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Sinking of dense water from Antarctic coastal polynyas produces Antarctic Bottom Water (AABW), which is the densest water in the global overturning circulation and is a key player in climate change as a significant sink for heat and carbon dioxide. Very recent studies have suggested that landfast sea ice (fast ice) plays an important role in the formation and variability of the polynyas and possibly AABW. However, they have been limited to regional and case investigations only. This study provides the first coincident circumpolar mapping of Antarctic coastal polynyas and fast ice. The map reveals that most of the polynyas are formed on the western side of fast ice, indicating an important role of fast ice in the polynya formation. Winds diverging from a boundary comprising both coastline and fast ice are the primary determinant of polynya formation. The blocking effect of fast ice on westward sea ice advection by the coastal current would be another key factor. These effects on the variability in sea ice production for 13 major polynyas are evaluated quantitatively. Furthermore, it is demonstrated that a drastic change in fast ice extent, which is particularly vulnerable to climate change, causes dramatic changes in the polynyas and possibly AABW formation that can potentially contribute to further climate change. These results suggest that fast ice and precise polynya processes should be addressed by next-generation models to produce more accurate climate projections. This study provides the boundary and validation data of fast ice and sea ice production for such models.

Midocean ridge fracture zones channel bottom waters in the eastern Brazil Basin in regions of intensified deep mixing. The mechanisms responsible for the deep turbulent mixing inside the numerous midocean fracture zones, whether affected by the local or the nonlocal canyon topography, are still subject to debate. To discriminate those mechanisms and to discern the canyon mean flow, two moorings sampled a deep canyon over and away from a sill/contraction. A 2-layer exchange flow, accelerated at the sill, transports 0.04–0.10-Sv (1 Sv ≡ 10 6 m 3 s −1 ) up canyon in the deep layer. At the sill, the dissipation rate of turbulent kinetic energy ε increases as measured from microstructure profilers and as inferred from a parameterization of vertical kinetic energy. Cross-sill density and microstructure transects reveal an overflow potentially hydraulically controlled and modulated by fortnightly tides. During spring to neap tides, ε varies from O (10 −9 ) to O (10 −10 ) W kg −1 below 3500 m around the 2-layer interface. The detection of temperature overturns during tidal flow reversal, which almost fully opposes the deep up-canyon mean flow, confirms the canyon middepth enhancement of ε . The internal tide energy flux, particularly enhanced at the sill, compares with the lower-layer energy loss across the sill. Throughout the canyon away from the sill, near-inertial waves with downward-propagating energy dominate the internal wave field. The present study underlines the intricate pattern of the deep turbulent mixing affected by the mean flow, internal tides, and near-inertial waves.

A statistically significant reduction in Antarctic Bottom Water (AABW) volume is quantified between the 1980s and 2000s within the Southern Ocean and along the bottom-most, southern branches of the meridional overturning circulation (MOC). AABW has warmed globally during that time, contributing roughly 10% of the recent total ocean heat uptake. This warming implies a global-scale contraction of AABW. Rates of change in AABW-related circulation are estimated in most of the world’s deep-ocean basins by finding average rates of volume loss or gain below cold, deep potential temperature (θ) surfaces using all available repeated hydrographic sections. The Southern Ocean is losing water belowθ= 0°C at a rate of −8.2 (±2.6) × 10⁶ m³ s−1. This bottom water contraction causes a descent of potential isotherms throughout much of the water column until a near-surface recovery, apparently through a southward surge of Circumpolar Deep Water from the north. To the north, smaller losses of bottom waters are seen along three of the four main northward outflow routes of AABW. Volume and heat budgets below deep, coldθsurfaces within the Brazil and Pacific basins are not in steady state. The observed changes in volume and heat of the coldest waters within these basins could be accounted for by small decreases to the volume transport or small increases toθof their inflows, or fractional increases in deep mixing. The budget calculations and global contraction pattern are consistent with a global-scale slowdown of the bottom, southern limb of the MOC.

Zonal momentum input into the Antarctic Circumpolar Current (ACC) by westerly winds is ultimately removed via topographic form stress induced by large bathymetric features that obstruct the path of the current. These bathymetric features also support the export of Antarctic Bottom Water (AABW) across the ACC via deep, geostrophically balanced, northward flows. These deep geostrophic currents modify the topographic form stress, implying that changes in AABW export will alter the ocean bottom pressure and require a rearrangement of the ACC in order to preserve its zonal momentum balance. A conceptual model of the ACC momentum balance is used to derive a relationship between the volume export of AABW and the shape of the sea surface across the ACC’s standing meanders. This prediction is tested using an idealized eddy-resolving ACC/Antarctic shelf channel model that includes both the upper and lower cells of the Southern Ocean meridional overturning circulation, using two different topographic configurations to obstruct the flow of the ACC. Eliminating AABW production leads to a shallowing of the sea surface elevation within the standing meander. To quantify this response, the authors introduce the “surface-induced topographic form stress,” the topographic form stress that would result from the shape of the sea surface if the ocean were barotropic. Eliminating AABW production also reduces the magnitude of the eddy kinetic energy generated downstream of the meander and the surface speed of the ACC within the meander. These findings raise the possibility that ongoing changes in AABW export may be detectable via satellite altimetry.

Internal waves close to the seafloor of abyssal oceans are the key energy suppliers driving near-bottom mixing and the upwelling branches of meridional overturning circulation, but their spatiotemporal variability and intrinsic mechanisms remain largely unclear. In this study, measurements from 10 long-term moorings were used to investigate the internal wave activities in the abyssal South China Sea, which is an important upwelling zone. Strong near-inertial internal waves (NIWs) with current velocity pulses exceeding 5 cm s −1 were observed to dominate the near-bottom internal wave field at approximately 14°N. These abyssal NIWs were phase-coupled with diurnal internal tides (D 1 ), and both displayed common seasonal variations that were larger in winter and summer, providing evidence of diurnal parametric subharmonic instability (PSI) near its critical latitudes (CLs). Emitted from the bottom, near-inertial kinetic energy rapidly decreased by one order of magnitude from depths of ∼120 to ∼620 m above the bottom. Near rough topographies, the abyssal PSI was shifted poleward to approximately 14.8°N by negative relative vorticities of passing anticyclonic eddies or topographic Rossby waves. Compared with flat topography, PSI near rough topography was significantly promoted by topographic-localized strong D 1 with high-mode structures, creating abyssal NIW bursts. Bottom-reaching shipboard conductivity–temperature–depth profiles revealed that the bottom mixed layers became much thicker when approaching CLs, suggesting that abyssal PSI potentially accelerates the ventilation and upwelling of bottom water. The observational results presented here illustrate notable spatiotemporal variations in abyssal NIWs regulated by PSI and call for consideration of PSI to better understand near-bottom mixing and upwelling.

Small-scale mixing drives the diabatic upwelling that closes the abyssal ocean overturning circulation. Indirect microstructure measurements of in-situ turbulence suggest that mixing is bottom-enhanced over rough topography, implying downwelling in the interior and stronger upwelling in a sloping bottom boundary layer. Tracer Release Experiments (TREs), in which inert tracers are purposefully released and their dispersion is surveyed over time, have been used to independently infer turbulent diffusivities—but typically provide estimates in excess of microstructure ones. In an attempt to reconcile these differences, Ruan and Ferrari (2021) derived exact tracer-weighted buoyancy moment diagnostics, which we here apply to quasi-realistic simulations. A tracer’s diapycnal displacement rate is exactly twice the tracer-averaged buoyancy velocity, itself a convolution of an asymmetric upwelling/downwelling dipole. The tracer’s diapycnal spreading rate, however, involves both the expected positive contribution from the tracer-averaged in-situ diffusion as well as an additional non-linear diapycnal distortion term, which is caused by correlations between buoyancy and the buoyancy velocity, and can be of either sign. Distortion is generally positive (stretching) due to bottom-enhanced mixing in the stratified interior but negative (contraction) near the bottom. Our simulations suggest that these two effects coincidentally cancel for the Brazil Basin Tracer Release Experiment, resulting in negligible net distortion. By contrast, near-bottom tracers experience leading-order distortion that varies in time. Errors in tracer moments due to realistically sparse sampling are generally small (< 20%), especially compared to the O (1) structural errors due to the omission of distortion effects in inverse models. These results suggest that TREs, although indispensable, should not be treated as “unambiguous” constraints on diapycnal mixing.

Along various stretches of the Antarctic margins, dense Antarctic Bottom Water (AABW) escapes its formation sites and descends the continental slope. This export necessarily raises the isopycnals associated with lighter density classes over the continental slope, resulting in density surfaces that connect the near-freezing waters of the continental shelf to the much warmer circumpolar deep water (CDW) at middepth offshore. In this article, an eddy-resolving process model is used to explore the possibility that AABW export enhances shoreward heat transport by creating a pathway for CDW to access the continental shelf without doing any work against buoyancy forces. In the absence of a net alongshore pressure gradient, the shoreward CDW transport is effected entirely by mesoscale and submesoscale eddy transfer. Eddies are generated partly by instabilities at the pycnocline, sourcing their energy from the alongshore wind stress, but primarily by instabilities at the CDW–AABW interface, sourcing their energy from buoyancy loss on the continental shelf. This combination of processes induces a vertical convergence of eddy kinetic energy and alongshore momentum into the middepth CDW layer, sustaining a local maximum in the eddy kinetic energy over the slope and balancing the Coriolis force associated with the shoreward CDW transport. The resulting slope turbulence self-organizes into a series of alternating along-slope jets with strongly asymmetrical contributions to the slope energy and momentum budgets. Cross-shore variations in the potential vorticity gradient cause the jets to drift continuously offshore, suggesting that fronts observed in regions of AABW down-slope flow may in fact be transient features.

The Strait of Georgia is a large and deep fjordlike basin on the northeastern Pacific coast whose bottom waters are dramatically renewed by a series of intermittent gravity currents in summer. Here, we analyze a dataset that includes moored observations from 2008 to 2021 and shipborne measurements from a 2018 field program to describe the vertical and cross-channel structure of these gravity currents. We show that the timing of these currents for more than a decade is well predicted by proxy measurements for both tidal mixing strength in the Haro Strait/Boundary Pass region and coastal upwelling on the west coast of Vancouver Island. Renewals occur as an ∼30-m-thick turbid layer extending along the right-hand slope of a broad V-shaped valley that forms the southern end of the strait. Currents are primarily along-isobath at speeds of up to 20 cm s −1 with a small downhill component. A diagnostic analytical model with a depth-dependent eddy viscosity is fitted to the observations and confirms a clockwise rotation of current vectors with height, partly driven by boundary layer dynamics over a scale of a few meters and partly driven by Coriolis forces in the near-bottom linear density gradient. Bottom drag and (small) entrainment parameters are similar to those found in other oceanic situations, and the current is “laminar” with respect to large-scale instabilities (with Froude number ≈1 and Ekman number ≈0.01), although subject to turbulence at small scales (Reynolds number of ∼10 6 ). The predictability and reliability of this accessible rotationally modified gravity current suggests that it is an ideal geophysical laboratory for future studies of such features.

The meridional overturning circulation (MOC) is composed of interconnected overturning cells that transport cold dense abyssal waters formed at high latitudes back to the surface. Turbulent diapycnal mixing plays a primary role in setting the rate and patterns of the various overturning cells that constitute the MOC. The focus of the analyses in this paper will be on the influence of sharp vertical variations in mixing on the MOC and ocean stratification. Mixing is enhanced close to the ocean bottom topography where internal waves generated by the interaction of tides and geostrophic motions with topography break. It is shown that the sharp vertical variations in mixing lead to the formation of three layers with different dynamical balances governing meridional flow. Specifically, an abyssal bottom boundary layer forms above the ocean floor where mixing is largest and hosts the northward transport of the heaviest waters from the southern channel to the closed basins. A deep layer forms above the bottom layer in which the upwelled waters return south. A third adiabatic layer lies above the other two. While the adiabatic layer has been studied in detail in recent years, the deep and bottom layers are less appreciated. It is shown that the bottom layer, which is not resolved or allowed for in most idealized models, must be present to satisfy the no flux boundary condition at the ocean floor and that its thickness is set by the vertical profile of mixing. The deep layer spans a considerable depth range of the ocean within which the stratification scale is set by mixing, in line with the classic view of Munk in 1966.

The abyssal overturning circulation is thought to be primarily driven by small-scale turbulent mixing. Diagnosed watermass transformations are dominated by rough topography “hotspots”, where the bottom-enhancement of mixing causes the diffusive buoyancy flux to diverge, driving widespread downwelling in the interior—only to be overwhelmed by an even stronger up-welling in a thin Bottom Boundary Layer (BBL). These watermass transformations are significantly underestimated by one-dimensional (1D) sloping boundary layer solutions, suggesting the importance of three-dimensional physics. Here, we use a hierarchy of models to generalize this 1D boundary layer approach to three-dimensional eddying flows over realistically rough topography. When applied to the Mid-Atlantic Ridge in the Brazil Basin, the idealized simulation results are roughly consistent with available observations. Integral buoyancy budgets isolate the physical processes that contribute to realistically strong BBL upwelling. The downwards diffusion of buoyancy is primarily balanced by upwelling along the sloping canyon sidewalls and the surrounding abyssal hills. These flows are strengthened by the restratifying effects of submesoscale baroclinic eddies and by the blocking of along-ridge thermal wind within the canyon. Major topographic sills block along-thalweg flows from restratifying the canyon trough, resulting in the continual erosion of the trough’s stratification. We propose simple modifications to the 1D boundary layer model which approximate each of these three-dimensional effects. These results provide local dynamical insights into mixing-driven abyssal overturning, but a complete theory will also require the non-local coupling to the basin-scale circulation.