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. At very high freshwater input the model approaches AMOC collapse — a scenario studied by climate scientists as a potential tipping point.
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. Ocean density is computed from a linearised equation of state, ρ = ρ₀[1 − α(T − T₀) + β(S − S₀)], using a thermal expansion coefficient α = 2.1×10⁻⁴ K⁻¹ and haline contraction β = 7.5×10⁻⁴ psu⁻¹. Cold, salty water sinks in the North Atlantic, driving the AMOC at a baseline of around 18 Sverdrups.
Three sliders set the scenario: a global temperature anomaly (−2 to +4°C), Arctic sea ice coverage (0–100%) and a freshwater flux from glacial melt (0–5 Sv). Each weakens deep-water formation, and the model recomputes AMOC strength, North Atlantic Deep Water flux, the tropics-to-pole temperature gap and the circulation period in real time. AMOC weakening is a key climate tipping point that regulates Europe's mild climate.
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. The simulation gives it a healthy baseline strength of about 18 Sverdrups, where one Sverdrup equals one million cubic metres per second. 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.
The Global Temperature Anomaly slider warms or cools the planet from −2 to +4°C, reducing the pole-to-tropics density contrast. The Arctic Sea Ice slider sets ice coverage from 0 to 100%; less ice means a fresher surface cap. The Freshwater Flux slider injects 0 to 5 Sv of glacial meltwater, diluting North Atlantic salinity. Each factor lowers the combined circulation strength.
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, which in the model directly reduces the AMOC and the dependent North Atlantic Deep Water flux.
AMOC strength is the total overturning in Sverdrups. NADW formation is the North Atlantic Deep Water flux, scaled at about 79% of the AMOC value. The ΔT figure is the temperature gap between tropics (~28°C) and poles, which narrows slightly as you warm the climate. Circulation period estimates how long one full loop takes, roughly 1000 years at full strength.
It is a qualitative teaching model rather than a research ocean model. The density equation, the roles of temperature and salinity, and the direction of cause and effect are physically faithful. However, the AMOC response is a smoothed empirical formula combining the three sliders, not a solved fluid-dynamics calculation, so exact numbers are illustrative rather than predictive.
As combined weakening drives AMOC below about 2 Sv, the status badge turns to Collapsed and the circulation period tends to infinity. 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.