Log-skew-normality of ocean turbulence

The statistics of intermittent ocean turbulence is the key link between physical understanding of turbulence and its global implications. The log-normal distribution is the standard but imperfect assumed distribution for the turbulent kinetic energy dissipation rate. We argue that as turbulence is often generated by multiple changing sources, a log-skew-normal (LSN) distribution is more appropriate. We show the LSN distribution agrees excellently and robustly with observations. The heavy tail of the LSN distribution has important implications for sampling of turbulence in terrestrial and extraterrestrial analogous systems

Prandtl number effects on extreme mixing events in forced stratified turbulence

Relatively strongly stratified turbulent flows tend to self-organise into a 'layered anisotropic stratified turbulence' (LAST) regime, characterised by relatively deep and well-mixed density 'layers' separated by relatively thin 'interfaces' of enhanced density gradient. Understanding the associated mixing dynamics is a central problem in geophysical fluid dynamics. It is challenging to study 'LAST' mixing, as it is associated with Reynolds numbers $Re := UL/\nu \gg 1$ and Froude numbers $Fr :=(2\pi U)/(L N) \ll 1$, ($U$ and $L$ being characteristic velocity and length scales, $\nu$ being the kinematic viscosity and $N$ the buoyancy frequency). Since a sufficiently large dynamic range (largely) unaffected by stratification and viscosity is required, it is also necessary for the buoyancy Reynolds number $Re_{b} := \epsilon/(\nu N^{2}) \gg 1$ where $\epsilon$ is the (appropriately volume-averaged) turbulent kinetic energy dissipation rate. This requirement is exacerbated for oceanically relevant flows, as the Prandtl number $Pr := \nu/\kappa = \mathcal{O}(10)$ in thermally-stratified water (where $\kappa$ is the thermal diffusivity), thus leading (potentially) to even finer density field structures. We report here on four forced fully resolved direct numerical simulations of stratified turbulence at various Froude ($Fr=0.5, 2$) and Prandtl numbers ($Pr=1, 7$) forced so that $Re_{b}=50$, with resolutions up to $30240 \times 30240 \times 3780$. We find that, as $Pr$ increases, emergent 'interfaces' become finer and their contribution to bulk mixing characteristics decreases at the expense of the small-scale density structures populating the well-mixed 'layers'. However, extreme mixing events (as quantified by significantly elevated local destruction rates of buoyancy variance $\chi_0$) are always preferentially found in the (statically stable) interfaces, irrespective of the value of $Pr$.Comment: 10 pages, 4 figure

​​Observing Antarctic Bottom Water in the Southern Ocean​

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 AABW’s key role in regulating Earth’s 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

The Sensitivity of Southern Ocean Air‐Sea Carbon Fluxes to Background Turbulent Diapycnal Mixing Variability

Funder: UK Engineering and Physical Sciences Research CouncilAbstractThe Southern Ocean (SO) connects major ocean basins and hosts large air‐sea carbon fluxes due to the resurfacing of deep nutrient and carbon‐rich waters. While wind‐induced turbulent mixing in the SO mixed layer is significant for air‐sea fluxes, the importance of the orders‐of‐magnitude weaker background mixing below is less well understood. The direct impact of altering background mixing on tracers, as opposed to the response due to a longer‐term change in large‐scale ocean circulation, is also poorly studied. Topographically induced upward propagating lee waves, wind‐induced downward propagating waves generated at the base of the mixed layer, shoaling of southward propagating internal tides, and turbulence under sea ice are among the processes known to induce upper ocean background turbulence but typically are not represented in models. Here, we show that abruptly altering the background mixing in the SO over a range of values typically used in climate models ((10−4) m2 s−1‐ (10−5) m2 s−1) can lead to a ∼70% change in annual SO air‐sea CO2 fluxes in the first year of perturbations, and around a ∼40% change in annual SO air‐sea CO2 fluxes over the six‐year duration of the experiment, with even greater changes on a seasonal timescale. This is primarily through altering the temperature and the dissolved inorganic carbon and alkalinity distribution in the surface water. Given the high spatiotemporal variability of processes that induce small‐scale background mixing, this work demonstrates the importance of their representation in climate models for accurate simulation of global biogeochemical cycles.</jats:p

Enhanced upwelling of Antarctic Bottom Water by topographic interaction of water mass interfaces

The lower cell of the meridional overturning circulation (MOC) is sourced by dense Antarctic Bottom Water (AABW), which forms and sinks around Antarctica and subsequently fills the abyssal ocean. For the MOC to ‘overturn’, these dense waters must upwell through mixing with lighter waters above. Here, we investigate the processes underpinning such mixing, and the resulting water mass transformation, using an observationally forced, high-resolution numerical model of the Drake Passage in the Southern Ocean. In the Drake Passage, the mixing of dense AABW formed in the Weddell Sea with lighter deep waters transported from the Pacific Ocean by the Antarctic Circumpolar Current is catalysed by energetic flows impinging on rough topography. We find that multiple topographic interaction processes act to facilitate mixing of the two water masses, ultimately resulting in upwelling of waters with neutral density greater 28.19 kg m-3, and downwelling of the lighter waters above. In particular, we identify the role of sharp density interfaces between AABW and overlying waters, and find that the dynamics of the interfaces’ interaction with topography can enhance mixing. Such sharp interfaces between water masses have been observed in several parts of the global ocean, but are unresolved and unrepresented in ocean and climate models. We suggest that they are likely to play an important role in abyssal dynamics and mixing, and therefore require further exploration

Boundary upwelling of Antarctic Bottom Water by topographic turbulence

Software and data associated with the following publication: Baker, Mashayek, and Naveira Garabato (2023), Boundary upwelling of Antarctic Bottom Water by topographic turbulence (Accepted in AGU Advances)</span

Enhanced upwelling of Antarctic Bottom Water by topographic interaction of water mass interfaces: software and data

Software and data associated with the following publication: Baker, Mashayek, and Naveira Garabato (2022),&nbsp;Enhanced upwelling of Antarctic Bottom Water by topographic interaction of water mass interfaces.&nbsp; </span

Mountains to climb: on the role of seamounts in upwelling of deep ocean waters

Ocean turbulent mixing exerts an important control on the rate and structure of the overturning circulation. Recent observational evidence suggests, however, that there could be a mismatch between the observed intensity of mixing integrated over basin or global scales, and the net mixing required to sustain the overturning's deep upwelling limb. Here, we investigate the hitherto largely overlooked role of tens of thousands of seamounts in resolving this discrepancy. Dynamical theory indicates that seamounts may stir and mix deep waters by generating lee waves and topographic wake vortices. At low latitudes, this is enhanced by a layered vortex regime in the wakes. We consider three case studies (in the equatorial zone, Southern Ocean and Gulf Stream) that are predicted by theory to be representative of, respectively, a layered vortex, barotropic wake, and hybrid regimes, and corroborate theoretical scalings of mixing in each case with a realistic regional ocean model. We then apply such scalings to a global seamount dataset and an ocean climatology to show that seamount-generated mixing makes a leading-order contribution to the global upwelling of deep waters. Our work thus brings seamounts to the fore of the deep-ocean mixing problem, and urges observational, theoretical and modeling efforts toward incorporating the seamounts' mixing effects in conceptual and numerical models of the ocean circulation

The contribution of coastal blue carbon ecosystems to climate change mitigation and adaptation

This briefing paper explores the potential for marine coastal ecosystems that store carbon, blue carbon ecosystems (BCEs), to help both limit climate change and adapt to the impacts of a changing climate. It also considers the range of benefits BCEs bring to coastal communities, and makes recommendations for policy approaches