This MCQ module is based on: Ocean Temperature, Salinity & Exercises
TOPIC 25 OF 29
Ocean Temperature, Salinity & Exercises
🎓 Class 11
Social Science
CBSE
Theory
Ch 12 — Water (Oceans)
⏱ ~28 min
🌐 Language: [gtranslate]
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12.5 Temperature of Ocean Waters
This section deals with the spatial and vertical variations of temperature in the various oceans. Ocean waters get heated up by the solar energy just as land does — but the process of heating and cooling of oceanic water is much slower than on land. Water has a high specific heat capacity, so it absorbs and releases heat gradually. This single property is the reason oceans regulate the climate of the entire planet — they are gigantic thermal reservoirs that buffer the daily and seasonal swings of temperature on land.
Factors Affecting the Distribution of Temperature
NCERT lists four main factors that control how temperature is distributed across the surface oceans of the world. Understanding these factors is the key to reading any sea-surface temperature map.
(i) Latitude
Surface temperature decreases from the equator towards the poles because the amount of insolation decreases poleward. Vertical noon-sun rays at the equator deliver concentrated energy.
(ii) Unequal Land–Water Distribution
Oceans of the Northern Hemisphere are bordered by larger land masses than those of the Southern Hemisphere, so they absorb more heat. NH ocean averages ~ 19 °C, SH ~ 16 °C.
(iii) Prevailing Winds
Offshore winds drive warm surface water away, causing upwelling of cold water (longitudinal variation). Onshore winds pile warm water against the coast and raise the temperature.
(iv) Ocean Currents
Warm currents (e.g. Gulf Stream) raise temperature in cold areas; cold currents (e.g. Labrador) lower temperature in warm areas. Currents are the planet's thermal conveyor belt.
🌡️ Open Sea vs Enclosed Sea
All four factors influence the temperature of ocean currents locally. The enclosed seas in the low latitudes record relatively higher temperature than the open seas; whereas the enclosed seas in the high latitudes record lower temperature than the open seas — because limited mixing exaggerates the local climate.
Horizontal Distribution of Surface Temperature
The maximum temperature of the oceans is always at their surface, because the surface directly receives the heat from the Sun and the heat is transmitted to lower sections through the slow process of convection. The temperature falls very rapidly up to the depth of 200 m, and thereafter the rate of decrease of temperature is slowed down dramatically.
📊 Average Surface Temperatures (NCERT Numbers)
- Average over all ocean surfaces: about 27 °C.
- Rate of decrease: roughly 0.5 °C per latitude moving away from the equator.
- Around 20° latitudes: ≈ 22 °C.
- Around 40° latitudes: ≈ 14 °C.
- Near the poles: ≈ 0 °C.
- Hemisphere average: Northern ≈ 19 °C; Southern ≈ 16 °C — due to unequal distribution of land and water.
Figure 12.4 — Spatial Pattern of Surface Temperature of the Oceans
Vertical Distribution of Temperature — The Thermocline
The temperature-depth profile for ocean water shows how the temperature decreases with increasing depth. The profile reveals a special boundary region between the surface waters of the ocean and the deeper layers. This boundary usually begins around 100 - 400 m below the sea surface (in some sources extended to 200 - 1,000 m) and stretches several hundred metres downward. This boundary region — from where there is a rapid decrease of temperature — is called the thermocline?. About 90 per cent of the total volume of water lies below the thermocline, in the deep ocean. In this zone temperatures approach 0 °C — uniformly cold and dark.
📖 Definition — Thermocline
The thermocline is the boundary region in the open ocean where the temperature drops sharply with depth. It separates the warm sunlit surface layer from the deep cold abyssal layer. The thermocline is 500 - 1,000 m thick and acts as a thermal "wall" that keeps the deep ocean cold and the surface ocean warm.
The Three-Layer Thermal Structure (Middle and Low Latitudes)
The temperature structure of oceans over middle and low latitudes can be described as a three-layer system from surface to bottom:
- First layer (Mixed surface layer): the top layer of warm oceanic water, about 500 m thick, with temperatures ranging between 20 °C and 25 °C. Within the tropical region this layer is present throughout the year, but in mid-latitudes it develops only during summer.
- Second layer (Thermocline layer): lies below the first and is characterised by a rapid decrease of temperature with increasing depth. The thermocline is 500 - 1,000 m thick.
- Third layer (Deep cold layer): very cold and extends right up to the deep ocean floor. Temperatures are nearly uniform and close to 0 °C.
In the Arctic and Antarctic Circles, surface water temperatures are themselves close to 0 °C, so the temperature change with depth is very slight. Here only one layer of cold water exists, extending from surface to deep ocean floor — the three-layer structure collapses into a single cold pillar.
Figure 12.3 — Thermocline (Vertical Temperature Profile)
LET'S EXPLORE — Hemispheric Temperature Asymmetry
L4 Analyse
The Northern Hemisphere's oceans average 19 °C; the Southern Hemisphere's oceans average 16 °C. The Sun shines equally on both halves of the globe across the year. Why then is one hemisphere's ocean 3 °C warmer than the other?
The asymmetry comes from the unequal distribution of land and water. The Northern Hemisphere has roughly twice as much land as the Southern Hemisphere. Land warms up faster and to higher temperatures than water during summer; the warm land then bleeds heat into adjoining oceans through the air and through coastal currents. The Southern Hemisphere ocean, surrounded by the cold continent of Antarctica and lacking large warm landmasses, has fewer "heat injections" of this kind. NCERT lists "unequal distribution of land and water" as the second factor controlling ocean temperature, and this 3 °C gap is its most direct global signature.
12.6 Salinity of Ocean Waters
All waters in nature — whether rain water or ocean water — contain dissolved mineral salts. Salinity? is the term used to define the total content of dissolved salts in sea water. It is calculated as the amount of salt (in grams) dissolved in 1,000 g (1 kg) of seawater, and is usually expressed as parts per thousand — written ‰ or ppt. Salinity of 24.7 ‰ has been considered as the upper limit to demarcate "brackish water" — anything below 24.7 ‰ is called brackish.
📖 Definition — Salinity
Salinity is the total content of dissolved salts in seawater, measured in grams of salt per 1,000 grams of seawater (parts per thousand, ‰). The world ocean's average salinity is approximately 35 ‰ — meaning every kilogram of seawater contains about 35 grams of dissolved mineral salts.
Factors Affecting Ocean Salinity
(i) Evaporation & Precipitation
High evaporation removes pure water and leaves salts behind, raising salinity. Heavy rainfall dilutes seawater and lowers it. The surface salinity of oceans depends mainly on this balance.
(ii) Freshwater Inflow / Ice
Surface salinity is greatly influenced in coastal regions by the fresh water flow from rivers, and in polar regions by the freezing and thawing of ice (sea-ice formation traps less salt; melting ice dilutes seawater).
(iii) Wind
Winds influence salinity of an area by transferring water (and the dissolved load) to other areas — and by accelerating evaporation in dry windy conditions.
(iv) Ocean Currents
Currents redistribute salinity. Salinity, temperature and density are interrelated — any change in temperature or density influences the salinity of water in an area, and vice-versa.
Horizontal Distribution of Salinity
The salinity for a normal open ocean ranges between 33 ‰ and 37 ‰. Specific basins, however, depart sharply from this average — controlled by the local balance of evaporation, river inflow and connectivity to the open sea.
📊 Salinity Hot-Spots — Highs & Lows
- Land-locked Red Sea: as high as 41 ‰ (high evaporation, almost no fresh inflow).
- Hot & dry enclosed seas: can reach 70 ‰ in some places.
- Mediterranean Sea: ~ 38 ‰ — high evaporation, narrow connection to Atlantic.
- North Sea: high salinity despite a high-latitude position because of saline water brought by the North Atlantic Drift.
- Baltic Sea: very low — about 7 ‰ — due to large river inflow.
- Black Sea: very low because of enormous freshwater influx by rivers.
- Estuaries & the Arctic: seasonally fluctuates between 0 ‰ and 35 ‰.
- Inland water bodies (not part of ocean): Lake Van (Turkey) 330 ‰, Dead Sea 238 ‰, Great Salt Lake 220 ‰.
Salinity in the Three Major Oceans
Pacific Ocean. Salinity variation in the Pacific is mainly due to its shape and large areal extent. Salinity decreases from 35 ‰ to 31 ‰ on the western parts of the northern hemisphere because of the influx of melted water from the Arctic region. In the same way, after 15° - 20° south, it decreases to 33 ‰.
Atlantic Ocean. The average salinity of the Atlantic Ocean is around 36 ‰. The highest salinity is recorded between 15° and 20° latitudes. Maximum salinity (37 ‰) is observed between 20° N and 30° N and 20° W - 60° W. It gradually decreases towards the north. The North Sea, in spite of its location in higher latitudes, records higher salinity due to more saline water brought by the North Atlantic Drift; the Baltic Sea records low salinity due to influx of river waters in large quantity. The Mediterranean records higher salinity due to high evaporation, but salinity is very low in the Black Sea owing to enormous freshwater influx.
Indian Ocean. The average salinity of the Indian Ocean is 35 ‰. The low-salinity trend is observed in the Bay of Bengal due to influx of river water (Ganga, Brahmaputra, Mahanadi, Godavari, Krishna, Kaveri). On the contrary, the Arabian Sea shows higher salinity due to high evaporation and low fresh-water influx.
Salinity by Region (parts per thousand) — Highs and Lows
Figure 12.5 — World Ocean Surface Salinity Map
Vertical Distribution of Salinity — The Halocline
Salinity changes with depth, but the way it changes depends on the location of the sea. Salinity at the surface is increased by the loss of water to ice or evaporation, or decreased by the input of fresh waters from rivers. Salinity at depth, on the other hand, is very much fixed — because there is no way water can be "lost", or salt can be "added" once we are below the surface mixed layer.
There is a marked difference in the salinity between the surface zones and the deep zones of the oceans. The lower salinity water rests above the higher salinity dense water. Salinity, generally, increases with depth, and there is a distinct zone called the halocline?, where salinity increases sharply.
⚖️ Salinity → Density → Stratification
Other factors being constant, increasing salinity of seawater causes its density to increase. High-salinity seawater, generally, sinks below the lower salinity water. This vertical sorting by salt content leads to stratification by salinity — the ocean is layered like a parfait, with denser, saltier water at the bottom and lighter, fresher water at the top. The halocline is the sharp boundary between the two.
THINK ABOUT IT — Why Does the Bay of Bengal Have Lower Salinity Than the Arabian Sea?
L4 Analyse
Both the Bay of Bengal and the Arabian Sea are part of the Indian Ocean and lie at similar latitudes. Yet the Bay of Bengal has significantly lower salinity than the Arabian Sea. Use NCERT's four salinity factors to explain this contrast.
The contrast comes mainly from factor (i) evaporation–precipitation and factor (ii) freshwater inflow. (a) The Bay of Bengal receives the great Indian rivers — Ganga, Brahmaputra, Mahanadi, Godavari, Krishna, Kaveri — pouring vast volumes of fresh water into it; the Himalaya-fed Ganga–Brahmaputra system alone delivers more freshwater than almost any other estuary on Earth. (b) The Bay also receives heavy summer-monsoon rainfall directly on its surface, again diluting it. (c) The Arabian Sea, by contrast, has only one major fresh inflow (the Indus from the north) and is bordered by hot, dry deserts (Arabian and Thar) that drive intense evaporation. The result is the salinity gap that NCERT records — Bay of Bengal "low salinity" against Arabian Sea "high salinity" — even though both lie in the same ocean and the same latitude band.
DISCUSS — Why Does the Dead Sea Reach 238 ‰?
L5 Evaluate
The world's average ocean salinity is 35 ‰; the Red Sea reaches 41 ‰; but inland water bodies like the Dead Sea (238 ‰) and Lake Van (330 ‰) reach near-saturation. Discuss in pairs why these inland lakes are so much saltier — and why they are technically not part of the ocean salinity chart.
The Dead Sea sits in the lowest land basin on earth, in a hot dry climate, and has no outflow to the sea. Rivers (the Jordan) bring in water carrying tiny dissolved salts; the water then evaporates under the desert Sun and leaves the salt behind. Over thousands of years the salt has concentrated to nearly 238 ‰ — about seven times the open ocean average. The same logic explains Lake Van and the Great Salt Lake. They are not shown on ocean salinity charts because they are terminal lakes on land, not branches of the world ocean — they have no marine connection. Yet they make a perfect natural laboratory: floating effortlessly in the Dead Sea is a vivid demonstration of how salinity controls water density.
🎯 Competency-Based Questions — Temperature & Salinity
Case Stem. An oceanographic ship surveys the Indian Ocean. Sensors record the following readings off the coast of Kochi (10° N): surface temperature 28 °C, surface salinity 34.5 ‰, and a sharp temperature drop between 200 m and 800 m. At a second station off the coast of Aden (Gulf of Aden, Red Sea entry), surface temperature is 30 °C, surface salinity 39 ‰. A third station at 60° S records surface temperature 2 °C, salinity 33 ‰, and almost no temperature change with depth. Use this scenario for Q1–Q4.
Q1. The sharp temperature drop between 200 m and 800 m at the Kochi station is the —
L2 UnderstandAnswer: (b) Thermocline. The thermocline is the boundary region where temperature falls rapidly with depth. NCERT places it commonly between 100 - 400 m and 500 - 1,000 m thick — exactly matching the survey reading of 200 m to 800 m.
Q2. The Aden station shows higher salinity (39 ‰) than the Kochi station (34.5 ‰) despite being only a few hundred kilometres apart. Identify the dominant factor and justify your answer using NCERT's list.
L4 AnalyseDominant factor: evaporation–precipitation balance (factor i). The Aden station lies near the entrance of the Red Sea — a hot, almost-enclosed basin with virtually no major river inflow and intense desert evaporation, pushing salinity towards 41 ‰ inside. The Kochi station, by contrast, sits in the path of monsoon rain and receives freshwater from the Periyar and other rivers, keeping salinity near the 35 ‰ open-ocean average. NCERT specifically notes the Red Sea's 41 ‰ as the textbook example of high salinity by evaporation.
Q3. At 60° S the survey records "almost no temperature change with depth". Why does the typical three-layer thermal structure collapse here?
L5 EvaluateBecause the surface itself is already cold. NCERT explains that in the Arctic and Antarctic Circles, surface water temperatures are close to 0 °C. There is therefore no warm "first layer" to begin with, no big thermal contrast to maintain a thermocline, and the entire water column is essentially one cold pillar of water from surface to abyssal floor. The familiar three-layer structure of the tropics simply does not exist at high latitudes.
HOT Q. A class-11 student claims that "warmer water always sinks because heat makes molecules move faster". Critically evaluate this claim using your knowledge of salinity, temperature and density.
L6 CreateThe claim is wrong — the opposite is true. Cold water is denser than warm water (because cold molecules pack closer together), so cold water tends to sink and warm water tends to rise. NCERT confirms this: "cold water is denser than warm water… denser water tends to sink, while relatively lighter water tends to rise". Salinity adds a second control: water with high salinity is denser than water with low salinity — so a parcel that is both cold and salty sinks fastest. This is exactly what happens at the poles: surface water becomes very cold and salty (because sea ice ejects salt into the surrounding water), it sinks, and drives the global thermohaline circulation that ventilates the deep ocean. So density depends on both temperature and salinity together — never on temperature alone.
⚖️ Assertion–Reason Questions — Class 11
Options:
(A) Both A and R are true, and R is the correct explanation of A.
(B) Both A and R are true, but R is NOT the correct explanation of A.
(C) A is true, but R is false.
(D) A is false, but R is true.
(A) Both A and R are true, and R is the correct explanation of A.
(B) Both A and R are true, but R is NOT the correct explanation of A.
(C) A is true, but R is false.
(D) A is false, but R is true.
Assertion (A): About 90 per cent of the total volume of ocean water is found below the thermocline.
Reason (R): The thermocline is a 500 - 1,000 m thick layer of rapid temperature decrease that separates warm surface water from the deep cold ocean.
Answer: (B) — Both statements are true, but R does not explain A. R describes what the thermocline is; A describes where most of the water lies relative to it. The two facts are linked by the geometry of the ocean basin (most ocean depth is well below the thermocline) rather than by the thermocline's own properties. Hence the relationship is true but not explanatory.
Assertion (A): The Red Sea records salinity as high as 41 ‰.
Reason (R): The Red Sea is a land-locked basin in a hot, dry region with high evaporation and very little freshwater inflow.
Answer: (A) — Both true and R correctly explains A. NCERT cites the Red Sea as the textbook example of evaporation-driven salinity. Restricted exchange with the open Indian Ocean traps salts inside, and the desert climate strips water out by evaporation faster than rain or rivers can replace it.
Assertion (A): The Northern Hemisphere oceans are warmer than the Southern Hemisphere oceans.
Reason (R): The Northern Hemisphere has a larger area of land than the Southern Hemisphere, and contact with land contributes additional heat to bordering oceans.
Answer: (A) — Both true and R is the correct explanation. NCERT records NH ocean average ≈ 19 °C and SH ocean average ≈ 16 °C, attributing the difference directly to "the unequal distribution of land and water in the Northern and Southern Hemispheres".
📚 NCERT Exercises — Chapter 12 (Water — Oceans)
1. Multiple Choice Questions
(i)Identify the element which is not a part of the hydrological cycle:
Answer: (b) Hydration. Hydration is a chemical process (a mineral combining with water) that does not move water between reservoirs. The three core phases of the hydrological cycle are evaporation, condensation and precipitation.
(ii)The average depth of continental slope varies between:
Answer: (b) 200 - 2,000 m. NCERT states the depth of the slope region varies between 200 and 3,000 m; option (b) is the closest match within the given choices and matches the lower part of that range.
(iii)Which one of the following is not a minor relief feature in the oceans:
Answer: (c) Oceanic Deep. Oceanic deeps (trenches) are listed as one of the four major divisions of the ocean floor. Seamounts, atolls and guyots are minor relief features.
(iv)Salinity is expressed as the amount of salt in grams dissolved in sea water per:
Answer: (b) 1,000 g. Salinity is calculated as grams of salt per 1,000 g (= 1 kg) of seawater — that is, "parts per thousand" or ‰.
(v)Which one of the following is the smallest ocean:
Answer: (b) Arctic Ocean. The Arctic Ocean is by far the smallest of the world's oceans — also the shallowest, and largely covered by sea-ice.
2. Answer the following questions in about 30 words
(i)Why do we call the earth a Blue Planet?
Model Answer. About 71 per cent of the earth's surface is covered by oceans of liquid water. From space the planet appears overwhelmingly blue because deep water absorbs red wavelengths and reflects blue ones — hence the name "Blue Planet".
(ii)What is a continental margin?
Model Answer. The continental margin is the submerged outer edge of a continent that grades from the shoreline down to the deep ocean basin. It comprises the continental shelf (≤ 200 m), the continental slope (200 - 3,000 m, gradient 2-5°) and the continental rise.
(iii)List out the deepest trenches of various oceans.
Model Answer. Pacific Ocean — the Mariana Trench (over 11,000 m, the deepest in the world). Atlantic Ocean — the Puerto Rico Trench. Indian Ocean — the Java (Sunda) Trench. Southern Ocean — the South Sandwich Trench. Of 57 deeps explored, 32 lie in the Pacific, 19 in the Atlantic and 6 in the Indian Ocean.
(iv)What is a thermocline?
Model Answer. The thermocline is the boundary region in the ocean — about 500 to 1,000 m thick, beginning roughly 100 - 400 m below the surface — where the temperature drops rapidly with increasing depth, separating warm surface water from cold deep water.
(v)When you move into the ocean, what thermal layers would you encounter? Why does the temperature vary with depth?
Model Answer. One would encounter three layers — a warm mixed surface layer (~ 500 m thick, 20 - 25 °C), the thermocline (500 - 1,000 m thick, rapid drop), and a deep cold layer (≈ 0 °C reaching the floor). Temperature varies because the surface absorbs solar heat and convection slowly carries heat downward; depths beyond 200 m receive little heat, so temperature falls rapidly through the thermocline.
(vi)What is salinity of sea water?
Model Answer. Salinity is the total content of dissolved mineral salts in sea water, calculated as grams of salt per 1,000 grams of seawater and expressed in parts per thousand (‰). The world ocean averages about 35 ‰; salinity below 24.7 ‰ is termed brackish.
3. Answer the following questions in about 150 words
(i)How are various elements of the hydrological cycle interrelated?
Model Answer (≈ 150 words). The hydrological cycle is a closed loop in which the atmosphere, oceans, land surface and subsurface continuously exchange water in three states — liquid, solid and vapour. The first link is evaporation: solar energy converts liquid water in oceans, lakes, soils and plants (evapotranspiration) into invisible vapour that rises into the atmosphere. The second link is condensation: as moist air ascends and cools, vapour changes into clouds. The third link is precipitation: cloud droplets coalesce into rain or freeze into snow and hail and fall to the surface. Once on land, water either runs off into streams that return it to the ocean, infiltrates the ground to recharge groundwater, or is locked into glaciers and icecaps. Ground water emerges through springs back into rivers; rivers feed the sea; the sea feeds evaporation again. Each element acts both as a sink and a source — together they keep the planetary water budget in balance.
(ii)Examine the factors that influence the temperature distribution of the oceans.
Model Answer (≈ 150 words). Four major factors shape ocean temperature distribution. (i) Latitude: insolation is most intense near the equator and weakest near the poles, so surface temperature falls roughly 0.5 °C per latitude — about 27 °C average at the equator down to 0 °C at the poles. (ii) Unequal land-water distribution: the Northern Hemisphere has more land bordering its oceans than the Southern Hemisphere, transferring more heat to NH waters; NH ocean averages 19 °C, SH 16 °C. (iii) Prevailing winds: offshore winds drive warm surface water away and trigger upwelling of cold water; onshore winds pile warm water against the coast and raise temperature. (iv) Ocean currents: warm currents like the Gulf Stream raise temperature near eastern North America and Western Europe, while cold currents like the Labrador lower the temperature of the north-east coast of North America. Enclosed seas amplify whichever local effect dominates.
Project Work
(i)Consult the atlas and show ocean floor relief on the outline of the world map.
Project Guide. On a blank world map outline, mark and label: (a) the Continental Shelf zones (shade light blue along all coastlines), highlighting the Siberian Shelf, North Sea shelf and Sundarbans-Bay of Bengal shelf; (b) the Mid-Atlantic Ridge (running N-S through the Atlantic, passing Iceland); (c) the East Pacific Rise; (d) the Ninety East Ridge in the Indian Ocean; (e) the Mariana Trench, Java Trench, Peru-Chile Trench and Puerto Rico Trench; (f) major abyssal plains in dark blue. Use a colour key — light blue (shelf, < 200 m), medium blue (slope, 200-3,000 m), dark blue (abyssal plain, 3,000-6,000 m) and red (trenches, > 6,000 m). Add a north arrow and scale bar.
(ii)Identify the areas of mid-oceanic ridges from the Indian Ocean.
Project Guide. The Indian Ocean carries three connected mid-oceanic ridge segments: (a) the Central Indian Ridge, running roughly N-S between Africa and India; (b) the Carlsberg Ridge in the north-west, between the Arabian Peninsula and India; and (c) the South-East Indian Ridge running south-east towards Antarctica/Australia. Together they form a Y-shaped mid-oceanic ridge system meeting at the Rodrigues Triple Junction. Mark these on an Indian Ocean outline map with red dashed lines for ridge crests and small triangles for major volcanic islands (Iceland-style examples in the Indian Ocean include Reunion and Mauritius, both on hot-spot trails). Cross-reference with the atlas plate-tectonics map.
📝 Chapter Summary — Water (Oceans)
- Blue Planet: About 71 per cent of the earth's surface is covered by oceans, hence the nickname.
- Hydrological cycle: Continuous circulation of water in three phases (liquid, solid, vapour) through three core processes — evaporation, condensation, precipitation. Earth's renewable water is constant, but demand keeps rising, causing local water crises.
- Distribution: Oceans hold ≈ 97% of all water; glaciers and icecaps ≈ 2.5%; groundwater ≈ 0.7%; surface waters and atmosphere ≈ 0.3%.
- Five oceans: Pacific (largest, 32 deeps), Atlantic (19 deeps, "S"-shape, Mid-Atlantic Ridge), Indian (6 deeps, monsoon currents), Southern, Arctic (smallest, Siberian shelf).
- Major relief divisions: Continental Shelf (~ 200 m, 1° gradient, fishing & fossil fuels) → Continental Slope (200-3,000 m, 2-5°) → Continental Rise → Abyssal/Deep Sea Plain (3,000-6,000 m) → Trenches (3-5 km below surrounding floor; Mariana > 11,000 m).
- Minor features: Mid-Oceanic Ridges (twin chains, peaks 2,500 m, e.g. Iceland), Seamounts (3,000-4,500 m, Emperor Seamount), Submarine Canyons (Hudson Canyon), Guyots (flat-topped, > 10,000 in Pacific), Atolls (coral rings).
- Temperature: Average surface ≈ 27 °C; falls ~ 0.5 °C/latitude poleward (22 °C at 20°, 14 °C at 40°, 0 °C at poles). NH avg 19 °C; SH avg 16 °C. Three-layer profile: warm mixed (500 m, 20-25 °C) → thermocline (500-1,000 m, rapid drop) → deep cold (≈ 0 °C). At polar latitudes the column is uniformly cold.
- Salinity: Average ≈ 35 ‰; brackish < 24.7 ‰. Highs — Red Sea 41 ‰, Mediterranean ≈ 38 ‰, Atlantic peak 37 ‰. Lows — Baltic 7 ‰, Black Sea, Bay of Bengal. Inland: Dead Sea 238 ‰, Lake Van 330 ‰, Great Salt Lake 220 ‰.
- Vertical salinity: Surface salinity is variable (evaporation, ice, rivers); deep salinity is fixed. The halocline is the sharp zone where salinity rises with depth. Higher salinity → higher density → sinks below lower-salinity water (stratification).
🔑 Key Terms
Blue PlanetEarth, named for its dominant blue ocean cover (71%) seen from space.
Hydrological CycleContinuous circulation of water through ocean, atmosphere, land and subsurface in three phases.
Continental ShelfShallow margin of a continent ending around 200 m, gradient ≤ 1°, world average width ~ 80 km.
Continental SlopeSteeper transition (2-5°, 200-3,000 m) marking the true edge of continental crust.
Abyssal / Deep Sea PlainFlat ocean floor at 3,000-6,000 m, covered with fine clay and silt.
Mid-Oceanic RidgeTwin-chain submarine mountain system at divergent plate margins (e.g. Iceland).
SeamountVolcanic peak rising 3,000-4,500 m from the seabed but not reaching the surface.
GuyotFlat-topped seamount; a former volcanic island eroded flat and then submerged.
Submarine CanyonDeep underwater valley cutting across shelf and slope; carved by turbidity currents.
Trench (Oceanic Deep)Narrow, steep-sided basin 3-5 km deeper than surrounding floor; subduction zone.
Mariana TrenchThe deepest trench on earth, > 11,000 m, in the western Pacific Ocean.
AtollRing of coral reefs surrounding a central lagoon, found in tropical seas.
SalinityTotal dissolved salts in seawater, in parts per thousand (‰); world avg ≈ 35 ‰.
Thermocline500-1,000 m thick zone of rapid temperature decrease; separates warm surface from cold deep water.
HaloclineVertical zone of rapid salinity increase with depth; produces stratification.
Brackish WaterWater with salinity below 24.7 ‰; typical of estuaries and coastal lagoons.
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