🌊 Ocean Currents — Thermohaline Circulation & Wind-Driven Gyres

Global Temp sets mean ocean temperature from 5 to 25°C; warmer water is less dense and weakens sinking. Wind Strength scales surface stress from 0.1 to 2.0, intensifying the wind-driven gyres. Freshwater Forcing, from 0 to 1, represents meltwater diluting North Atlantic salinity, which directly reduces the modelled AMOC strength.

The diagnostic uses a simplified formula: base value 18 Sv multiplied by wind strength, by a freshwater factor (1 minus freshwater times 1.2), and by a warming factor that reduces flow as temperature rises above 15°C. The result is clamped at zero. It is an illustrative scaling, not a physical ocean model, designed to convey the right qualitative trends.

Ekman transport is the net movement of surface water caused by wind acting through the rotating Earth. Because of the Coriolis effect, the depth-averaged transport flows at right angles to the wind — to the right in the Northern Hemisphere and to the left in the Southern. Converging Ekman transport piles up water in gyre centres, driving the swirling subtropical circulations.

Four presets set the sliders to representative states. Modern uses present-day values; Ice Age applies a colder, windier ocean with stronger overturning; +2°C warming weakens the AMOC with some freshwater; and Collapse adds heavy freshwater forcing to push the system toward shutdown so you can see the conveyor stall.

Toggling the deep layer hides the surface currents and instead draws North Atlantic Deep Water sinking near the North Atlantic and returning at depth toward the Pacific and Indian oceans. Its visibility scales with the freshwater-dependent overturning factor, so as you increase meltwater the deep return flow visibly fades, mirroring a weakening conveyor.

It is an educational visualisation, not a numerical ocean general circulation model. The current paths are stylised Bezier curves, the continents are simplified shapes, and the AMOC figure comes from a heuristic scaling rather than solving the governing equations. It captures real concepts and trends correctly but should not be used for quantitative prediction.

Many climate models suggest the AMOC has tipping behaviour: enough freshwater from melting Greenland ice can reduce surface density past a threshold, shutting down deep convection. The exact risk and timing are debated, but a substantial weakening this century is considered plausible. This is why the Freshwater Forcing slider and Collapse preset are central to the simulation.

Water has a very high heat capacity, so currents move enormous amounts of energy from the tropics toward the poles, complementing the atmosphere. The Gulf Stream and AMOC alone carry over a petawatt of heat northward. This redistribution moderates regional climates, which is why changes in circulation can have outsized effects on temperature and weather far from the ocean itself.