Temperature ,Salinity and Density of Oceans

Temperature ,Salinity and Density of Oceans

Temperature distribution of Oceans-

• Oceans have varying temperature due to different factors responsible for such variability.

Source of Heat in Oceans-

• Primary and major source of heating of oceans is Insolation or energy received from The Sun.

Process-

• Absorption of Sun’s radiation ->Circulation of heat received in ocean water due to difference in temperature of top and lower layers of sea ->Further distribution of this heat.

• Also Oceans gets cooled because of Radiation loss due to long wave Infrared radiations ,Evaporation etc.

Factors Affecting Temperature Distribution of Oceans-

Latitude -

• The temperature of surface water tends to decrease as one moves from the equator towards the poles.

• This phenomenon occurs primarily due to a reduction in the amount of insolation that is received in regions that are located farther away from the equator.

Prevailing wind-

• When offshore winds blow from land towards oceans and seas, it creates an upwelling phenomenon by pushing warm surface water away from the coast and causing cold bottom water to rise to the surface.

• This exchange of warm and cold water results in longitudinal variation in temperature.

• Contrary to this, the onshore winds pile up warm water near the coast and thus raise the temperature.

Ocean currents-

• Warm currents raise the temperature of the that area of ocean whereas cool currents lower down the temperature.

Unequal distribution of land and water-

• The northern hemisphere's oceans receive more heat due to their proximity to a larger expanse of land than the southern hemisphere's oceans. As a result, the surface water temperature is relatively higher in the northern hemisphere than in the southern hemisphere.

The shape of the ocean-

• The seas located in low latitude regions that extend latitudinally are characterized by warmer surface water than the seas that extend longitudinally.

Horizontal and Vertical Distribution of Temperature-

Horizontal Distribution of Temperature-

• The average temperature of the Earth's ocean surface water is recorded as 26.7°C. It follows a gradual decrease in temperature from the equator towards the poles.

• The distribution of temperature in a horizontal plane is represented by isothermal lines.

• These lines connect different points having the same temperature, and they indicate the variation in temperature across different locations.

• The decrease in temperature with increasing latitudes is usually observed to be around 0.5°C per latitude.

• The temperatures observed in the oceans of the northern hemisphere tend to be comparably higher in contrast to the oceans located in the southern hemisphere.

• The difference in distribution of land and water between the northern and southern hemispheres is the reason behind this variation.

• The highest temperature is not recorded precisely at the equator, rather it is observed slightly towards the north of it.

Vertical Distribution of Temperature-

In oceans temperature decreases with depth of oceans

Thermocline -

• The zone at which there is a sudden change or drop in temperature is referred to as the thermocline boundary region.

• Approximately 90% of the entire water volume is situated beneath the thermocline within the deep ocean.

• Typically, the boundary starts at a depth of 100 – 400 meters below the sea level and extends several hundred meters downwards.

Photic or euphotic zone-

• It extends from the upper surface to 200 m.

• The photic zone receives adequate solar insolation. ·

Aphotic zone -

• It extends from 200 m to the ocean bottom; this zone does not receive adequate sunrays.

Three-Layer System -

The temperature profile of oceans located at middle and low latitudes can be classified as a three-layer system from the surface to the seafloor.

The first layer -

• It represents the top layer of warm oceanic water and it is about 500m thick with temperatures ranging between 20° and 25° C.

• The layer mentioned here is present all year long in the tropical region, but it only develops during the summer months in mid-latitudes.

The second layer -

• It is called the thermocline layer lies below the first layer and is characterized by rapid decrease in temperature with increasing depth.

• The thermocline is 500 -1,000 m thick. 

The third layer- 

• It is very cold and extends up to the deep ocean floor. Here the temperature becomes almost stagnant.

Some facts related to Ocean Temperature-

• Diurnal and annual ranges of temperature cease after a depth of 30 feet and 600 feet respectively.

• The high temperatures of waters are found in enclosed seas in the tropics e.g. the Red Sea.

• The enclosed seas of low latitudes like the Sargasso Sea, the Red Sea and the Mediterranean Sea have high bottom temperatures due to high insolation throughout the year and lesser mixing of the warm and cold’ waters.

• The equator exhibits a more significant rate of temperature reduction with increasing depth compared to the poles.

Salinity of Oceans-

• Ocean water salinity is calculated based on the amount of dissolved salts present in it.

• These salts are predominantly composed of sodium chloride, but there are also other salts that contain sulfur, magnesium, calcium, and potassium.

• It is the sum of these salts that determines the salinity of ocean water.

• Salinity of approximately 25 % has been considered as the upper limit to demarcate ‘brackish water’.

• Typical salinity level of seawater comprises roughly 35 parts per thousand, which is equivalent to 3.5 percent of its overall mass.

• The salinity of seawater is determined by measuring the amount of salt (in grams) dissolved in 1,000 grams (1 kilogram) of water, which is referred to as the salinity of seawater.

Share of Different Salts -

• 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-

• When the rate of evaporation is greater, the level of salinity increases accordingly, and the converse is also true: if the rate of evaporation is lower, the salinity level will also decrease.

Precipitation-

• More the precipitation lesser will be Salinity because precipitation will add fresh water to the saline water and thus reducing its salinity.

The Influx of Fresh water-

• More the influx of fresh water lesser will be salinity.

• In coastal region there is influx of fresh water ,therefore there is less salinity as comparison to areas far from coasts.

• In the Polar region, the surface salinity is influenced by the processes of freezing and melting of ice which adds freshwater to the oceans and thus decreasing its salinity.

Ocean Currents and winds -

• The wind and ocean currents also influences the salinity of an area by moving water from one area to other area.

For example-

• The movement of saline waters from the western coasts of the continents (or eastern margins of the oceans) to the eastern coasts (or western margins of the oceans) is driven by the trade winds.

• This phenomenon results in low salinity in the former area and high salinity in the latter.

• This is why the Gulf of Mexico records 36  o/oo  to 37 o/oo salinity whereas it is only 34 o/oo in the Gulf of California.

• The eastern edges of oceans tend to have lower salinity levels due to the influence of warm currents in the equatorial region, which push salts away from these margins. As a result, the salts tend to accumulate near the western margins of the oceans.

• Likewise, oceanic currents in the temperate zones elevate the salinity of ocean waters that are closer to the eastern coasts. The Gulf Stream in the North Atlantic Ocean is an example of this phenomenon, as it boosts the salinity of ocean waters that lie along the western edges of the Atlantic Ocean.

Distribution of Ocean Salinity-

1.Horizontal distribution-

A.Latitudinal Distribution

• In general salinity decreases as we move from equator to pole.

• Also Equator have less salinity as comparison to tropical region because of more precipitation in equator as comparison to tropics which helps to add more fresh water in oceanic water of equator.

B.Regional distribution

• To maintain a healthy marine environment, it is essential to regulate the salinity levels of the open ocean. Typically, the salinity range for normal open ocean water is between 33 parts per thousand (ppt) and 37 ppt.

• The salinity level in the western parts of the northern hemisphere experiences a reduction from 35 o/oo to 31 o/oo. This decrease is primarily attributed to the inflow of melted water that comes from the Arctic region.

• In the same way, after 15° - 20° south, it decreases to 33 o/oo .

Pacific ocean-

• There is variation in salinity of pacific ocean and this variation is mainly due to its shape and larger areal extent.

• Salinity remains 34.85  o/oo  near the equator.

• It increases to 35 o/oo  between 15 -20 degree latitudes in the northern hemisphere but it becomes still higher (36 o/oo ) in the southern Pacific Ocean between the same latitudes

Atlantic ocean-

• The average salinity of the Atlantic Ocean is around 36 o/oo.

• There is a maximum concentration of salt in the ocean water, which is observed between latitudes 15° and 20°.

• Maximum salinity (37 o/oo) is observed between 20° N and 30° N and 20° W - 60° W.

• It gradually decreases towards the north.

The North Sea-

• In spite of its location in higher latitudes, records higher salinity due to more saline water brought by the North Atlantic Drift.

Baltic Sea -

• It records low salinity due to influx of river waters in large quantity.

The polar areas-

• It experience very little evaporation and receive large amounts of fresh water from the melting of ice.

• This leads to low levels of salinity.

Indian Ocean-

• The distribution of salinity in the Indian Ocean is comparatively more intricate and diverse than that of the Pacific and Atlantic oceans in terms of its spatial patterns.

• An average salinity of 35o/oo is found between 0-10 degree N but it gradually decreases northward in the Bay of Bengal (33.5 o/oo at 10N lat to 30 o/oo at the mouth of the Ganga) because of influx of immense volume of freshwater brought by the Ganga river.

• On the other hand, the Arabian Sea records higher salinity (36 o/oo ) than the Bay of Bengal because there is higher rate of evaporation due to relatively less humid conditions and low influx of freshwater as compared to the Bay of Bengal.

The Red Sea -

• It records the highest salinity  because of low precipitation and very high evaporation.

Inland Seas and Lakes:

• The salinity levels of lakes and inland seas can be impacted by the introduction of fresh water brought in by rivers.

• Caspian sea also have variation in its salinity.

• Low salinity of the northern part of Caspian Sea (14 o/oo) is because of addition of enormous volume of water brought by the rivers like Volga, Ural etc. but it becomes as high as 170 o/oo in the southern part i.e. the Gulf of Karabugas.

Highest salinity in water bodies-

• Lake Van in Turkey (330 o/oo ).

• Dead Sea (238 o/oo ).

• Great Salt Lake (220 o/oo ).

Salinity of some of the seas(in o/oo )

Seas having salinity above normal:

• Red Sea (34-41).

• Persian Gulf (37-38).

• Mediterranean Sea (37-39).

Seas having normal salinity-

• Caribbean Sea and Gulf of Mexico (35-36).

• Bass Strait (35).

• Gulf of California (25-36).

Seas having salinity below normal-

• Arctic Ocean (20-35).

• North Australian Sea(33-34).

• Bering Sea (28-33).

• North Sea (31-35).

• Baltic Sea (3-15).

• Hudson Bay (3-15).

2.Vertical Distribution-

• Salinity increases with increasing depth in high latitudes and after some depth it becomes constant.

• Whereas in low latitude areas there is more salinity at the surface and it decreases with depth till certain point and than after it becomes constant.

Halocline-

• It is a vertical region  in the oceanic water column where salinity undergoes a rapid transformation with increasing depth. This region is situated beneath the surface water layer that is well-mixed and has a uniform salinity.

Density of ocean water-

• Density refers to the amount of mass per unit volume.

• The density of fresh water is 1 g/cm3 at 4o C but addition of salts increases the density.

Factors affecting density of sea water-

Temperature-

• Temperature and density are inversely proportional.
• Increase in the temperature leads to expansion of water which leads to increase in volume and decrease in density.

Salinity-

• Salinity is directly proportional to the density of the ocean water.

Pressure-

• Pressure is directly proportional to the density of the ocean water and as water is not that much compressible therefore change in pressure does not have that much high effect on density of the ocean water.

Distribution of density with depth-

• Density of ocean water increases with depth.

Pycnocline Zone -

• This region of the ocean is characterized by a sharp increase in water density with depth, caused by alterations in temperature and salinity.

• After some depth and just below Pycnocline density of ocean water becomes constant.

Important one liner on Temperature ,Salinity and Density of Oceans.

What is the average salinity of the ocean?

Answer: The average salinity of the ocean is 35 ppt (parts per thousand).

What is the thermocline in the ocean?

Answer: The thermocline is the layer of the ocean where temperature changes rapidly with depth.

What is the temperature of the surface layer of the ocean?

Answer: The temperature of the surface layer of the ocean varies between 0°C to 30°C depending on location.

What is the average depth of the ocean?

Answer: The average depth of the ocean is around 12,100 feet (3,688 meters).

What is the density of seawater?

Answer: The density of seawater ranges from 1020 to 1030 kg/m³.

What is the halocline in the ocean?

Answer: The halocline is the layer of the ocean where salinity changes rapidly with depth.

What is the average temperature of the ocean?

Answer: The average temperature of the ocean is around 3.9°C (39°F).

What is the pycnocline in the ocean?

Answer: The pycnocline is the layer of the ocean where density changes rapidly with depth.

What is the freezing point of seawater?

Answer: The freezing point of seawater is -1.9°C (28.6°F).

What is the density of the ocean at the surface layer?

Answer: The density of the ocean at the surface layer ranges from 1020 to 1025 kg/m³.

What is the salinity of the ocean in the polar regions?

Answer: The salinity of the ocean in the polar regions is lower due to melting ice.

What is the relationship between salinity and density of seawater?

Answer: The density of seawater increases with increasing salinity.