Salinity of the Ocean: Factors, Distribution & Dynamic

Introduction

The oceans cover most of our planet, and most of their water is salty. Salinity, the measure of saltiness in seawater, plays a crucial role in shaping our oceans. It reflects the balance between evaporation and precipitation, and the influence of factors like temperature, currents, and water mixing. Understanding salinity helps us comprehend the dynamics of oceanic ecosystems and their impact on climate.

Salinity of the Ocean

• Salinity: It is the total content of dissolved salts in seawater. It is usually expressed as parts per thousand (o/oo) or ppt. 
  • Average Salinity: of seawater is about 35 grams per kilogram (g/kg) of seawater or 35 ppt.
• Ocean Water Composition: 77.7% sodium chloride, 10.9% magnesium chloride, 4.7% magnesium sulphate, 3.6% calcium sulphate, and 2.5% potassium sulphate. 

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Factors affecting Salinity of the Ocean

• Evaporation increases and precipitation decreases the salinity. 
  • Examples: Mediterranean Sea, Red Sea (high evaporation, high salinity).
• Decreases in coastal regions by the freshwater flow from rivers (Ganges and Brahmaputra in Bay of Bengal) and in polar regions by the processes of freezing (increases salinity) and melting (reduces salinity) of ice.
• Wind: influences salinity by transferring water to other areas.
• Ocean Currents: Warm currents elevate salinity in higher latitudes; Cold currents decrease salinity near the equator. 
  • Example: The salinity of the North Sea increases due to more saline water brought by the North Atlantic Drift.
• Water Mixing and Enclosed Seas: Limited freshwater mixing increases salinity. 
  • Examples: Black Sea, Caspian Sea, Red Sea, and Persian Gulf have high salinity.
• Salinity, temperature, and density of water are interrelated. 
  • Any change in temperature or density influences the salinity of water in an area.

Horizontal Distribution of Salinity of the Ocean

• Salinity Range in the Open Ocean: The salinity of the normal open ocean ranges between 33 o/oo and 37 o/oo. 
  • In general, salinity gradually decreases towards the poles.
  • Equatorial Regions: have higher salinity due to evaporation, but this is countered by precipitation.
  • Subtropical High-pressure Zones: experience the highest salinity due to high insolation (clear sky) and low precipitation due to subsiding winds.
  • Polar Regions and Mid-latitudes: have lower surface salinity.
  • Land-locked Sea (Red sea): High in  land-locked Sea (Red sea) as there is not much mixing of water, and in  hot and dry regions, where evaporation is high.
• Salinity decreases in the western parts of the northern hemisphere because of the influx of melted water from the Arctic region.

Vertical Distribution of Salinity

• Halocline: Salinity generally increases with depth and there is a distinct zone called the halocline (compare this with thermocline), where salinity increases sharply. 
• Factors Affecting Surface Salinity: Salinity at the surface increases with the loss of water to ice or evaporation or decreases with the input of fresh water.
  • Lower salinity water rests above the higher salinity dense water, leading to stratification by salinity.
• Vertical Variation: Salinity changes with depth, but the way it changes depends upon the location of the sea.
• High latitudes: Increases with depth. 
• Middle latitudes: Increases up to 35 metres and then decreases. 
• Equator: Surface salinity is lower due to high precipitation and low evaporation due to cloudiness.

Salinity of Marginal Seas

• 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: records low salinity due to the influx of river waters in large quantities.
• Red Sea: has high salinity due to high evaporation and limited freshwater input.
• Mediterranean Sea: records higher salinity due to high evaporation. Salinity is, however, very low in the Black Sea due to enormous freshwater influx by rivers.
• Bay of Bengal: has low salinity due to the influx of river water. 
  • On the contrary, the Arabian Sea shows higher salinity due to high evaporation and low influx of fresh water.

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Conclusion

Studying deeper into the Salinity of the Ocean reveals a world of interconnectedness. From the composition of seawater to its distribution around the globe, salinity tells us a lot about how oceans work. By understanding these salty waters, we gain insights into the dynamics of our planet’s blue oceans, guiding our efforts to protect and preserve our precious oceans.

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