Variability of Subpolar Mode Water Volume and Formation in the North Atlantic during 1993-2018

Subpolar Mode Water (SPMW) represents a variety of near-surface waters that occupy a large volume in the upper 1000 m of the water column of the Subpolar North Atlantic (SPNA). Originating in the eastern and northeastern SPNA through late winter water mass formation, SPMW acts as a precursor to the formation of the North Atlantic Deep Water, which is an important ingredient of the Atlantic Meridional Overturning Circulation (AMOC). In this study we address spatial and temporal changes in the SPMW layer thickness and volume. We relate these changes to variability in the water mass formation estimated through the net subduction/obduction rates along predefined isopycnal bins between σ_θ = 27.05 kg m^-3 and σ_θ = 27.55 kg m^-3 with 0.1 kg m^-3 interval. We use two observation-based gridded 3D products from the Copernicus Marine Environmental Monitoring Service (CMEMS), i.e., the ARMOR3D and the OMEGA3D datasets. The first one provides 3D temperature and salinity fields and is available on a weekly 0.25° regular grid from 1993 to present. The second one provides observation-based quasi-geostrophic vertical and horizontal velocity fields with the same temporal and spatial resolution as ARMOR3D, but for the period 1993 to 2018. Throughout this period of 27 years of observations, the analysis reveals not only pronounced interannual variability in the SPMW formation and volume but also a strong spatial variability, which is caused by spatial changes of the main SPMW formation area within the northeastern SPNA.