Isopycnal Stirring and Diapycnal Mixing in the Eastern North Atlantic Inferred from Argo Observations

Abstract The variability of isopycnal stirring and diapycnal mixing associated with mesoscale and microscale turbulence, respectively, is assessed using Argo float observations in the eastern subtropical North Atlantic. A new method is introduced combining a finescale parameterization with a triple decomposition framework to separate isopycnal stirring and diapycnal mixing. Our approach is validated by comparing with microstructure measurements from an independent North Atlantic tracer release experiment in the 1990s. Both sets of diagnostics reveal that diapycnal mixing dominates in the upper thermocline and that isopycnal stirring prevails in deeper, intermediate layers, influenced by Mediterranean Outflow Water. The analysis of Argo data for years 2014–24 reveals substantial interannual variations in stirring and mixing rates and in the relative contributions of mesoscale and microscale processes to thermohaline variance dissipation, which persisted in time (∼3 yr) and across isopycnal layers (26.75–27.5 kg m −3 ). A shift in the relative contributions to the variance dissipation occurred over the period, with more prevalent isopycnal stirring in the thermocline after 2018. This enhancement of isopycnal stirring, which concurred with lower isopycnal diffusivities, is attributed to increased property gradient along-isopycnals induced by a large-scale cooling and freshening of the eastern subtropical North Atlantic. Our results highlight how property contrasts in the thermocline, induced originally by ocean ventilation, are modified by isopycnal stirring and diapycnal mixing. This application of finescale parameterizations constitutes a powerful tool that has the potential to capture and quantify the temporal evolution of isopycnal stirring and diapycnal mixing rates from Argo observations across the global ocean. Significance Statement Ocean ventilation is vital in the climate system, transferring fluid and properties from the surface to the ocean interior. Yet, the ways in which stirring and mixing modulate ventilation are poorly understood, due to the sparseness of small-scale ocean observations. By applying a novel approach to Argo float data, we diagnose stirring and mixing processes in the subtropical North Atlantic over a recent decade. Our results reveal a shift after 2018, with mesoscale stirring becoming more dominant in response to a major cooling and freshening event. The enhanced stirring dampens temperature and salinity contrasts, thus distributing the effects of externally forced, large-scale changes across the North Atlantic. Our study opens new opportunities to diagnose stirring and mixing across the global ocean.