Oceanography

Core concept and process

Ocean relief

The ocean floor is not flat. From the shore outward and downward:

• Continental shelf: the shallow, gently sloping submerged edge of the continent (to about 200 m deep). It is the richest zone for fisheries and the main source of offshore oil and gas, and it defines much of the Exclusive Economic Zone.
• Continental slope: the steep drop at the shelf edge to the deep floor; submarine canyons cut through it.
• Continental rise: the gentler apron of sediment at the foot of the slope.
• Abyssal plain: the vast, flat, deep-ocean floor (about 3,000 to 6,000 m).
• Oceanic deeps and trenches: the deepest features, formed at subduction zones. The deepest known point on earth is the Challenger Deep in the Mariana Trench (western Pacific), at about 11,000 m. Other major trenches include the Tonga, Philippine, Java (Sunda) and Puerto Rico trenches.
• Mid-oceanic ridges: undersea mountain chains at divergent plate boundaries (the Mid-Atlantic Ridge), and isolated seamounts and guyots (flat-topped seamounts) rise from the plains.

Salinity

Salinity is the total dissolved salt per thousand parts of water (parts per thousand, ppt), averaging about 35 ppt in the open ocean. It is higher in hot, enclosed seas with high evaporation and little freshwater inflow, and lower where rivers add fresh water or rainfall is heavy. The Dead Sea (about 240 ppt) and Lake Van are hypersaline; among large seas the Red Sea and Persian Gulf are highly saline; the Baltic Sea is among the least saline (cold, enclosed, heavy river inflow). Salinity is generally highest in the subtropics (high evaporation, low rainfall) and lower at the equator (heavy rain) and the poles (ice melt).

Temperature and the thermocline

Ocean temperature is highest at the surface in the tropics and falls with depth and toward the poles. The vertical structure has a warm, well-mixed surface layer, then the thermocline (a layer of rapid temperature decrease with depth), then a near-freezing deep layer. The thermocline is sharpest in the tropics and weak or absent in the polar oceans.

Ocean currents

Currents are large, persistent flows of surface water driven mainly by the prevailing planetary winds, modified by the Coriolis force, salinity and temperature differences (density), and the shape of the coasts and ocean basins. They organise into great circular gyres: clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere.

• Warm currents flow from the tropics toward the poles, raising the temperature of the coasts they wash. The Gulf Stream and its extension the North Atlantic Drift keep north-west Europe mild and ice-free far north.
• Cold currents flow from the poles toward the tropics, or rise as cold upwelling along western coasts, lowering coastal temperatures and bringing nutrient-rich water that supports great fisheries (the Humboldt or Peru Current off western South America).
• Where a warm current meets a cold current, dense fog forms and plankton thrives, creating some of the world's best fishing grounds, classically the Grand Banks off Newfoundland (Gulf Stream meeting the cold Labrador Current).

A deep, slow, density-driven global circulation (the thermohaline circulation or "great ocean conveyor belt") links the surface currents and redistributes heat over centuries.

Tides

Tides are the periodic rise and fall of sea level caused by the gravitational pull of the moon (the dominant influence, being closer) and the sun, plus the centrifugal effect of the earth-moon system. There are normally two high and two low tides each lunar day (semi-diurnal).

• Spring tides: when the sun, earth and moon are in a line (at new moon and full moon, called syzygy), the pulls add, giving the highest high tides and lowest low tides. These occur about twice a month and are unrelated to the season of spring.
• Neap tides: when the sun and moon are at right angles to the earth (the first and third quarter moons), the pulls partly cancel, giving the lowest tidal range.

The Bay of Fundy in Canada records the world's highest tidal range. High tidal ranges support tidal energy (Gulf of Khambhat and Gulf of Kutch in India have potential).

Marine deposits and coral reefs

The ocean floor is blanketed by sediments. Terrigenous deposits (gravel, sand, mud, washed from land) cover the shelves and slopes; pelagic deposits cover the deep floor and are of two kinds: oozes (organic remains of tiny organisms, calcareous or siliceous) and red clay (inorganic, in the deepest basins). Coral reefs are built by tiny coral polyps that need warm (above about 20° Celsius), shallow, clear, salty, sediment-free tropical water, which is why they fringe the tropics. Charles Darwin's classification gives three forms: fringing reefs (attached to the shore), barrier reefs (separated from shore by a lagoon, the Great Barrier Reef off Australia being the largest), and atolls (ring-shaped reefs round a lagoon where an island has subsided, common in the Maldives and Lakshadweep). Coral bleaching, the loss of the symbiotic algae under heat stress, is a growing climate concern.

Waves and maritime zones

Waves are wind-driven surface oscillations of energy. Tsunamis (from submarine earthquakes) are not wind waves and travel as long, fast waves across the ocean. Under the United Nations Convention on the Law of the Sea, a coastal State has a territorial sea to 12 nautical miles, a contiguous zone to 24 nautical miles, and an Exclusive Economic Zone (EEZ) to 200 nautical miles, within which it controls fishing, mineral and energy resources.

Ocean resources and the blue economy

The oceans hold biotic resources (fisheries, the great catch zones lying over the continental shelves and at cold-current upwellings), mineral resources (offshore petroleum and gas on the shelf, placer deposits of monazite and other minerals in coastal sands, and polymetallic or manganese nodules of nickel, copper and cobalt scattered on the deep abyssal floor), and energy resources (tidal, wave and ocean-thermal). India holds a deep-sea exploration allotment for polymetallic nodules in the Central Indian Ocean Basin and runs a Deep Ocean Mission to develop these technologies. The framework of using the sea sustainably for growth is the "blue economy", which is why the EEZ and the continental-shelf claim matter economically as well as for security.

El Nino, La Nina and the IOD

Normally the equatorial Pacific trade winds pile warm water in the west (Indonesia) and allow cold upwelling in the east (Peru), with rising air in the west, the Walker circulation.

• El Nino is the periodic warming of the central and eastern equatorial Pacific, with weakened or reversed trade winds and a disrupted Walker circulation. El Nino years are associated with weak or deficient Indian summer monsoons, drought in Australia and Indonesia, and floods on the Pacific coast of South America.
• La Nina is the opposite cool phase, with intensified trades and stronger cold upwelling; La Nina years generally favour a stronger Indian monsoon.
• The whole oscillation is ENSO (El Nino Southern Oscillation), the "Southern Oscillation" being the see-saw of air pressure between the eastern and western Pacific.
• The Indian Ocean Dipole (IOD) is the analogous see-saw in the Indian Ocean: a positive IOD (warmer western Indian Ocean) tends to strengthen the Indian monsoon and can offset an El Nino.