ALBERT

Description: Sub-Antarctic Mode Waters (SAMW), forming in the deep winter mixed layers in the Sub-Antarctic Zone (SAZ) to the north of the Antarctic Circumpolar Current (ACC), connect the ocean thermocline with the atmosphere, contributing to ocean carbon and heat uptake and transporting high-latitude nutrients northward, to fuel primary production at low latitudes. The important climatic role of SAMW is controlled by the rate of fluid subduction from the deep winter mixed layers and the concentration of heat, carbon and nutrients at the end of winter. These concentrations depend on a range of processes, both physical (air-sea exchange, transport of Antarctic waters across the ACC, along ACC advection, eddy fluxes, diapycnal mixing, etc.) and biogeochemical (biological uptake, export and remineralisation), whose relative contributions are very poorly understood. With a Lagrangian particle-tracking experiment in a data-assimilative coupled physico-biogeochemical model of the Southern Ocean (B-SOSE), we assess the origin of the water masses reaching SAMW formation regions and the physico- and biogeochemical transformations occurring along their transport pathways. Our results underline the importance of the advection of subtropical waters along the ACC for the sequestration of heat and anthropogenic carbon and in modulating the fertilization of the low-latitude thermocline.

Description: It is thought that the primary mechanism driving the basal melting of Antarctic ice shelves is the intrusion of relatively warm circumpolar deep water (CDW) onto the continental shelf. However, the exact set of processes by which this happens – and the routes along which they occur - is often uncertain. The formation of dense shelf water (DSW) at the ocean-ice interface is a key factor in controlling the amount of heat that can cross the shelf break. Coarse resolution climate models struggle to explicitly resolve DSW due to its highly localised formation areas. We use an eddy-resolving formulation of MITgcm (SOHI), with 1/24〈sup〉th〈/sup〉 degree horizontal resolution, 225 vertical levels and realistic bathymetry and sub-glacial cavities. We investigate DSW formation and CDW intrusion near the shelf using an OMP inversion to characterise water masses. Shelf regimes are classified and areas of high mixing that are important for heat transfer to the shelf are identified. We characterise the seasonality of these processes, showing the pathways that enable CDW to reach the shelf in summer when the slope front is weaker. OMP analysis of the 1/6〈sup〉th〈/sup〉 degree Southern Ocean MITgcm model (SOSE) reveals that shelf water is more poorly represented at lower resolution. A lack of cavities and a coarser representation of bathymetry mean that SOSE also cannot simulate some of the topographically constrained pathways of CDW to the shelf. These results highlight the importance of model resolution for understanding and projecting future Antarctic melt rates.

Description: The deep oceans play a crucial role in regulating the Earth’s climate on long timescales by exchanging heat and chemical compounds with the atmosphere and moving them globally. However, our knowledge about deep circulation is in the initial stage. For instance, twenty years ago, in the Madagascar Basin, chlorofluorocarbons (CFCs) concentration in the Antarctic Bottom Water (AABW) layer was insignificant. But newer measurements in 2018 revealed an increased concentration of these chemical compounds, which counters our previous knowledge. A possible explanation is that deep currents, as we understood them, may have changed course and strength in the last twenty years. To begin solving this puzzle, we put together the 2019 DMB Experiment, a project funded by the US National Science Foundation and endorsed by the International Indian Ocean Expedition. The project aims to measure the deep currents in this region for the first time using shipboard instruments, RAFOS, and deep Argo floats. Combining the novel measurements with computer simulations, the study will identify the pathways that deep waters travel in the Madagascar Basin and examine what causes such circulation patterns. Due to COVID-19, our cruise has been on hold since 2020. After four years of waiting, we are sailing from South Africa to Mauritius in April-May this year. In this talk, we will present the first results of the fieldwork component of this project.

Description: The 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, driven by strong surface winds. Vertical mixing in the SO, induced by breaking waves excited by strong surface winds and interaction of tides, jets and eddies with rough topography, has been considered of secondary importance for the global meridional overturning circulation. Its importance for biogeochemical cycles has largely been assumed to be due to the role of mixing in changing the underlying dynamics on a centennial timescale. Using an eddy-resolving ocean model that assimilates an extensive array of observations, we show that altered mixing can cause biological productivity to be highly altered, with strong regional and seasonal variations in the sensitivity and response to enhanced mixing. This altered biological productivity could lead to alterations in the biological carbon pump over longer time scales. The high sensitivity of biological productivity shown over short time scales is due to high vertical gradients in nutrients and temperature found in the upper waters of the SO, and the sensitivity of the mixed layer depth to mixing. Enhanced mixing may be induced by the propagation of tidal waves from around the globe to the SO as well as the flux of wave energy from the deep SO to shallow depths. Such processes are unresolved in climate models, yet essential for the modelling of SO carbon cycles.