Ocean Salinity
Learn the Concept
AllSalinity is the total amount of dissolved salts in sea water, measured in parts per thousand (‰ or ppt). Along with temperature, it is a master property of the ocean — it controls density, currents and marine life. Fix the central idea: average sea-water salinity is about 35‰; it rises where evaporation exceeds precipitation and falls where fresh water is added (rain, rivers, melting ice), so the subtropics are saltiest and the equator and poles are fresher.
- Definition: dissolved salts per 1000 g of sea water; average ~35‰.
- Main control: evaporation − precipitation (E > P raises it).
- Horizontal: highest in the subtropics (~20–30°), lower at equator & poles.
- Extremes: enclosed hot seas very salty (Red Sea, Dead Sea); fresher near rivers/ice.
- Salinity: grams of dissolved salt per kilogram of sea water (‰).
- Halocline: the layer where salinity changes rapidly with depth.
- Isohaline: a line joining points of equal salinity.
- Constancy of composition: the proportions of the major salts are nearly constant everywhere.
- Brine: very salty water (e.g. left behind when sea ice forms).
1. Composition & average
- Average salinity is ~35‰ (35 g of salt per kg of water). The dominant salt is sodium chloride (common salt, ~77%), then magnesium chloride, magnesium & calcium sulphates, etc. Crucially, the proportions of the major ions stay nearly constant everywhere (the law of constancy of composition / Forchhammer's Principle) — only the total concentration varies.
2. Controls of salinity
- Evaporation (raises salinity) vs precipitation + river inflow + ice melt (lower it) — the master balance.
- Ocean currents (mix & redistribute salt); winds; enclosed vs open seas (enclosed seas reach extremes); and ice formation (which expels brine, raising salinity nearby).
3. Horizontal distribution
- Subtropical highs (~20–30°): hot, dry, descending air → high evaporation → highest open-ocean salinity (~36–37‰).
- Equator: heavy rain lowers it slightly (~34–35‰).
- Poles: low evaporation + ice melt → low salinity.
- Enclosed seas: the Dead Sea (~250‰) and Lake Van are extreme; the Red Sea (~40‰) is the saltiest open sea; the Baltic (~7‰) is very fresh (much river inflow, little evaporation).
4. Vertical distribution
- Salinity changes with depth through a halocline. In low latitudes salinity is high at the surface (evaporation) and falls with depth; in high latitudes it can be low at the surface (ice melt) and rises with depth. Salinity, with temperature, fixes the density that drives the deep circulation.
| Water body | Salinity |
|---|---|
| Ocean average | ~35‰ |
| Red Sea | ~40‰ (saltiest open sea) |
| Dead Sea | ~250‰ (extreme) |
| Baltic Sea | ~7‰ (very fresh) |
| Subtropical open ocean | ~36–37‰ (highest) |
- Average salinity ~35‰; dominant salt NaCl; proportions nearly constant (Forchhammer).
- Main control = evaporation − precipitation; subtropics saltiest, equator & poles fresher.
- Extremes: Dead Sea ~250‰, Red Sea ~40‰, Baltic ~7‰.
- Vertical change through a halocline.
- Salinity + temperature → density (drives deep circulation).
- Ocean temperature (previous leaf): the partner property.
- Ocean density (next leaf): salinity's main effect.
- Water masses / thermohaline circulation (next): salt & heat together.
- Salt budget (later): sources & sinks of ocean salt.
Prepare for the Exam — Prelims
PrelimsTested on the 35‰ average, the E−P control, the subtropical maximum, extremes (Dead/Red/Baltic), and the halocline.
- Average ~35‰; NaCl dominant.
- Control = evaporation − precipitation.
- Subtropics saltiest; equator dip.
- Dead Sea ~250‰; Baltic ~7‰.
- Forchhammer: constant proportions.
- Highest in subtropics, not equator.
- Red Sea = saltiest open sea (~40‰).
- Halocline = salinity layer (not thermocline).
- Ice formation raises nearby salinity (brine).
Prepare for the Exam — Mains
MainsGS Paper I. The examiner rewards the controls and both distributions with examples.
- Discuss the factors controlling ocean salinity and its distribution. (15)
- Explain the horizontal distribution of salinity. (10)
- Why is the subtropical ocean the saltiest? (10)
- Explain salinity extremes with examples. (10)
- Composition & average.
- Controls.
- Horizontal & vertical.
- Extremes & significance.
- Forchhammer's Principle.
- Dead/Red/Baltic figures.
- Link to density & circulation.
How to Write the Answer
Mains“Discuss the distribution of salinity in the oceans.” (15)
Give controls, then horizontal (latitudinal) and vertical patterns with examples.
- Saying equator is saltiest.
- Confusing halocline & thermocline.
- No examples.
Prelims Questions
Prelims30 interactive MCQs — previous-year questions are included and tagged with their year. Select an answer for instant feedback and an explanation; your score is tracked and you can review wrong answers at the end.
Mains Questions
Mains15 mains questions — previous-year questions are included and tagged. Write a full answer, then click Evaluate My Answer for paragraph-level feedback. Reveal the model structure after you attempt.
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Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
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Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
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Body 3: limitations or the counter-view (balanced).
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Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
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Body 1: conceptual / historical basis with a named fact, date or example.
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Body 3: limitations or the counter-view (balanced).
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Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.