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The surface mixed layer of the world ocean regulates global climate by controlling heat and carbon exchange between the atmosphere and the oceanic interior.sup.1-3. The mixed layer also shapes marine ecosystems by hosting most of the ocean's primary production.sup.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 to 2018 the density contrast across the base of the mixed layer increased and that the mixed layer itself became deeper. Using a physically based definition of upper-ocean stability that follows different dynamical regimes across the global ocean, we find that the summertime density contrast increased by 8.9 [plus or minus] 2.7 per cent per decade (10.sup.-6-10.sup.-5 per second squared per decade, depending on region), more than six times greater than previous estimates. Whereas prior work has suggested that a thinner mixed layer should accompany a more stratified upper ocean.sup.5-7, we find instead that the summertime mixed layer deepened by 2.9 [plus or minus] 0.5 per cent per decade, or several metres per decade (typically 5-10 metres per decade, 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.sup.8,9, accompanied by a wind-driven intensification of upper-ocean turbulence.sup.10,11. Our findings are based on a complex dataset with incomplete coverage of a vast area. Although our results are robust within a wide range of sensitivity analyses, important uncertainties remain, such as those related to sparse coverage in the early years of the 1970-2018 period. 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. Oceanographic observations from 1970-2018 reveal substantial changes in the summer upper-ocean structure, showing a thickening of the mixed layer and a density gradient increase at its base., Author(s): Jean-Baptiste Sallée [sup.1] , Violaine Pellichero [sup.2] [sup.3] , Camille Akhoudas [sup.1] , Etienne Pauthenet [sup.1] , Lucie Vignes [sup.1] , Sunke Schmidtko [sup.4] , Alberto Naveira Garabato [sup.5] [...]

The characteristics of the CISE-LOCEAN sea water isotope data set (δ18O, δ²H, later designed as δD) are presented. This data set covers the time period from 1998 to 2021 and currently includes close to 8000 data entries, all with δ18O, three quarters of them also with δD, associated with a time and space stamp and usually a salinity measurement. Until 2010, samples were analysed by isotopic ratio mass spectrometry, and since then mostly by cavity ring-down spectroscopy (CRDS). Instrumental uncertainty on individual data in this dataset is usually with a standard deviation as low as 0.03 / 0.15 ‰ for δ18O and δD. An additional uncertainty is related to uncertain isotopic composition of the in-house standards that are used to convert daily data into the VSMOW scale. Different comparisons suggest that since 2010 the latter have remained within at most 0.03 / 0.20 ‰ for δ18O and δD. Therefore, combining the two suggests a standard deviation of at most 0.05 / 0.25 ‰ for δ18O / δD. Finally, for some samples, we find that there has been evaporation during collection and storage, requiring adjustment of the isotopic data produced by CRDS, based on d-excess. This adds an uncertainty on the adjusted data of roughly 0.05 / 0.10 ‰ on δ18O and δD. This issue of conservation of samples is certainly a strong source of quality loss for parts of the database, and 'small' effects may have remained undetected. The internal consistency of the database can be tested for subsets of the dataset, when time series can be obtained (such as in the southern Indian Ocean or North Atlantic subpolar gyre). These comparisons suggest that the overall uncertainty of the spatially (for a cruise) or temporally (over a year) averaged data is on the order of or less than 0.03 / 0.15 ‰ for δ18O / δD. On the other hand, 17 comparisons with duplicate sea water data analysed in other laboratories or with other data sets in deep regions suggest a larger scatter. When averaging the 17 comparisons done for δ18O, we find a difference close to the adjustment applied at LOCEAN to convert salty water data from the activity to the concentration scale. Such a difference is expected, but the scatter found suggests that care is needed when merging datasets from different laboratories. Examples of time series in the surface North Atlantic subpolar gyre illustrate the temporal changes in water isotope composition that can be detected with a carefully validated dataset. [ABSTRACT FROM AUTHOR]

The Weddell Sea is a key region in the global climate system as it fuels the global overturning circulation by producing the world's densest water mass, the Antarctic Bottom Water (AABW), which ultimately ventilates and sets the characteristics of the world's deep oceans. AABW is formed by the production of very saline and dense waters on the continental shelves, as well as by entrainment and mixing with overlying old Circumpolar Deep Water. While the relative importance of the process by which AABW is formed is still imperfectly determined, it is key to our understanding of the overturning circulation, and of the observed variability and trends of AABW characteristics. In this study, we use a new set of observations of water stable isotopes on the southern Weddell Sea continental shelf, along with hydrographic transects at the northern edge of the Weddell Sea to disentangle the rate of production of dense shelf water which ultimately fuel the bottom water exported in the gyre. We also able to quantify the net volume of dense water produced by entrainment in the Weddell Gyre. In addition, we highlight the formation process of dense shelf water outflowing from the southern Weddell Sea continental shelf, by pinpointing the role of ocean/ice-shelf interactions in altering the characteristics of dense shelf water. Indeed, the region hosts the largest Antarctic ice-shelf, Filchner-Ronne Ice Shelf (FRIS) where ocean-ice interactions in the cavities are pivotal for our understanding of future climate change, and impact the overturning circulation. We identify the main locations where water containing glacial meltwater flows out from the cavity, quantify the rate of melt and freeze in the cavity, and pinpoint the source waters that originally entered the cavity. In summary, we provide a detailed decomposition of the life history of AABW before it is exported at the northern edge of the Weddell Gyre. [ABSTRACT FROM AUTHOR]

The Southern Ocean upper-layer freshwater balance exerts a global climatic influence by modulating density stratification and biological productivity, and hence the exchange of heat and carbon between the atmosphere and the ocean interior. It is thus important to understand and quantify the time-varying freshwater inputs, which is challenging from measurements of salinity alone. Here we use seawater oxygen isotopes from samples collected between 2016 and 2021 along a transect spanning the Scotia and northern Weddell Seas to separate the freshwater contributions from sea ice and meteoric sources. The unprecedented retreat of sea ice in 2016 is evidenced as a strong increase in sea ice melt across the northern Weddell Sea, with surface values increasing approximately two percentage points between 2016 and 2018 and column inventories increasing approximately 1 to 2 m. Surface meteoric water concentrations exceeded 4% in early 2021 close to South Georgia due to meltwater from the A68 megaberg; smaller icebergs may influence meteoric water at other times also. Both these inputs highlight the importance of a changing cryosphere for upper-ocean freshening; potential future sea ice retreats and increases in iceberg calving would enhance the impacts of these freshwater sources on the ocean and climate. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'.