The oceanography community is deeply engaged in a debate about how much weight the Labrador Sea actually carries in maintaining the Atlantic Overturning Circulation (AMOC) system. According to the OSNAP (Overturning in the Subpolar North Atlantic Program) data, the Labrador Sea’s contribution to the AMOC is surprisingly small (around 3–4 Sv) while the eastern subpolar North Atlantic (Irminger and Iceland Basins) contributes roughly 15 Sv (Zou et al., 2020). This has been a major paradigm shift in oceanography over the last few years. The so-called “salinity compensation” is the primary reason for this discrepancy. Here is the detailed breakdown of how it works. This blog post and the “Deep Dive” podcast were generated by Google Gemini Pro and NotebookLM.
1. The Density Equation: T vs. S
To understand the “compensation,” you have to remember that the density of seawater is a tug-of-war between Temperature (T) and Salinity (S):
- Cold water is denser (it wants to sink).
- Salty water is denser (it wants to sink).
- Fresh water is lighter (it wants to stay on top).
In the Labrador Sea, these two factors work against each other. While the water is extremely cold (which should make it sink), it is also relatively fresh compared to the rest of the Atlantic. This freshness acts as a “buoyancy brake,” partially offsetting the density gain from the cold temperatures.
2. Why the Labrador Sea is “Compensated”
When deep convection occurs in the Labrador Sea, the surface water loses heat to the atmosphere, becomes dense, and sinks. However, because this water is fresh:
- The Net Density is Lower: Even after cooling, the “Labrador Sea Water” (LSW) is not as dense as the water masses formed further east.
- The Pressure Gradient is Weak: The “drive” of the AMOC depends on density differences across the ocean. Because the Labrador Sea Water isn’t “heavy” enough (due to its low salt content), it doesn’t create a strong enough pressure gradient at depth to directly push a large volume of water southward.
3. Labrador Sea vs. The “Eastern Engine”
The reason the eastern subpolar North Atlantic (Irminger Sea and Iceland Basin) contributes so much more (~15 Sv) is that it lacks this level of salinity compensation.
| Feature | Labrador Sea (The “Volume” Maker) | Eastern Subpolar Gyre (The “Strength” Maker) |
| Salinity | Low (Fresh Arctic influence) | High (Salty subtropical influence) |
| Temperature | Extremely Cold | Cold |
| Net Density | High, but “compensated” by freshness | Very High (Cold + Salty) |
| AMOC Contribution | ~3–4 Sv | ~15 Sv |
| Role | Mixes nutrients and oxygen deep down | Provides the “pressure head” to drive the flow |
4. Convection ≠ Overturning
This is the most confusing part of the science: Deep convection does not always equal overturning.
- Convection is a vertical mixing process (like a blender). The Labrador Sea is great at this; it creates huge volumes of LSW.
- Overturning is a horizontal transport process (the actual conveyor belt).
Because of salinity compensation, the Labrador Sea is like a blender that is very efficient at mixing but isn’t actually “hooked up” to the main pump of the conveyor belt as strongly as we thought. Most of the water that sinks in the Labrador Sea actually recirculates locally within the subpolar region rather than being exported immediately through the AMOC.
Why this matters for the future slowdown the AMOC
If the Labrador Sea only contributes 3–4 Sv to the total ~18 Sv of the AMOC, then a “shutdown” of Labrador Sea convection (due to melting Greenland ice) might not be the total catastrophe previously feared. It would mean we lose the oxygen/carbon “breathing” of the ocean, but the physical flow of the AMOC might be more resilient because its “main engine” is actually further east, where the water is saltier.
The infographic was generated by Notebook LM.
Zou, S., Lozier, M.S., Li, F. et al. Density-compensated overturning in the Labrador Sea. Nat. Geosci. 13, 121–126 (2020). https://doi.org/10.1038/s41561-019-0517-1
Ocean to Climate 
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