The Labrador Sea as a conduit for AMOC lower-limb transport anomalies on multiple timescales

Abstract The Atlantic Meridional Overturning Circulation (AMOC) is an essential element of the global climate system, yet its variability on timescales longer than those observed remains unsettled. Recent studies show that transport variability in the lower AMOC limb is maximized on density surfaces well below the density of maximum overturning. To explore the source and impact of this feature, we analyze lower-limb transport variability using climate simulations and observations from the Overturning in the Subpolar North Atlantic Program. We find that enhanced variability at select densities reflects transport redistribution within the lower limb and is not strongly linked with local or downstream AMOC variability on monthly to multidecadal timescales. Furthermore, the strong transport variability in dense waters leaving the Labrador Sea is tightly linked to variability entering the basin, with the opposing inflow and outflow largely canceling in basin-integrated diagnostics. These results underscore the need to account for density-space compensation and cancellation of opposing flows when interpreting the climatic relevance of strong subpolar transport variability.