Properties of Ocean Water: Salinity and Temperature

(Horizontal and Vertical Distribution)

Pritiranjan Das
Assistant Professor,
Dept. of Geography,
Shaheed Bhagat Singh Evening College,
University of Delhi
Delhi-110030
 Salinity
(Horizontal and Vertical Distribution)
 Ocean Salinity

•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 gm) dissolved in 1,000 gm (1 kg)
of seawater.
•It is usually expressed as parts per thousand or ppt.
 Role of Ocean Salinity

•Salinity determines thermal expansion, temperature,

density, evaporation and humidity.
•It also influences the composition and movement of the
sea: water and the distribution of fish and other marine
resources.

Share of different salts is as shown

below—
•sodium chloride — 77.7%
•magnesium chloride—10.9%
•magnesium sulphate —4.7%
•calcium sulphate — 3.6%
•potassium sulphate — 2.5%
 Factors Affecting Ocean Salinity

•Evaporation: The salinity of water in the surface layer of

oceans depend mainly on evaporation. Where the
evaporation is greater, the salinity is higher, for example,
Mediterranean sea.
•Freshwater flow influx: Surface salinity is greatly
influenced in coastal regions by the freshwater flow from
rivers, and in polar regions by the processes of freezing and
Melting of ice.
• Where the freshwater flow into the oceans is
greater, the salinity is lower.
• For instance, at the mouths of rivers such as Amazon,
Congo, Ganga etc., the ocean surface salinity is
found to be lower than the average surface
salinity.
•Temperature and density: Salinity, temperature and
density of water are interrelated. Hence, any change in the
temperature or density influences the salinity of an area.
• In general, regions with high temperatures are also,
regions with high salinity.
•Ocean Currents: They play an important role in the spatial
distribution of dissolved salts in ocean waters.
• The warm currents near the equatorial region push
away the salts from the eastern margins of the oceans
and accumulate them near the western margins.
• Similarly, ocean currents in the temperate regions
increase the salinity of ocean waters near the eastern
margins. For instance, Gulf Stream in the North
Atlantic Ocean increases the salinity of ocean waters
along the western margins of the Atlantic Ocean.
•Precipitation: Precipitation and salinity share an inverse
relationship.
• In general, regions with higher levels of precipitation
have lower levels of salinity. This is the reason why
though the equatorial region is as hot as the sub-
tropics, it records lower salinity than the sub-tropics
since the former receives heavy precipitation in a day.
•Atmospheric pressure and Wind direction: anti-cyclonic
conditions with stable air and high temperature increase salinity
of the surface water of oceans
• winds help is redistribution of salinity, as they drive
away saline waters to less saline areas resulting into
decrease of salinity in the former and increase in the
latter.
 Salinity Distribution

Surface seawater salinities largely reflect the local balance between evaporation and precipitation.
•Low salinities occur near the equator due to rain from rising atmospheric circulation.
•Salinity can also be affected by sea ice formation/melting (e.g. around Antarctica)
•The surface N. Atlantic is saltier than the surface N. Pacific, making surface water denser in the N. Atlantic
at the same temperature and leading to down-welling of water in this region this difference is because on
average N. Atlantic is warmer (10.0 °C) than N. Pacific (6.7 °C).
•This is mostly because of the greater local heating effect of the Gulf Stream, as compared to the Kuroshio
Current. Warmer water evaporates more rapidly, creating a higher residual salt content.
•The influence of surface fluctuations in salinity due to changes in evaporation and precipitation is generally
small below 1000 m, where salinities are mostly between about 34.5 and 35.0 at all latitudes.
•Zones where salinity decreases with depth are typically found occur at low latitudes and mid latitudes,
between the mixed surface layer and the deep ocean. These zones are known as haloclines.

The spatial distribution of salinity across oceans, is studied in two ways:

•Horizontal Distribution of Salinity
•Vertical Distribution of Salinity
 Horizontal Distribution of Salinity
•On an average, salinity decreases from equator towards the poles. However, it is
important to note that the highest salinity is seldom recorded near the equator though this
zone records high temperature and evaporation but high rainfall reduces the relative
proportion of salt. Thus, the equator accounts for only 35‰ salinity
•The highest salinity is observed between 20° N and 30° N (36‰) because this zone is
characterized by high temperature, high evaporation but relatively low rainfall
•The average salinity is 35‰.
•The zone between 40 deg -60 deg latitudes in both the hemispheres records low salinity
where it is 31‰ and 33‰ in the northern and the southern hemispheres respectively.
•Salinity further decreases in the polar zones because of influx of Glacial melt-water. On
an average, the northern and the southern hemispheres record average salinity of 35‰ and
34‰ respectively
 Horizontal Distribution of Salinity

1.The salinity for normal Open Ocean ranges between 33 to 37 grams.

2.The highest salinity is recorded between 15° and 20° latitudes.
3.Maximum salinity (37 o/oo) is observed between 20° N and 30° N and 20° W – 60° W.
4.The salinity gradually decreases towards the pole.
5.The salinity sometimes reaches up to 70 o/oo in the hot and dry regions where evaporation is high.
6.The salinity variation in the Pacific Ocean is largely due to its shape and larger areal stretch.
7.In the landlocked Red Sea, the salinity is 41o/oo which considerably high.
8.Due to the influx of melted water from the Arctic region, the salinity decreases from 35 o/oo – 31 o/oo on the
western parts of the northern hemisphere.
9.Due to the influx of river waters in the large amount, the Baltic Sea records low salinity.
10.The Mediterranean Sea accounts for the higher salinity due to high evaporation.
11.Salinity is very low in the Black Sea due to massive freshwater influx by rivers.
12.The average salinity of the Indian Ocean is 35 o/oo.
13.The low salinity trend in the Bay of Bengal is due to the influx of river water.
14.But the Arabian Sea displays higher salinity due to the low influx of fresh water and high evaporation.
 Vertical Distribution of Salinity
•Salinity at the surface of the sea is decreased by the input of fresh waters or increased by the
loss of water to ice or evaporation. Thus both the trends of increase and decrease of salinity
with increasing depths have been observed
•Salinity increases with increasing depth from 300 meters to 1000 meters in high latitudes
i.e. there is positive relationship between the amount of salinity and depth because of denser
water below but salinity becomes more or less constant beyond 1000 m depth
•Salinity decreases with increasing depth from 300 meters to 1000 meters in the low latitudes
but it becomes more or less constant beyond 1000 m depth
•It appears from the above mentioned trends of vertical distribution of salinity that there is
rapid rate of change of salinity (both increase and decrease) in the depth zone of 300m-
1000m. This zone of steep gradient of salinity is called halocline
•Maximum salinity is found in the upper layer of the oceanic water. Salinity decreases with
increasing depth. Thus, the upper zone of maximum salinity and the lower zone of minimum
salinity is separated by a transition zone which is called as halocline, on an average above
which high salinity is found in the low latitudes while low salinity is found in the high
latitudes
A halocline is a layer where salinity changes rapidly with depth, while a thermocline is a layer
where temperature changes rapidly with depth. Both are types of "clines", which are layers where
properties change rapidly with depth. The pycnocline encompasses both the halocline and the
thermocline, referring to the rapid change in density with depth.
1. Halocline in High Latitudes:
 Freshwater Inputs: High latitudes receive significant freshwater input from melting ice, glaciers, and rivers, which
lowers surface salinity. This creates a strong halocline as salinity increases rapidly with depth.
 Low Evaporation: Cold temperatures in high latitudes result in minimal evaporation, which helps maintain the lower
salinity of surface waters.
 Sea Ice Formation and Melting: When sea ice forms, it expels salt (a process called brine rejection) into the
surrounding water, increasing salinity in deeper layers. Conversely, melting sea ice adds fresh water to the surface,
enhancing the halocline.
 Stratification: The strong density difference caused by low surface salinity and higher salinity at depth creates stable
stratification, which strengthens the halocline.

2. Halocline in Low Latitudes:

 High Evaporation: In tropical and subtropical regions, high temperatures cause significant evaporation, increasing
surface salinity. As a result, the halocline is weaker or even absent in some areas.
 Precipitation: While low latitudes experience heavy rainfall in certain regions (e.g., near the equator), it often offsets
evaporation only partially, leading to moderately saline surface waters.
 Mixing Processes: Strong winds and ocean currents in low latitudes can mix surface and subsurface waters, reducing
the salinity gradient and weakening the halocline.
 Regional distribution of water salinity across the oceans

•Indian Ocean: The average salinity of the Indian Ocean is 35 parts per thousand . The low salinity is observed
in the Bay of Bengal due to the influx of river water by the river Ganga. On the other hand, the Arabian
Sea shows higher salinity due to high evaporation and a low influx of freshwater.
•Pacific Ocean: The salinity variation in the Pacific Ocean is mainly due to its shape and larger areal extent.
•Atlantic Ocean: The salinity in the Atlantic ocean varies between 20 to 37 parts per thousand according to the
location.
• For example, Near the equator, there is heavy rainfall, high relative humidity, cloudiness and calm air of
the doldrums.
• Whereas, The polar areas experience very little evaporation and receive large amounts of freshwater
from the melting of ice. This leads to low levels of salinity, ranging between 20 and 32 parts per
thousand
•North Sea: In spite of its location in higher latitudes, it records higher salinity due to more saline water
brought by the North Atlantic Drift.
•The Mediterranean Sea: The Mediterranean Sea records higher salinity due to high evaporation. Surface
waters average about 38 parts per thousand in this sea
•The Baltic Sea: Baltic Sea records low salinity due to influx of river waters in large quantity, averaging around
35 parts per thousand
 Temperature
(Horizontal and Vertical Distribution)
Source of Heat in Oceans
•The sun is the principal source of energy (Insolation).
•The ocean is also heated by the inner heat of the ocean itself (earth’s interior is
hot. At the sea surface, the crust is only about 5 to 30 km thick). But this heat is
negligible compared to that received from sun.

Why is diurnal range of ocean temperatures too small?, Why oceans take
more time to heat or cool?
•The process of heating and cooling of the oceanic water is slower than land due
to vertical and horizontal mixing and high specific heat of water.
•More time required to heat up a Kg of water compared to heating the same unit
of a solid at same temperatures and with equal energy supply.
Introduction
•The Distribution of Temperature and salinity of
oceans play crucial roles in various phenomena
such as the vertical and horizontal circulation of
ocean water, the movement of surface
and subsurface currents, and the climate of
different locations.
•These factors vary across the globe, resulting in
diverse impacts on different regions.
Average Temperature of Ocean water

•The sea surface typically has an average

temperature of approximately 20°C (68°F), although it
can vary depending on the location.
•In warm tropical regions, the temperature can exceed
30°C (86°F), while at high latitudes, it can drop below
0°C. Generally, as you go deeper into the ocean, the
water becomes colder.
General Horizontal Trends:
1.Tropical Oceans:
1.Generally uniform and warm (24–30°C or 75–86°F).
2.Low variability due to consistent solar radiation throughout the year.
2.Subtropical Oceans:
1.Temperatures begin to decline with increasing latitude.
2.Influenced heavily by warm and cold currents.
3.Temperate Oceans:
1.More variable due to stronger seasonal changes.
2.Cooler than tropical waters, typically ranging from 5–20°C (41–68°F).
4.Polar Oceans:
1.Extremely cold, often near or below 0°C (32°F).
2.Sea ice formation and melting strongly influence temperature.
Factors of Horizontal Distribution of Ocean Temperature
 Latitudinal Variation:
1. Equator: Surface temperatures are highest near the equator due to direct solar radiation,
often exceeding 27°C (80°F).
2. Poles: Surface temperatures decrease significantly toward the poles, where they may
approach freezing (−2°C) due to limited solar energy and ice cover.
3. Tropics and Subtropics: Warm waters dominate, particularly in regions influenced by
currents like the Gulf Stream and Kuroshio Current.
 Influence of Ocean Currents:
1. Warm Currents (e.g., Gulf Stream): Transport warm water from the equator toward
higher latitudes, raising temperatures in these regions.
2. Cold Currents (e.g., California Current): Bring cooler waters from polar or deep
regions toward lower latitudes.
 Regional Differences:
1. Eastern vs. Western Coasts: Western coasts of continents often have cooler surface
waters due to upwelling and cold currents, while eastern coasts tend to have warmer
waters due to warm currents.
2. Enclosed Seas: Smaller bodies like the Mediterranean and Red Seas can have higher
temperatures due to limited circulation and high evaporation rates.
 Wind Patterns
 Trade Winds: Persistent wind
systems influence the surface
movement of water, causing warm or
cold water to be transported
horizontally.
 Upwelling: Wind-driven upwelling in
certain coastal regions brings cooler,
nutrient-rich waters to the surface,
significantly impacting horizontal
temperature distribution.
5. Proximity to Land
 Continental Influence: Coastal waters are influenced by land temperatures, runoff, and freshwater inputs, which
can moderate or amplify local ocean temperatures.
 Enclosed Seas: Semi-enclosed seas and gulfs, such as the Mediterranean or the Gulf of Mexico, may experience
temperature anomalies due to restricted water circulation.

6. Seasonal Variations
 Summer vs. Winter: Seasonal changes in solar radiation affect ocean temperatures, especially in mid-latitude and
high-latitude regions.
 Monsoonal Effects: In regions influenced by monsoons, seasonal winds and precipitation can alter ocean
temperatures.

7. Salinity
 Density Differences: Variations in salinity affect water density, which can influence the horizontal movement and
mixing of warm or cold water masses.
8. Ocean Basin Configuration
 Geographical Features: The shape and size of ocean basins, the presence of underwater ridges, and coastal
boundaries impact the flow of currents and the distribution of heat.
Vertical Distribution of Ocean Temperature
1.Layers of the Ocean:
1. Surface Layer (Mixed Layer):
1.Extends from the surface to about 200 meters.
2.Uniform temperature due to mixing caused by winds and waves.
3.Temperature depends on solar radiation and atmospheric conditions.
2. Thermocline:
1.Lies between 200–1000 meters.
2.Rapid temperature decrease with depth.
3.Most pronounced in tropical regions; less defined in polar regions.
3. Deep Ocean:
1.Below the thermocline, temperatures are nearly uniform.
2.Typically range from 0°C to 4°C, with slight variations based on location.
Factors of vertical Distribution of Ocean Temperature
1. Solar Radiation (Insulation)
 Surface Heating: The sun heats the ocean's surface, leading to higher temperatures in the upper layers.
 Latitude: The intensity of solar radiation varies with latitude, causing equatorial waters to be warmer compared to
polar regions.
2. Thermal Stratification
 Surface Layer (Mixed Layer): The uppermost layer is well-mixed due to wind and wave action, maintaining a nearly
uniform temperature.
 Thermocline: Below the mixed layer, there is a rapid decrease in temperature with depth. The thermocline varies in
thickness and steepness depending on location.
 Deep Layer: The bottom-most layer has cold, stable temperatures, typically near 0°C to 4°C.

3. Depth
 Pressure: With increasing depth, water is compressed slightly, affecting its thermal properties, though the impact on
temperature is minimal compared to other factors.
 Lack of Sunlight: Beyond a certain depth, no solar radiation penetrates, leading to cold temperatures in deeper layers.
4. Ocean Currents
 Warm and Cold Currents: Surface and subsurface currents redistribute heat. For example, warm currents like the Gulf
Stream raise surface temperatures, while cold currents like the California Current lower them.
 Upwelling and Downwelling: Upwelling brings cold, nutrient-rich water to the surface, while downwelling pushes
warmer surface water to deeper layers.
5. Geographical Location
 Proximity to Land: Coastal areas often have varying temperature distributions due to land-sea interactions, river
discharge, and shallow depths.
 Basin Characteristics: Ocean basins with unique bathymetric features, like mid-ocean ridges and trenches, influence
water movement and heat distribution.
6. Salinity
 Thermohaline Circulation: Variations in salinity influence density and, in turn, vertical mixing and temperature profiles.
7. Seasonal Changes
 Summer Heating: In temperate regions, summer heating can cause a stronger thermocline due to increased surface
temperatures.
 Winter Mixing: Cooler surface temperatures in winter promote vertical mixing, reducing the thermocline's steepness.
Thermohaline circulation (THC), also known as the global conveyor belt, is a large-scale ocean circulation driven by
differences in water density, which are controlled by temperature (thermo-) and salinity (-haline). It plays a crucial role in
regulating Earth's climate and redistributing heat, nutrients, and gases across the oceans.
8. Wind and Weather Conditions
 Wind-Induced Mixing: Strong winds enhance the mixing of surface layers, distributing heat more
evenly.
 Storms and Hurricanes: Intense weather events can cause temporary disruptions in thermal
stratification.
9. Tectonic and Volcanic Activity
 Hydrothermal Vents: Volcanic activity on the seafloor releases heat, locally warming the deep ocean.
10. Global Climate and Long-Term Changes
 Climate Change: Increased greenhouse gases are causing the upper ocean to warm more rapidly.
 El Niño and La Niña Events: These phenomena alter sea surface temperatures and affect the vertical
distribution.
Vertical Distribution of
Temperature
•The maximum
temperature of the oceans is
consistently found at their
surface because it directly
absorbs sunlight, and the heat
is then transferred to the deeper
layers of the oceans through
conduction.

•Sunlight is able to penetrate up to a depth of 200 meters in the ocean and rarely
goes beyond 200 meters.
•The upper surface of the ocean, known as the Photic or Euphotic zone, extends
up to a depth of 200 meters and is where solar radiation is received.