Oceanography

Overview

The ocean is best understood as a single connected body of water divided by convention into basins, not as separate seas. Its average depth is far greater than the average height of land above sea level, so the volume of the ocean dwarfs the volume of the land that rises above it. The ocean floor is not flat. It has mountains higher than many land ranges, plains flatter than any land plain, and trenches deeper than any land valley is high. Mapping that relief, first by echo sounding and later by satellite and sonar surveys, was one of the great discoveries of modern geography, because it revealed mid-ocean ridges and trenches that confirmed plate tectonics and explained the distribution of earthquakes and volcanoes in the oceans. Ocean relief, ocean chemistry, and ocean movement are therefore not three separate topics but three views of one system: the shape of the basin guides the currents, the currents carry heat and salt, and heat and salt set density, which in turn drives deeper movement.

Relief and composition of oceans

The continental shelf is the gently sloping, shallow extension of a continent under the sea, generally ending at the shelf break where the slope steepens sharply. Shelves are economically important out of proportion to their area: they hold most marine life, most fisheries, and many mineral and hydrocarbon deposits, because sunlight reaches their shallow waters and nutrients are supplied from land and by mixing. Beyond the shelf break, the continental slope falls steeply to the deep ocean floor, often cut by submarine canyons that carry sediment from the shelf into the deep sea. At the foot of the slope, the continental rise is a gentle apron of sediment accumulated from the slope above. Together, shelf, slope, and rise form the continental margin, the transition from continental to oceanic crust. Passive margins, where a continent meets ocean without a plate boundary, tend to have wide shelves and thick sediment; active margins, where subduction occurs, tend to have narrow shelves, steep slopes, and deep trenches close to the coast.

The deep ocean floor is dominated by abyssal plains, vast flat areas covered by fine sediment, interrupted by abyssal hills, seamounts, and guyots — flat-topped seamounts whose tops were planed by waves when they stood above sea level and later subsided. Mid-ocean ridges are long, globe-encircling mountain chains where new oceanic crust is created; their central rift valleys, volcanic activity, and shallow earthquakes mark divergent plate boundaries. Deep-sea trenches are narrow, very deep depressions where oceanic crust is subducted, marking convergent boundaries; they are the deepest places on Earth and are associated with strong earthquakes and volcanic island arcs. Oceanic islands fall into two main kinds: volcanic islands built by eruptions, often in chains over hotspots or along ridges, and coral islands and atolls built by coral growth on subsiding volcanic foundations, a relationship first explained by the subsidence theory of atoll formation.

Seawater is a solution, not pure water. Its salinity, the total dissolved salts, averages a little above three and a half percent by weight, with sodium chloride dominant, followed by magnesium, sulphur, calcium, and potassium compounds. Salinity varies with the balance of evaporation, precipitation, river inflow, and freezing: it is high in hot, dry subtropical regions where evaporation exceeds rainfall, and where enclosed seas lose water by evaporation with little inflow; it is low near the mouths of large rivers, in regions of heavy rainfall, and where melting ice dilutes the surface. Temperature also varies: surface water is warmest near the equator and coldest near the poles, and temperature falls with depth, rapidly through the thermocline, the layer of rapid change, and then slowly toward the cold deep water. Density depends on both temperature and salinity: cold, salty water is densest and sinks, driving the deep circulation, while warm, fresh water is lightest and stays near the surface. This density structure explains why the deep ocean is cold almost everywhere, even beneath warm tropical surface waters.

Ocean circulation, waves and tides

Ocean circulation has two linked parts. Surface currents, flowing in the upper layers, are driven mainly by wind, deflected by the Coriolis force, and shaped by the continents they meet. In each ocean basin, surface currents form large circular gyres: clockwise in the northern hemisphere and anticlockwise in the southern hemisphere, matching the deflection of the winds that drive them. Western boundary currents, on the western side of ocean basins, are narrow, deep, and fast, carrying warm water poleward; eastern boundary currents, on the eastern side, are broad, shallow, and slow, carrying cold water equatorward. This asymmetry matters for climate: warm western boundary currents moderate the climates of coasts they pass, while cold eastern boundary currents cool and stabilise the air above them, suppressing rainfall on adjacent coasts and favouring fog and aridity. Where surface water is driven away from a coast or from the equator, deep water rises to replace it, in upwelling. Upwelled water is cold and rich in nutrients, so upwelling zones support exceptionally productive fisheries, even though they cover a small fraction of the ocean area. Downwelling occurs where surface waters converge and sink.

Deep circulation, sometimes called thermohaline circulation because it depends on temperature and salinity, moves water through the whole ocean depth over long timescales. Cold, salty water formed in high latitudes sinks and spreads along the ocean floor, eventually rising again in other basins, in a slow global conveyor that transports heat, salt, oxygen, and nutrients. Because this circulation is slow, a change in surface temperature or salinity in the sinking regions can affect the deep ocean for a very long time, and a weakening of deep circulation would have far-reaching climatic consequences. The ocean and atmosphere are coupled: winds drive surface currents, currents redistribute heat, heat affects atmospheric pressure and winds, and the cycle continues. That coupling is why an ocean anomaly such as a warm event in the tropical Pacific is never only an ocean event.

Waves are oscillations of the water surface caused mainly by wind. In deep water, the water itself moves in small circles and returns almost to its starting point; it is the wave form, the energy, that travels forward. Wave size depends on wind speed, duration, and fetch, the distance over which the wind blows. As waves approach the shore, they feel the bottom, slow, steepen, and break, and their energy is spent eroding cliffs, moving sediment along the shore, and building beaches. Longshore drift, the zigzag movement of sediment along a beach by waves approaching at an angle, is the main mechanism of coastal sediment transport. Tides are the periodic rise and fall of sea level caused by the gravitational attraction of the Moon, and to a lesser extent the Sun, combined with the Earth's rotation. When Sun, Moon, and Earth are aligned, their pulls reinforce each other and produce spring tides with a large range; when the Sun and Moon pull at right angles, they partly cancel and produce neap tides with a small range. Tides matter for navigation, for port access, for intertidal ecosystems, and, in a few places with very large ranges, for tidal energy. Tsunamis are not tides at all, despite the old name tidal wave: they are waves generated by sudden displacement of the sea floor in earthquakes, landslides, or volcanic eruptions, travelling fast across deep water with low height and growing dangerously tall as they enter shallow water.

Sea level changes and ocean hazards

Sea level is not fixed. It changes because the volume of water in the oceans changes and because the land itself rises or falls. Eustatic change is a change in the global volume of ocean water, caused mainly by the growth or melting of ice sheets and glaciers and by thermal expansion as ocean water warms. Isostatic change is a change in the level of the land relative to the sea, caused by loading or unloading of the crust, sediment accumulation, or tectonic movement. A coast can therefore experience a relative sea-level rise even while global sea level is stable, if the land is subsiding, and a coast can experience a relative fall if the land is rebounding after ice removal. Evidence for past sea-level changes comes from raised beaches and marine terraces above present sea level, drowned river valleys and forests below it, coral reefs that grew upward to keep pace with rising water, and sediment records. Present sea-level rise, driven by warming and ice loss, threatens low-lying coasts, deltas, and islands with flooding, erosion, saltwater intrusion into groundwater and farmland, and the loss of wetlands that buffer storm surges. Adaptation includes coastal defences, managed retreat, restoration of mangroves and wetlands, and land-use planning that avoids new building in the most exposed places.

Ocean hazards include both sudden events and slow degradation. Storm surges, the rise of sea level driven by cyclone winds and low pressure, flood coasts at the moment of landfall and are often the deadliest part of a tropical cyclone. Tsunamis, generated by undersea earthquakes and landslides, can cross whole ocean basins and strike coasts with little warning where detection and communication are weak; warning systems, evacuation routes, and public education are the main defences, since the waves themselves cannot be prevented. Coastal erosion, accelerated by storms, sea-level rise, and human interference with sediment supply, such as damming rivers that once fed beaches, removes land and undermines buildings. Pollution threatens the ocean differently: oil spills coat wildlife and shores, plastic debris persists and is eaten by marine animals, excess nutrients from farmland cause algal blooms and oxygen-depleted dead zones, and overfishing depletes stocks faster than they can recover. Marine conservation, through protected areas, fishing limits, pollution control, and international agreements on the law of the sea, is therefore not an optional extra but a condition for the ocean to continue providing food, climate regulation, and livelihoods. The ocean's size can make its problems seem remote, but its circulation ensures that pollution, warming, and depletion in one basin eventually affect others.

Ocean floor features compared

Waves and tides compared