Ocean water is a complex mixture, and its properties vary widely with depth, latitude, and proximity to land. Two fundamental properties of ocean water that significantly influence its behavior and movement are temperature and salinity.
Temperature of Ocean Water
1. Variation with Depth:
- Surface Layer: The top layer of the ocean, up to about 200 meters deep, is known as the epipelagic or sunlight zone. Here, temperatures are warmer due to direct heating by the sun. This layer can reach up to 30°C (86°F) in tropical regions.
- Thermocline: Below the surface layer, there’s a region where the temperature drops rapidly with depth, called the thermocline. This zone can extend from about 200 meters to 1,000 meters. The temperature gradient in this layer is steep, with temperatures decreasing to around 5°C (41°F).
- Deep Layer: Below the thermocline, the temperature decreases more slowly and remains relatively constant. In the deep ocean (below 1,000 meters), temperatures are typically around 0-4°C (32-39°F).
2. Variation with Latitude:
- Tropical Regions: Near the equator, ocean surface temperatures are consistently high, often exceeding 25°C (77°F) throughout the year.
- Temperate Regions: In mid-latitudes, surface temperatures vary with the seasons. Summer temperatures can reach 20°C (68°F), while winter temperatures may drop to 5°C (41°F).
- Polar Regions: Near the poles, surface temperatures are much lower, often near the freezing point of seawater (-2°C to 0°C or 28.4°F to 32°F).
Example:
- The Gulf Stream: This warm ocean current originates in the Gulf of Mexico and flows along the eastern coast of the United States before crossing the Atlantic Ocean towards Europe. The Gulf Stream significantly warms the climate of nearby coastal regions, such as the southeastern United States and western Europe.
Salinity of Ocean Water
1. Measurement of Salinity:
- Salinity is the concentration of dissolved salts in seawater, usually measured in parts per thousand (ppt or ‰). The average salinity of ocean water is about 35 ppt, meaning there are 35 grams of dissolved salts in every liter of seawater.
2. Factors Influencing Salinity:
- Evaporation and Precipitation: High evaporation rates, such as in subtropical regions, increase salinity, while high precipitation rates, such as in the equatorial region, decrease salinity.
- River Discharge: Freshwater from rivers dilutes seawater, reducing salinity near river mouths. For example, the Amazon River significantly lowers the salinity in the adjacent Atlantic Ocean.
- Ice Formation and Melting: In polar regions, the formation of sea ice increases the salinity of the surrounding water, while melting ice reduces salinity.
3. Variation with Latitude:
- High Salinity: Subtropical regions (about 20° to 30° N and S) exhibit higher salinity levels due to high evaporation and low precipitation. Salinity can exceed 37 ppt in these areas.
- Low Salinity: Equatorial regions have lower salinity, around 34 ppt, due to high rainfall and river input.
- Polar Regions: Polar regions also have lower salinity, typically around 32 ppt, due to ice melt and lower evaporation rates.
Example:
- The Mediterranean Sea: This semi-enclosed sea has a higher salinity (around 38 ppt) compared to the average ocean salinity. This is due to high evaporation rates and limited freshwater input from rivers. The water exiting the Mediterranean into the Atlantic Ocean through the Strait of Gibraltar is saltier than the surrounding Atlantic water.
Interplay of Temperature and Salinity
The combination of temperature and salinity determines the density of seawater, which drives ocean circulation patterns:
- Thermohaline Circulation: This large-scale ocean circulation is driven by density differences created by variations in temperature (thermo) and salinity (haline). For example, in the North Atlantic, cold, salty water sinks, creating a deep-water current that is part of the global conveyor belt.
Example:The North Atlantic Deep Water (NADW): In the North Atlantic, surface water cools and increases in salinity due to evaporation and ice formation. This dense water sinks and flows southward as part of the deep ocean current, contributing to the thermohaline circulation
