Upstream governors of the Warm Deep Water inflows towards the Filchner-Ronne Ice Shelf

Importance of the Weddell Sea for the global ocean circulation is irrefutable as it is a hotspot of dense water formation, a precursor of the Antarctic Bottom Water (AABW), which dominates the abyssal ocean ventilation. Furthermore, the region hosts the largest by volume floating ice shelf, Filchner-Ronne Ice Shelf (FRIS), which is vulnerable to episodic Warm Deep Water (WDW) inflows that induce basal melting at the ice shelf front and within its cavities. The resulting meltwater enters the system as freshwater forcing and alters local density gradients and water mass distribution, ultimately contributing to global sea level rise and potentially disrupting the global overturning circulation. Hence, knowledge about the seasonality and magnitude of the WDW inflows carries significance both locally and globally.

In 2017 and 2018, the anomalously warm inflows were observed yet there is still no agreement on the mechanism for their trigger. The on-shore propagation of WDW inherently depends on several factors, such as e.g circulation at the continental margins, the seasonal cycle of sea ice, the thermocline depth, position and magnitude of the Antarctic Slope Front (ASF). The intricate interplay between these factors yields a fairly complex system, which is difficult to unravel with the scarce available observations. Albeit the Weddell sea remains fairly undersampled up to this day, significant efforts are being undertaken to supplement data gaps with numerical modeling.  

The aim of my research is to shed light on seasonal and interannual variability of the coastal circulation upstream of the FRIS and its sensitivity to external forcings, using existing in-situ observations, reanalyses and the high-resolution eddy-permitting Finite-Element/Volume Sea Ice-Ocean Model (FESOM), in order to investigate the impact of wind and buoyancy forcing. Preliminary results indicate that the maximum thermocline depth exhibits a distinct seasonal cycle which is not consistent with sea ice formation, as previously speculated. Apparently, it reaches its deepest position in the Dronning Maud Land region in austral summer, then downstream at Kapp Norvegia in austral autumn and further downstream next to the FRIS – in austral winter, which rather implies generation and along-coast propagation of a large-scale baroclinic signal and further accentuates the importance of along-coast connectivity. Furthermore, the character of thermocline also needs to be considered. For instance, in 2017-2018, when the anomalous warm inflows were observed at the FRIS, thermocline thickness was increased rather than its depth, which raises the question of necessary and sufficient conditions for said inflows to occur.