Oceans and Ocean Currents: Tides, Salinity and PYQs

Oceans cover about seventy-one per cent of the surface of the earth and hold roughly ninety-seven per cent of all the water on the planet. Because that water is constantly in motion, this single topic supplies questions on ocean relief, salinity, tides, currents and El Nino in almost every general studies paper. The facts below are the ones that repeat.

The five oceans

The International Hydrographic Organization recognises five oceans. The Pacific is the largest and the deepest, roughly triangular, and it holds the Mariana Trench, whose Challenger Deep is the deepest known point on earth at about eleven kilometres below sea level. The rim of the Pacific is the Ring of Fire, a horseshoe of volcanoes and earthquake zones. The Atlantic is the second largest and is S-shaped; its heavily indented coastline and the traffic between Europe and North America make it the busiest trading ocean, and the Mid-Atlantic Ridge runs down its centre. The Indian Ocean is the only ocean named after a country, and it is the only one whose northern currents reverse with the monsoon. The Southern Ocean surrounds Antarctica, and the Arctic Ocean is the smallest and shallowest.

Relief of the ocean floor

Moving seaward from the coast, the ocean floor is divided into the continental shelf, the continental slope, the continental rise, the abyssal plain and the deeps or trenches. The continental shelf is the shallow submerged extension of the land, usually taken to a depth of about two hundred metres. It is the richest part of the ocean because sunlight reaches the bottom, so most of the world's great fishing grounds and most offshore oil lie there. Beyond the abyssal plains rise mid-oceanic ridges, submarine volcanic peaks called seamounts, flat-topped seamounts called guyots, and coral atolls. Trenches are the deepest features and lie close to volcanic island arcs.

Salinity and temperature

Salinity is the amount of dissolved salt in sea water, measured in parts per thousand, and the average for the oceans is about thirty-five parts per thousand. It is highest in the subtropics near twenty to forty degrees latitude, where evaporation is strong and rainfall low, and lower at the equator, where heavy rain dilutes the water, and in polar seas, where melting ice does the same. Among open seas the Red Sea is the most saline, the Baltic Sea the least. Enclosed water bodies such as Lake Van in Turkey and the Dead Sea are far more saline than any ocean. Sea temperature falls with depth, and the layer of rapid fall between the warm surface layer and the cold deep water is called the thermocline.

Waves, tides and tsunamis

Waves are made by wind dragging on the surface; the water itself moves very little while the energy travels forward. Tides are the periodic rise and fall of sea level caused by the gravitational pull of the moon and the sun together with the centrifugal force of the earth-moon system. When the sun, the earth and the moon lie in a straight line, at new moon and full moon, their pulls add up and produce the highest tides, called spring tides. When the sun and the moon are at right angles to the earth, at the first and the third quarter, the pulls partly cancel and give the lowest tides, called neap tides. The moon's pull is the stronger of the two because it is much nearer. The highest tidal range in the world is recorded in the Bay of Fundy in Canada. A tsunami is not a tide at all: it is a train of long waves set off by an undersea earthquake, landslide or volcanic eruption, and the Indian Ocean tsunami of December 2004 is the standard example.

Ocean currents

Currents are steady movements of a large mass of ocean water in a definite direction. They are driven by the planetary winds, by differences in temperature and salinity and therefore in density, and are deflected by the Coriolis force, which turns them to the right in the northern hemisphere and to the left in the southern. The result is a set of large circular gyres, clockwise north of the equator and anticlockwise south of it. Currents flowing from the equator towards the poles are warm; those flowing from the poles towards the equator are cold.

What currents do

Where a warm and a cold current meet, plankton multiplies and the world's best fishing grounds appear, along with dense fog. The Grand Banks off Newfoundland, where the Gulf Stream meets the Labrador current, and the seas off Hokkaido, where the Kuroshio meets the Oyashio, are the two examples every paper uses. The North Atlantic Drift keeps the ports of north-western Europe free of ice in winter although they lie far north. Cold currents flowing along western coasts in the tropics chill the air above them and help create coastal deserts such as the Atacama beside the Peru current and the Namib beside the Benguela current. El Nino is the periodic appearance of unusually warm water off the coast of Peru and Ecuador, which weakens the trade winds, ruins the anchovy fishery and is generally associated with a weak Indian monsoon; the opposite, cooler phase is called La Nina, and the linked pressure see-saw is the Southern Oscillation.