Buoyancy redistribution within the lower limb of the Atlantic Meridional Overturning Circulation revealed by Deep Argo

The steady-state buoyancy budget underpinning the Atlantic Meridional Overturning Circulation (AMOC) implies a balance between the surface-forced and mixing-driven transformation of North Atlantic Deep Water (NADW) and its meridional export. Here, we employ a climatology of ocean temperature and salinity data, a contemporary Deep-Argo array, and an atmospheric reanalysis to assess this balance in the subpolar North Atlantic and Nordic Seas over interannual, decadal, and bidecadal timescales. We quantify the residual of this balance - the rate of water mass volume change - and its role in the water mass transformation budget. The analysis reveals that the magnitude and density range of local volume trends decrease with longer timescales. On decadal and bidecadal scales, trends are confined to upper NADW with minimal changes in AMOC limb volumes, suggesting that water mass transformation and AMOC may be interchangeable. On interannual scales, trends are larger and span lighter and denser density ranges in the eastern basins, aligning with surface-forced transformation patterns. Here, the volume of the AMOC limbs is impacted and AMOC intensity will lag transformation rates by the southward export timescales of transformed water masses.