The subpolar North Atlantic is a key region where the deep water of the Atlantic Meridional Overturning Circulation (AMOC) is formed. Understanding the variability of deep water produced in this region and its thermodynamic attribution is thus of great importance to understand the AMOC change. In this study, we investigate the seasonal variability of deep water mass transformation (WMT) with respect to density, potential temperature and salinity space in the subpolar North Atlantic using Estimating the Circulation and Climate of the Ocean version 4 release 4 (ECCOv4r4) product. We find that the seasonal cycle of WMT in density space is primarily associated with transformation in temperature space: near-surface waters start to cool and densify in winter and are progressively transformed into the cold and dense water in late spring. In summer-fall, the opposite occurs. These processes are dominated by surface buoyancy/heat forcing, with a minor contribution from interior mixing. Transformation in the salinity space is rather complex. During winter, salinification occurs for relatively fresher water masses (salinity < 34.8), while saltier water masses (> 34.8) experience freshening. This seasonal variability of salinity transformation is dominated by mixing throughout the year, while surface freshwater forcing has a negligible impact. Ongoing work targets at understanding the spatial distribution of transformations and the relationships of water mass transformations among the three coordinates.

water mass transformation, seasonal variation