47 research outputs found
Sliding or stumbling on the staircase: numerics of ocean circulation along piecewise-constant coastlines
Coastlines in most ocean general circulation models are piecewise constant. Accurate representation of boundary currents along staircase-like coastlines is a long-standing issue in ocean modelling. Pioneering work by Adcroft and Marshall (1998) revealed that artificial indentation of model coastlines, obtained by rotating the numerical mesh within an idealized square basin, generates a \textit{spurious form drag} that slows down the circulation. Here, we revisit this problem and show how this spurious drag may be eliminated. First, we find that \textit{physical} convergence (i.e. the main characteristics of the flow are insensitive to the increase of the mesh resolution) allows simulations to become independent of the mesh orientation. An advection scheme with a wider stencil also reduces sensitivity to mesh orientation from coarser resolution. Second, we show that indented coastlines behave as straight and slippery shores when a true mirror boundary condition on the flow is imposed. This finding applies to both symmetric and rotational-divergence formulations of the stress tensor, and to both flux and vector-invariant forms of the equations. Finally, we demonstrate that the detachment of a vortex flowing past an outgoing corner of the coastline is faithfully simulated with exclusive implementation of impermeability conditions. These results provide guidance for a better numerical treatment of coastlines (and isobaths) in ocean general circulation models
Deep-ocean mixing driven by small-scale internal tides
Turbulent mixing in the ocean is key to regulate the transport of heat, freshwater and biogeochemical tracers, with strong implications for Earthâs climate. In the deep ocean, tides supply much of the mechanical energy required to sustain mixing via the generation of internal waves, known as internal tides, whose fateâthe relative importance of their local versus remote breaking into turbulenceâremains uncertain. Here, we combine a semi-analytical model of internal tide generation with satellite and in situ measurements to show that from an energetic viewpoint, small-scale internal tides, hitherto overlooked, account for the bulk (>50%) of global internal tide generation, breaking and mixing. Furthermore, we unveil the pronounced geographical variations of their energy proportion, ignored by current parameterisations of mixing in climate-scale models. Based on these results, we propose a physically consistent, observationally supported approach to accurately represent the dissipation of small-scale internal tides and their induced mixing in climate-scale models
Climate recorded in seawater: A workshop on water-mass transformation analysis for ocean and climate studies
First workshop on water-mass transformation analysis for ocean physics, biogeochemistry, and climate. 4â6 February 2019, Sydney, New South Wales, AustraliaInternational audienceAn international cohort of oceanographers, marine biogeochemists, and climate modelers gathered to expand the use of water-mass transformation diagnostics in studies of ocean physics, biogeochemistry, and climate. Led by early-career scientists, the group laid out avenues to leverage growing oceanic observational databases and new model capabilities, using fundamental understanding of the oceanâs layering
ââObserving Antarctic Bottom Water in the Southern Oceanâ
Dense, cold waters formed on Antarctic continental shelves descend along the Antarctic continental margin, where they mix with other Southern Ocean waters to form Antarctic Bottom Water (AABW). AABW then spreads into the deepest parts of all major ocean basins, isolating heat and carbon from the atmosphere for centuries. Despite AABWâs key role in regulating Earthâs climate on long time scales and in recording Southern Ocean conditions, AABW remains poorly observed. This lack of observational data is mostly due to two factors. First, AABW originates on the Antarctic continental shelf and slope where in situ measurements are limited and ocean observations by satellites are hampered by persistent sea ice cover and long periods of darkness in winter. Second, north of the Antarctic continental slope, AABW is found below approximately 2 km depth, where in situ observations are also scarce and satellites cannot provide direct measurements. Here, we review progress made during the past decades in observing AABW. We describe 1) long-term monitoring obtained by moorings, by ship-based surveys, and beneath ice shelves through bore holes; 2) the recent development of autonomous observing tools in coastal Antarctic and deep ocean systems; and 3) alternative approaches including data assimilation models and satellite-derived proxies. The variety of approaches is beginning to transform our understanding of AABW, including its formation processes, temporal variability, and contribution to the lower limb of the global ocean meridional overturning circulation. In particular, these observations highlight the key role played by winds, sea ice, and the Antarctic Ice Sheet in AABW-related processes. We conclude by discussing future avenues for observing and understanding AABW, impressing the need for a sustained and coordinated observing system
On the lifecycle of Antarctic Bottom Water
L'Eau Antarctique de Fond constitue la principale masse d'eau océanique par son volume, et nourrit la composante la plus profonde et la plus lente de la circulation océanique. Les processus qui régissent son cycle de vie sont donc clé pour la capacité de stockage de l'océan en carbone et chaleur aux échelles centennales à multi-millénaires. Cette thÚse tente de caractériser et quantifier les principaux processus responsables de la destruction (synonyme d'allÚgement et de remontée) de l'Eau Antarctique de Fond dans l'océan abyssal. A partir d'une estimée issue d'observations de la structure thermohaline de l'océan mondial et de diagnostics fondés sur le budget de densité des eaux profondes, les rÎles respectifs du chauffage géothermal, du mélange turbulent par déferlement d'ondes internes et de la géométrie des bassins sont évalués. Il est montré que la géométrie de l'océan gouverne la structure de la circulation de l'Eau Antarctique de Fond. La contribution du déferlement des ondes internes, bien que mal contrainte, est estimée insuffisante pour maintenir un rythme de destruction de l'Eau Antarctique de Fond comparable à celui de sa formation. Le chauffage géothermal a quant à lui un rÎle important pour la remontée des eaux recouvrant une large surface du lit océanique. Les résultats suggÚrent une réévaluation de l'importance du mélange au niveau des détroits et seuils profonds, mais aussi du rÎle fondamental de la forme des bassins, pour l'allÚgement et le transport des eaux abyssales.Antarctic Bottom Water is the most voluminous water mass of the World Ocean, and it feeds the deepest and slowest component of ocean circulation. The processes that govern its lifecycle are therefore key to the ocean's carbon and heat storage capacity on centennial to multi-millennial timescales. This thesis aims at characterizing and quantifying processes responsible for the destruction (synonymous of lightening and upwelling) of Antarctic Bottom Water in the abyssal ocean. Using an observational estimate of the global ocean thermohaline structure and diagnostics based on the density budget of deep waters, we explore the roles of basin geometry, geothermal heating and mixing by breaking internal waves for the abyssal circulation. We show that the shape of ocean basins largely controls the structure of abyssal upwelling. The contribution of mixing powered by breaking internal waves, though poorly constrained, is estimated to be insufficient to destroy Antarctic Bottom Water at a rate comparable to that of its formation. Geothermal heating plays an important role for the upwelling of waters covering large seafloor areas. The results suggest a reappraisal of the role of mixing in deep straits and sills, but also of the fundamental role of basin geometry, for the lightening and transport of abyssal waters
Slowing of the ocean's deep breath
International audienceThe deepest reaches of the ocean are ventilated by sinking of cold and relatively saline seawater around Antarctica. Observations from the Australian sector of the Southern Ocean reveal a decline in sinking and abyssal ventilation, linked to dropping ocean salinity on the Antarctic shelf
Eléments du cycle de vie de l'Eau Antarctique de Fond
Antarctic Bottom Water is the most voluminous water mass of the World Ocean, and it feeds the deepest and slowest component of ocean circulation. The processes that govern its lifecycle are therefore key to the ocean's carbon and heat storage capacity on centennial to multi-millennial timescales. This thesis aims at characterizing and quantifying processes responsible for the destruction (synonymous of lightening and upwelling) of Antarctic Bottom Water in the abyssal ocean. Using an observational estimate of the global ocean thermohaline structure and diagnostics based on the density budget of deep waters, we explore the roles of basin geometry, geothermal heating and mixing by breaking internal waves for the abyssal circulation. We show that the shape of ocean basins largely controls the structure of abyssal upwelling. The contribution of mixing powered by breaking internal waves, though poorly constrained, is estimated to be insufficient to destroy Antarctic Bottom Water at a rate comparable to that of its formation. Geothermal heating plays an important role for the upwelling of waters covering large seafloor areas. The results suggest a reappraisal of the role of mixing in deep straits and sills, but also of the fundamental role of basin geometry, for the lightening and transport of abyssal waters.L'Eau Antarctique de Fond constitue la principale masse d'eau océanique par son volume, et nourrit la composante la plus profonde et la plus lente de la circulation océanique. Les processus qui régissent son cycle de vie sont donc clé pour la capacité de stockage de l'océan en carbone et chaleur aux échelles centennales à multi-millénaires. Cette thÚse tente de caractériser et quantifier les principaux processus responsables de la destruction (synonyme d'allÚgement et de remontée) de l'Eau Antarctique de Fond dans l'océan abyssal. A partir d'une estimée issue d'observations de la structure thermohaline de l'océan mondial et de diagnostics fondés sur le budget de densité des eaux profondes, les rÎles respectifs du chauffage géothermal, du mélange turbulent par déferlement d'ondes internes et de la géométrie des bassins sont évalués. Il est montré que la géométrie de l'océan gouverne la structure de la circulation de l'Eau Antarctique de Fond. La contribution du déferlement des ondes internes, bien que mal contrainte, est estimée insuffisante pour maintenir un rythme de destruction de l'Eau Antarctique de Fond comparable à celui de sa formation. Le chauffage géothermal a quant à lui un rÎle important pour la remontée des eaux recouvrant une large surface du lit océanique. Les résultats suggÚrent une réévaluation de l'importance du mélange au niveau des détroits et seuils profonds, mais aussi du rÎle fondamental de la forme des bassins, pour l'allÚgement et le transport des eaux abyssales
Response of a comprehensive climate model to a broad range of external forcings: relevance for deep ocean ventilation and the development of late Cenozoic ice ages
International audienc
Diffusion controls the ventilation of a Pacific Shadow Zone above abyssal overturning
International audienceMid-depth North Pacific waters are rich in nutrients and respired carbon accumulated over centuries. The rates and pathways with which these waters exchange with the surface ocean are uncertain, with divergent paradigms of the Pacific overturning: one envisions bottom waters upwelling to 1.5 km depth; the other confines overturning beneath a mid-depth Pacific shadow zone (PSZ) shielded from mean advection. Here global inverse modelling reveals a PSZ where mean ages exceed 1400 years with overturning beneath. The PSZ is supplied primarily by Antarctic and North-Atlantic ventilated waters diffusing from below and from the south. Half of PSZ waters re-surface in the Southern Ocean, a quarter in the subarctic Pacific. The abyssal North Pacific, despite strong overturning, has mean re-surfacing times also exceeding 1400 years because of diffusion into the overlying PSZ. These results imply that diffusive transportsdistinct from overturning transportsare a leading control on Pacific nutrient and carbon storage