Diapycnal mixing shapes the distribution of climatically important tracers, such as heat and carbon, as these are carried by dense water masses in the ocean interior. Here, we analyze a suite of ...observation‐based estimates of diapycnal mixing to assess its role within the Atlantic Meridional Overturning Circulation (AMOC). The rate of water mass transformation in the Atlantic Ocean's interior shows that there is a robust buoyancy increase in the North Atlantic Deep Water (NADW, neutral density γn ≃ 27.6–28.15), with a diapycnal circulation of 0.5–8 Sv between 48°N and 32°S in the Atlantic Ocean. Moreover, tracers within the southward‐flowing NADW may undergo a substantial diapycnal transfer, equivalent to a vertical displacement of hundreds of meters in the vertical. This result, confirmed with a zonally averaged numerical model of the AMOC, indicates that mixing can alter where tracers upwell in the Southern Ocean, ultimately affecting their global pathways and ventilation timescales. These results point to the need for a realistic mixing representation in climate models in order to understand and credibly project the ongoing climate change.
Plain Language Summary
The Atlantic Ocean meridional overturning circulation plays a key role in regulating the global heat and carbon budgets by inter‐hemispheric transport of anthropogenic and natural tracers as well as water masses. While most of this transport occurs along nearly horizontal density surfaces in the ocean interior, vertical transport across density levels is key to bringing deep waters back to the surface. Such cross‐density transport is facilitated mainly by the internal waves breaking into turbulence and near boundary processes. This work employs a host of observation‐based estimates of turbulence in the Atlantic Ocean to (a) better quantify the contribution of cross‐density mixing to the inter‐hemispheric Atlantic circulation, and (b) discuss the potential implications for pathways and residence times of tracers carried from the North Atlantic to the Southern Ocean. This work calls for a more careful representation of turbulence‐induced vertical mixing within the Atlantic Ocean in climate models to better understand and project the ongoing climate change.
Key Points
The cross‐density mixing of water and tracers is quantified from observation‐based estimates and numerical simulations in the Atlantic Ocean
A net 0.5–8 Sv of North Atlantic Deep Water upwells diapycnally in the Atlantic Ocean (48°N–32°S), comprised of larger regional up/downwelling fluxes
Tracer mixing in the deep Atlantic Ocean can significantly modify pathways and ventilation rates of tracers upwelling in the Southern Ocean
•Tides enhance simulated ice-shelf melting by 1–39% depending on the ice shelf.•Enhanced melting is mostly due to strengthened turbulence near the ice/ocean interface.•Tidal processes occurring ...seaward are less important than those occurring underneath ice shelves.•A methodology to prescribe tidal effects on ice shelf melt is proposed.
The representation of tides in regional ocean simulations of the Amundsen Sea enhances ice-shelf melting, with weakest effects for Pine Island and Thwaites (<+10%) and strongest effects for Dotson, Cosgrove and Abbot (>+30%). Tides increase vertical mixing throughout the water column along the continental shelf break. Diurnal tides induce topographically trapped vorticity waves along the continental shelf break, likely underpinning the tidal rectification (residual circulation) simulated in the Dotson–Getz Trough. However, the primary effect by which tides affect ice-shelf melting is the increase of ice/ocean exchanges, rather than the modification of water masses on the continental shelf. Tide-induced velocities strengthen turbulent heat fluxes at the ice/ocean interface, thereby increasing melt rates. Approximately a third of this effect is counterbalanced by the resulting release of cold melt water that reduces melt downstream along the meltwater flow. The relatively weak tide-induced melting underneath Pine Island and Thwaites could be partly related to their particularly thick water column, which limits the presence of quarter wavelength tidal resonance. No sensitivity to the position of Pine Island and Thwaites with respect to the M2 critical latitude is found. We refine and evaluate existing methodologies to prescribe the effect of tides on ice-shelf melt rates in ocean models that do not explicitely include tidal forcing. The best results are obtained by prescribing spatially-dependent tidal top-boundary-layer velocities in the melt equations. These velocities can be approximated as a linear function of existing barotropic tidal solutions. A correction factor needs to be applied to account for the additional melt-induced circulation associated with tides and to reproduce the relative importance of dynamical and thermodynamical processes.
The physical oceanographic environment, water mass characteristics, and distribution in the area adjacent to Larsen C Ice Shelf (LCIS) are investigated using hydrographic data collected during the ...2019 Weddell Sea Expedition. The results shed light on the ocean conditions adjacent to a thinning LCIS, on a continental shelf that is a source region for Weddell Sea Deep Water (WSDW), a precursor of the globally important Antarctic Bottom Water. Modified Warm Deep Water (MWDW), a water mass of circumpolar origin, is identified on the continental shelf and is observed to mix with Ice Shelf Water (ISW) and High Salinity Shelf Water (HSSW), both source waters of WSDW. A source water type decomposition analysis reveals high levels of mixing in the area, with much spatial variability. Heat content anomalies indicate an introduction of heat, presumed to be associated with MWDW, into the area via Jason Trough. Furthermore, candidate parent sources for ISW are identified in the region, indicating the potential for a flow of continental shelf waters into the ice shelf cavity; however, the impact on LCIS cannot be surmised from the available observations. ISW and HSSW are observed to make dominant contributions to the densest layers within Jason Trough, where waters are likely en route to feed the deep layers of the Antarctic Slope Current. This cross‐shelf flux of water masses links the region of the Weddell Sea adjacent to northern LCIS to global ocean circulation and Bottom Water characteristics via its contribution to ISW and HSSW, and hence WSDW properties.
Plain Language Summary
A voyage to the Antarctic Weddell Sea in 2019 has enabled a diagnosis of the water mass characteristics adjacent to the northern sector of Larsen C Ice Shelf (LCIS). This region is of interest due to its contribution to the properties of the principal source waters of Antarctic Bottom Water (AABW), a water mass that constitutes the deep limb of the global ocean circulation system. LCIS has experienced considerable variability in thickness and extent during the satelite observational period, yet the role of the ocean in these changes remains unclear. It is, therefore, necessary that we improve our understanding of the ocean region adjacent to LCIS. Hydrographic results presented here reveal the presence of a warm water mass in the area, termed Modified Warm Deep Water (MWDW), which has its origins in the Antarctic Circumpolar Current. MWDW appears to thoroughly mix with the local shelf waters such as Ice Shelf Water (ISW) and High Salinity Shelf Water (HSSW), both precursors of AABW. An alteration of water properties on the continental shelf could have important implications for future regional and global ocean circulation, meaning that knowledge regarding the water masses in this area is highly relevant.
Key Points
Oxygen measurements enable the use of a linear mixing model to quantify contributions from principal source waters
High levels of water mass mixing reveal the potential for transformation of the precursors of Weddell Sea Deep Water
The observed presence of Modified Warm Deep Water signals the transport of heat to the area
Over the past few million years, the Earth descended from the relatively warm and stable climate of the Pliocene into the increasingly dramatic ice age cycles of the Pleistocene. The influences of ...orbital forcing and atmospheric CO
2
on land-based ice sheets have long been considered as the key drivers of the ice ages, but less attention has been paid to their direct influences on the circulation of the deep ocean. Here we provide a broad view on the influences of CO
2
, orbital forcing and ice sheet size according to a comprehensive Earth system model, by integrating the model to equilibrium under 40 different combinations of the three external forcings. We find that the volume contribution of Antarctic (AABW) vs. North Atlantic (NADW) waters to the deep ocean varies widely among the simulations, and can be predicted from the difference between the surface densities at AABW and NADW deep water formation sites. Minima of both the AABW-NADW density difference and the AABW volume occur near interglacial CO
2
(270–400 ppm). At low CO
2
, abundant formation and northward export of sea ice in the Southern Ocean contributes to very salty and dense Antarctic waters that dominate the global deep ocean. Furthermore, when the Earth is cold, low obliquity (i.e. a reduced tilt of Earth’s rotational axis) enhances the Antarctic water volume by expanding sea ice further. At high CO
2
, AABW dominance is favoured due to relatively warm subpolar North Atlantic waters, with more dependence on precession. Meanwhile, a large Laurentide ice sheet steers atmospheric circulation as to strengthen the Atlantic Meridional Overturning Circulation, but cools the Southern Ocean remotely, enhancing Antarctic sea ice export and leading to very salty and expanded AABW. Together, these results suggest that a ‘sweet spot’ of low CO
2
, low obliquity and relatively small ice sheets would have poised the AMOC for interruption, promoting Dansgaard–Oeschger-type abrupt change. The deep ocean temperature and salinity simulated under the most representative ‘glacial’ state agree very well with reconstructions from the Last Glacial Maximum (LGM), which lends confidence in the ability of the model to estimate large-scale changes in water-mass geometry. The model also simulates a circulation-driven increase of preformed radiocarbon reservoir age, which could explain most of the reconstructed LGM-preindustrial ocean radiocarbon change. However, the radiocarbon content of the simulated glacial ocean is still higher than reconstructed for the LGM, and the model does not reproduce reconstructed LGM deep ocean oxygen depletions. These ventilation-related disagreements probably reflect unresolved physical aspects of ventilation and ecosystem processes, but also raise the possibility that the LGM ocean circulation was not in equilibrium. Finally, the simulations display an increased sensitivity of both surface air temperature and AABW volume to orbital forcing under low CO
2
. We suggest that this enhanced orbital sensitivity contributed to the development of the ice age cycles by amplifying the responses of climate and the carbon cycle to orbital forcing, following a gradual downward trend of CO
2
.
Internal tides power much of the observed small-scale turbulence in the ocean interior. To represent mixing induced by this turbulence in ocean climate models, the cascade of internal tide energy to ...dissipation scales must be understood and mapped. Here, we present a framework for estimating the geography of internal tide energy sinks. The mapping relies on the following ingredients: (i) a global observational climatology of stratification; (ii) maps of the generation of M2, S2 and K1 internal tides decomposed into vertical normal modes; (iii) simplified representations of the dissipation of low-mode internal tides due to wave-wave interactions, scattering by small-scale topography, interaction with critical slopes and shoaling; (iv) Lagrangian tracking of low-mode energy beams through observed stratification, including refraction and reflection. We thus obtain a global map of the column-integrated energy dissipation for each of the four considered dissipative processes, each of the three tidal constituents and each of the first five modes. Modes ≥6 are inferred to dissipate within the local water column at the employed half-degree horizontal resolution. Combining all processes, modes and constituents, we construct a map of the total internal tide energy dissipation, which compares well with observational inferences of internal wave energy dissipation. This result suggests that tides largely shape observed spatial contrasts of dissipation, and that the framework has potential in improving understanding and modelling of ocean mixing. However, sensitivity to poorly constrained parameters and simplifying assumptions entering the parameterized energy sinks calls for additional investigation. The attenuation of low-mode internal tides by wave-wave interactions needs particular attention.
•A framework for estimating the global geography of internal tide energy sinks is presented.•The column-integrated dissipation rate of internal tides is mapped globally.•The estimated internal tide dissipation compares well with finestructure observations.•The maps pave the way toward comprehensive 3D mapping and parameterization of tidal mixing.
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 thèse tente de caractériser et quantifier les principaux processus responsables de la destruction (synonyme d'allègement 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 rôles 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 rôle important pour la remontée des eaux recouvrant une large surface du lit océanique. Les résultats suggèrent une réévaluation de l'importance du mélange au niveau des détroits et seuils profonds, mais aussi du rôle fondamental de la forme des bassins, pour l'allègement 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.
Changes in Pacific tracer reservoirs and transports are thought to be central to the regulation of atmospheric CO
2
on glacial–interglacial timescales. However, there are currently two contrasting ...views of the circulation of the modern Pacific; the classical view sees southern sourced abyssal waters upwelling to about 1.5 km depth before flowing southward, whereas the bathymetrically constrained view sees the mid-depths (1–2.5 km) largely isolated from the global overturning circulation and predominantly ventilated by diffusion. Furthermore, changes in the circulation of the Pacific under differing climate states remain poorly understood. Through both a modern and a Last Glacial Maximum (LGM) analysis focusing on oxygen isotopes in seawater and benthic foraminifera as conservative tracers, we show that isopycnal diffusion strongly influences the mid-depths of the Pacific. Diapycnal diffusion is most prominent in the subarctic Pacific, where an important return path of abyssal tracers to the surface is identified in the modern state. At the LGM we infer an expansion of North Pacific Intermediate Water, as well as increased layering of the deeper North Pacific which would weaken the return path of abyssal tracers. These proposed changes imply a likely increase in ocean carbon storage within the deep Pacific during the LGM relative to the Holocene.
This study presents the global climate model IPSL‐CM6A‐LR developed at Institut Pierre‐Simon Laplace (IPSL) to study natural climate variability and climate response to natural and anthropogenic ...forcings as part of the sixth phase of the Coupled Model Intercomparison Project (CMIP6). This article describes the different model components, their coupling, and the simulated climate in comparison to previous model versions. We focus here on the representation of the physical climate along with the main characteristics of the global carbon cycle. The model's climatology, as assessed from a range of metrics (related in particular to radiation, temperature, precipitation, and wind), is strongly improved in comparison to previous model versions. Although they are reduced, a number of known biases and shortcomings (e.g., double Intertropical Convergence Zone ITCZ, frequency of midlatitude wintertime blockings, and El Niño–Southern Oscillation ENSO dynamics) persist. The equilibrium climate sensitivity and transient climate response have both increased from the previous climate model IPSL‐CM5A‐LR used in CMIP5. A large ensemble of more than 30 members for the historical period (1850–2018) and a smaller ensemble for a range of emissions scenarios (until 2100 and 2300) are also presented and discussed.
Plain Language Summary
Climate models are unique tools to investigate the characteristics and behavior of the climate system. While climate models and their components are developed gradually over the years, the sixth phase of the Coupled Model Intercomparison Project (CMIP6) has been the opportunity for the Institut Pierre‐Simon Laplace to develop, test, and evaluate a new configuration of its climate model called IPSL‐CM6A‐LR. The characteristics and emerging properties of this new model are presented in this study. The model climatology, as assessed from a range of metrics, is strongly improved, although a number of biases common to many models do persist. The equilibrium climate sensitivity and transient climate response have both increased from the previous climate model IPSL‐CM5A‐LR used in CMIP5.
Key Points
The IPSL‐CM6A‐LR model climatology is much improved over the previous version, although some systematic biases and shortcomings persist
A long preindustrial control and a large number of historical and scenario simulations have been performed as part of CMIP6
The effective climate sensitivity of the IPSL model increases from 4.1 to 4.8 K between IPSL‐CM5A‐LR and IPSL‐CM6A‐LR
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