Thermohaline Circulation

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The part of ocean circulation driven by density rather than by wind. Seawater density rises as it cools and as salt is concentrated in it — by evaporation, or by sea ice forming and rejecting brine — and where surface water becomes denser than the water beneath it, it sinks. That happens in only a few small places: the Labrador and Nordic seas in the north, and a handful of shelf regions around Antarctica in the south. The sunk water spreads at depth, mixes slowly, and returns to the surface diffusely over the rest of the world ocean, so the circuit is grossly asymmetric — sinking is concentrated and rising is spread out. Radiocarbon ages give a sense of the timescale: deep water in the North Pacific is roughly two thousand years older than deep water in the North Atlantic. Wallace Broecker's 1987 "great ocean conveyor" cartoon made the idea famous and, by simplifying it into a single looping belt, also made it easy to misunderstand.