Assessment of the seasonal cycle in Atlantic density flux

Abstract. An analysis of the seasonal/sub-seasonal cycles of density flux is now possible thanks to advances in satellite oceanography. The kinematic density flux framework, developed to infer the buoyancy-driven ocean circulation using high-resolution satellite datasets, was applied at 1/4° resolution to monthly maps of satellite-derived Sea Surface Salinity, Temperature, and Currents (SSS, SST and SSC) over 2011–2020. Combining them with a blended satellite/in-situ Mixed Layer Depth (MLD) dataset, we derived density flux estimates throughout the Atlantic. We also performed a harmonic analysis to the density flux estimates, to diagnose the contribution of thermal and haline processes to density flux. We find that the sub-tropics and mid-latitude annual cycle explains 70–80 % of the variability in net density flux. With the addition of a semi-annual cycle, the explained variance reaches 80–85 %, suggesting density flux is sensitive to other atmospheric/oceanic processes with higher/lower temporal frequencies. Haline processes dominate density flux variability in the Denmark Strait, and parts of the Labrador and Norwegian Seas – all crucial areas for the Atlantic Meridional Overturning Circulation. The subpolar North Atlantic density flux is primarily governed by haline variability, with freshwater forcing driving most monthly extremes and exhibiting a quasi-symmetric pattern of alternating positive and negative events. Anomalous thermal contributions and localized salinification in December 2020 mark a striking departure from prior years, raising the question of whether this signals a regime shift or a singular event.