Temperature, Salinity & Heat/Salt Budgets

1. Temperature of Ocean Water

The temperature of ocean water is fundamental in determining ocean currents, marine life distribution, and global climate.

Vertical Distribution

• Surface Layer (Epipelagic): Warm, well-mixed by wind and waves, typically 500m thick. Temperatures vary from 30°C at the equator to -2°C at the poles.
• Thermocline: A transition layer between 500m to 1000m where temperature drops rapidly with increasing depth.
• Deep Water: From 1000m down to the ocean floor, water temperature is uniformly cold, hovering around 1°C to 4°C, regardless of latitude.

Horizontal Distribution

Generally, surface temperature decreases from the equator (average 26°C) towards the poles. It is influenced by:

• Latitudes: Insolation decreases poleward.
• Ocean Currents: Warm currents raise the temperature of cold regions (e.g., Gulf Stream warms NW Europe); cold currents lower the temperature of warm regions (e.g., Peru Current).
• Unequal distribution of Land and Water: Oceans in the Northern Hemisphere (land-dominated) are slightly warmer (avg 19.4°C) than in the Southern Hemisphere (avg 16.1°C) because landmasess heat up and warm the surrounding oceans.

2. Salinity of Ocean Water

Salinity is the total amount of dissolved solid material in seawater, expressed as Parts Per Thousand (‰ or ppt). The average oceanic salinity is ~35 ppt. The main salts are Sodium Chloride (77.8%), Magnesium Chloride (10.9%), and Magnesium Sulphate (4.7%).

Factors Affecting Salinity

• Evaporation (+): Increases salinity. High in sub-tropical regions (20°-30° latitudes) characterized by clear skies, high temps, and descending dry air (Horse Latitudes). Thus, sub-tropics have the highest salinity, not the equator.
• Precipitation (-): High rainfall dilutes seawater. The equatorial region has lower salinity despite high heat because of daily heavy convectional rainfall.
• Freshwater Influx (-): Large rivers discharging into the ocean lower salinity locally (e.g., Bay of Bengal has much lower salinity than the Arabian Sea due to Ganga/Brahmaputra rivers).
• Ice Melting/Freezing: Melting icebergs (polar regions) decrease salinity. Freezing of sea ice increases the salinity of the remaining unfrozen water beneath it.

Vertical Distribution of Salinity

Salinity changes with depth, but differently at different latitudes. There is a sharp gradient called the Halocline (similar to thermocline) between 300m and 1000m where salinity changes rapidly. Below the halocline, the deep ocean has a incredibly uniform salinity.

3. Heat & Salt Budgets of the Oceans

• Heat Budget: Similar to the Earth's heat budget. The ocean surface gains heat primarily from solar insolation and loses heat through evaporation (mainly), back-radiation, and sensible heat conduction. The net balance keeps ocean temperatures relatively stable over time.
• Salt Budget: The oceans are not getting progressively saltier. A steady state exists. Inputs of salts (from rivers eroding continental rocks and submarine volcanism) are perfectly balanced by outputs (precipitation as chemical sediments, biological utilization by calcareous/siliceous planktons, and trapping in evaporite basins).