Presented at the 2016 Ocean Sciences MeetingHorizontal mixing by mesoscale eddies has been proposed to be a significant term of the mean surface freshwater budget in the North Atlantic sea surface salinity (SSS) maximum, balancing part of the excess evaporation in the subtropics. Seasonal to inter annual variability in SSS as well as eddy kinetic energy make the use of a constant eddy diffusivity and mean salinity fields problematic to diagnose eddy mixing over time. The choice of Lagrangian framework (salinity coordinates) eliminates the variability due to advective processes which do not form or destroy water masses.Here we present estimates of seasonal transformation rates in salinity coordinates due to mesoscale stirring. Transformation rates are diagnosed in an idealized model setup with a 1/10 degree global surface layer (two dimensional). Initial surface salinity fields from Argo are passively advected with geostrophic velocities derived from altimetry and related to observed changes in SSS-maximum extent and intensity.