Oceanography — Quick Revision

A last-week revision sheet for oceanography. Most Prelims questions here are matching (current to ocean, feature to definition) or statement-based (salinity, tides, ENSO).

Relief of the Ocean Floor

Feature	What it is	Remember
Continental shelf	Gently sloping, shallow extension of the continent	Average width ~80 km (NCERT); richest fishing grounds; offshore oil (Mumbai High); largest — Siberian Shelf
Continental slope	Steep descent from the shelf edge to the deep ocean	Submarine canyons cut into it
Continental rise	Gentle apron of sediment at the base of the slope	Merges into the abyssal plain
Abyssal (deep-sea) plain	Flattest, smoothest areas on Earth, covered with fine sediment	Between ~3,000 and 6,000 m deep
Oceanic trench (deep)	Long, narrow, deepest parts of the ocean at convergent margins	Mariana Trench (Challenger Deep) — about 11 km deep, the deepest
Mid-ocean ridge	Underwater mountain chain at divergent margins	Longest mountain system on Earth; Iceland is part of the Mid-Atlantic Ridge
Seamount	Submerged volcanic mountain that does not reach the surface	—
Guyot	Flat-topped seamount	Top flattened by erosion when it stood at the surface
Atoll	Ring-shaped coral reef enclosing a lagoon	Lakshadweep islands are atolls

Temperature and Salinity

High-yield facts

Ocean temperature decreases with depth; the thermocline is the layer of rapid fall.
Average salinity ~35‰; open ocean 33–37‰ (NCERT).
Highest: Lake Van (~330‰) > Dead Sea (~238‰) > Great Salt Lake (~220‰); among seas, the Red Sea (~41‰).
Low: Baltic Sea, Black Sea; the Bay of Bengal is less saline than the Arabian Sea.
Open-ocean salinity peaks in the subtropics (high evaporation, low rain), not at the Equator.

Tides

Term	Meaning
Cause	Gravitational pull of the Moon (mainly) and the Sun, plus centrifugal force
Spring tide	Highest high tides and lowest low tides; Sun, Moon and Earth in line — new moon and full moon
Neap tide	Smallest tidal range; Sun and Moon at right angles relative to the Earth — first and third quarter
Interval	High tides about 12 hours 25 minutes apart in semi-diurnal tides
Perigee / apogee	Moon nearest / farthest — larger / smaller tidal ranges
Highest tidal range	Bay of Fundy (Canada)
India	Gulf of Khambhat and Gulf of Kutch — high tidal ranges, tidal-energy potential

Ocean Currents

Ocean	Warm	Cold
North Atlantic	Gulf Stream, North Atlantic Drift, Florida	Canary, Labrador
South Atlantic	Brazil	Benguela, Falkland
North Pacific	Kuroshio, Alaska, North Pacific Drift	California, Oyashio
South Pacific	East Australian	Humboldt (Peru)
Indian	Agulhas, Mozambique	West Australian
Southern	—	West Wind Drift

Gyres: clockwise in NH, anticlockwise in SH.
Low latitudes: warm on east coasts, cold on west coasts.
North Indian Ocean currents reverse with the monsoon.
Fishing grounds: Grand Banks (Gulf Stream + Labrador), off Japan (Kuroshio + Oyashio), Peru (upwelling).

Ocean–Atmosphere Links

Term	One-liner
Upwelling	Cold, nutrient-rich water rising along west coasts as offshore winds push surface water away
El Niño	Warming of central/eastern equatorial Pacific; trades weaken; tends to weaken the Indian monsoon
La Niña	Cooling of central/eastern equatorial Pacific; trades strengthen; tends to strengthen the monsoon
Southern Oscillation	Pressure see-saw between Tahiti and Darwin
Walker circulation	East–west air loop over the equatorial Pacific
Indian Ocean Dipole	West–east SST difference in the Indian Ocean; positive phase supports the monsoon
Thermohaline circulation	Density-driven deep circulation; the AMOC is its Atlantic branch

Maritime Zones under UNCLOS (1982)

Zone	Extent from baseline	Rights of the coastal state
Territorial sea	Up to 12 nautical miles	Sovereignty over water, seabed and airspace; foreign ships have the right of innocent passage
Contiguous zone	Up to 24 nautical miles	Control to prevent and punish violations of customs, fiscal, immigration and sanitary laws
Exclusive Economic Zone	Up to 200 nautical miles	Sovereign rights over living and non-living resources; other states keep freedom of navigation and overflight
Continental shelf	At least 200 nautical miles; can extend further up to limits set by UNCLOS	Rights over seabed resources
High seas	Beyond national jurisdiction	Open to all states

The International Seabed Authority (Kingston, Jamaica) regulates mineral activity on the deep seabed beyond national jurisdiction. The Agreement on Marine Biological Diversity of Areas Beyond National Jurisdiction (BBNJ, the “High Seas Treaty”) was adopted in June 2023.

Common Traps

Traps to avoid

Spring tides have nothing to do with the season of spring — they occur at new and full moon.
The EEZ gives sovereign rights over resources, not full sovereignty; navigation and overflight remain free.
The Equator is not the most saline part of the ocean.
El Niño raises the risk of a weak monsoon; it does not guarantee drought.