The Overturning in the Subpolar North Atlantic Program (OSNAP) observing system, which was launched in the summer of 2014 comprises an integrated coast-to-coast array of two sections: OSNAP West, extending from the southeastern Labrador shelf to the southwestern tip of Greenland, and OSNAP East, extending from the southeastern tip of Greenland to the Scottish shelf. The highlight of this new record is that the overturning circulation across OSNAP East (15.6 ± 0.8 Sv) is much larger than that across OSNAP West (2.1 ± 0.3 Sv). This result defied the traditional view that deep water formation in the Labrador Sea dominates the AMOC and its variability.
A new study published in Nature Communications by Claus Boening and his colleagues proposed a new perspective that may reconcile the disagreement between the traditional and modern views of the AMOC prompted by OSNAP. This study used two sets of high-resolution ocean model experiments. The first experiment was forced by full heat flux fields that produced increased deep convection in the Labrador Sea in the 1990s (FULL). The second experiment was carried out with the same heat flux forcing everywhere except in the Labrador Sea where climatological heat flux is prescribed (SENS). As expected, deep convection in the Labrador Sea increased significantly in the 1990s in FULL, but no increase is observed in SENS. The model results showed very little change in the direct contribution of the Labrador Sea to the AMOC across OSNAP West in both FULL and SENS, consistent with observations from OSNAP. However, the study found an increase in downwelling into the lower limb in the Irminger basin in FULL, but not in SENS. This suggests a remote influence of the Labrador Sea convection on the Irminger basin. Specifically, the study showed that enhanced deep conviction in the 1990s produced a thickened layer of extremely dense Labrador Sea Water (LSW), and then the thickened LSW invaded (or spread to) the Irminger basin, facilitating its downwelling into the lower limb there and thus increasing the AMOC.
In summary, this study proposed that excessive deep convection in the Labrador Sea during the 1990s increased the AMOC by exporting a layer of extremely dense LSW to the Irminger basin, facilitating its conversion into deeper waters that move southward. So, this study suggests that deep convection in the Labrador Sea does matter and solely explains the large amplitude decadal AMOC signal in the 1990s. So, the next question is if and to what extent deep convection variability in the Labrador Sea explains AMOC variability prior to and after the 1990s.
Figure 1 from Boening et al. (2023). Snapshot of surface speed in the high-resolution model VIKING20X, illustrating the meandering flow of the North Atlantic Current and the narrow boundary current emerging south of the Denmark Strait along the eastern continental shelf of Greenland. Shaded in grey is the area where convection exceeded 1800 m depth during the winters of 1990–1994. The black lines indicate the trans-Atlantic transport sections denoted SILL (dotted), FULL (solid), and its eastern portion EAST (dashed), as well as 48N (dashdot). The blue frame encloses the area for which the annually repeated heat flux is applied in experiment SENS.
Böning, C.W., Wagner, P., Handmann, P. et al. Decadal changes in Atlantic overturning due to the excessive 1990s Labrador Sea convection. Nat Commun 14, 4635 (2023). https://doi.org/10.1038/s41467-023-40323-9
Ocean to Climate 
Comment from Arne Biastoch
By generating higher densities that were later entrained into the overflow water east of Greenland. This North Atlantic Deep Water constitutes the lower limb of the AMOC