The distribution of salinity in the ocean is not uniform; it varies both horizontally (across the surface) and vertically (with depth).
Horizontal Distribution of Salinity
The salinity of the open ocean typically ranges from 33 to 37 parts per thousand (o/oo) or practical salinity units (PSU). Key factors influencing surface salinity distribution include:
- Evaporation and Precipitation: Areas with high evaporation rates tend to have higher salinity, while areas with significant precipitation have lower salinity.
- Freshwater Input: River runoff and melting ice introduce freshwater, decreasing salinity in coastal areas and high-latitude regions.
- Ice Formation: When seawater freezes, the salt is largely excluded, increasing the salinity of the remaining water.
- Ocean Currents: Currents transport water with different salinity levels, influencing regional salinity patterns.
Generally:
- Latitudinal Variation: The highest salinity levels are usually found between 15° and 30° latitude in both hemispheres due to high evaporation rates and low precipitation in these subtropical regions. Specifically, maximum salinity (around 37 o/oo) is observed between 20° N and 30° N and 20° W – 60° W.
- Equatorial Region: Salinity is typically lower near the equator due to high precipitation.
- Polar Regions: Salinity is also lower in polar regions due to freshwater input from melting ice.
Vertical Distribution of Salinity
The vertical distribution of salinity is also layered and influenced by:
- Surface Processes: Evaporation, precipitation, and freshwater runoff affect the surface salinity.
- Density Differences: Denser, saltier water tends to sink, while less dense, fresher water stays at the surface.
- Thermocline and Halocline: A thermocline is a zone of rapid temperature change with depth, while a halocline is a zone of rapid salinity change with depth. These layers can act as barriers, preventing mixing between surface and deep waters.
- Deep Ocean Salinity: The deep ocean generally has a more uniform salinity, usually between 34 and 35 o/oo.
Specific vertical salinity profiles can vary significantly depending on location and oceanographic conditions:
- High Latitudes: Salinity may increase with depth due to the formation of dense, salty water during sea ice formation (halocline).
- Mid-Latitudes: Salinity may decrease with depth due to a balance between surface evaporation and deeper mixing.
- Estuaries: Salinity varies greatly with depth and distance from the river mouth, creating a strong halocline.
