Ocean Salinity and Temperature
~8 min read
- Avg salinity: ~35 parts per thousand (35 ‰). Varies with evaporation, rainfall, river inflow.
- Dead Sea: ~340 ‰ — most saline. Red Sea ~40‰. Baltic ~10‰ (river-diluted).
- Temperature: Decreases with depth; thermocline (sharp drop zone) at ~200-1000 m. Surface temp drops from equator to poles.
Ocean salinity averages 35 ‰ but varies hugely. Temperature decreases with depth in three layers — surface mixed, thermocline, deep cold. NDA tests salinity extremes (Dead Sea, Red Sea) and the thermocline concept.
Ocean Salinity
Salinity = grams of dissolved salts per 1000 grams of seawater. Average ocean salinity = 35 ‰. NaCl is the dominant salt (~78% of dissolved solids).
| Water body | Salinity (‰) |
|---|---|
| Dead Sea | ~340 |
| Great Salt Lake (Utah) | ~280 |
| Red Sea | ~40 |
| Mediterranean | ~38 |
| Atlantic / Pacific / Indian Ocean (avg) | ~35 |
| Bay of Bengal | ~31 (river-diluted) |
| Arabian Sea | ~36 (high evaporation) |
| Baltic Sea | ~10 (heavy river inflow) |
Factors: evaporation (↑ salinity), rainfall and river inflow (↓ salinity), latitude (max salinity near 20°-30° — hot deserts on coast).
Ocean Temperature
Three vertical layers:
- Surface (mixed) layer — ~0-200 m. Stirred by wind/waves, nearly uniform temperature. Tropical: ~27°C; polar: 0°C.
- Thermocline — ~200-1000 m. Temperature drops sharply with depth.
- Deep ocean — > 1000 m. Cold (~2-4°C) and uniform everywhere on Earth.
Horizontally: surface temperature decreases from equator (~28°C) to poles (-2°C). Warm currents (Gulf Stream) raise temps locally; cold currents (Labrador) lower them.
NDA PYQ Examples
Q: Which sea has the highest salinity?
(a) Mediterranean (b) Red Sea (c) Dead Sea (d) Baltic Sea
Answer: (c) Dead Sea — ~340 ‰, nearly 10× normal.
Q: The thermocline is the layer where:
(a) Salinity is highest (b) Temperature drops sharply with depth (c) Currents reverse (d) Light penetrates deepest
Answer: (b) Temperature drops sharply with depth — usually 200-1000 m.
Drill Ocean Salinity and Temperature for NDA
NDA-pattern items on Ocean Salinity and Temperature with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why is the Dead Sea so salty?
It is a terminal lake (no outflow) in an arid climate. High evaporation and inflow of mineral-rich Jordan River water concentrate salts to ~340 ‰ — nearly 10× normal ocean salinity. You float effortlessly.
Why is the Bay of Bengal less saline than the Arabian Sea?
Bay of Bengal receives huge freshwater from Ganga, Brahmaputra, Mahanadi, Krishna. Arabian Sea has fewer rivers and high evaporation under hot dry winds.
What is the average ocean salinity?
35 grams of dissolved salt per 1,000 grams of seawater = 35 ‰ or 3.5%. NaCl is ~78% of the dissolved solids; the rest are sulphates, magnesium, calcium.
How does salinity affect ocean density?
Higher salinity = higher density. Cold + salty water sinks (drives the global thermohaline circulation, the 'great ocean conveyor belt'). The Atlantic's Gulf Stream is part of this system.