PRECIP DECREASES
(300m TO 1000m)
THERMOHALINE SINK
Ocean Salinity
Evaporation vs Precipitation · Halocline Profile · Isohalines · Temperature-Salinity Density Engine (28 Micro-Realms)
The Chemical Balance of the Hydrosphere (NCERT Chapter 13)
Salinity is the total content of dissolved salts in seawater, calculated as parts per thousand (‰). The normal average salinity of the open ocean is 35‰, primarily composed of Sodium Chloride ($\text{NaCl}$). The primary factors dictating surface salinity are the balance between Evaporation (which concentrates salt) and Precipitation/Freshwater influx (which dilutes it). Due to this balance, the highest salinities are not found at the Equator (where heavy rainfall dilutes the water) but at the Subtropics ($20^\circ\text{–}30^\circ$) where evaporation is maximum and rainfall is minimal. Vertically, salinity changes rapidly in a transition zone called the Halocline. Together, Temperature and Salinity dictate the Density of seawater (mapped on T-S diagrams), driving the massive, slow-moving deep ocean currents known as the Thermohaline Circulation.
I. Definition, Composition & The Isohaline Grid
Salinity Definition (NCERT)
- The total content of dissolved salts in seawater
- Calculated as amount of salt (in grams) dissolved in 1,000g (1 kg) of seawater
- Expressed as parts per thousand (‰) or ppt
Primary Salt: Sodium Chloride
- Sodium Chloride ($\text{NaCl}$) is common table salt
- It comprises the vast majority of ocean salinity (77.8%)
- Making ocean water largely undrinkable for land mammals
Other Dissolved Salts
- NCERT lists the descending order of salts
- Magnesium chloride, Magnesium sulphate
- Calcium sulphate, Potassium sulphate
Isohalines
- Lines drawn on a map joining places
- Having an equal degree of salinity
- Used to map horizontal salinity distribution
II. Factors Affecting Salinity (The E-P Balance)
Factor 1: Evaporation
- Evaporation turns pure liquid water into gas
- The dissolved salts cannot evaporate
- Therefore, high evaporation INCREASES surface salinity
Factor 2: Precipitation
- Rainfall adds pure, distilled freshwater to the surface
- This dilutes the concentration of salts
- Therefore, high precipitation DECREASES surface salinity
Factor 3: River Influx
- Massive rivers (Amazon, Congo, Ganga, Niger)
- Dump enormous volumes of freshwater into the ocean
- Significantly lowering salinity at river mouths/estuaries
Factor 4: Freezing & Melting
SALINITY ↑
SALINITY ↓
- When sea ice freezes, it expels salt into the water below (increases salinity)
- When sea ice/glaciers melt, they add freshwater (decreases salinity)
Factor 5: Wind Mixing
- Wind transfers water to other areas
- It also causes vertical mixing (turbulent flow)
- Which redistributes concentrated surface salts downwards
Factor 6: Ocean Currents
(BRINGS SALT)LABRADOR
(DILUTES)
- Ocean currents act as massive conveyor belts
- Warm currents from subtropics carry highly saline water poleward
- Cold polar currents bring fresher, melted water equatorward
III. Horizontal Distribution (The Equatorial Trap & Enclosed Seas)
The Equatorial Trap
- UPSC Trap: The equator is the hottest, BUT it is NOT the saltiest!
- Despite high evaporation, heavy daily convectional rainfall
- Adds immense freshwater, keeping salinity relatively low
Subtropical Maximum
- Maximum open-ocean salinity occurs in Subtropics ($20^\circ\text{–}30^\circ$)
- Skies are clear (high evaporation) and air is sinking
- There is almost zero precipitation to dilute the salt
Polar Minimum
- Salinity decreases sharply from subtropics toward poles
- Due to very low temperatures (minimal evaporation)
- And massive influx of freshwater from melting ice caps/glaciers
Enclosed Hot Seas
PERSIAN GULF
- Partially enclosed seas in hot, dry regions
- Experience extreme evaporation with zero river influx
- Resulting in exceptionally high surface salinities
Enclosed Cold Seas
BLACK SEA
- Partially enclosed seas in temperate/cool regions
- Low evaporation, massive influx from large rivers
- Resulting in exceptionally low surface salinities (brackish)
Extreme Inland Lakes (NCERT)
- Landlocked lakes with no outlet become super-concentrated
- Lake Van in Turkey (330‰) is the highest listed in NCERT
- Dead Sea (280‰) and Great Salt Lake (220‰)
IV. Vertical Distribution, Density & Thermohaline Circulation
Surface Salinity Variation
- Salinity at the surface changes by changing water/ice
- It is highly variable depending on latitude and season
- Must be transferred downwards to affect the deep ocean
The Halocline Layer
- A distinct zone (usually 300m-1000m deep)
- Where salinity changes RAPIDLY with depth
- Acts as a density barrier preventing easy vertical mixing
Deep Ocean Uniformity
HIGHLY SALINE & DENSE
- Salinity at depth is very much fixed (NCERT)
- Because water is neither lost nor added down there
- High salinity water is dense and generally sinks
Density Definition
- Density is the mass of water per unit volume
- It determines whether a parcel of water will float or sink
- It is the master key to deep ocean circulation
The T-S Diagram
- Temperature-Salinity diagram
- Used by oceanographers to identify specific "Water Masses"
- Density is a function of BOTH Temperature and Salinity
Temperature vs. Density
DENSITY ↓
DENSITY ↑
- Inverse relationship
- Warm water expands, becoming less dense (floats)
- Cold water contracts, becoming highly dense (sinks)
Salinity vs. Density
DENSITY ↑
DENSITY ↓
- Direct relationship
- Adding dissolved salts increases the mass per volume
- Highly saline water is dense and sinks below fresh water
The Pycnocline
- The layer where water density increases rapidly with depth
- Usually coincides with the Thermocline and Halocline
- Acts as a "floor" to the surface mixing zone
Thermohaline Circulation
- Thermo (Temp) + Haline (Salinity)
- The global deep-ocean "conveyor belt"
- Driven entirely by density differences, not surface winds
North Atlantic Deep Water (NADW)
- Salty water from the Gulf Stream moves to the Arctic
- It cools rapidly, becoming extremely dense
- Sinks to the bottom, pulling more water behind it (The Engine!)
Antarctic Bottom Water (AABW)
- As Antarctic sea ice forms, it expels pure salt into the water below
- This creates freezing, ultra-saline brine
- The absolute densest water mass, creeping along the ocean floor globally
Master T-S / Salinity Matrix
- Completes the chemical and density properties of the ocean
- Locks the Thermohaline engine mechanics
- Transitions to Realm 90 (Ocean Movements: Waves, Tides & Currents)
Core Distinctions
Do NOT assume the Equator is the saltiest part of the ocean just because it is hot. While evaporation is high, heavy daily convectional rainfall adds massive amounts of freshwater, diluting the surface. The highest salinity is actually found in the Subtropics ($20^\circ\text{–}30^\circ$), where air subsides, skies are clear, evaporation is maximum, and rainfall is nearly zero.
2. Thermocline vs. Halocline vs. Pycnocline• Thermocline: The transition layer where temperature drops rapidly with depth.
• Halocline: The transition layer where salinity changes rapidly with depth.
• Pycnocline: The transition layer where density increases rapidly with depth (caused by the combined effects of the thermocline and halocline).
Enclosed seas exaggerate their latitudinal traits. The Red Sea is in the hot, dry subtropics; high evaporation and zero river input make it extremely saline (41‰). The Baltic Sea is in the cool temperate zone; low evaporation and massive freshwater input from rivers make it exceptionally fresh/brackish (~7‰).
Salinity & Density Matrix
NCERT Chapter 13 Reference Guide