On the generation of Maud Rise polynyas in the Weddell Sea

Abstract

The Weddell Sea is a region prone to the formation of open-ocean polynyas. Open-ocean polynyas are offshore openings of sea ice in the winter season, often accompanied by deep convection. Deep convection allows for the homogenization of the water column and the intense exchange of heat and gasses between the deep ocean and the atmosphere making them important for the forma- tion of deep waters. In the 1970s, large polynyas were observed in the central Weddell Sea, called Weddell Sea polynyas. The Weddell Sea polynya has not reoccurred since, sparking questions about their frequency and climate relevance. Instead smaller and short-lived polynyas have irregularly occurred in the vicinity of Maud Rise, a seamount in the eastern Weddell Sea, most recently in 2016 and 2017. The interaction of Maud Rise with the large-scale ocean circulation in the Weddell Sea, the Weddell Gyre, forms two prominent hydrographic features: a warm-water Halo and a Taylor Cap. The Taylor Cap sits above Maud Rise and the warm-water Halo encircles it. Our knowledge of the processes and dynamics in the region of Maud Rise is based on a scarce number of observations. Various processes for the generation of the Maud Rise polynya have been proposed, and the adequate representation of Maud Rise polynyas in ocean models remains a challenge. This thesis proposes to investigate the mechanisms leading to the generation of the 2016 and 2017 polynyas, and provides a modeling approach for the adequate representation of these events in ocean models.

In this thesis, publicly available observations and newly gained SO-CHIC observations from 2022 are used, as well as two reanalyses and two new regional ocean model configurations. This thesis revealed large interannual variability of the Halo and Taylor Cap with a period of a near-vanishing Taylor Cap in the years preceding the polynya opening. The variability is largely controlled remotely through the advection of anomalous water masses from the Weddell Gyre. The flow-topography interaction of the Weddell Gyre and Maud Rise enhances the chances of polynya opening by generating anomalies in the stratification. These anomalies are found to be related to the Taylor Cap and depend on the strength of the impinging flow. Further, the role of thermobaric effects on ambient stratification is important in triggering the 2016 polynya. The numerical simulation highlighted that the 2016 polynya preconditioned the region for the 2017 polynya. The 2017 polynya was enabled by an Ekman transport of salinity from the Taylor Cap to the Halo destratifying the region immediately north of the rise. In summary, this thesis highlights the complexity of processes at play in the most recent polynya events at Maud Rise and emphasizes the role of the ocean. Moreover, it shows the importance of improving the available convection parameterizations to improve deep convection and polynyas in ocean models.