Ocean conveyor belt · AMOC · Deep water formation · Climate sensitivity
A planet-scale heat engine driven by tiny density differences. Warm salty water flows poleward, cools, sinks, and returns as cold deep current — redistributing heat and regulating Earth's climate.
Ocean density depends on both temperature (cold = denser) and salinity (salty = denser): ρ = ρ₀[1 − α(T − T₀) + β(S − S₀)]. In the North Atlantic, the Gulf Stream delivers warm salty water from the tropics. As it cools near Greenland it becomes dense enough to sink — this is North Atlantic Deep Water (NADW) formation. The sinking drives a return flow of cold bottom water through the deep ocean, completing the Atlantic Meridional Overturning Circulation (AMOC) with a strength of ~18 Sverdrups (1 Sv = 10⁶ m³/s).
Drag the Global Temperature slider to warm the planet. As Arctic temperatures rise, Greenland ice melts, adding fresh (low-density) water to the North Atlantic surface — this freshwater "cap" prevents deep water formation and weakens the AMOC. Watch the AMOC strength gauge and circulation arrows slow down. The Freshwater Flux slider lets you simulate a catastrophic glacial melt event. Push it past about 1.2 Sv and the two-box model crosses its tipping point: the overturning collapses and even reverses. It is a genuine tipping point, not a fade-out — pulling the freshwater back off does not restart the circulation until you drop below about 0.5 Sv, the hysteresis that makes an AMOC shutdown so hard to undo. Warming the planet moves the threshold closer: at +4 °C the model collapses at only 0.8 Sv.
The Gulf Stream carries ~100 times more water than all the world's rivers combined. Without thermohaline circulation, Northern Europe would be 5–10°C colder — similar to Labrador in Canada at the same latitude. Recent studies (2023) suggest AMOC could collapse between 2025 and 2095 under high emissions scenarios — which would cause rapid cooling in Europe even as the global average warms. The last AMOC collapse, ~12 900 years ago during the Younger Dryas, triggered a 1 000-year cold snap in the Northern Hemisphere.
This simulation maps the global ocean conveyor belt as animated current particles flowing over a simplified world map. Underneath it runs Stommel's two-box model (Stommel 1961), the standard teaching model of the thermohaline circulation. Two well-mixed boxes — the high-latitude North Atlantic and the equatorial Atlantic — exchange heat and salt. Each box's density comes from the linearised equation of state, ρ = ρ₀[1 − α(T − T₀) + β(S − S₀)], with thermal expansion α = 2.1×10⁻⁴ K⁻¹ and haline contraction β = 7.5×10⁻⁴ psu⁻¹, and the overturning is driven directly by the density contrast between them: q = k(ρ₁ − ρ₂)/ρ₀ = k[αΔT − βΔS]. Nothing else sets the AMOC. With the observed forcing (a 0–28 °C pole-to-tropics restoring contrast and a 0.15 Sv net freshwater input to the subpolar North Atlantic) the model settles at 17.3 Sverdrups, inside the observed 15–20 Sv range (the RAPID array measures about 17 Sv at 26°N).
Three sliders set the scenario: a global temperature anomaly (−2 to +4°C, applied with 2.5× Arctic amplification so warming shrinks the driving temperature contrast), Arctic sea ice coverage (0–100%, whose loss adds meltwater and polar warming) and a freshwater flux from glacial melt (0–5 Sv). They are inputs to the box equations, which are integrated forward in time — so the ocean takes model-centuries to respond, exactly as the real one does. Because the freshwater flux enters the salt budget divided by the very circulation it suppresses, the steady state is a quadratic in q with two stable branches: a vigorous circulation and a collapsed one. Past about 1.2 Sv of meltwater the model tips from the first to the second, and it will not tip back until the freshwater drops below about 0.5 Sv. That two-state behaviour, not a smooth fade, is what makes AMOC shutdown a tipping point.
What is thermohaline circulation?
Thermohaline circulation is the global, density-driven movement of ocean water, where "thermo" refers to heat and "haline" to salt. Warm, salty surface water flows poleward, cools, becomes dense enough to sink, and returns as cold deep current. This slow overturning redistributes heat around the planet and helps regulate Earth's climate over centuries.
What does AMOC mean in this simulator?
AMOC stands for the Atlantic Meridional Overturning Circulation, the Atlantic limb of the global conveyor belt. In the box model its equilibrium strength at the default settings is 17.3 Sverdrups, where one Sverdrup equals one million cubic metres per second — close to the ~17 Sv the RAPID array measures at 26°N. As you warm the planet or add freshwater, the AMOC strength gauge falls and its status badge moves from Stable through Weakening and Critical to Collapsed.
How does the density equation work?
The model uses a linearised seawater equation of state: density rises when water cools and rises when it gets saltier. With α = 2.1×10⁻⁴ per kelvin and β = 7.5×10⁻⁴ per psu about a reference of 10°C and 35 psu, it captures why cold, salty North Atlantic water becomes dense enough to sink, while warm or freshened water stays buoyant at the surface.
They are the forcings of the two-box model. The Global Temperature Anomaly slider warms or cools the planet from −2 to +4°C; because the Arctic warms about 2.5× faster than the global mean while the tropics warm about 0.9×, warming shrinks the pole-to-tropics temperature contrast ΔT that drives the sinking. The Arctic Sea Ice slider sets ice coverage from 0 to 100%; losing it adds about 0.03 Sv of meltwater and a further 2 K of polar warming through the ice-albedo feedback. The Freshwater Flux slider injects 0 to 5 Sv of glacial meltwater on top of the 0.15 Sv natural budget, diluting North Atlantic salinity. All three feed the same density equation.
Deep-water formation depends on surface water being dense enough to sink. Freshwater from melting Greenland ice or sea ice lowers salinity, and because salinity raises density through the haline term, a fresher surface stays buoyant. This freshwater "cap" suppresses sinking in the Nordic and Labrador Seas. In the model it is not a fudge factor: the meltwater enters the polar box's salt budget, lowers S₁, lowers ρ₁, and so lowers the density contrast ρ₁ − ρ₂ that is the only thing driving the overturning q.
AMOC strength is the overturning q solved from the box model, in Sverdrups; it goes negative if the cell reverses. NADW formation is the North Atlantic Deep Water flux, scaled at about 79% of the AMOC value. ΔT and ΔS are the model's live temperature and salinity contrasts between the equatorial and polar boxes, and Δρ is the density contrast they produce — that number, about 5.8 kg/m³ at the default settings, is what actually drives q. Circulation period estimates how long one full loop takes, roughly 1000 years at full strength.
It is a teaching model, not a research ocean model — but it is a real one. The AMOC readout is not an empirical formula fitted to the sliders: it is the overturning q = k(ρ₁ − ρ₂)/ρ₀ of Stommel's 1961 two-box model, integrated forward in time from the same density equation the page teaches, and it reproduces the model's famous two stable states and collapse threshold. What it cannot do is resolve ocean geometry, eddies, wind forcing or the real three-dimensional overturning, and the hydraulic constant k is calibrated so the salt-free circulation matches the observed ~18 Sv. Treat the trends and the tipping behaviour as sound, and the exact numbers as a box-model estimate rather than a forecast.
Once the freshwater forcing passes the model's threshold (about 1.2 Sv at present-day temperatures, 0.8 Sv at +4 °C), the vigorous branch of the box model ceases to exist: the overturning falls through zero within a few model-centuries and settles into a weak reversed cell. The status badge turns to Collapsed and the circulation period tends to infinity. Turning the freshwater back down does not undo it — the circulation only restarts below about 0.5 Sv. Physically, an AMOC shutdown would stop the northward heat transport that warms Europe, potentially cooling parts of the North Atlantic region even as the global average rises. It is studied as a major climate tipping point.
Surface currents such as the Gulf Stream move at around 1.5 m/s, while the deep return limbs in the model run at roughly 0.01 to 0.02 m/s. Deep water travels slowly through the abyss, which is why a full conveyor loop takes on the order of 1000 years. Hover any current in the simulation to read its labelled flow speed.
The conveyor's past behaviour is tied to abrupt climate shifts. The Younger Dryas, roughly 12,900 years ago, is thought to involve an AMOC slowdown that plunged the Northern Hemisphere back into cold for about a thousand years. Today, scientists monitor the AMOC closely because accelerating Greenland melt could push it toward the kind of weakening this simulator lets you explore.