Interannual Variability of Subpolar Mode Water in the Subpolar North Atlantic

Abstract

Subpolar Mode Water (SPMW) is an important water mass originating in the eastern North Atlantic. Its formation, subject to modification through oceanic interior mixing, can directly influence the volume of water contributing to the Atlantic meridional overturning circulation. Utilizing observation-based data sets spanning from 1993 to 2018, we estimated the formation rates and volume of SPMW within isopycnal layers and examined its temporal variability. Two complementary approaches were used to estimate the formation rate: a thermodynamic approach focusing on the air-sea interactions and a kinematic approach involving volume transport from the mixed layer to the ocean's interior, including the entrainment/detrainment of the mixed layer itself. This is the first time that thermodynamic and kinematic approaches are applied to observation-based data in the North Atlantic. Our results suggest a substantial role of diapycnal mixing in diluting the dense waters formed by air-sea fluxes toward the range of SPMW densities. The study reveals a complex interplay of processes, with entrainment being the primary driver of subduction/obduction rates, while advection contributes to the overall small-scale dynamics. Variations in the volume and location of SPMW formation are observed from year to year. Notably, when SPMW forms extensively in lighter isopycnal layers, the volume occupied by denser isopycnals decreases and vice versa. We attributed this compensation effect to a propagation signal, where formation in the lightest isopycnal bins influences the formation in denser isopycnal bins with a delay of a few years, emphasizing the circulation's role in shaping the SPMW distribution.