Factors Controlling Ocean Salinity and its Distribution

• Salinity is the measure used to define the collective content of dissolved salts in seawater. 
• It is calculated as the quantity of salt (in grams) dissolved in 1,000 grams (1 kilogram) of seawater. 
• Salinity is typically expressed as parts per thousand (ppt).
• A salinity of 24.7 (24.7 o/oo) has been traditionally recognized as the upper threshold to delineate 'brackish water'. 
• This demarcation helps classify water bodies based on their salt content, where salinity values beyond this point signify a more saline or concentrated saltwater environment.    

ROLE OF OCEAN SALINITY

Salinity is a critical factor that influences various properties and characteristics of seawater, including compressibility, thermal expansion, temperature, density, absorption of insolation, evaporation, and humidity. 

Moreover, it significantly impacts the composition and dynamics of ocean water and the distribution of marine life and resources.

The distribution of different salts in seawater is as follows:

• Sodium chloride constitutes approximately 77.7% of the salts found in seawater.
• Magnesium chloride accounts for around 10.9%.
• Magnesium sulphate makes up about 4.7%.
• Calcium sulphate comprises roughly 3.6%.
• Potassium sulphate contributes approximately 2.5% to the total salt content in seawater.

FACTORS AFFECTING OCEAN SALINITY

• The salinity of surface ocean water is primarily affected by evaporation and precipitation. 
• Coastal regions experience significant influence on surface salinity due to freshwater inflow from rivers, while polar regions are affected by the freezing and thawing processes of ice.
• Wind also plays a role in determining the salinity of a particular area by moving water to different regions. 
• Furthermore, ocean currents contribute to variations in salinity levels.
• Salinity, temperature, and water density are interconnected. Changes in temperature or density can significantly impact the salinity of a particular area. 
• This interrelationship demonstrates how alterations in these elements can influence the overall salinity of a given region.

HORIZONTAL DISTRIBUTION OF SALINITY

Salinity Levels in Different Regions:

• Normal Open Ocean: Ranges between 33 and 37 in most areas.

High Salinity Regions:

• Red Sea: Salinity around 41 (Note: Not to be confused with the Dead Sea, which has much greater salinity).
• Hot and Dry Regions: In regions with high evaporation, salinity can reach as high as 70.

Comparatively Low Salinity Regions:

• Estuaries and Arctic: Salinity fluctuates from 0 to 35 seasonally due to the mixing of fresh and saline waters in estuaries, and the influx of freshwater from ice caps in the Arctic.
• Pacific Ocean:
  • Salinity Variations: Mainly influenced by the ocean's shape and extensive area.
• Atlantic Ocean:
  • Average Salinity: Around 36-37 in most areas.
• Equatorial Region: Salinity around 35 due to heavy rainfall, high humidity, cloudiness, and calm air in the doldrums.
• Polar Areas: Salinity ranges between 20 and 32 due to little evaporation and a large influx of freshwater from melting ice.
• Indian Ocean:
  • Average Salinity: Around 35.
• Bay of Bengal: Lower salinity due to the influx of river water, especially from the Ganga River.
• Arabian Sea: Higher salinity due to increased evaporation and lower freshwater input.
• Marginal Seas:
  • North Sea: Records higher salinity due to more saline water from the North Atlantic Drift.
  • Baltic Sea: Low salinity due to the large influx of river waters.
  • Mediterranean Sea: Higher salinity due to increased evaporation.
• Black Sea: Very low salinity due to the significant influx of freshwater from rivers.
• Inland Seas and Lakes:
  • High Salinity: Inland seas and lakes due to consistent salt supply from rivers, and increasing salinity due to evaporation.
  • Examples: Great Salt Lake (Utah, USA) - 220, Dead Sea - 240, Lake Van (Turkey) - 330.

Trends in Salinity Change:

• Oceans and Salt Lakes: Becoming saltier over time due to increased salt deposition by rivers and freshwater loss through evaporation.

Cold and Warm Water Mixing Zones:

• Western Northern Hemisphere: Salinity decreases from 35 to 31 due to melted water influx from the Arctic region.

Sub-Surface Salinity:

• Latitudinal Influence: In high latitudes, salinity increases with depth. In middle latitudes, it increases up to 35 metres and then decreases. 
  • At the equator, surface salinity is lower.
• Influence of Cold and Warm Currents: Sub-surface salinity variation is influenced by these currents.
  • This revised breakdown offers a clear insight into the diverse salinity levels and their influencing factors in various oceanic and coastal regions around the world.

VERTICAL DISTRIBUTION OF SALINITY

• Salinity within the ocean varies with depth, the alterations influenced by the specific characteristics of each sea.
• At the surface, salinity levels can increase due to water loss through processes like ice formation or evaporation, or decrease with the influx of freshwater from sources like rivers.
• However, at depth, salinity remains relatively stable as water is neither lost nor added, leading to a pronounced disparity between surface and deep ocean zones.
• Lower salinity water typically resides above higher salinity, denser water.
• Generally, salinity tends to increase with depth, culminating in a notable zone known as the halocline, akin to the thermocline, where salinity sharply rises.
• Given constant conditions, elevating seawater salinity results in increased density. Consequently, higher salinity water typically sinks beneath lower salinity water, fostering stratification based on salinity levels. 
• This process, known as stratification by salinity, contributes to the layering within the ocean.

FAQs: Salinity and its Impact on Oceans

1. What is salinity, and how is it measured in seawater?

Ans. Salinity is the measure of dissolved salts in seawater and is calculated as the quantity of salt (in grams) dissolved in 1,000 grams (1 kilogram) of seawater. The salinity of seawater is commonly expressed as parts per thousand (ppt).

2. What is the benchmark for 'brackish water' concerning salinity?

Ans. The upper threshold to delineate 'brackish water' is traditionally recognized as 24.7 parts per thousand (24.7 ppt). Salinity values beyond this point indicate a more saline or concentrated saltwater environment.

3. What role does ocean salinity play in seawater characteristics?

Ans. Salinity significantly influences various properties and characteristics of seawater. It affects compressibility, thermal expansion, temperature, density, absorption of insolation, evaporation, and humidity. Moreover, it impacts the composition, dynamics, and distribution of marine life and resources in oceans.

4. How are different salts distributed in seawater?

Ans. The composition of salts in seawater includes sodium chloride (77.7%), magnesium chloride (10.9%), magnesium sulphate (4.7%), calcium sulphate (3.6%), and potassium sulphate (2.5%).

5. What factors affect the salinity of oceans?

Ans. Salinity in surface ocean water is primarily influenced by evaporation, precipitation, freshwater input from rivers in coastal regions, and freezing/thawing processes of ice in polar regions. Wind movements, ocean currents, and the interconnected nature of salinity, temperature, and water density also affect salinity levels.

6. What are the salinity levels in different ocean regions?

Ans. Salinity ranges between 33 and 37 in the normal open ocean. Specific areas vary widely; for instance, the Red Sea has a salinity around 41, while the Baltic Sea has lower salinity due to a large influx of river waters.

7. How does ocean salinity vary in different seas and inland water bodies?

Ans. Salinity in different seas and lakes varies due to specific conditions, such as the Mediterranean Sea's higher salinity due to increased evaporation, while the Black Sea has very low salinity due to significant freshwater influx from rivers.

8. What trends in salinity change are observed in oceans and salt lakes?

Ans. Oceans and salt lakes are becoming saltier over time due to increased salt deposition by rivers and freshwater loss through evaporation.

9. How does salinity change with depth in the ocean?

Ans. Salinity varies significantly with depth, creating a distinct zone known as the halocline, akin to the thermocline. Generally, salinity tends to increase with depth. High salinity water sinks beneath lower salinity water, leading to stratification by salinity.

10. How is subsurface salinity affected in different latitudes and by cold and warm currents?

Ans. In high latitudes, salinity increases with depth. In middle latitudes, it increases up to 35 metres and then decreases. Sub-surface salinity variation is influenced by the presence of cold and warm currents.