The Great Ocean Conveyor — How the Sea Moves Heat Around the Whole Planet

It is a nickname for the thermohaline circulation — the global, density-driven loop that connects all the oceans. The name was popularised by the climate scientist Wallace Broecker in 1991. In the simplest picture, warm water flows along the surface toward the far North Atlantic, cools, becomes dense and sinks, then travels back along the deep seafloor through every ocean basin before slowly rising again. 'Thermohaline' just means it is driven by heat ('thermo', temperature) and salt ('haline', salinity), the two things that set how dense seawater is. The real circulation is a tangle of branches rather than a single tidy belt, but the belt picture captures the essential idea.

Roughly a thousand years for the deep limb, with estimates spanning about 1,000 to 2,000 years depending on the path traced (Broecker, 1991; introductory oceanography texts). Water that sinks in the North Atlantic today may not surface again in the North Pacific for ten centuries. That slowness is exactly why the deep ocean is such a powerful store of heat and carbon — and why changes to the circulation play out over decades and centuries, not days.

Two things make the water there dense enough to sink. First, surface water arriving from the tropics has already lost a lot of fresh water to evaporation on the way, so it is salty. Second, in the far north it gives up its heat to the cold air and becomes cold as well. Cold and salty means dense — denser than the water beneath it — so it plunges kilometres down and begins flowing south along the bottom as North Atlantic Deep Water. The same thing happens around Antarctica, where freezing sea ice leaves behind extra-salty water that forms the densest water in the world ocean, Antarctic Bottom Water.

The AMOC — the Atlantic Meridional Overturning Circulation — is the Atlantic part of the conveyor: the warm northward surface flow (which includes the Gulf Stream) balanced by the cold southward deep return. It carries about 15 sverdrups of overturning at 26°N, measured continuously by the RAPID mooring array (Smeed et al., 2018; 1 Sv = one million cubic metres per second). Scientists watch it closely because a warming climate adds fresh water to the northern seas (from rain and melting ice), which makes the surface water less salty and harder to sink — and that could slow the whole loop. A major slowdown would have outsized effects on European and North Atlantic climate. The size and timing of any future change is an active area of research.

The northward surface limb carries an enormous amount of tropical heat into the North Atlantic — and as that water cools and sinks, it pulls more warm water up behind it. The heat released to the air on the way is one reason northwest Europe is far milder than other places at the same latitude: Lisbon, New York and the cold heart of Canada all sit near 40–45°N, yet Europe's Atlantic edge stays remarkably temperate. The ocean does part of the work a continent's worth of heating otherwise could not.

No — it is an authored schematic, and the map says so. The real circulation has many branches and varies over time; no single drawn loop is 'the' conveyor. What is real and sourced are the figures attached to it: the roughly thousand-year circuit time, the ~15 Sv AMOC strength from the RAPID array, and the locations where deep water forms. The live, measured companion to this view is the surface-current particle layer and the Argo float profiles, which show the actual moving ocean rather than a diagram.