The recent shift in understanding (the OSNAP results) doesn’t mean the Labrador Sea is irrelevant; it just changes its job description. It is less of a “pump” and more of a “conduit” and a “collector.” Here is the detailed breakdown of how it works. This blog post was generated by Google Gemini Pro and NotebookLM.
1. The Path of the “Overflows”
The water that actually drives the AMOC strength starts even further north than the Irminger Sea, in the Nordic Seas. This water (Denmark Strait Overflow Water and Iceland-Scotland Overflow Water) is incredibly dense.
- The Transit: This dense water spills over the submarine ridges between Greenland, Iceland, and Scotland.
- The Irminger Connection: As it enters the Irminger Sea, it is at the bottom of the ocean. It doesn’t just sit there; it follows the bathymetry (the shape of the sea floor).
- Rounding the Cape: This deep water hugs the continental slope of Greenland, flows south, and then rounds Cape Farewell (the southern tip of Greenland).
2. Entering the Labrador Sea
Yes, this deep water must enter the Labrador Sea. Because of the Earth’s rotation (the Coriolis effect), these deep currents want to keep the “coast on their right” (in the Northern Hemisphere).
- The Loop: The deep water from the Irminger Sea flows into the Labrador Sea, traveling northwest along the West Greenland slope.
- The U-Turn: It then loops around the northern end of the Labrador Sea and joins the Deep Western Boundary Current (DWBC) flowing south along the coast of Labrador and Newfoundland.
- The Pick-up: As it passes through, it “picks up” the locally formed Labrador Sea Water (LSW). The LSW gets entrained into the faster-moving, denser overflow waters coming from the east.
3. The “Engine” vs. The “Track”
This is the best way to visualize the difference:
- The Engine (Irminger/Nordic Seas): This is where the Potential Energy is created. The water is so dense here that it creates the “pressure head” that pushes the entire AMOC system.
- The Track (Labrador Sea DWBC): This is the Kinetic Pathway. The Labrador Sea hosts the physical boundary current that organizes these disparate deep water masses into a single, cohesive “river” of water heading toward the equator.
| Feature | Eastern Basins (Irminger/Iceland) | Labrador Sea |
| Primary Role | Dynamic Forcing (The Pump) | Gathering & Transit (The Track) |
| Water Mass | DSOW / ISOW (The Densest) | LSW (The Mid-Depth Ventilation) |
| Velocity | High | High (in the boundary current) |
| Contribution to Sv | ~15 Sv | ~3-4 Sv |
4. Why the DWBC is “Western”
You might wonder why it doesn’t just flow south from the Irminger Sea. The rotation of the Earth forces these intensified currents to the western side of ocean basins. This is why the Deep Western Boundary Current is so much stronger than any flow on the eastern side. To get to the western boundary of the North Atlantic, the water formed in the east has to travel through the Labrador Sea to find that western “wall” to lean against as it flows south.
Summary
The deep water does not skip the Labrador Sea. It uses the Labrador Sea’s western slope as a guide. Even if the local “convection” in the Labrador Sea stopped tomorrow, the DWBC would still flow through it, carrying the denser water from the Irminger Sea toward the South Atlantic.
Fig. 1 from Zhang and Thomas (2021): Schematic of long-term mean AMOC pathways. Colors of arrows indicate seawater density (light to dense: yellow-red-purple-violet-blue-dark blue). The density of Atlantic inflow increases along the pathways of the northeastern subpolar gyre, the gyre extended into Nordic Seas, and the branches entering the Arctic through BSO and east FS. Dark blue arrows: dense outflow through the Nordic Seas. The overflows become lighter after passing through the GSR due to entrainments. Yellow arrows: light (cold fresh) surface currents. In addition to the non-Ekman depth-space AMOC component linked to the density contrast across a section, the northeastern subpolar gyre and the gyre extended into the Nordic Seas moving with changing densities also contribute to the density-space AMOC. The density contrast across OSNAP East is much larger than that across OSNAP West, consistent with a much stronger AMOC across OSNAP East.
Zhang, R., Thomas, M. Horizontal circulation across density surfaces contributes substantially to the long-term mean northern Atlantic Meridional Overturning Circulation. Commun Earth Environ 2, 112 (2021). https://doi.org/10.1038/s43247-021-00182-y
Ocean to Climate 
Leave a comment