Seen from space, our planet’s surface appears to be dominated by the color blue. The Earth appears blue because large bodies of saline water known as the oceans dominate the surface. Oceans cover approximately 70.8% or 361 million square kilometers (139 million square miles) of Earth’s surface (Table 1) with a volume of about 1370 million cubic kilometers (329 million cubic miles). The average depth of these extensive bodies of sea water is about 3.8 kilometers (2.4 miles). Maximum depths can exceed 10 kilometers (6.2 miles) in a number of areas known as ocean trenches.
The oceans contain 97% of our planet's available water. The other 3% is found in atmosphere, on the Earth's terrestrial surface, or in the Earth's lithosphere in various forms and stores (see the Hydrologic Cycle).
Table 1: Surface area of our planet covered by oceans and continents.
|
Surface |
Percent of Earth’s Total Surface Area |
Area Square Kilometers |
Area Square Miles |
|
Earth’s Surface Area Covered by Land |
29.2% |
148,940,000 |
57,491,000 |
|
Earth’s Surface Area Covered by Water |
70.8% |
361,132,000 |
139,397,000 |
|
Pacific Ocean |
30.5% |
155,557,000 |
60,045,000 |
|
Atlantic Ocean |
20.8% |
76,762,000 |
29,630,000 |
|
Indian Ocean |
14.4% |
68,556,000 |
26,463,000 |
|
Southern Ocean |
4.0% |
20,327,000 |
7,846,000 |
|
Arctic Ocean |
2.8% |
14,056,000 |
5,426,000 |
The spatial distribution of ocean regions and continents is unevenly arranged across the Earth's surface. In the Northern Hemisphere, the ratio of land to ocean is about 1 to 1.5. The ratio of land to ocean in the Southern Hemisphere is 1 to 4. This greater abundance of ocean surface has some fascinating effects on the environment of the southern half of our planet. For example, climate of Southern Hemisphere locations is often more moderate when compared to similar places in the Northern Hemisphere. This fact is primarily due to the presence of large amounts of heat energy stored in the oceans.
The International Hydrographic Organization has divided and named the interconnected oceans of the world into five main regions: Atlantic Ocean, Arctic Ocean, Indian Ocean, Pacific Ocean, and the Southern Ocean. Each one of these regions is different from the others in some specific ways.
Atlantic Ocean
The Atlantic Ocean is a relatively narrow body of water that snakes between nearly parallel continental masses covering about 21% of the Earth’s total surface area (Figure 1). This ocean body contains most of our planet’s shallow seas, but it has relatively few islands. Some of the shallow seas found in the Atlantic Ocean include the Caribbean, Mediterranean, Baltic, Black, North, Baltic, and the Gulf of Mexico.
The average depth of the Atlantic Ocean (including its adjacent seas) is about 3300 meters (10,800 feet). The deepest point, 8605 meters (28,232 feet), occurs in the Puerto Rico Trench. TheMid-Atlantic Ridge, running roughly down the center of this ocean region, separates the Atlantic Ocean into two large basins.
Figure 1. Atlantic Ocean region
Many streams empty their fresh water discharge into the Atlantic Ocean. In fact, the Atlantic Ocean receives more freshwater from terrestrial runoff than any other ocean region. This ocean region also drains some of the Earth’s largest rivers including the Amazon, Mississippi, St. Lawrence, and Congo. The surface area of the Atlantic Ocean is about 1.6 times greater than the terrestrial area providing runoff.
Arctic Ocean
The Arctic Ocean is the smallest of the world’s five ocean regions, covering about 3% of the Earth’s total surface area. Most of this nearly landlocked ocean region is located north of the Arctic Circle (Figure 2). The Arctic Ocean is connected to the Atlantic Ocean by the Greenland Sea, and the Pacific Ocean via the Bering Strait. The Arctic Ocean is also the shallowest ocean region with an average depth of 1050 meters (3450 feet).
The center of the Arctic Ocean is covered by a drifting persistent icepack that has an average thickness of about 3 meters (10 feet). During the winter months, this sea ice covers much of the Arctic Ocean surface. Higher temperatures in the summer months cause the icepack to seasonally shrink in extent by about 50%.
Figure 2 Arctic Ocean region
Indian Ocean
The Indian Ocean covers about 14% of the Earth’s surface area. This ocean region is enclosed on three sides by the landmasses of Africa, Asia, and Australia (Figure 8o-3). The Indian Ocean’s southern border is open to water exchange with the much colder Southern Ocean. Average depth of the Indian Ocean is 3900 meters (12,800 feet). The deepest point in this ocean region occurs in the Java Trench with a depth of 7258 meters (23,812 feet) below sea level. The Indian Ocean region has relatively few islands.
Continental shelf areas tend to be quite narrow and not many shallow seas exist. Relative to the Atlantic Ocean, only a small number of streams drain into the Indian Ocean. Consequently, the surface area of the Indian Ocean is approximately 400% larger than the land area supply runoff into it. Some of the major rivers flowing into the Indian Ocean include the Zambezi, Arvandrud/Shatt-al-Arab, Indus, Ganges, Brahmaputra, and the Irrawaddy. Sea water salinity ranges between 32 and 37 parts per 1000. Because much of the Indian Ocean lies within the tropics, this basin has the warmest surface ocean temperatures.
Figure 3: Indian Ocean region
Pacific Ocean
The Pacific Ocean is the largest ocean region (Figure 4) covering about 30% of the Earth’s surface area (about 15 times the size of the United States). The ocean floor of the Pacific is quite uniform in depth having an average elevation of 4300 meters (14,100 feet) below sea level. This fact makes it the deepest ocean region on average. The Pacific Ocean is also home to the lowest elevation on our planet. The deepest point in the Mariana Trench lies some 10,911 meters (35,840 feet) below sea level as recorded by the Japanese probe, Kaiko, on March 24, 1995. About 25,000 islands can be found in the Pacific Ocean region.
This is more than the number for the other four ocean regions combined. Many of these islands are actually the tops of volcanic mountains created by the release of molten rock from beneath the ocean floor.
Figure 4: Pacific Ocean region
Relative to the Atlantic Ocean, only a small number of rivers add terrestrial freshwater runoff to the Pacific Ocean. In fact, the surface area of the Pacific is about 1000% greater than the land area that drains into it. Some of the major rivers flowing into this ocean region include the Colorado, Columbia, Fraser, Mekong, Río Grande de Santiago, San Joaquin, Shinano, Skeena, Stikine, Xi Jiang, and Yukon. Some of larger adjacent seas connected to the Pacific are Celebes, Tasman, Coral, East China, Sulu, South China, Yellow, and the Sea of Japan.
Southern Ocean
The Southern Ocean surrounds Antarctica extending to the latitude 60° South (Figure 5). This ocean region occupies about 4% of the Earth’s surface or about 20,327,000 square kilometers (7,846,000 square miles). Relative to the other ocean regions, the floor of the Southern Ocean is quite deep ranging from 4000 to 5000 meters (13,100 to 16,400 feet) below sea level over most of the area it occupies. Continental shelf areas are very limited and are mainly found around Antarctica. But even these areas are quite deep with an elevation between 400 to 800 meters (1300 to 2600 feet) below sea level. For comparison, the average depth of the continental shelf for the entire planet is about 130 meters (425 feet).
The Southern Ocean’s deepest point is in the South Sandwich Trench at 7235 meters (23,737 feet) sea level. Seas adjacent to this ocean region include the Amundsen Sea, Bellingshausen Sea, Ross Sea, Scotia Sea, and the Weddell Sea. By about September of each year, a mobile icepack situated around Antarctic reaches its greatest seasonal extent covering about 19 million square kilometers (7 million square miles). This icepack shrinks by around 85% six months later in March.
Figure 5: Southern Ocean region
- Heat fluxes, evaporation, rain, river in flow, and freezing and melting of sea ice all influence the distribution of temperature and salinity at the ocean's surface.
- Changes in temperature and salinity can increase or decrease the density of water at the surface, which can lead to convection.
- If water from the surface sinks into the deeper ocean, it retains a distinctive relationship between temperature and salinity which helps oceanographers track the movement of deep water.
- In addition, temperature, salinity, and pressure are used to calculate density.
- The distribution of density inside the ocean is directly related to the distribution of horizontal pressure gradients and ocean currents.
For all these reasons, we need to know the distribution of temperature, salinity, and density in the ocean.
Definition
- At the simplest level, salinity is the total amount of dissolved material in grams in one kilogram of sea water.
- Thus salinity is a dimensionless quantity.
- It has no units.
The variability of dissolved salt is very small, and we must be very careful to define salinity in ways that are accurate and practical.
A Simple Definition
Originally salinity was defined to be the "Total amount of dissolved material in grams in one kilogram of sea water."
This is not useful because the dissolved material is almost impossible to measure in practice. For example, how do we measure volatile material like gasses? Nor can we evaporate sea-water to dryness because chlorides are lost in the last stages of drying.
A More Complete Definition
To avoid these difficulties, the International Council for the Exploration of the Sea set up a commission in 1889 which recommended that salinity be defined as the...
"Total amount of solid materials in grams dissolved in one kilogram of sea water when all the carbonate has been converted to oxide, the bromine and iodine replaced by chlorine and all organic matter completely oxidized."
An ocean current can be defined as a horizontal movement of seawater at the ocean's surface. Ocean currents are driven by the circulation of wind above surface waters. Frictional stress at the interface between the ocean and the wind causes the water to move in the direction of the wind. Large ocean currents are a response of the atmosphere and ocean to the flow of energy from the tropics to polar regions. In some cases, currents are transient features and affect only a small area. Other ocean currents are essentially permanent and extend over large horizontal distances.
On a global scale, large ocean currents are constrained by the continental masses found bordering the three oceanic basins. Continental borders cause these currents to develop an almost closed circular pattern called a gyre. Each ocean basin has a large gyre located at approximately 30° North and South latitude in the subtropical regions. The currents in these gyres are driven by the atmospheric flow produced by the subtropical high pressure systems. Smaller gyres occur in the North Atlantic and Pacific Oceans centered at 50° North. Currents in these systems are propelled by the circulation produced by polar low pressure centers. In the Southern Hemisphere, these gyre systems do not develop because of the lack of constraining land masses.
A typical gyre displays four types of joined currents: two east-west aligned currents found respectively at the top and bottom ends of the gyre; and two boundary currents oriented north-south and flowing parallel to the continental margins. Direction of flow within these currents is determined by the direction of the macro-scale wind circulation. Boundary currents play a role in redistributing global heat latitudinally.
Surface Currents of the Subtropical Gyres
On either side of the equator, in all ocean basins, there are two west flowing currents: the North and South Equatorial (Figure 1). These currents flow between 3 and 6 kilometers per day and usually extend 100 to 200 meters in depth below the ocean surface. The Equatorial Counter Current, which flows towards the east, is a partial return of water carried westward by the North and South Equatorial currents. In El Niño years, this current intensifies in the Pacific Ocean.
Flowing from the equator to high latitudes are the western boundary currents. These warm water currents have specific names associated with their location: North Atlantic - Gulf Stream; North Pacific - Kuroshio; South Atlantic - Brazil; South Pacific - East Australia; and Indian Ocean - Agulhas. All of these currents are generally narrow, jet like flows that travel at speeds between 40 and 120 kilometers per day. Western boundary currents are the deepest ocean surface flows, usually extending 1000 meters below the ocean surface.
Flowing from high latitudes to the equator are the eastern boundary currents. These cold water currents also have specific names associated with their location: North Atlantic - Canary; North Pacific - California; South Atlantic - Benguela; South Pacific - Peru; and Indian Ocean - West Australia. All of these currents are generally broad, shallow moving flows that travel at speeds between 3 and 7 kilometers per day.
In the Northern Hemisphere, the east flowing North Pacific Current and North Atlantic Drift move the waters of western boundary currents to the starting points of the eastern boundary currents. The South Pacific Current, South Indian Current and South Atlantic Current provide the same function in the Southern Hemisphere. These currents are associated with the Antarctic Circumpolar (West Wind Drift). Because of the absence of landmass at this latitude zone, the Antarctic Circumpolar flows in continuous fashion around Antarctica and only provides a partial return of water to the three Southern Hemispheric ocean basins.
Surface Currents of the Polar Gyres
The polar gyres exist only in the Atlantic and Pacific basins in Northern Hemisphere. They are propelled by the counterclockwise winds associated with the development of permanent low pressure centers at 50° of latitude over the ocean basins. Note that the bottom west flowing current of the polar gyres is the topmost flowing current of the subtropical gyres. Other currents associated with these gyres are shown on Figure 1.
Subsurface Currents
The world's oceans also have significant currents that flow beneath the surface (Figure 2). Subsurface currents generally travel at a much slower speed when compared to surface flows. The subsurface currents are driven by differences in the density of sea water. The density of sea water deviates in the oceans because of variations in temperature and salinity. Near surface sea water begins its travel deep into the ocean in the North Atlantic. The downwelling of this water is caused by high levels of evaporation which cools and increases the salinity of the sea water located here. The high levels of evaporation take place in between Northern Europe and Greenland and just north of of Labrador, Canada. This sea water then moves south along the coast of North and South America until it reaches Antarctica. At Antarctica, the cold and dense sea water then travels eastward joining another deep current that is created by evaporation occuring between Antarctica and the southern tip of South America. Slightly into its eastward voyage the deep cold flow splits off into two currents, one of which moves northward. In the middle of the North Pacific and in the Indian Ocean (off the east coast of Africa), these two currents move from the ocean floor to its surface creating upwellings. The current then becomes near surface moving eventually back to the starting point in the North Atlantic or creating a shallow warm flow that circles around Antarctica. One complete circuit of this flow of sea water is estimated to take about 1,000 years.
| Figure 2: The following illustration describes the flow pattern of the major subsurface ocean currents. Near surface warm currents are drawn in red. Blue depicts the deep cold currents. Note how this system is continuously moving water from the surface to deep within the oceans and back to the top of the ocean. (Source: Arctic Climate Impact Assessment -ACIA). |
- An ocean tide refers to the cyclic rise and fall of seawater.
- Tides are caused by slight variations in gravitational attraction between the Earth and the moon and the Sun in geometric relationship with locations on the Earth's surface.
- Tides are periodic primarily because of the cyclical influence of the Earth's rotation.
- The moon is the primary factor controlling the temporal rhythm and height of tides (Figure 1).
- The moon produces two tidal bulges somewhere on the Earth through the effects of gravitational attraction.
- The height of these tidal bulges is controlled by the moon's gravitational force and the Earth's gravity pulling the water back toward the Earth.
- At the location on the Earth closest to the moon, seawater is drawn toward the moon because of the greater strength of gravitational attraction.
- On the opposite side of the Earth, another tidal bulge is produced away from the moon. However, this bulge is due to the fact that at this point on the Earth the force of the moon's gravity is at its weakest. Considering this information, any given point on the Earth's surface should experience two tidal crests and two tidal troughs during each tidal period.
| Figure 1: The moon's gravitational pull is the primary force responsible for the tides on the Earth. Photo taken by the Galileo spacecraft from a distance of about 6.2 million kilometers from Earth, on December 16, 1992. (Source:NASA). |
Spring tides
- The timing of tidal events is related to the Earth's rotation and the revolution of the moon around the Earth.
- If the moon was stationary in space, the tidal cycle would be 24 hours long. However, the moon is in motion revolving around the Earth. One revolution takes about 27 days and adds about 50 minutes to the tidal cycle. As a result, the tidal period is 24 hours and 50 minutes in length.
- The second factor controlling tides on the Earth's surface is the Sun's gravity.
- The height of the average solar tide is about 50% the average lunar tide.
- At certain times during the moon's revolution around the Earth, the direction of its gravitational attraction is aligned with the Sun's (Figure 2).
- During these times the two tide producing bodies act together to create the highest and lowest tides of the year.
- These springtides occur every 14-15 days during full and new moons.
Figure 2: Forces involved in the formation of a spring tide.
Neap tides
- When the gravitational pull of the moon and Sun are at right angles to each other, the daily tidal variations on the Earth are at their least (Figure 3).
- These events are called neap tides and they occur during the first and last quarter of the moon.
Figure 3: Forces involved in the formation of a neap tide.
