Hold, stir or sink?
Set the water at the surface and the water underneath it, then find out which of three things happens. The densities are real, and they are closer together than you would expect.
Relative level, 0 to 100. Dissolved oxygen and mineral nutrients, shown beside the layer they belong to.
The layers hold. The wind is not strong enough to mix them.
Surface water is lighter than the deep water.
The wind stirs the column. It does not create upwelling.
20.0 °C
998.207 kg/m³
6.0 °C
999.943 kg/m³
The layers hold. The layers hold. The wind is not strong enough to mix them. Surface water is lighter than the deep water. The mixed layer is 998.207 kilograms per cubic metre, the deep water 999.943, a difference of 1.736.
Densities are real values from the international standard for seawater (TEOS-10). The wind rule and the oxygen and nutrient changes are simplified to show the direction of change, not real amounts.
- A summer heatwave on a deep lake. Lake mode. Push the surface up towards 30 °C and watch what happens to the difference between the layers, and then to the oxygen in the deep water when you run the seasons.
- Meltwater reaching the North Atlantic. Ocean mode, surface at 8 °C. Drag the salinity down to 32 g/kg. Then put the wind on a gale and see whether it helps.
- Strong evaporation over the North Atlantic. Same starting point, but take the salinity up to 37 g/kg instead. The verdict changes completely, and the wind has nothing to do with it.
- Cool the surface all the way to freezing. Do it in Lake mode first, then in Ocean mode. The two water bodies answer this differently, and the reason is the whole lesson.
Fresh water is densest at about 4 °C
Cool a lake surface below 4 °C and it gets lighter again, not heavier, so it stays where it is and freezes there. That is why lakes ice over from the top and why the water underneath keeps its 4 °C even in the hardest winter. Nothing in the simulation is told to behave that way: it falls out of the real densities.
Salt water never gets the chance
Add enough salt and the temperature of maximum density drops below the freezing point, so it can never be reached. Ocean surface water therefore keeps getting denser the colder it gets, right down to freezing, and keeps sinking. That is what drives the deep circulation of the ocean.
Very small differences decide it
Water at 2 °C and water at 4 °C differ by 0.032 kg/m³, about three parts in a hundred thousand. That is the entire margin holding one layer above another, and it is why the densities here are looked up from TEOS-10, the international standard for the properties of seawater, rather than worked out on the page. Once the layers hold, the deep water is cut off: its oxygen is used up and not replaced, while the nutrients that would feed the producers at the surface accumulate out of reach below.
Then try “Will it mix?”
This simulation asks whether the layers survive. Will it mix? takes the next question, on a lake that has been measured for decades: how far down does the mixing actually reach, and what has been happening to that number.