Chapter 3 - The Physical Setting

Chapter 3 Contents

Earth is a oblate ellipsoid, an ellipse of rotation, with an equatorial radius of

Re = 6,378.1349 km (West, 1982)

which is slightly greater than the polar radius of

Rp = 6,356.7497 km.

The small equatorial bulge is due to Earth's rotation.

Distances on Earth are measured in many different units, the most common are degrees of latitude or longitude, meters, miles, and nautical miles.

  1. Latitude is the angle between the local vertical and the equatorial plane. A meridian is the intersection at Earth's surface of a plane perpendicular to the equatorial plane and passing through Earth's axis of rotation.
  2. Longitude is the angle between the standard meridian and any other meridian, where the standard meridian is that which passes through a point at the Royal Observatory at Greenwich, England. Thus longitude is measured east or west of Greenwich.
  3. A degree of latitude is not the same length as a degree of longitude except at the equator. Latitude is measured along great circles with radius R, where R is the mean radius of Earth. Longitude is measured along circles with radius R cos(φ), where φ is latitude. Thus 1° latitude = 111 km, and 1° longitude = 111 cos(φ) km. For careful work, remember that Earth is not a sphere, and latitude varies slightly with distance from the equator. The values listed here are close enough for our discussions of the ocean.

Because distance in degrees of longitude is not constant, oceanographers measure distance on maps using degrees of latitude.

Nautical miles and meters are connected historically to the size of Earth. Gabriel Mouton, who was vicar of St. Paul's Church in Lyons, France, proposed in 1670 a decimal system of measurement based on the length of an arc that is one minute of a great circle of Earth. This eventually became the nautical mile. Mouton's decimal system eventually became the metric system based on a different unit of length, the meter, which was originally intended to be one ten millionth the distance from the Equator to the pole along the Paris meridian. Although the tie between nautical miles, meters, and Earth's radius was soon abandoned because it was not practical, the approximations are still useful. For example, the Earth's polar circumference is approximately 40,008 km. Therefore one ten-millionth of a quadrant is 1.0002 m. Similarly, a nautical mile should be 1.8522 km, which is very close to the official definition of the international nautical mile: 1 nm 1.8520 km.

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3.1 Ocean and Seas

There is only one ocean. It is divided into three named parts by international agreement: the Atlantic, Pacific, and Indian Ocean (International Hydrographic Bureau, 1953). The seas, which are part of the ocean, are defined in several ways, and we will consider two.

Figure 3.1 The Atlantic Ocean viewed with an Eckert VI equal-area projection. Depths, in meters, are from the ETOPO 30´ data set. The 200m contour outlines continental shelves.

The Atlantic Ocean extends northward from Antarctica and includes all of the Arctic Sea, the European Mediterranean, and the American Mediterranean more commonly known as the Caribbean sea (Figure 3.1). The boundary between the Atlantic and Indian Oceans is the meridian of Cape Agulhas (20°E) . The boundary between the Atlantic and Pacific Oceans is the line forming the shortest distance from Cape Horn to the South Shetland Islands. In the north, the Arctic Sea is part of the Atlantic Ocean, and the Bering Strait is the boundary between the Atlantic and Pacific.

Figure 3.2 The Pacific Ocean viewed with an Eckert VI equal-area projection. Depths, in meters, are from the ETOPO 30´ data set. The 200m contour outlines continental shelves.

The Pacific Ocean extends northward from Antarctica to the Bering Strait (Figure 3.2). The boundary between the Pacific and Indian Oceans follows the line from the Malay Peninsula through Sumatra, Java, Timor, Australia at Cape Londonderry, and Tasmania. From Tasmania to Antarctica it is the meridian of South East Cape on Tasmania 147°E.

Figure 3.3 The Indian Ocean viewed with an Eckert VI equal-area projection. Depths, in meters, are from the ETOPO 30´ data set. The 200m contour outlines continental shelves.

The Indian Ocean extends from Antarctica to the continent of Asia including the Red Sea and Persian Gulf (Figure 3.3). Some authors use the name Southern Ocean to describe the ocean surrounding Antarctica.

Mediterranean Seas are mostly surrounded by land. By this definition, the Arctic and Caribbean Seas are both Mediterranean Seas, the Arctic Mediterranean and the Caribbean Mediterranean.

Marginal Seas are defined by only an indentation in the coast. The Arabian Sea and South China Sea are marginal seas.

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