Thermohaline Circulation
Thermohaline circulation is a global system of ocean currents driven by differences in water density, which is controlled by temperature (thermo) and salinity (haline). Cold, salty water is denser and sinks in the North Atlantic and around Antarctica, driving a slow, deep circulation that connects all ocean basins in what is often called the "global ocean conveyor belt." This circulation system plays a critical role in regulating Earth's climate by transporting heat from the tropics to higher latitudes and cycling nutrients through the ocean depths.
Key Formula
rho = rho_0 * [1 - alpha*(T - T_0) + beta*(S - S_0)]
LaTeX: \rho = \rho_0 [1 - \alpha(T - T_0) + \beta(S - S_0)]
| Symbol | Meaning | Unit |
|---|---|---|
| ρ | Seawater density | kg/m³ |
| ρ₀ | Reference density | kg/m³ |
| α | Thermal expansion coefficient | °C⁻¹ |
| T | Water temperature | °C |
| β | Haline contraction coefficient | ppt⁻¹ |
| S | Salinity | ppt (parts per thousand) |
Worked Example
Problem
Compare the density of two seawater samples: Sample A at T=5°C and S=35 ppt, and Sample B at T=20°C and S=34 ppt. Use ρ₀ = 1025 kg/m³, α = 1.5×10⁻⁴ °C⁻¹, β = 8×10⁻⁴ ppt⁻¹, T₀ = 10°C, S₀ = 35 ppt. Which sample sinks?
Solution
For Sample A (T=5°C, S=35 ppt): ρ_A = 1025 × [1 − 1.5×10⁻⁴×(5−10) + 8×10⁻⁴×(35−35)] ρ_A = 1025 × [1 − 1.5×10⁻⁴×(−5) + 0] ρ_A = 1025 × [1 + 7.5×10⁻⁴] ρ_A = 1025 × 1.00075 = 1025.77 kg/m³ For Sample B (T=20°C, S=34 ppt): ρ_B = 1025 × [1 − 1.5×10⁻⁴×(20−10) + 8×10⁻⁴×(34−35)] ρ_B = 1025 × [1 − 1.5×10⁻³ − 8×10⁻⁴] ρ_B = 1025 × [1 − 0.0015 − 0.0008] ρ_B = 1025 × 0.9977 = 1022.64 kg/m³
Answer
Sample A (ρ = 1025.77 kg/m³) is denser than Sample B (ρ = 1022.64 kg/m³) and will sink, driving thermohaline downwelling.
Key Features of Thermohaline Circulation
| Feature | Deep Water Formation | Deep Current Speed | Timescale | Region |
|---|---|---|---|---|
| North Atlantic Deep Water | Labrador & Greenland Seas | ~1 cm/s | ~1000 years | North Atlantic |
| Antarctic Bottom Water | Weddell Sea | ~0.5 cm/s | ~1000 years | Southern Ocean |
| Mediterranean Outflow | Mediterranean Sea | ~5 cm/s | ~100 years | Atlantic |
| Pacific Deep Water | Southern Ocean | <1 cm/s | ~2000 years | Pacific |
| Indian Ocean Deep Water | Southern Ocean | <1 cm/s | ~1500 years | Indian Ocean |
Interactive Tools
NOAA Thermohaline Circulation
Tutorial on the global ocean conveyor belt and thermohaline processes
Open ToolNASA GISS Ocean Circulation
NASA research overview of thermohaline circulation and climate connections
Open ToolKhan Academy: Thermohaline Circulation
Lesson on density-driven ocean circulation and climate regulation
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Ocean Current
An ocean current is a continuous, directed movement of seawater generated by forces acting upon the water, including wind, the Coriolis effect, temperature and salinity differences, and tidal forces. Surface currents, driven primarily by wind, affect the upper 10% of the ocean, while deep-water currents are driven by density differences related to temperature and salinity. Ocean currents play a vital role in regulating global climate by redistributing heat from the tropics toward the poles and influencing weather patterns on nearby landmasses.
Ocean Salinity
Ocean salinity is the concentration of dissolved salts in seawater, primarily sodium chloride (NaCl), along with chloride, sulfate, magnesium, calcium, and potassium ions. Average ocean salinity is approximately 35 parts per thousand (ppt) or 35 g of salt per kilogram of seawater, though it varies regionally due to evaporation, precipitation, river input, sea ice formation, and melting. Salinity directly affects seawater density and is a key driver of thermohaline circulation, marine organism physiology, and the freezing point of seawater.
Thermocline
The thermocline is a distinct layer in the ocean — typically found between 200 and 1000 meters depth — where water temperature decreases rapidly with increasing depth, separating the warm, well-mixed surface layer from the cold, deep ocean. The main thermocline is a permanent feature of the tropical and mid-latitude oceans, with temperature dropping from about 20°C at the surface to 5°C at 1000 m depth, while seasonal thermoclines can form and dissipate in response to summer heating. The thermocline acts as a physical barrier that limits the exchange of nutrients, gases, and heat between the surface and deep ocean.
From Greek "thermos" (heat) and "halos" (salt), combined to describe circulation driven by both temperature and salinity gradients. The concept was developed extensively by Henry Stommel in the 1950s–1960s, and the term "conveyor belt" was popularized by Wallace Broecker in 1987.