Maarg Manthan · Topic 6.6

Density of Ocean Waters

Geography › Oceanography · Topic 6.6

The density of sea water is its mass per unit volume, about 1.025 g per cubic cm at the surface, and slightly more than that of fresh water. It increases as the water gets colder, as it gets saltier and as pressure rises with depth. Differences in density make water masses sink or float, and layer the ocean into a light warm surface layer and a dense cold deep layer separated by a pycnocline. Density differences also drive the slow deep circulation called the thermohaline circulation.

5 min read · UPSC Prelims · Geography · Oceanography

At a Glance

Average density About 1.025 g per cubic cm
Controls Temperature, salinity, pressure
Cold water Denser
Salty water Denser
Deep circulation Thermohaline
Exam link UPSC Prelims, Geography: Oceanography

Where Does It Fit?

This topic follows the topics on temperature and salinity and combines the two. Density decides how water layers and sinks, and it is the key to the vertical motion of the ocean that forms part of the topic on ocean currents. Previous topic: Salinity of Ocean Water: Factors Affecting Salinity, Vertical Distribution. Next topic: Ocean Currents: Horizontal and Vertical Motion; Cold and Warm Currents.

What Controls the Density of Sea Water?

In short: cold, salty and deep water is dense; warm, fresh and shallow water is light.

Factor	Effect on density	Where it matters
Temperature	Falls in temperature raise density, the strongest control in most of the ocean	Polar waters are dense; tropical surface water is light
Salinity	More salt gives higher density	Dry subtropics with high evaporation; the Red Sea and the Mediterranean
Pressure	Increases density slightly with depth, since water is only slightly compressible	The deep ocean

Freshwater is densest at about 4 °C, then expands as it freezes. For sea water with a salinity above about 24.7 parts per thousand, density increases right down to its freezing point of about -1.9 °C, so cold salty water keeps sinking instead of floating. Ice formed from sea water leaves most of the salt behind, which makes the water below denser; this brine rejection is important around Antarctica. The factors behind temperature and salinity are in Temperature of Ocean Water: Vertical and Spatial Variations, the Thermocline and Salinity of Ocean Water: Factors Affecting Salinity, Vertical Distribution.

How Does Density Layer the Ocean?

Surface mixed layer: stirred by wind and waves, warm and light, tens to a few hundred metres deep.
Pycnocline: the zone in which density increases fast with depth. It often coincides with the thermocline (rapid temperature change) and halocline (rapid salinity change). It acts as a barrier that limits mixing between the surface and the deep, and so traps nutrients below.
Deep layer: cold and dense, about four-fifths of the ocean’s volume, with little change.
Stratification: the ocean is stably layered in most places, since heavy water lies under light water. In polar regions the layering is weak and water can sink.

Density also explains why a ship floats higher in sea water than in a river, and why the load line (Plimsoll line) on its hull differs for fresh, tropical and winter water. Lakes and inland seas show the extremes: the Dead Sea is so salty that its density is above 1.2 g per cubic cm and swimmers float with ease, while the Baltic Sea, fed by many rivers, is light and only weakly salty.

What Is the Thermohaline Circulation?

Where surface water becomes cold and salty enough to be denser than the water below, it sinks. This happens in the North Atlantic (Labrador and Nordic Seas), forming North Atlantic Deep Water, and around Antarctica, mainly in the Weddell and Ross Seas, forming Antarctic Bottom Water. These dense waters spread slowly along the ocean floor, rise gradually in the Indian and Pacific Oceans, and return at the surface. The whole loop, often called the global conveyor belt, takes roughly a thousand years. The name comes from thermo (heat) and haline (salt), the two controls of density.

Climate role: the circulation carries heat, carbon dioxide and oxygen to the deep ocean and keeps the climate steady over long periods.
Nutrients: upwelling of deep water brings nutrients to the surface, supporting fisheries, as in Major Ocean Currents of the World and Their Effects.
Atlantic branch: the part that includes the Gulf Stream is the subject of Atlantic Meridional Overturning Circulation (AMOC).

As an analytical point, the circulation depends on very small differences in density, and fresh water from melting ice or heavy rain can reduce the sinking and slow the circulation.

Why Is This Topic Important?

Concept Links temperature and salinity
Circulation Basis of deep ocean currents
Climate Heat and carbon transport

What Came Next?

The next topic describes ocean currents, including the vertical motion that density differences produce. Next topic: Ocean Currents: Horizontal and Vertical Motion; Cold and Warm Currents.

Key People and Terms

Pycnocline Zone of rapid density change
Halocline Zone of rapid salinity change
Thermocline Zone of rapid temperature change
Brine rejection Salt left behind when sea ice forms
Bottom water Densest water on the ocean floor
Conveyor belt Global thermohaline circulation

Exam Corner

Points to Remember

Cold and salty water is denser.
Average surface density is about 1.025 g per cubic cm.
The pycnocline separates light surface water from the deep.
Deep water forms in the North Atlantic and around Antarctica.
The conveyor belt takes about 1,000 years.

Do Not Confuse With

Pycnocline, thermocline and halocline Density, temperature and salinity change
Thermohaline and wind-driven circulation Density driven deep flow versus surface flow driven by wind
Density and salinity Salinity is only one of the controls of density

Memory Hook

Cold and salt make water dense; dense water sinks, and the deep belt thinks.

Mains Angle

Questions link density to ocean circulation and climate. These points add depth.

Controls Temperature and salinity
Layers Pycnocline
Climate Heat and carbon storage

A Question You May Face

An original practice question, not a past paper question.

“Explain how temperature and salinity control the density of ocean water and how density differences drive deep ocean circulation.”

How to Answer

Controls Temperature, salinity, pressure.
Layers Pycnocline.
Sinking North Atlantic and Antarctica.
Effect Climate and nutrients.

GS Relevance

Prelims: factors that control density, pycnocline, thermohaline circulation, deep water formation. GS Paper 1: oceanography.

Frequently Asked Questions

What is the density of sea water?

The density of sea water at the surface is about 1.025 grams per cubic centimetre, slightly higher than fresh water, which is 1.0. It rises as the water gets colder or saltier and with increasing pressure at depth, and it varies from place to place in the ocean.

What factors control the density of ocean water?

The density of ocean water is controlled by temperature, salinity and pressure. Colder water is denser, saltier water is denser, and pressure increases the density slightly with depth. In most of the ocean, temperature is the dominant factor, while salinity is important in dry and polar regions.

What is the pycnocline?

The pycnocline is a layer in the ocean where density increases rapidly with depth. It often coincides with the thermocline and the halocline and acts as a barrier between the warm, light surface water and the cold, dense deep water, limiting mixing and the supply of nutrients to the surface.

What is the thermohaline circulation?

The thermohaline circulation is the slow, global circulation of the ocean driven by differences in density, which depend on temperature and salinity. Cold, salty water sinks in the North Atlantic and near Antarctica, flows along the ocean floor and eventually rises elsewhere, a cycle taking roughly a thousand years.

Where is the deep water of the oceans formed?

Deep water forms where surface water becomes cold and dense enough to sink. The two main areas are the North Atlantic, in the Labrador and Nordic Seas, which makes North Atlantic Deep Water, and the seas around Antarctica, such as the Weddell Sea, which make Antarctic Bottom Water.

PYQ Practice — Statement Analysis

1 Cold water is denser than warm water of the same salinity.

True

Density rises as temperature falls.

2 The pycnocline is a zone of rapid change of density with depth.

True

It separates surface and deep water.

3 Deep water forms mainly in the tropics.

False

It forms in the North Atlantic and around Antarctica.

4 Sea water is less dense than fresh water.