The Antarctic Ice Sheet (AIS) is out of equilibrium with the current anthropogenic‐enhanced climate forcing. Paleo‐environmental records and ice sheet models reveal that the AIS has been tightly coupled to the climate system during the past, and indicate the potential for accelerated and sustained Antarctic ice mass loss into the future. Modern observations by contrast suggest that the AIS has only just started to respond to climate change in recent decades. The maximum projected sea level contribution from Antarctica to 2100 has increased significantly since the IPCC 5^th Assessment Report, although estimates continue to evolve with new observational and theoretical advances. This review brings together recent literature highlighting the progress made on the known processes and feedbacks that influence the stability of the AIS. Reducing the uncertainty in the magnitude and timing of the future sea‐level response to AIS change requires a multi‐disciplinary approach that integrates knowledge of the interactions between the ice sheet, solid Earth, atmosphere, and ocean systems, and across timescales of days to millennia. We start by reviewing the processes affecting AIS mass change, from atmospheric and oceanic processes acting on short timescales (days‐decades), through to ice processes acting on intermediate timescales (decades‐centuries) and the response to solid Earth interactions over longer timescales (decades‐millennia). We then review the evidence of AIS changes from the Pliocene to the present, and consider the projections of global sea‐level rise, and their consequences. We highlight priority research areas required to improve our understanding of the processes and feedbacks governing AIS change.

A holistic review is given of the Southern Ocean dynamic system, in the context of the crucial role it plays in the global climate and the profound changes it is experiencing. The review focuses on connections between different components of the Southern Ocean dynamic system, drawing together contemporary perspectives from different research communities, with the objective of closing loops in our understanding of the complex network of feedbacks in the overall system. The review is targeted at researchers in Southern Ocean physical science with the ambition of broadening their knowledge beyond their specific field, and aims at facilitating better‐informed interdisciplinary collaborations. For the purposes of this review, the Southern Ocean dynamic system is divided into four main components: large‐scale circulation; cryosphere; turbulence; and gravity waves. Overviews are given of the key dynamical phenomena for each component, before describing the linkages between the components. The reviews are complemented by an overview of observed Southern Ocean trends and future climate projections. Priority research areas are identified to close remaining loops in our understanding of the Southern Ocean system.

Topography influences the circulation in the Southern Ocean, generating stationary meanders in the lee of topographic features, triggering hot spots of mesoscale eddy kinetic energy, and modifying jets and fronts. However, the relationship between topography and submesoscale flows (with length scales of order 1–10 km) has not yet been explored. The first submesoscale‐resolving (1/80° resolution) ocean model, with realistic topography of the Indian sector of the Southern Ocean, is used to investigate this interaction. The results show spatial inhomogeneity in submesoscale activity that is correlated with topography. Topographic influence is primarily indirect: topography controls mesoscale flows, which in turn generate submesoscale activity. Mesoscale eddy kinetic energy and strain rate can be used, to first order, to infer submesoscale vertical velocity, implying a possible route to parameterise submesoscale activity in coarser resolution models.

Significance: Ocean-driven melting of ice shelves is a leading cause of mass loss from Antarctica. However, the small-scale ocean processes responsible for melting are poorly understood due to the difficulty of making direct measurements in these hard to reach environments. Here, we use a high-resolution ocean model to fill this knowledge gap. At low current speeds and relatively warm ocean temperatures, we show that a small-scale ocean process called double-diffusive convection controls ice shelf melt rates and turbulent mixing, forming a unique “staircase” structure beneath the ice. This process is currently missing from ocean–climate models, which consider only turbulent melting due to ocean currents.

Recent results have shown that convection forced by a temperature gradient along one horizontal boundary of a rectangular domain at a large Rayleigh number can be turbulent in parts of the flow field. However, the conditions for onset of turbulence, the dependence of flow and heat transport on Rayleigh number, and the roles of large and small scales in the flow, have not been established. We use three-dimensional direct numerical simulation (DNS) and large-eddy simulation (LES) over a wide range of Rayleigh numbers, $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}Ra\sim 10^8\mbox{--}10^{15}$ , for Prandtl number $Pr=5$ and a small aspect ratio, and show that a sequence of several stability transitions at $Ra \sim 10^{10}\mbox{--} 10^{11}$ defines a change from laminar to turbulent flow. The Prandtl number dependence too is examined at $Ra = 5.86 \times 10^{11}$ . At the smallest $Ra$ considered the thermal boundary layer is characterized by a balance of viscous stress and buoyancy, whereas inertia and buoyancy dominate in the large- $Ra$ regime. The change in the momentum balance is accompanied by turbulent enhancement of the overall heat transfer, although both laminar and turbulent regimes give $Nu\sim Ra^{1/5}$ . The results support both viscous and inviscid theoretical scaling models from previous work. The mechanical energy budget for an intermediate range of Rayleigh numbers above onset of instability ( $10^{10}<Ra<10^{13}$ ) reveals that the small scales of motion are produced predominantly by thermal convection, whereas at $Ra \ge 10^{14}$ shear instability of the large-scale flow begins to play a dominant role in sustaining the small-scale turbulence. Extrapolation to ocean conditions requires knowledge of the inertial regime identified here, but the simulations show that the corresponding asymptotic balance has not been fully realized by $Ra \sim 10^{15}$ .

We investigate the convection and dissolution rate generated when a wall of ice dissolves into seawater under Antarctic Ocean conditions. In direct numerical simulations three coupled interface equations are used to solve for interface temperature, salinity and ablation velocity, along with the boundary layer flow and transport. The main focus is on ambient water temperatures between $-1\,^{\circ }\text{C}$ and $6\,^{\circ }\text{C}$ and salinities around 35 ‰, where diffusion of salt to the ice–water interface depresses the freezing point and enhances heat diffusion to the ice. We show that fluxes of both heat and salt to the interface are significant in governing the dissolution of ice, and the ablation velocity agrees well with experiments and a recent theoretical prediction. The same turbulent flow dynamics and ablation rate are expected to apply at any depth in a deeper ocean water column (after choosing the relevant pressure coefficient for the liquidus temperature). At Grashof numbers currently accessible by direct numerical simulation, turbulence is generated both directly from buoyancy flux and from shear production in the buoyancy-driven boundary layer flow, whereas shear production by the convective flow is expected to be more important at geophysical scales. The momentum balance in the boundary layer is dominated by buoyancy forcing and wall stress, with the latter characterised by a large drag coefficient.