Mr. Tom's Blog

The AMOC and other MOCs

AMOC stands for Atlantic Meridional Overturning Circulation. That sounds simple enough, in 25 words or less you might say it is ‘the flow of warm water north where it gets cooled, sinks and flows back south at depth.’ But as soon as you start to look under the hood, as it were, things get a lot more involved – there are so many aspects to it.

There was a time when I thought that the AMOC shut down completely during the ice ages. After all the North Atlantic was filled with water from Antarctica up to nearly 2 km depth. But a paper by Curry and Oppo (2005) made it clear that there was North Atlantic water below the warm upper ocean in the South Atlantic. That came as a surprise, but they made a persuasive case that the AMOC had not shut down – entirely – in glacial times. This would be consistent with the idea of two return paths at depth: one is the overturning in the Nordic Seas, what I used to think was the source of all deep water, and the other, a shallower overturning in the Labrador and the Irminger Seas, the latter east of Greenland and southwest of Iceland. The distinction between the two is fundamental, the Nordic Seas produce very dense water that is pooled behind the massive ridge that runs from Greenland to Iceland and from Iceland past the Faroes to Scotland. Its greatest sill depth is about 800 m. Behind, north of this ridge, the water is very cold and dense. As new dense water is produced in the Norwegian and Greenland Seas farther north, some of this pooled water spills over the ridge into the deep North Atlantic. This water is at such a depth that it can flow freely along the deep western margin underneath the North Atlantic Current and the Gulf Stream out into the global abyss. The water produced by wintertime convection in the Labrador and Irminger Seas can during cold winters sink at most about 2 km depth, and often to less. Some of this dense water spreads east forming part of the subpolar circulation, while the rest flows south along the Canadian margin where it can continue west after rounding the Grand Banks or get stirred across the meandering Gulf Stream into the Sargasso Sea and continue west and south. This flow pattern differs greatly from the unhindered deep flow from the Nordic Seas. By distinguishing between these two sources of deep water, North Atlantic Deep Water from the Nordic Seas, and North Atlantic Intermediate Water (NAIW) from the Labrador and Irminger Seas, we can appreciate the different roles they play: the former shuts down in glacial times, the latter not.

During glacial times the subpolar North Atlantic expanded greatly such that the paleo-Gulf Stream (paleo-GS) went essentially straight across the North Atlantic much like the Kuroshio Current does in the Pacific. Which makes me wonder if the North Pacific today might be seen as an analog for the glacial North Atlantic such that the production of North Pacific Intermediate Water (NPIW) would be seen as the return limb of the PMOC. (Pacific – MOC). It is formed in the mixing region between the east-flowing Kuroshio and Oyashio currents, the latter roughly 5° farther north. It spreads east and stirs and subducts across the Kuroshio into the subtropical North Pacific at depth. But there is no production of deep water in the North Pacific. It is always filled with water from the Antarctic, although now, in interglacial times, it also includes water from the deep North Atlantic ocean.

It is tempting to speculate further. During glacial times the subpolar North Atlantic was probably rather fresh being surrounded by continental ice in North America, Greenland, and Europe. There would have been a south-flowing Labrador Current, and ice-rafted debris found across the northern North Atlantic could be an indication that it separates and flows east somewhere to the north of the paleo-GS. Can the region in between these two zonal flows be the Atlantic equivalent of the mixing region in the Pacific? This might make sense: very cold, but fresh water from the north mixes with the salty water in the paleo-GS to produce what might called be paleo-North Atlantic Intermediate Water, the source of the shallow lower limb MOC during glacial times.