The surface mixed layer of the world ocean regulates global climate by controlling heat and carbon exchanges between the atmosphere and the oceanic interior 1 – 3 . The mixed layer also shapes marine ecosystems by hosting most of the ocean’s primary production 4 and providing the conduit for oxygenation of deep oceanic layers. Despite these important climatic and life-supporting roles, possible changes in the mixed layer during an era of global climate change remain uncertain. Here, we use oceanographic observations to show that from 1970-2018 the density contrast across the mixed-layer base increased and that the mixed layer itself deepened. The summertime density contrast increased by 8.9±2.7% dec -1 (10 -6 -10 -5 s -2 dec -1 , depending on region), more than six times greater than previous estimates due to our use of a more physically-based definition of mixed layer stability following the differing dynamical regimes across the global ocean. While prior work has suggested that a thinner mixed layer should accompany a more stratified ocean 5 – 7 , we instead find that the summertime mixed layer deepened by 2.9±0.5% dec -1 or several meters per decade (typically 5-10m dec -1 , depending on region). A detailed mechanistic interpretation is challenging, but the concurrent stratification and deepening of the mixed layer are related to an increase in stability associated with surface warming and high latitude surface freshening 8 , 9 , accompanied by a wind-driven intensification of upper-ocean turbulence 10 , 11 . Our results are based on a complex dataset with incomplete coverage of a vast area; we found our results to be robust within a wide range of sensitivity analyses, but important uncertainties remain, such as sparse coverage in the early years. Nonetheless, our work calls for reconsideration of the drivers of ongoing shifts in marine primary production, and reveals stark changes in the world’s upper ocean over the past five decades.

The oceanic mixed layer is the gateway for the exchanges between the atmosphere and the ocean; in this layer, all hydrographic ocean properties are set for months to millennia. A vast area of the Southern Ocean is seasonally capped by sea‐ice, which alters the characteristics of the ocean mixed layer. The interaction between the ocean mixed layer and sea‐ice plays a key role for water mass transformation, the carbon cycle, sea‐ice dynamics, and ultimately for the climate as a whole. However, the structure and characteristics of the under‐ice mixed layer are poorly understood due to the sparseness of in situ observations and measurements. In this study, we combine distinct sources of observations to overcome this lack in our understanding of the polar regions. Working with elephant seal‐derived, ship‐based, and Argo float observations, we describe the seasonal cycle of the ocean mixed‐layer characteristics and stability of the ocean mixed layer over the Southern Ocean and specifically under sea‐ice. Mixed‐layer heat and freshwater budgets are used to investigate the main forcing mechanisms of the mixed‐layer seasonal cycle. The seasonal variability of sea surface salinity and temperature are primarily driven by surface processes, dominated by sea‐ice freshwater flux for the salt budget and by air‐sea flux for the heat budget. Ekman advection, vertical diffusivity, and vertical entrainment play only secondary roles. Our results suggest that changes in regional sea‐ice distribution and annual duration, as currently observed, widely affect the buoyancy budget of the underlying mixed layer, and impact large‐scale water mass formation and transformation with far reaching consequences for ocean ventilation.

The oceans are traversed by a large-scale overturning circulation, essential for the climate system as it sets the rate at which the deep ocean interacts with the atmosphere. The main region where deep waters reach the surface is in the Southern Ocean, where they are transformed by interactions with the atmosphere and sea-ice. Here, we present an observation-based estimate of the rate of overturning sustained by surface buoyancy fluxes in the Southern Ocean sea-ice sector. In this region, the seasonal growth and melt of sea-ice dominate water-mass transformations. Both sea-ice freezing and melting act as a pump, removing freshwater from high latitudes and transporting it to lower latitudes, driving a large-scale circulation that upwells 27 ± 7 Sv of deep water to the surface. The upwelled water is then transformed into 22 ± 4 Sv of lighter water and 5 ± 5 Sv into denser layers that feed an upper and lower overturning cell, respectively.