In fact, the ocean has few dependable natural divisions and is only one great mass of water. The Pacific and Atlantic Oceans and the Mediterranean and Baltic Seas, so named for our convenience, are in reality only temporary features of a single world ocean. In this chapter, I refer to the ocean as a single entity, with subtly different characteristics at different locations but with very few natural partitions. Such a view emphasizes the interdependence of the ocean with land, life, water, atmospheric and oceanic circulation, and natural and human-made environments.
Earth’s ocean exists because of a fortuitous combination of circumstances. Our planet’s orbit is roughly circular around a relatively stable star. Earth is large enough to hold an atmosphere, but not so large that its gravity would overwhelm. Its neighborhood is tranquil—supernovae have not seared its surface with ionizing radiation. Our planet generates enough warmth to recycle its interior and generate the raw materials of atmosphere and ocean, but is not so hot that lava fills vast lowlands or roasts complex molecules. Best of all, our distance from the sun allows the earth’s abundant surface water to exist in the liquid state. Ours is a clement ocean world; surely Oceanus would be a better name for our watery home.
The ocean moderates temperature and dramatically influences weather. The ocean borders most of the planet’s largest cities. It is a primary shipping and transportation route that provides much of our food. From its floor is pumped more than one-third of the world’s supply of petroleum and natural gas. The dry land on which nearly all of human history has unfolded is hardly visible from space, because nearly three-quarters of the planet is covered by water.
The ocean did not prevent the spread of humanity. By the time European explorers set out to “discover” the world, native peoples met them at nearly every landfall. Ocean transportation offers people the benefits of mobility and greater access to food supplies. Any coastal culture skilled at raft building or small-boat navigation would have economic and nutritional advantages over less-skilled competitors. Thus, from the earliest period of human history, understanding and appreciating the ocean and its life-forms benefited those patient enough to learn.
Systematic application of marine science began at the Library of Alexandria in Egypt. Founded during the third century BC at the behest of Alexander the Great, the library and the adjacent museum could be considered the first university in the world. When any ship entered the harbor, the books (actually scrolls) it contained were by law removed and copied; the copies were returned to the owner and the originals kept for the library. Caravans arriving over land were also searched. Manuscripts describing the Mediterranean coast were of great interest, and traders quickly realized the competitive benefit of this information.
The second librarian at Alexandria (from 235 to 192 BC) was the Greek astronomer, philosopher, and poet Eratosthenes of Cyrene. This remarkable man was the first to calculate the circumference of the earth with the use of geometry. The Greek Pythagoreans had realized that the earth was spheric by the sixth century BC, but Eratosthenes was the first to estimate its true size.
Scientific oceanography began with the departure of HMS Challenger from Plymouth, England, in 1872. Conceived by Wyville Thomson, a professor of natural history at Scotland’s University of Edinburgh, and his Canadian-born student, John Murray, HMS Challenger’s 4-year cruise was the first research expedition devoted completely to marine science, and it also holds the record as the longest such voyage. It should be noted that other scientific expeditions had been launched previously. The voyages of Captain James Cook, RN, and of the United States Exploring Expedition under Charles Wilkes were prominent among them, but these were hybrid military and scientific undertakings; HMS Challenger is notable for being the first purely scientific oceanographic endeavor.
Stimulated by their own curiosity and with the inspiration of Charles Darwin’s voyage in HMS Beagle, these individuals convinced the Royal Society and British government to provide a Royal Navy ship and trained crew for a prolonged and arduous voyage of exploration across the oceans of the world. Thomson and Murray even coined a word for their enterprise: oceanography. Although the term literally implies only marking or charting, it has come to refer to the science of the ocean.
The demands of scientific oceanography have become greater than the capability of any single voyage. Modern oceanography depends on an interlocking suite of terrestrial and space-based sensors. Among the most interesting are the radar altimeters borne by TOPEX/Poseidon/Jason, as the project is known, a train of satellites orbiting 1336 km (830.2 miles) above Earth in an orbit that allows coverage of 95% of the ice-free ocean every 10 days. Experiments that are occurring as part of this 5-year program include sensing water vapor over the ocean, determining the precise location of ocean currents, and determining wind speed and direction. The most revolutionary devices are the satellites’ TOPography Experiment, which use radar positioning devices to allow researchers to determine position to within 1 cm of Earth’s center. Computers can then determine the height of the sea surface with unprecedented accuracy.
Disregarding waves, tides, and currents, researchers have found the ocean surface can vary from the ideal smooth (ellipsoid) shape by as much as 200 m (656.2 feet). The reason is that the pull of gravity varies across Earth’s surface depending on the nearness or farness of massive parts of the earth. An undersea mountain or ridge “pulls” water toward it from the sides, forming a mount of water over itself. For example, a typical undersea volcano with a height of 2000 m (6561.7 feet) above the seabed and a radius of 20 km (12.4 miles) would produce a 2-m (6.6-feet) rise in the ocean surface. This mound cannot be seen with the unaided eye because the slope of the surface is very gradual. The large features of the seabed are amazingly and accurately reproduced in the subtle standing irregularities of the sea surface. Hundreds of previously unknown features have been discovered using the data provided by this project.
Water is so familiar and abundant that we do not always appreciate its unusual characteristics. Two big concepts are reviewed here. The first is the influence of water on global temperatures. Liquid water’s thermal characteristics prevent broad swings of temperature during day and night and, through a longer span, during winter and summer. Heat is stored in the ocean during the day and released at night. A much greater amount of heat is stored through the summer and given off during the winter. Liquid water has an important thermostatic balancing effect—an oceanless Earth would be much colder in winter and much hotter in summer than the moderate climates we experience. The second is the influence of density on ocean structure. Ocean structure and large-scale movement depend on changes in the density of seawater, with this density dependent on temperature and salt content.
Perhaps the most important physical properties of water are related to its behavior as it absorbs or loses heat. Water’s unusual thermal characteristics prevent wide temperature variation from day to night and from winter to summer, permit vast amounts of heat to flow from equatorial to polar regions, and power Earth’s great storms, wind waves, and ocean currents.
Heat and temperature are related concepts, but they are not the same thing. Heat is energy produced by random vibration of atoms or molecules. On average, water molecules in hot water vibrate more rapidly than do water molecules in cold water. Heat is a measure of how many molecules are vibrating and how rapidly they are vibrating. Temperature records only how rapidly the molecules of a substance are vibrating. Temperature is an object’s response to the input (or removal) of heat. The amount of heat required to bring a substance to a certain temperature varies with the nature of that substance.
Heat capacity is a measure of the heat required to raise the temperature of 1 gram of a substance by 1° C. Different substances have different heat capacities, and not all substances respond to identical inputs of heat by rising in temperature the same number of degrees (Table 74-1). Heat capacity is measured in calories per gram per degree centigrade.
|Substance||Heat Capacity† in Calories/Gram/° C|
|Ice (not freezing or thawing)||0.51|
|Pure liquid water||1.00|
† Different substances have different heat capacities. Note how little heat is required to raise the temperature of 1 gram of silver by 1° C. Of all common substances, only liquid ammonia has a higher heat capacity than liquid water.
Because of the great strength and large number of the hydrogen bonds between water molecules, more heat energy must be added to speed up molecular movement and raise water’s temperature than would be necessary in a substance held together by weaker bonds. Liquid water’s heat capacity is therefore among the highest of all known substances. This means that water can absorb (or release) large amounts of heat while changing relatively little in temperature.
The uniqueness of water becomes even more apparent when one considers the effect of temperature change on water’s density. Most substances become denser as they get colder. Pure water generally becomes denser as heat is removed and its temperature falls, but water’s density behaves in an unexpected way as its temperature approaches the freezing point. As the water continues to cool, its framework of hydrogen bonds becomes more rigid; this causes the liquid to expand slightly because the molecules are held slightly farther apart. Water becomes slightly less dense as cooling continues, until 0° C (32° F) is reached; this is the point at which water begins to freeze and change state by crystallizing into ice. At this point, the density of the water decreases abruptly. Ice is therefore lighter than an equal volume of water. Ice increases in density as it gets colder than 0° C; however, no matter how cold it gets, ice never reaches the density of liquid water. Because it is less dense than water, ice “freezes over” as a floating layer instead of “freezing under” like the solid forms of virtually all other liquids.
The progressive transition from liquid water to ice crystals requires continued removal of heat energy; the change in state does not occur instantly throughout the mass when the cooling water reaches 0° C (32° F). The removal of heat does not stop when some of the water in the freezing water mass reaches the freezing point, but the decline in temperature stops. Although heat continues to be removed, the water will not get colder until the water mass has changed state from liquid (water) to solid (ice). Heat may therefore be removed from water when it is changing state (i.e., when it is freezing) without the water dropping in temperature. Indeed, continued removal of heat is what makes the change in state possible. Heat is released as hydrogen bonds form to make ice, and that heat must be removed to allow more ice to form. This heat is called the latent heat of fusion (from the Latin latere, meaning “to be hidden”).
The implications of this odd thermal behavior are striking. More than 18,000 km3 (11,185 miles3) of polar ice that covers as many as 20 million km2 (12,400,000 miles2) of surface thaws and refreezes in the Southern Hemisphere each year; this is an area of ocean larger than South America. The annual change in sea ice cover is less in the Arctic, averaging about 5 million km2 (3,100,000 mi2). Incoming solar heat melts ice in the local polar summer, but the ocean’s temperature does not change. The situation reverses during the winter: the water freezes, and again the temperature does not change. Models suggest that without this thermostatic effect—or if ice “froze under” rather than “froze over”—Earth would be a much different planet, perhaps roiled by near-transonic winds peaking about a month after the equinoxes at equatorial latitudes.
The total quantity or concentration of dissolved inorganic solids in water is its salinity. The ocean’s salinity varies from about 3.3% to 3.7% by mass, depending on such factors as evaporation, precipitation, and freshwater runoff from the continents. However, the average salinity is usually given as 3.5%. Most of the dissolved solids in seawater are salts that have been separated into ions. Sodium and chloride are the most abundant of these.
These four properties, which vary with the quantity of solutes dissolved in water, are called water’s colligative properties (from the Latin colligatus, meaning “to bind together”). Because colligative properties are the properties of solutions, the more concentrated (saline) the solute, the more important these properties become. Because it is not a solution, pure water has no colligative properties.
Because about 3.5% of seawater consists of dissolved substances, boiling away 100 kg of seawater theoretically produces a residue with a mass of 3.5 kg. Because variations of 0.1% are significant, however, oceanographers prefer to use the parts-per-thousand notation (‰) rather than percent (%, parts-per-hundred notation) when discussing these materials. The seven ions listed below oxygen and hydrogen in Table 74-2 make up more than 99% of this residual material; sodium and chloride make up 85% of the total. When seawater evaporates, its ionic components combine in many different ways to form table salt, Epsom salts, and other mineral salts.
|Constituent||Concentration in Parts per Thousand (‰) or Grams per Kilogram (g/kg)||Percent by Mass|
|Most Abundant Ions|
Seawater also contains minor constituents. The ocean is sort of an “Earth tea;” nearly every element present in Earth’s crust and atmosphere is also present in the oceans, although sometimes in extremely small amounts. Only 14 elements have concentrations in seawater of more than 1 part per million. Elements present in amounts less than 0.001‰ (1 part per million) are known as trace elements.
Remembering the effectiveness of water as a solvent, one might think that the ocean’s saltiness has resulted from the ability of rain, groundwater, or crashing surf to dissolve crustal rock. Much of the sea’s dissolved material originated in that way, but is crustal rock the source of all the ocean’s solutes? An easy way to find out would be to investigate the composition of salts in river water and compare this to that of the ocean as a whole. If crustal rock is the only source, then the salts in the ocean should be like those of concentrated river water; however, they are not. River water is usually a dilute solution of bicarbonate and calcium ions, whereas the principal ions in seawater are sodium and chloride. The magnesium content of seawater would be higher if seawater were simply concentrated river water. The proportions of salts in isolated salty inland lakes (e.g., Utah’s Great Salt Lake, the Dead Sea) are much different from the proportions of salts in the ocean. Thus, weathering and erosion of crustal rock cannot be the only source of sea salts.
The components of ocean water with proportions that are not accounted for by the weathering of surface rocks are called excess volatiles. The sources of these excess volatiles are Earth’s deeper layers. The upper mantle appears to contain more of the substances found in seawater (including the water itself) than are found in surface rocks, and their proportions are about the same as found in the ocean. As noted earlier, convection currents slowly churn Earth’s mantle, causing the movement of tectonic plates. Because of this activity, some deeply trapped volatile substances escape to the exterior, outgassing through volcanoes and rift vents. These excess volatiles include carbon dioxide (CO2), chlorine, sulfur, hydrogen, fluorine, nitrogen, and water vapor. This material, along with residue from surface weathering, accounts for the chemical constituents of today’s ocean.
Some of the ocean’s solutes are hybrids of the two processes of weathering and outgassing. Table salt (sodium chloride) is an example of this. Sodium ions come from the weathering of crustal rocks, whereas chloride ions come from the mantle by way of volcanic vents and outgassing from mid-ocean rifts. As for the lower-than-expected quantity of magnesium and sulfate ions in the ocean, research at a spreading center east of the Galápagos Islands suggests that the chemical composition of seawater percolating through mid-ocean rifts is altered by contact with fresh crust. The water that circulates through new ocean floor at these sites is stripped of magnesium as well as of a few other elements. The magnesium seems to be incorporated into mineral deposits, but calcium is added as hot water dissolves adjacent rocks.
Recent research has shown that temperature and density gradients inside seamounts also drive great quantities of water into close association with hot geologic bits. The ocean contains about 15,000 seamounts, and the volume of seawater circulated through them may exceed the amounts associated with ridges. Astonishingly, all the water in the ocean is thought to cycle through the seabed at rift zones every 1 to 2 million years.
Heat combines with salinity to define ocean structure. A liter of seawater weighs between 2% and 3% more than a liter of pure water because of the solids (often called salts) dissolved in seawater. The density of seawater is thus between 1.020 and 1.030 g/cm3 compared with 1.000 g/cm3 for pure water at the same temperature. Cold, salty water is denser than warm, less salty water. Seawater’s density increases with increasing salinity, increasing pressure, and decreasing temperature. Figure 74-1 shows the relationship between temperature, salinity, and density. Notice that two samples of water can have the same density at different combinations of temperature and salinity.
FIGURE 74-1 The complex relationship among the temperature, salinity, and density of seawater. Note that two samples of water can have the same density at different combinations of temperature and salinity.
Much of the ocean is divided into three density zones: the surface zone, pycnocline, and deep zone. The surface zone, or mixed layer, is the upper layer of ocean. Temperature and salinity are relatively constant with depth in the surface zone because of the action of waves and currents. The surface zone consists of water in contact with the atmosphere and exposed to sunlight; it contains the ocean’s least dense water and accounts for only about 2% of total ocean volume. This layer typically extends to a depth of about 150 m (500 feet), but, depending on local conditions, may reach a depth of 1000 m (3300 feet) or be absent entirely.
The pycnocline (from the Greek pyknos, meaning “strong,” and the Latin clinare, meaning “to slope” or “to lean”) is a zone in which density increases with increasing depth. This zone isolates surface water from the denser layer below. The pycnocline contains about 18% of all ocean water.
The deep zone lies below the pycnocline at depths of more than about 1000 m (3300 feet) in mid latitudes (40 degrees S to 40 degrees N). There is little additional change in water density with increasing depth through this zone. This deep zone contains about 80% of all ocean water.
The pycnocline’s rapid density increase with depth is mainly the result of a decrease in water temperature. The surface zone is well mixed, with little decrease in temperature with depth. In the next layer, temperature drops rapidly with depth. Beneath it lies the deep zone of cold, stable water. The middle layer, the zone in which temperature changes rapidly with depth, is called the thermocline. Falling temperature is the major contributor to the formation of the pycnocline.
Thermoclines are not identical in form in all areas or latitudes. Surface temperature is proportional to available sunlight. More solar energy is available in the tropics than in the polar regions, so the water there is warmer. The ocean’s sunlit upper layer is thicker in the tropics, both because the solar angle there is more nearly vertical and because water in the open tropical ocean contains fewer suspended particles and is therefore clearer than water in open temperate or polar regions. Because the ocean is heated to a greater depth, the tropical thermocline is deeper and much more pronounced than thermoclines at higher latitudes. The transition to the colder, denser water below is more abrupt in the tropics than at high latitudes.
Polar waters, which receive relatively little solar warmth, are not stratified by temperature and generally lack a thermocline because surface water in the polar regions is nearly as cold as water at great depths.
Figure 74-2 contrasts polar, tropical, and temperate thermal profiles, showing that the thermocline is primarily a mid- and low-latitude phenomenon. Thermocline depth and intensity vary with season, local conditions (e.g., storms), currents, and many other factors.
The vertical movement of large volumes of water from the surface to great depths (and vice versa) is possible only where surface-water density is similar to deep-water density. The great difference in temperature—and therefore density—between surface water and deep water in the tropics makes the water column very stable and prevents an exchange of surface and deep water. This stability is maintained despite the fact that the surface of the tropical ocean is in constant horizontal motion, churned by tropical cyclones and stirred by currents.
Vertical movement of water in the northern polar ocean is also limited. There, however, the stratification is caused largely by a salinity difference between surface water and water at great depths. The surface of the Arctic Ocean receives a large volume of freshwater runoff from Siberian and Canadian rivers. Continental masses block the formation of large currents, and the landlocked northern ocean communicates sluggishly with other ocean areas, so the surface water tends not to mix with deeper water or to flow to lower latitudes.
By contrast, the southern polar ocean is only weakly stratified. The cold temperature of southern ocean surface water closely matches that of deep water, so no thermocline divides surface water from deep water (see Figure 74-2). The absence of confining continental margins and mixing at the boundaries of the Antarctic Circumpolar Current minimize salinity differences. Turbulence and weak stratification encourage a huge volume of deep-water upwelling, which contributes to high surface nutrient levels and high biologic productivity.
Layering by density traps dense water masses at great depths, where they are not exposed to daily heating and cooling, surface circulation driven by winds and storms, or light. The pycnocline effectively isolates 80% of the world ocean’s water from the 20% involved in surface circulation. Dense water masses form near polar continental shelves (as cold water freezes and excludes salt) or in enclosed areas such as the Mediterranean Sea (where evaporation exceeds precipitation and river input, raising salinity). These heavy water masses sink, sometimes overlapping one another and often retaining their identity for long periods. Separate water masses below the pycnocline tend not to merge, because little energy is available for mixing in these quiet depths.
However, water does circulate in the ocean. The mass flow of ocean water in currents occurs in two forms: (1) surface currents are wind-driven movements of water at or near the ocean’s surface; and (2) thermohaline currents (so named because they depend on density differences caused by variations in water’s temperature and salinity) are the slow, deep currents that affect the vast bulk of seawater beneath the pycnocline. Both have very important influences on Earth’s temperature, climate, and biologic productivity, and will change as Earth’s climate varies.
A small fraction of the water in the world ocean is involved in surface currents, comprised of water that flows horizontally in the uppermost 400 m (1300 feet) of the ocean’s surface, driven mainly by wind friction. Most surface currents move water above the pycnocline.
The primary force responsible for surface currents is wind. Surface winds form global patterns within latitude bands. Most of Earth’s surface wind energy is concentrated in each hemisphere’s trade winds (i.e., easterlies) and westerlies. Waves on the sea surface transfer some of the energy from the moving air to the water using friction. This tug of wind on the ocean surface begins a mass flow of water, and the water flowing beneath the wind forms a surface current.
Because of the Coriolis effect, Northern Hemisphere surface currents flow to the right and Southern Hemisphere currents flow to the left of the wind direction. Continents and basin topography often block continuous flow and help deflect the moving water into a circular pattern, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. This flow around the periphery of an ocean basin is called a gyre (from the Greek gyros, meaning “a circle”).
There are six great current circuits in the world ocean, two in the Northern Hemisphere and four in the Southern Hemisphere. Five are geostrophic gyres–gyres that flow around the periphery of an ocean basin: the North Atlantic gyre, the South Atlantic gyre, the North Pacific gyre, the South Pacific gyre, and the Indian Ocean gyre. Although it is a closed circuit, the sixth and largest current is technically not a gyre because it does not flow around the periphery of an ocean basin. The West Wind Drift, or Antarctic Circumpolar Current, as this exception is called, flows endlessly eastward around Antarctica, driven by powerful, nearly ceaseless westerly winds. This greatest of all the surface ocean currents is never deflected by a continent. Figure 74-3 shows the major surface currents of the world ocean.
FIGURE 74-3 A chart showing the names and usual directions of the world ocean’s major surface currents. The powerful western boundary currents flow along the western boundaries of ocean basins in both hemispheres.