Description: The Antarctic Slope Current (ASC) and Antarctic Coastal Current advect heat, freshwater, nutrients, and biological organisms westward around the Antarctic margin, providing a connective link between different sectors of the continental shelf. However, the timescales over which these currents transport water along the shelf, and the strength and pathways of connectivity around the continent, remain poorly understood. We use daily velocity fields from a global high-resolution ocean-sea ice model, combined with Lagrangian particle tracking, to provide a baseline estimate of advection timescales and improve our understanding of circumpolar connectivity around Antarctica. Analysis of particle trajectory experiments shows that advection around the continent is typically rapid with peak transit times of 1–5 years for particles to travel 90° of longitude downstream. The ASC plays a key role in driving connectivity in East Antarctica and the Weddell Sea, while the Coastal Current controls connectivity in West Antarctica, the eastern Antarctic Peninsula, and along the continental shelf east of Prydz Bay. Crucially, the West Antarctic sector, which has experienced rapid melting, has widespread connectivity with all regions of the Antarctic shelf. These findings assist us in understanding the locations and timescales over which anomalies, such as meltwater from the Antarctic Ice Sheet, can be redistributed downstream.

Description: The global ocean plays a major role in moderating atmospheric temperature rise, thereby buffering climate change. Amongst the various oceanic regions undergoing warming, the Southern Ocean is a primary heat sink in the climate system. Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) are the dominant water masses in the upper Southern Ocean, and play a fundamental role in ocean ventilation and the uptake of heat and carbon into the ocean interior. This talk will first focus on understanding the geographic variability in the formation of SAMW and AAIW in the Southern Ocean based on a volume budget analysis, as well as the advection of heat and freshwater by SAMW and AAIW along the Antarctic Circumpolar Current (ACC), using observationally based hydrographic and eddy diffusivity datasets. Our results suggest that the distribution of SAMW and AAIW is set by their formation due to subduction and mesoscale and small-scale turbulent mixing, which shows strong regional variability with hotspots of large subduction and water-mass transformation. Their circulation eastward along the ACC transports temperature and salinity anomalies and preconditions the mixed-layer formation further downstream in the ACC. To better understand how and where the anthropogenic heat is stored in the world ocean, we further analyzed the warming of a set of regional mode and intermediate waters over the subtropical oceans and in the Southern Ocean. Warming of these mode and intermediate waters explains nearly half net global ocean warming during the Argo era, despite occupying just 24% of the total ocean volume.

Description: A fundamental divide exists between previous studies which conclude that polar amplification does not occur without sea ice and studies which find that polar amplification is an inherent feature of the climate system independent of sea ice. We hypothesise that a representation of climatological ocean heat transport is key for simulating polar amplification in ice-free climates. To investigate this we run a suite of targeted experiments in the slab ocean aquaplanet configuration of CESM2-CAM6 with different profiles of prescribed q-fluxes. In simulations without climatological ocean heat transport, polar amplification does not occur. In contrast, in simulations with climatological ocean heat transport, robust polar amplification occurs in all seasons. What is causing this dependence of polar amplification on ocean heat transport? Energy-balance model theory is incapable of explaining our results and in fact would predict that introducing ocean heat transport leads to less polar amplification. We instead demonstrate that shortwave cloud radiative feedbacks can explain the divergent polar climate responses simulated by CESM2-CAM6. Targeted cloud locking experiments produce robust polar amplification in the zero ocean heat transport simulations solely by prescribing high latitude cloud radiative feedbacks from the simulations with realistic climatological ocean heat transport. We conclude that polar amplification is an inherent response of the atmosphere-ocean system with an important role for cloud radiative feedbacks. In addition to reconciling previous disparities, these results have important implications for interpreting past equable climates and climate projections under high emissions scenarios.

Description: Between 2014 and 2017, the Antarctic experienced a drastic loss of annual-mean sea ice extent (SIE) of 2 million square kilometers. This precipitous decline in SIE was a five-sigma event compared to previously observed variability since 1979. A number of studies have suggested a range of potential causes of this rare event including fluctuations in the Southern Annular Mode and teleconnections to Tropical Pacific variability. It is imperative to understand the underlying drivers of Antarctic SIE interannual variability because it is unclear whether the recent decline in SIE is due to internal variability or is instead the belated emergence of the global warming signal. Here we explore the fundamental processes contributing to the interannual variability of Antarctic SIE in a state of the art global climate model with idealized boundary conditions. We employ CESM2-CAM6 coupled to a slab ocean in a hierarchy of idealised configurations to systematically quantify the role of seasonal insolation, ice dynamics, ice thermodynamics, and the shape of the Antarctic continent. We retain as much of the simplicity of an aquaplanet setup as possible by employing a double Antarctica configuration. Our model can broadly capture the characteristics of modern day Antarctic sea ice cover. In particular we investigate the potential of each mechanism for driving extreme levels of interannual variability as was seen in 2014-2017. As such, each simulation was integrated for over 100 years. Our approach adds physical understanding of the drivers of Antarctic SIE variability absent the longer timescale interaction with oceanic dynamics.

Description: The abyssal ocean circulation is a key component of the global meridional overturning circulation, cycling heat, carbon, oxygen and nutrients throughout the world ocean. The strongest historical trend observed in the abyssal ocean is warming at high southern latitudes, yet it is unclear what processes have driven this warming, and whether it is linked to a slowdown in the ocean's overturning circulation. Furthermore, future change in the abyssal overturning remains uncertain, with the latest CMIP6 projections not accounting for dynamic ice-sheet melt. In this talk I will present new transient forced high-resolution coupled ocean – sea-ice model simulations to show that under a high emissions scenario, abyssal warming is set to accelerate over the next 30 years. We find that meltwater input around Antarctica drives a contraction of Antarctic Bottom Water (AABW), opening a pathway that allows warm Circumpolar Deep Water greater access to the continental shelf. The reduction in AABW formation results in warming and ageing of the abyssal ocean, consistent with recent measurements. In contrast, projected wind and thermal forcing has little impact on the properties, age, and volume of AABW. These results highlight the critical importance of Antarctic meltwater in setting the abyssal ocean overturning, with implications for global ocean biogeochemistry and climate that could last for centuries.

Description: Antarctic Bottom Water (AABW) is a major component of the global overturning circulation, originating around the Antarctic continental margin. In recent decades AABW has both warmed and freshened, but there is also evidence of large interannual variability. The causes of this underlying variability are not yet fully understood, in part due to a lack of ocean and air-sea-ice flux measurements in the region. Here, we simulate the formation and export of AABW from 1958 to 2018 using a global, eddying ocean–sea-ice model in which the four AABW formation regions and transports agree reasonably well with observations. The simulated formation and export of AABW exhibits strong interannual variability which is not correlated between the different formation regions. Reservoirs of very dense waters at depth in the Weddell and Ross Seas following 1-2 years of strong surface water mass transformation can lead to higher AABW export for up to a decade. In Prydz Bay and at the Adélie Coast in contrast, dense water reservoirs do not persist beyond 1 year which we attribute to the narrower shelf extent in the East Antarctic AABW formation regions. The main factor controlling years of high AABW formation are weaker easterly winds, which reduce sea ice import into the AABW formation region, leaving increased areas of open water primed for air-sea buoyancy loss and convective overturning. Our study highlights the variability of simulated AABW formation in all four formation regions, with potential implications for interpreting trends in observational data using only limited duration and coverage.

Description: Comprehensive climate model simulations with perturbed sea ice covers have been extensively used to assess the impact of future sea ice loss, suggesting substantial climate changes both in the high latitudes and beyond. However, previous work using an idealized energy balance model calls into question the methodologies that are used to perturb sea ice cover, demonstrating a consistent overestimate of the warming due to sea ice loss, while the large complexity gap between the idealized and comprehensive models makes the implications of this result unclear. To bridge this gap we have performed simulations with a new implementation of the CESM2 model in a slab-ocean aquaplanet configuration coupled with thermodynamic sea ice, which is able to capture the realistic seasonal characteristics of polar climate change. Using this model setup, we perform a suite of experiments to systematically quantify the spurious climate responses associated with melting sea ice without a CO2 forcing. We find that using the sea ice ghost flux method overestimates many aspects of the climate response by ~25%, including the polar warming, the mini global warming signal and the increase in equatorial precipitation. In contrast, the midlatitude circulation response may be underestimated, due to the location of the latitudinal band of heating applied to melt the sea ice relative to the midlatitude jet. This work advances our ability to isolate the true climate response to sea ice loss, and provides a framework for conducting coupled sea ice loss simulations absent the spurious impacts from the addition of artificial heating.

Description: Southern Ocean surface freshening has been observed in recent decades and is projected to continue over the 21st Century. Surface freshening due to precipitation and sea ice changes are represented in coupled climate models, however Antarctic ice sheet/shelf meltwater contributions are not. As Antarctic melting is projected to accelerate over the 21st Century this constitutes a fundamental shortcoming in present-day projections of high-latitude climate. Southern Ocean surface freshening has been shown to cause a surface cooling by reducing both ocean convection and the entrainment of warm subsurface waters to the surface. Over the 21st Century Antarctic meltwater is expected to alter the pattern of projected surface warming as well as having other climatic effects. However, there remains considerable uncertainty in projected Antarctic meltwater amounts, and previous findings could be model-dependent. Here, we use the ACCESS-ESM1.5 coupled model to investigate global climate responses to low and high Antarctic meltwater additions over the 21st Century under a high-emissions climate scenario. Our high meltwater simulations produce anomalous surface cooling, increased Antarctic sea ice, subsurface warming and hemispheric differences in precipitation. Our low meltwater simulations suggest the magnitude of surface temperature and Antarctic sea ice responses are strongly dependent on the applied meltwater amount. These findings highlight the importance of constraining Antarctic melt projections to better project global surface climate changes over the 21st Century. Our work also motivates the Southern Ocean freshwater release model experiments initiative (SOFIA), a standardised meltwater intercomparison designed to improve our understanding of Antarctic meltwater impacts on climate.

Description: Ocean heat transport towards Antarctica directly drives the melting of Antarctic ice shelves, modulating sea level rise and the formation of Antarctic Bottom Water. A common dynamical assumption is that heat transport across the Antarctic continental slope is modulated by the strength of the Antarctic Slope Current (ASC), which is thought to act as a barrier to cross-slope heat transport. However, observations of the ASC are too scarce to investigate its relationship to poleward heat transport across large circumpolar spatial scales, or over long temporal scales. Also, until recently, ocean models lacked the spatial resolution required to accurately represent the ASC or the eddy heat transport onto the Antarctic shelf. In this study, we analyze the relationship between the ASC and the cross-slope heat transport in a circumpolar, eddy-rich ocean and sea ice simulation. We find that the local strength of the time-mean ASC is not a good predictor of local cross-slope heat transport, i.e., spatial variability in the ASC is not related to spatial variability in poleward heat transport. However, there is a relationship between ASC strength and cross-slope heat transport in the temporal domain. We quantify the strength of the relationship across different time scales (sub-seasonal, seasonal and interannual) and across varying model resolution (from 1/10º to 1/20º to 1/40º).

Description: The Weddell Gyre is one of the largest features of the ocean circulation adjacent to the Antarctic margins. The gyre is a dynamically complex region and participates in several processes relevant to global climate. For example, the gyre’s circulation and its strength have been linked to changes in the properties and rates of export of Antarctic Bottom Water into the global abyssal ocean. However, the dynamic controls of the Weddell Gyre’s variability are largely unknown, possibly due to the complexities of the region: the interplay of the Weddell Gyre with an overturning circulation, strong buoyancy fluxes associated with sea ice formation and melt, and open and permeable boundaries which allow for significant inflows and outflows. In this work we analyse the mechanisms controlling the Weddell Gyre’s variability using a barotropic vorticity budget of a MOM6 simulation coupled with SIS2 and forced with a repeat year 1990-91 atmospheric state derived from JRA55-do. Unlike past studies that focus on the stationary state of a control simulation, we focus on the evolution of our simulation and the response to different wind and buoyancy perturbations. We find that a balance is achieved between surface stress and bottom pressure torque, bottom drag curl and the curl of horizontal viscosity.