The volume of the world ocean according to Google equals 1.3x10^9 m3 or 43 thousand Sverdrup-years. The Atlantic meridional overturning circulation (AMOC) rate is 20 Sv. Thus, for the AMOC ‘to fill the ocean’ would take something like 2000 years. This is enormously interesting, because of its similarity to the roughly 1500-year timing of the Dansgaard-Oeschger (D-O) oscillations. What are these?
Second to the major glacial-interglacial cycle with its roughly 100,000-year period, the D-O oscillations stand out as the next largest climate signal – predominantly in the northern hemisphere. They have their sharpest expression in the Greenland ice record, which shows that the air temperature between the warm and cold periods varied differed by 10-15°C. Precipitation also increased during the warm periods. It is generally understood that the source of the heat and water must come from a reorganization of the North Atlantic circulation. How might that express itself and why would that happen?
But first a couple of important details about the D-O oscillations. These occur only during glacial times, not now during the Holocene. This matters because there is a tendency to think those sudden transitions can be applied to modern times. We need to be careful about that. Second, and more importantly, the rise and fall in temperature as seen in the Greenland ice record have completely different characteristics. Following the sharp, virtually instantaneous rise (in only a few decades!) the air temperature starts to decrease and roughly a half-cycle later (500-1000 years) it tends to drop back to its cold state, sometimes rather rapidly sometimes more gradually. But common to all D-O events is the very sharp rise in temperature at the start and gradual decline over time, never a corresponding abrupt collapse.
The heat and moisture seen in the Greenland ice record come from a major reorganization of the North Atlantic circulation. In the cold state, the Gulf Stream flows essentially straight across the Atlantic, kind of like the Kuroshio (the Pacific equivalent of the Gulf Stream) does today. The sudden turn-on in a couple of decades results from a portion of warm Gulf Stream water turning north after it passes the Grand Banks south of Newfoundland and spreading across the subpolar North Atlantic with some continuing into the Nordic Seas. This means a sudden warming of the North Atlantic which of course will impact the atmosphere – transferring heat and moisture to the atmosphere which it then transports in part toward Greenland and in part toward Europe. (My April 16, 2024 offers an explanation of the sudden reorganization.)
The 1500-year period can be explained in terms of an oceanic relaxation oscillation. Let’s suppose the sudden warming has just started. Heat and moisture spread out across the North Atlantic. This causes melting of ice across Greenland and the ice-covered Europe. This run-off reduces the salinity of the surface water, but these do not immediately come into contact with the warm water, why? The Greenland meltwater will flow south and around Greenland making only very limited contact with the warm Gulf Stream water. At least that is what we observe today. The run-off from the European ice mass will flow along the Scandinavian coastline north into Barents Sea and the Arctic making little contact with the warm salty water inflow into the Nordic Seas, allowing it to cool, sink and flow back into the deep North Atlantic. This is important, if you want to bring warm salty water into the Nordic Seas, the water that leaves the Nordic Seas must be dense enough that it can sink and get out of the way of the warm salty water flowing north. But as time goes on the salinity of the surface water decreases, perhaps due to the continual supply of melt water, perhaps because the salinity of the surface waters returning into the Atlantic is decreasing. This will weaken and eventually shut down the production of North Atlantic Deep Water.
The Antarctic with its continuous production of bottom water gradually replaces the loss of North Atlantic Deep Water filling the entire deeper half of the Atlantic and indeed the world ocean. The salinity of these waters will decrease due to increased melt in the south (the shutdown of heat transport to the north means more heat transport to the south). We don’t know the rate of production but assuming it comparable to the AMOC, it would take ~1000 years to fill the replenish the deep half of the ocean with lower salinity water. This is what sets the time scale for the D-O oscillations. When the salinity contrast between the surface and deep water increases to the point where export of dense water from the Nordic Seas can resume, the process starts over.