Summertime increases in upper-ocean stratification and mixed-layer depth Jean-Baptiste Sallée, Violaine Pellichero, Camille Akhoudas, Etienne Pauthenet, Lucie Vignes, Sunke Schmidtko, Alberto Naveira Garabato, Peter Sutherland, Mikael Kuusela To cite this version: Jean-Baptiste Sallée, Violaine Pellichero, Camille Akhoudas, Etienne Pauthenet, Lucie Vignes, et al.. Summertime increases in upper-ocean stratification and mixed-layer depth. Nature, Nature Publishing Group, 2021, 591 (7851), pp.592-598. 10.1038/s41586-021-03303-x. hal-03184114 HAL Id: hal-03184114 https://hal.archives-ouvertes.fr/hal-03184114 Submitted on 1 Apr 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Article Summertime increases in upper-ocean stratification and mixed-layer depth https://doi.org/10.1038/s41586-021-03303-x Jean-Baptiste Sallée1 ✉, Violaine Pellichero2,3, Camille Akhoudas1, Etienne Pauthenet1, Lucie Vignes1, Sunke Schmidtko4, Alberto Naveira Garabato5, Peter Sutherland6 & Received: 8 December 2019 Mikael Kuusela7 Accepted: 26 January 2021 Published online: 24 March 2021 The surface mixed layer of the world ocean regulates global climate by controlling Check for updates heat and carbon exchange between the atmosphere and the oceanic interior1–3. The mixed layer also shapes marine ecosystems by hosting most of the ocean’s primary production4 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 defnition of upper-ocean stability that follows diferent dynamical regimes across the global ocean, we fnd that the summertime density contrast increased by 8.9 ± 2.7 per cent per decade (10−6–10−5 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 stratifed upper ocean5–7, we fnd instead that the summertime mixed layer deepened by 2.9 ± 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 stratifcation and deepening of the mixed layer are related to an increase in stability associated with surface warming and high-latitude surface freshening8,9, accompanied by a wind-driven intensifcation of upper-ocean turbulence10,11. Our fndings 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 fve decades. The fundamental vertical structure of the world ocean consists of three will expectedly weaken surface-to-depth exchanges as enhanced den- main layers: the surface mixed layer, which continually exchanges sity gradients decouple surface and subsurface waters, act to shoal heat, freshwater, carbon and other climatically important gases with the surface mixed layer, and result in reduced air–sea gas transfer, the atmosphere; the pycnocline, characterized by its pronounced deep-ocean ventilation and biological productivity3,13–15. Detecting stratification—that is, an enhanced density contrast between shallower and understanding physical changes in the ocean’s surface and pyc- and deeper layers, which inhibits cross-layer vertical mixing; and the nocline layers is thus essential to determine the role of the ocean in deep ocean, which is largely isolated from the atmosphere (Fig. 1; climate, and predict climate change and its ecosystem impacts. The some regions have an additional layer between the mixed layer and latest Special Report on Ocean and Cryosphere in a Changing Climate the pycnocline, which is termed ‘barrier layer’ and is associated with from the Intergovernmental Panel on Climate Change (IPCC)16 clearly an enhanced vertical salinity gradient12). Changes in the surface and identifies this aspect of oceanic evolution as highly policy-relevant. pycnocline layers can have widespread consequences for climate, as Changes in the surface mixed-layer depth and pycnocline stratifica- they may alter the rates at which exchanges occur between the surface tion feature prominently in the report’s summary for policymakers, and the deep ocean. For example, increased pycnocline stratification and in multiple contexts including ocean de-oxygenation, nutrient 1Sorbonne Université, CNRS/IRD/MNHN, LOCEAN, IPSL, Paris, France. 2Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Tasmania, Australia. 3CSIRO Oceans and Atmosphere, Hobart, Tasmania, Australia. 4GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany. 5Ocean and Earth Science, National Oceanography Centre, University of Southampton, Southampton, UK. 6Ifremer, Université Brest, CNRS, IRD, Laboratoire d’Océanographie Physique et Spatiale (LOPS), IUEM, Brest, France. 7Department of Statistics and Data Science, Carnegie Mellon University, Pittsburgh, PA, USA. ✉e-mail: [email protected] 592 | Nature | Vol 591 | 25 March 2021 Buoyancy uxes Winds (heat and freshwater) 50-year deepening of the summer Surface mixed-layer at a rate of ~2.9% dec–1 Seasonal mixed layer 50-year strengthening of the pycnocline easonal pycn S ocl –1 500 m ine stratication at a rate of ~8.9% dec 2,000 m Deep ocean 4,000 m South Equator North Fig. 1 | The three-layer structure of the world ocean. Schematic of an between surface and deep waters. The deep ocean is largely insulated from the idealized meridional section across the world ocean illustrating the ocean’s atmosphere, but climate signals propagate from and to the deep ocean three-layer structure. The upper seasonal mixed layer is stirred by a range of through mixing across the seasonal pycnocline and/or through direct contact turbulent processes driven by wind and buoyancy forcings (see Methods with the mixed layer as seasonal pycnocline stratification is eroded in winter. section ‘Dynamical forcing of changes in mixed-layer depth’). The seasonal Here, we present 50-year trends in both mixed-layer depth and pycnocline pycnocline emerges from the density contrast (that is, stratification) between stratification, with impacts on upper-ocean structure and deep-ocean surface and deep waters, and acts as a barrier that reduces communication ventilation. supply to living organisms in the mixed layer, and the global energy estimates. Data selection is based on a rigorously tested data-mapping budget. procedure21,23–25, temporal and spatial decorrelation scales used in the Despite its far-reaching climatic effects, the variability in the regression model are estimated from the data26, and uncertainties mixed-layer depth and pycnocline strength have not been examined in a associated with each individual observation are propagated through systematic fashion from observations. A few studies have documented the model to produce standard error maps for the climatology and the changes in upper-ocean stratification, but they have done so by focus- associated trends (see Methods for details). Because of the large sea- ing on mixed-layer depth variations at specific locations17 or on changes sonal cycle that characterizes the upper ocean, all our results are pre- in stratification averaged over a fixed depth range (generally 0–200 m, sented by season, where ‘summer’ (‘winter’) refers to August–October 0–1,000 m or 0–2,000 m) that conflates the distinct dynamical regimes in the Northern (Southern) Hemisphere and to January–March in the of the mixed layer, pycnocline and deep ocean1,8,9,18,19. For instance, Southern (Northern) Hemisphere. The fields referred to hereafter as stratification over 0–200 m, which has been widely used in past studies, ‘climatological fields’ are seasonal means estimated for year 2000, entirely misses pycnocline changes in regions where the mixed layer is computed from the monthly weighted local linear regression (see Meth- deeper than 200 m (typically at high latitudes in the Southern Ocean ods). In regions where salinity-driven barrier layers are present between and North Atlantic) and can underestimate pycnocline changes where the mixed layer and the pycnocline (mostly in the tropics12), the vari- the mixed layer is shallower than 200 m (such as in the tropics and the able referred to as ‘pycnocline strength’—that is, the density gradient subtropical ocean), especially when the mixed-layer depth also evolves at the base of the density-defined mixed layer (see Methods)—is actu- in time (see Methods, Extended