2.2 Temperature & Salinity
The Master Variables
Temperature and salinity are the two most important physical properties of seawater. Together, they determine density, which drives ocean circulation. T-S diagrams are fundamental tools for identifying water masses.
Ocean Temperature
Surface Temperature
- • Range: -2°C to 30°C
- • Warmest in tropics, coldest at poles
- • Seasonal variation strongest at mid-latitudes
- • Western boundary currents transport heat poleward
Vertical Structure
- • Mixed layer: 20-200m, uniform T
- • Thermocline: Rapid T decrease
- • Deep water: Cold (0-4°C), uniform
- • 75% of ocean is below 4°C
Salinity
$$S = \frac{\text{mass of dissolved salts}}{\text{mass of seawater}} \times 1000$$
Practical Salinity Units (PSU) ≈ parts per thousand (‰)
What Increases Salinity?
- • Evaporation (removes fresh water)
- • Sea ice formation (brine rejection)
- • Limited river input
What Decreases Salinity?
- • Precipitation
- • River discharge
- • Sea ice melting
Global Patterns
Highest in subtropics (evaporation exceeds precipitation). Low at equator (rain) and high latitudes (melt/runoff). Average: 34.7 PSU.
T-S Diagrams
Temperature-Salinity diagrams plot T vs S with isopycnals (lines of constant density). Each water mass has a characteristic T-S signature, making T-S diagrams essential for tracing water mass origins and mixing.
AABW
Antarctic Bottom Water
T: -0.4°C, S: 34.66
NADW
North Atlantic Deep Water
T: 2-4°C, S: 34.9-35.0
AAIW
Antarctic Intermediate Water
T: 3-7°C, S: 34.2-34.4