Platelet ice, the Southern Ocean’s hidden ice: a review

Basal melt of ice shelves is not only an important part of Antarctica’s ice-sheet mass budget, but it is also the origin of one of the most peculiar types of sea ice found in the polar oceans: platelet ice. In many regions around coastal Antarctica, tiny ice crystals form and grow in supercooled plumes of Ice Shelf Water, releasing heat into the surrounding ocean. They usually rise towards the surface, eventually becoming trapped under an ice shelf as marine ice. Frequently, masses of those crystals are advected out of the ice-shelf cavity, and accumulate below a solid sea-ice cover to form a semiconsolidated layer. When the overlying sea ice grows into this so-called sub-ice platelet layer, the loose crystals are consolidated, adding additional thickness to the sea ice. These phenomena are generally referred to as platelet ice, although confusion about the terminology is widespread in the literature. The presence of platelet ice has a profound impact on sea-ice properties and processes in several regions of Antarctica, with numerous implications for the local polar marine biosphere. Most notably, sub-ice platelet layers provide a stable, sheltered, nutrient- and food-rich habitat which usually results in a highly productive and uniquely adapted ecosystem. It has also been hypothesised that platelet ice may be an indicator of the state of an ice shelf, although comprehensive time series are limited to the Ross Sea. This paper clears up the terminology by providing exact definitions of the relevant terms.We review platelet-ice formation, observational methods as well as geographical and seasonal occurrence. The physical properties and ecological implications are merged in a way understandable for physicists and biologists alike, to lay the foundation for the interdisciplinary research that is necessary to tackle the current knowledge gaps

Drivers and Dynamics of the Ross Sea

The seasonal control mechanisms of the circulation in the Ross Sea are studied here by utilising a climatological three-dimensional numerical model based on the Regional Ocean Model System (ROMS). Of interest are the physical processes that facilitate exchange across the continental shelf break and the ice shelf front, as these act as effective barriers between different oceanographic regimes in the Ross Sea. In this thesis details of the system of cyclonic and anticyclonic flow structures and their transport variability are described. The analysis identifies external and internal forcing mechanisms, their region of influence and corresponding seasonal evolution. This study determines the temporal scales on which the seasonal signal propagates throughout the ice shelf cavity. Simulations show significant annual variation in volume fluxes of 2-10 Sv in the Antarctic Slope Current (ASC). Away from the coast the ASC’s transport is dominated by zonal differences in sea surface height (SSH) with wind induced transport playing only a minor role. SSH is generally higher over the continental shelf and its variability is not correlated with the annual sea ice cycle. While the isopycnal structure of the spatially-coincident Antarctic Slope Front is found to modulate current speeds within the ASC it has no significant effect on the transport magnitude. Over the continental shelf a system of three anticyclonic and one cyclonic circulation cells have been identified that facilitate the dvection in the interior, including the ice shelf cavity. Individual currents carry up to 2 Sv seasonally. Constrained by the banks and depressions, the cells are spatially persistent but experience individually different temporal changes. Horizontal differences in density and subsequent baroclinic pressure gradients are found to be the main control of their dynamics. Two competing mechanisms are found in the model that reinforce the salinity dominated density gradients. Circumpolar Deep Water (CDW), sourced from the ASC, sits next to northward-flowing High Salinity Shelf Water (HSSW) and other dense Shelf Water. CDW resupply events seem triggered by a zonal shift of the ASC on the order of ∼10 km that occurs at different times along the shelf break. The second seasonal process to strengthen density gradients is HSSW production through intense winter sea ice formation in the polynyas of the south-western Ross Sea. The HSSW formation is synchronised with the atmosphere. The polynya initiates two additional advection drivers. One is the gravity-driven bottom flows that establish the western band of the large anticyclonic circulation cell that ventilates the western cavity up to 175◦ W. Main inflows are located east and west of Ross Island. Approximately 74 % of the 0.87 Sv HSSW exported annually from the Ross Sea Polynya (RSP) and the McMurdo Sound Polynya (MMSP) are advected into remote parts of the cavity, and 10 % exits the shelf along the western coast. No significant HSSW quantities from the Terra Nova Bay Polynya (TNBP) intrude southward underneath the RIS, instead HSSW formed here drains north. The second advection driver, which has not been described previously, is caused by the significant depression of SSH, and is localized to the deep convection cell, where the entire water column is saturated with high density HSSW. Regional SSH gradients are large enough that the associated barotropic pressure gradient overwrites the counteracting baroclinic pressure gradient. This process drives an additional cyclonic circulation cell in the region of the RSP during winter. SSH gradients are also responsible for driving the Victoria Land Coastal Current, where the meridional density gradient is strengthened by abundant Ice Shelf Water (ISW) supplied from beneath the McMurdo Ice Shelf. Oceanographic conditions were also derived for heat intrusion under the Ross Ice Shelf at intermediate depth over the eastern continental shelf. Conditions that allow a change in the relevant potential vorticity of flows are a function of the local isopycnal structure and SSH with warm Circumpolar Deep Water (CDW) over Hayes Bank adjacent to Low Salinity Shelf Water (LSSW)

Variability in high-salinity shelf water production in the Terra Nova Bay polynya, Antarctica

Terra Nova Bay in Antarctica is a formation region for high-salinity shelf water (HSSW), which is a major source of Antarctic Bottom Water. Here, we analyze spatiotemporal salinity variability in Terra Nova Bay with implications for the local HSSW production. The salinity variations in the Drygalski Basin and eastern Terra Nova Bay near Crary Bank in the Ross Sea were investigated by analyzing hydrographic data from instrumented moorings, vessel-based profiles, and available wind and sea-ice products. Near-bed salinity in the eastern Terra Nova Bay (similar to 660 m) and Drygalski Basin (similar to 1200 m) increases each year beginning in September. Significant salinity increases (> 0.04) were observed in 2016 and 2017, which is likely related to active HSSW formation. According to velocity data at identical depths, the salinity increase from September was primarily due to advection of the HSSW originating from the coastal region of the Nansen Ice Shelf. In addition, we show that HSSW can also be formed locally in the upper water column (< 300 m) of the eastern Terra Nova Bay through convection supplied by brine from the surface, which is related to polynya development via winds and ice freezing. While the general consensus is that the salinity of the HSSW was decreasing from 1995 to the late 2000s in the region, the salinity has been increasing since 2016. In 2018, it returned to values comparable to those in the early 2000s.Y