Warm surface water drifts toward the poles, gives up its heat, and—being cold and salty—grows denser than the water beneath it and plunges to the deep. That sinking pulls the whole global current along: the thermohaline conveyor.
Density is a tug-of-war between temperature and salt. Cold and salty both make water heavier. Pour freshwater on the pole—from rain or a melting ice sheet—and you cap it with a light lid the cold can't overcome. Nothing sinks, and the conveyor stalls.
This isn't only theory. About 12,900 years ago a flood of glacial meltwater is thought to have shut this current down, and Europe plunged back into cold for a thousand years—the Younger Dryas. The modern Atlantic version (the AMOC) is the same engine, and it is weakening.
Re-salt the pole and watch the plunge—and the whole loop—come back to life.
Something in the simulation stopped unexpectedly — the lesson continues without it. Nothing you did was wrong; you can move on.