Water and Oceans

The One Thing

Oceans are dynamic systems rather than uniform bodies of water: their temperature, salinity, density, relief and movements vary across space and depth. These variations regulate climate, support marine ecosystems and influence human activities such as fishing, navigation and resource extraction.

Who and What

• Ocean: A large, continuous body of saline water covering about 71% of Earth’s surface. Oceans contain about 97% of Earth’s total water and strongly influence climate, weather, ecosystems and human activities.
• Hydrological cycle: The continuous circulation of water through evaporation, condensation, precipitation, runoff, infiltration and collection in oceans and other water bodies.
• Ocean salinity: The total amount of dissolved salts in seawater, commonly expressed in parts per thousand (‰). Average ocean salinity is about 35‰, meaning that approximately 35 grams of dissolved salts occur in 1 kilogram of seawater.

Salinity (‰) = Mass of dissolved salts ÷ Mass of seawater × 1000.

• Temperature of ocean water: The degree of heat in seawater. It is generally highest near the surface in the tropics and decreases toward the poles and with increasing depth.
• Density of ocean water: The mass of seawater per unit volume.

Density = Mass ÷ Volume. Average seawater density is approximately 1.025 g/cm³ and increases mainly when temperature decreases or salinity increases.

• Thermocline: A layer where temperature decreases rapidly with increasing depth.
• Halocline: A layer where salinity changes rapidly with depth.
• Pycnocline: A layer where seawater density changes rapidly with depth, often because of temperature and salinity changes.
• Continental shelf: The gently sloping, shallow submerged extension of a continent, generally ending at the shelf break. It is economically important because it contains fisheries, petroleum, natural gas and other mineral resources.
• Continental slope: The steep descent from the edge of the continental shelf to the deep ocean floor.
• Continental rise: A gentle accumulation of sediments at the base of a continental slope, gradually merging with the deep-sea floor.
• Deep-sea plain: A broad, nearly level area of the ocean floor, usually covered by fine sediments.
• Mid-oceanic ridge: A long underwater mountain chain formed mainly at divergent plate boundaries. It is associated with seafloor spreading and the creation of new oceanic crust and forms the longest mountain system on Earth.
• Ocean trench: A long, narrow and very deep depression in the ocean floor, commonly associated with subduction zones. The Mariana Trench is the deepest known ocean trench, reaching about 11 km in the Challenger Deep region.
• Seamount: An underwater volcanic mountain that does not rise above sea level.
• Guyot: A flat-topped seamount, usually shaped by erosion and later submerged.
• Coral reef: A marine structure built mainly from coral skeletons, usually in warm, shallow and clear tropical waters.
• Wave: A rhythmic movement of ocean water, mainly produced by wind. Water particles generally move in orbital paths rather than travelling forward with the wave.
• Wave length: The horizontal distance between two successive wave crests or two successive wave troughs.
• Wave height: The vertical distance between a wave crest and the adjacent trough.
• Tide: The regular periodic rise and fall of sea level caused mainly by the gravitational forces of the Moon and the Sun, together with Earth’s rotation.
• Spring tide: A tide with the greatest tidal range, occurring near the new Moon and full Moon when the Sun, Moon and Earth are nearly aligned.
• Neap tide: A tide with the smallest tidal range, occurring near the first- and third-quarter phases of the Moon when the Sun and Moon act at roughly right angles.
• Ocean current: A relatively persistent, directed movement of ocean water caused by winds, density differences, Earth’s rotation, pressure differences and the shapes of coastlines and ocean basins.
• Surface current: A current mainly driven by prevailing winds and influenced by the Coriolis effect and the arrangement of continents.
• Deep ocean current: A current driven mainly by differences in seawater density caused by variations in temperature and salinity. It forms part of global thermohaline circulation.
• Warm current: A current carrying relatively warm water from low latitudes toward higher latitudes, often raising coastal temperatures and increasing humidity.
• Cold current: A current carrying relatively cold water from high latitudes or deep ocean regions toward lower latitudes, often lowering coastal temperatures and producing dry coastal conditions.
• Upwelling: The rising of cold, nutrient-rich deep water to the surface. It supports high biological productivity and rich fisheries, especially along some western continental margins.
• Coriolis effect: The apparent deflection of moving air and water caused by Earth’s rotation: toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere.
• Thermohaline circulation: The large-scale movement of ocean water driven by differences in temperature and salinity, linking surface and deep waters across ocean basins.

Causes and Consequences

• The global distribution of water establishes the importance of oceans. Water covers about 71% of Earth’s surface, while land covers about 29%. About 97% of Earth’s total water is saline water in oceans and seas; most remaining freshwater is stored in glaciers, ice sheets and groundwater. Through the hydrological cycle, oceans exchange water with the atmosphere and land.

• Dissolved salts produce ocean salinity. Major dissolved salts include chlorides and sulphates of sodium, magnesium, calcium and potassium, with sodium chloride the most abundant. Average salinity is approximately 35‰. Salinity increases where evaporation is high and freshwater supply is low, and decreases where rainfall, river discharge, melting ice or freshwater inflow is high.

• Temperature varies horizontally and vertically. Ocean temperature generally decreases from the equator toward the poles and from the surface downward. The rapid temperature-change zone is the thermocline. Variations in temperature contribute to changes in seawater density and influence the formation of deep ocean currents.

• Salinity, temperature and pressure determine density. Seawater becomes denser when temperature decreases or salinity increases. Rapid changes in density form a pycnocline, while rapid salinity changes form a halocline. These layers contribute to the separation and movement of different water masses.

• Ocean relief is shaped by several geological processes. Plate tectonics, volcanism, sediment deposition, erosion and subduction produce a varied ocean floor consisting of continental margins, deep-sea basins, mid-oceanic ridges, trenches, seamounts, guyots and coral reefs. The continental margin includes the continental shelf, continental slope and continental rise.

• The continental shelf connects ocean relief with human economic activity. Its shallow waters support fisheries and contain petroleum, natural gas and other mineral resources. Beyond the shelf, the continental slope descends toward the deep-sea floor, while sediments accumulate on the continental rise.

• Mid-oceanic ridges are associated with divergent plate boundaries. Seafloor spreading at these ridges creates new oceanic crust. By contrast, subduction zones commonly produce ocean trenches, including the Mariana Trench and its Challenger Deep region.

• Wind produces most waves and transfers energy across the ocean. Stronger winds, longer duration and greater fetch generally produce larger waves. Wave water particles usually move in orbital paths rather than travelling forward with the wave. Wave length measures the distance between successive crests or troughs, while wave height measures the vertical distance between a crest and the adjacent trough.

• Gravity and Earth’s rotation produce tides. The Moon’s tidal force is stronger than the Sun’s because the Moon is much closer to Earth, even though the Sun is much larger. In many coastal areas, tides occur approximately twice daily, producing two high tides and two low tides in about 24 hours and 50 minutes, although the pattern varies with coastline and basin shape.

• The alignment of the Sun, Moon and Earth controls tidal range. Spring tides occur during the new Moon and full Moon, when the three bodies are nearly aligned and the tidal range is greatest. Neap tides occur during the first-quarter and third-quarter phases, when the Sun and Moon act at roughly right angles and the tidal range is smallest.

• Ocean currents transport water over persistent paths. They are influenced by prevailing winds, the Coriolis effect, temperature and salinity differences, pressure gradients, coastlines and ocean-basin shape. Large-scale ocean gyres generally circulate clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere.

• Warm and cold currents modify coastal environments. Warm currents transport water from low to high latitudes, usually raising coastal temperatures and increasing humidity. Cold currents transport water from high latitudes or deep ocean regions toward lower latitudes, often lowering temperatures and producing dry coastal conditions. Ocean currents therefore influence coastal rainfall, fog formation and the climate of nearby land areas.

• Density differences generate deep ocean circulation. Variations in temperature and salinity produce deep ocean currents and contribute to thermohaline circulation, which links surface and deep waters across ocean basins. This circulation redistributes heat from low latitudes toward higher latitudes and helps regulate global climate.

• Upwelling links physical oceanography to biological productivity. When cold, nutrient-rich deep water rises to the surface, nutrients become available to marine organisms. This supports productive fishing grounds and rich fisheries, particularly along some western continental margins.

• Ocean movements have environmental and economic consequences. Waves, tides and currents influence navigation, fishing, coastal rainfall, fog, marine ecosystems and human livelihoods. Together, they redistribute heat and nutrients and connect distant ocean regions.

What Gets Asked

• Explain how ocean temperature, salinity and density vary horizontally and vertically, including the roles of the thermocline, halocline and pycnocline.
• Distinguish between waves, tides and currents in terms of their causes, movement and effects.
• Compare spring tides and neap tides, including the positions of the Sun, Moon and Earth and the resulting tidal range.
• Explain how plate tectonics, volcanism, sediment deposition, erosion and subduction produce continental margins, deep-sea plains, mid-oceanic ridges, trenches, seamounts and guyots.
• Assess how warm currents, cold currents, upwelling and thermohaline circulation influence climate, marine ecosystems, fisheries and coastal human activities.
• Apply and interpret the equations Density = Mass ÷ Volume and Salinity (‰) = Mass of dissolved salts ÷ Mass of seawater × 1000, using the given seawater values of approximately 1.025 g/cm³ and 35‰.