Oceanography: Relief, Temperature, Salinity, Currents and Tides

The blue planet

Oceans cover about 71 per cent of the Earth's surface and hold about 97 per cent of its water. They store and move heat, drive the monsoon and cyclones, absorb about a quarter of human carbon dioxide emissions, supply food and minerals, and carry most world trade. India, with a coastline of about 11,098 km (revised estimate, 2024, up from the earlier 7,516 km) and an Exclusive Economic Zone of over 2 million sq km, has a strong stake in the oceans – the basis of its Blue Economy and the Deep Ocean Mission. UPSC asks about ocean floor relief, the distribution of temperature and salinity, ocean currents and their effects, tides, and marine resources such as coral reefs.

Relief of the ocean floor

• Continental shelf – the gently sloping, shallow extension of the continent, usually up to about 200 metres deep. Shelves are rich in fish (sunlight and nutrients) and in oil and gas (for example, Mumbai High). Width varies: very wide off the Siberian coast and in the Gulf of Khambhat, narrow along the coasts of Chile and India's east coast.
• Continental slope – the steep edge where the shelf drops to the deep ocean, often cut by submarine canyons (such as the Hudson canyon).
• Continental rise – a gentle apron of sediment at the base of the slope.
• Abyssal plains – the flat, deep ocean floor (3,000–6,000 m), covered with fine sediment and polymetallic nodules of manganese, nickel, cobalt and copper.
• Mid-oceanic ridges – underwater mountain chains formed at divergent plate boundaries, with a central rift valley; the Mid-Atlantic Ridge is the longest, and Iceland is a part of it above sea level.
• Trenches – the deepest parts, at subduction zones; mostly in the Pacific, such as the Mariana Trench (Challenger Deep, about 10,900 m). The Sunda (Java) Trench is the deepest in the Indian Ocean.
• Seamounts and guyots (flat-topped seamounts), atolls and coral reefs.

Temperature of ocean water

Surface temperature is highest near the equator (about 26–30°C) and falls towards the poles. It is also affected by ocean currents, prevailing winds, enclosed seas (the Red Sea and Persian Gulf are very warm) and upwelling of cold water. Oceans in the Northern Hemisphere are slightly warmer because of more land.

Vertically, oceans have three layers:

1. A warm surface (mixed) layer about 500 m thick in the tropics, mixed by winds.
2. The thermocline, where temperature falls rapidly with depth (500–1,000 m).
3. A cold, dark deep layer close to 0–4°C.

Salinity

Salinity is the amount of dissolved salt in sea water, usually measured as parts per thousand (‰) or practical salinity units. The average is about 35‰. Sodium chloride is the main salt, followed by magnesium chloride and magnesium sulphate.

Factors: evaporation raises salinity; precipitation, river inflow and melting ice lower it; ocean currents and mixing redistribute it.

• Highest in the subtropics (about 20°–30°N and S) where evaporation is high and rain is low – not at the equator, where heavy rainfall dilutes the water.
• Enclosed seas in hot, dry regions have very high salinity: Red Sea (about 40‰), Persian Gulf, and especially inland water bodies such as Lake Van (Turkey, ~330‰), the Dead Sea (~238‰) and the Great Salt Lake (~220‰).
• Low salinity in the Baltic Sea and Bay of Bengal, which receive large river inflows and heavy rain; the Arabian Sea is saltier than the Bay of Bengal because of higher evaporation and lower river inflow.

Ocean currents

Ocean currents are large, steady movements of water, like rivers in the sea. They are caused by planetary winds, differences in temperature and salinity (density), the Coriolis force and the shape of the coastlines.

Warm currents flow from low to high latitudes, usually along the eastern coasts of continents; cold currents flow from high to low latitudes, usually along the western coasts in low and middle latitudes. Together they form large circular systems called gyres – clockwise in the Northern Hemisphere and anticlockwise in the Southern.

The North Indian Ocean is unique: its currents reverse with the monsoon, flowing clockwise in summer (south-west monsoon current) and anticlockwise in winter (north-east monsoon current).

Effects of currents:

• Climate: the Gulf Stream and North Atlantic Drift keep north-west Europe much warmer than other places at the same latitude and keep ports such as Hammerfest and Murmansk ice-free; cold currents help create coastal deserts.
• Fishing grounds: where warm and cold currents meet, plankton thrive, creating rich fishing grounds and frequent fog – the Grand Banks off Newfoundland (Gulf Stream + Labrador), the Dogger Bank (North Sea) and the waters off Japan (Kuroshio + Oyashio). Upwelling along Peru creates rich anchovy fisheries.
• Navigation: ships follow favourable currents; icebergs carried by the Labrador current sank the Titanic (1912).
• Thermohaline circulation – the global "conveyor belt" driven by density differences, sinking in the North Atlantic and around Antarctica. Scientists warn that warming and melting could weaken the Atlantic Meridional Overturning Circulation (AMOC).

Tides

Tides are the periodic rise and fall of sea level, caused mainly by the gravitational pull of the Moon and, to a lesser extent, the Sun, together with the centrifugal force of the Earth–Moon system. Most coasts have two high tides and two low tides roughly every 24 hours 50 minutes.

• Spring tides – the highest high tides and lowest low tides, when the Sun, Moon and Earth are in a line (new moon and full moon).
• Neap tides – smaller range, when the Sun and Moon are at right angles (first and last quarter).
• Perigee (Moon closest) brings higher tides; apogee lower ones.
• The Bay of Fundy (Canada) has the world's highest tidal range (about 16 m). In India, the Gulf of Khambhat and Gulf of Kutch have high tidal ranges, suitable for tidal energy; the Hooghly has a tidal bore.

Tides help navigation by deepening harbours (Kolkata, Kandla), flush pollutants from estuaries and support mangrove ecosystems.

Coral reefs

Corals are colonies of tiny marine animals (polyps) living in symbiosis with algae called zooxanthellae, which give them colour and food. They need warm (20–28°C), clear, shallow, salty water with sunlight. Types: fringing reefs (close to the shore), barrier reefs (separated by a lagoon – the Great Barrier Reef, Australia) and atolls (ring-shaped reefs around a lagoon – the Lakshadweep and Maldives). In India, reefs are found in the Gulf of Mannar, Gulf of Kutch, Palk Bay, Andaman and Nicobar Islands and Lakshadweep. Rising sea temperatures cause coral bleaching, when corals expel their algae; the fourth global bleaching event was declared in 2024. Ocean acidification, pollution and destructive fishing add to the threat.

Mains practice question

Explain the factors that control the distribution of ocean currents. How do they influence the climate and economy of coastal regions? (250 words)

Answer framework

• Intro: definition; warm and cold currents; gyres.
• Factors: planetary winds, density (temperature, salinity), Coriolis, coastlines; monsoon reversal in North Indian Ocean.
• Influence: climate (Gulf Stream, coastal deserts), fisheries (Grand Banks, Peru), navigation, fog.
• Conclusion: risks from warming to thermohaline circulation.