Seawater is not simply "salt water" but a complex solution containing various dissolved salts. Its temperature and density also differ greatly with depth.
Basic Knowledge
Salinity of seawater: The mass of salts dissolved in 1 kg of seawater is expressed as salinity (‰ = psu). The global average is approximately 35‰ (35 psu). The main component is sodium chloride (NaCl), which accounts for about 77%, followed by magnesium chloride and magnesium sulfate.
Regions with high salinity: tropical and subtropical areas with high evaporation (Mediterranean Sea: approximately 38‰). Regions with low salinity: high latitudes and areas near river mouths with much precipitation and river inflow (Baltic Sea: approximately 10‰).
Vertical temperature structure of seawater:
① Surface mixed layer (0〜100〜200 m): The water is thoroughly mixed vertically by wind and waves. The surface water temperature is warmed by solar radiation and is approximately 25〜30℃ in the tropics.
② Thermocline (100〜1000 m): A layer in which water temperature decreases rapidly with depth. Density increases sharply, inhibiting convection.
③ Deep and bottom layers (deeper than 1000 m): Water temperature is stable at approximately 0〜4℃. Density is high.
Density increases as water temperature decreases and salinity increases.
Salinity (the mass of salts in 1 kg of seawater; global average approximately 35‰)
Surface mixed layer (the surface layer mixed by wind and waves; warm due to solar radiation)
Thermocline (the layer in which water temperature changes rapidly; inhibits convection)
Deep layer (water temperature 0〜4℃, high density, stable)
Pycnocline (the layer in which density changes rapidly; corresponds to the thermocline)
Further Exploration
Water temperature and salinity are the two major factors that determine the density of seawater. At high latitudes, surface water is cooled, increasing its density and making it likely to sink (deep-water formation). In the tropics, however, even if evaporation raises salinity, the high water temperature offsets the increase in density, so surface water is unlikely to sink.
In waters near Japan, some areas have surface temperatures approaching 30℃ in summer, providing an energy source for typhoons. Typhoons use the latent heat released when water vapor evaporated from warm seawater condenses as their energy source.
Because light does not reach the deep sea (aphotic zone: deeper than 200 m), photosynthesis cannot occur. However, ecosystems based on chemosynthetic bacteria exist around hydrothermal vents. The diversity of deep-sea fish and organisms is being rapidly clarified through recent ROVs (remotely operated vehicles).
- Average seawater salinity ≈ 35‰ (main component: NaCl)
- High salinity = regions where evaporation > precipitation
- Three-layer structure: surface mixed layer → thermocline → deep layer
- Thermocline = rapid temperature change and inhibited convection
- Density ∝ salinity and ∝ (1/temperature)
- Deep water = high density and low temperature (0〜4℃)
Points to Note
① The relationship between water temperature and density differs slightly between water (freshwater) and seawater (freshwater has its maximum density at 4℃, while seawater continues to increase in density down to its freezing point). ② The depth of the thermocline changes with season and latitude. It exists year-round in the tropics, but seasonal variation is large at high latitudes. ③ Salinity is expressed as a mass ratio (per mille = ‰), not a volume ratio.
Practice
- Approximately what is the average salinity of the world's seawater in ‰? Also name its main component.
- What is the thermocline? Explain it together with its role.
- What is the approximate temperature of deep water in ℃? Also state the latitudinal regions where deep water is likely to form.