This simulation models thermohaline circulation with Stommel's classic 2-box ocean model, where the driving force is a density difference between a warm/fresh tropical box and a cold/salty polar box: ρ = ρ₀(1 − α·ΔT + β·ΔS). The resulting overturning flow q = γ·Δρ/ρ₀ sets both the direction and strength of the circulation, and animated particles trace the loop from surface flow toward the pole, sinking as deep water, and returning as slow upwelling at the tropics.
An animated cross-section of an ocean basin split into a warm equatorial box and a cold polar box. Surface currents carry warm water poleward, it cools and sinks (deep water formation), then returns equatorward as a slow deep current before upwelling — the same loop that drives the real Atlantic Meridional Overturning Circulation (AMOC). Colour and particle speed track the overturning rate in Sverdrups (Sv), and the on-screen readout switches between "On (THC active)", "Weak/Off" and "Reversed" as the density balance changes.
Drag Temperature difference ΔT (0–35°C) and Salinity difference ΔS (0–5 psu) to change how much denser the polar box is than the tropical box — larger ΔT strengthens sinking, larger ΔS also strengthens it since saltier water is denser. Mixing rate γ (0.1–5.0 ×10⁻⁸ s⁻¹) scales how fast that density difference converts into flow. Freshwater forcing F (0–0.5 Sv) mimics ice-melt runoff at the pole: push it up and watch the circulation weaken and, past a threshold, reverse direction entirely.
In the real North Atlantic, sinking cold, salty water near Greenland and the Nordic Seas drives an overturning of roughly 15–20 Sv (1 Sv = one million cubic metres per second) and carries about 1 petawatt of heat northward — a major reason Western Europe is milder than other regions at the same latitude. Stommel showed in 1961 that this same 2-box system can have two stable circulation states, meaning enough freshwater input could flip the ocean from its current "on" mode to a weakened or shut-down state, exactly what the freshwater forcing slider demonstrates.
Thermohaline circulation is the large-scale ocean flow driven by differences in water density, which in turn depend on temperature ("thermo") and salinity ("haline"). Cold, salty water is denser and sinks at high latitudes, pulling warmer surface water poleward to replace it, and the sunken water eventually resurfaces elsewhere after slow mixing — completing a global conveyor belt that the Atlantic Meridional Overturning Circulation (AMOC) is the best-known part of.
It uses Stommel's 2-box model: density is ρ = ρ₀(1 − α·ΔT + β·ΔS) with ρ₀ = 1025 kg/m³, thermal expansion coefficient α = 1.5×10⁻⁴ per °C and haline contraction coefficient β = 8×10⁻⁴ per psu. The overturning flow is q = γ·Δρ/ρ₀, reduced by a freshwater forcing term, and its magnitude is reported in Sverdrups (Sv) after scaling.
Temperature difference ΔT and Salinity difference ΔS set how much denser the polar box is than the tropical box, which is what drives sinking in the first place. Mixing rate γ controls how efficiently that density difference is converted into actual transport — a stiffer, more responsive ocean. Freshwater forcing F adds a countervailing term that dilutes and lightens the polar box, directly opposing the sinking that salty cold water would otherwise produce.
The overturning rate q is proportional to the density difference Δρ. If freshwater forcing grows large enough, it can overwhelm the cooling-driven density increase at the pole, making the polar box lighter rather than heavier than the tropical box. Once that balance flips, q changes sign, and the animation shows the whole circulation loop running backwards — a simplified version of an AMOC collapse scenario driven by ice-sheet melt or increased precipitation at high latitudes.
The real AMOC transports roughly 1 petawatt of heat northward into the North Atlantic, moderating winter temperatures in Western Europe well beyond what its latitude alone would suggest. Because melting ice sheets and glaciers add freshwater to the polar ocean — exactly what the Freshwater forcing slider represents — climate scientists study whether continued warming could push the real AMOC toward the weakened or "off" state this simplified box model can also produce.