Observational evidence of along-shore exchange between Bransfield Strait and the central West Antarctic Peninsula

Abstract

Melting of Antarctic continental ice has a significant impact on rising global sea levels. Glacial retreat via basal melting is strongly modulated by ocean thermal forcing from shelf bottom waters. The temperature distribution of these shelf bottom waters helps categorize shelf regions around Antarctica as either warm or cold shelves. Considerable work has been undertaken in understanding the processes that set these independent thermal shelf regimes in isolation. However, less is known about along-shore exchange between warm and cold shelves around Antarctica. Using a novel mooring array deployed from December 2022 to December 2023 I investigate the exchange between the warm central West Antarctic Peninsula (WAP) shelf and the cold Bransfield Strait shelf. Using this data, I characterize the subtidal mean flow and its (seasonal) variability with depth through key pathways into Bransfield Strait, from the south through the Low-Hoseason Island Gap, and from offshore through Boyd Strait. In general, strong subtidal mean inflows in both the Low-Hoseason Island Gap and Boyd Strait are met with weak outflowing countercurrents, however there is a notable seasonality to these flow intensities. I supplement the subtidal mean flow characterization with a tidal analysis in which I find both significant semi-diurnal and diurnal tidal motions with small ellipticities often aligning with local bathymetry. I quantify exchange by calculating volume flux into Bransfield Strait and by conducting a water mass analysis at every mooring. Flux results through both Boyd Strait and the Low-Hoseason Island Gap show a reduction of flow into Bransfield Strait during winter. The weaker flows transporting water out of Bransfield Strait is more persistent, showing little seasonality. Significant seasonal variability is also evident in the results of the water mass analysis. Warm water masses characteristic of the central WAP dominate in southern Bransfield Strait in fall months with cool water masses characteristic of Bransfield Strait peaking in spring months. To understand the frequency and intensity of cold water intrusions in the central WAP I conduct an event-based analysis at a mooring off Anvers Island showing synoptic-scale deep cooling events consistent with the seasonal variability observed in Bransfield Strait. My results help confirm prior modeling work which had yet to be validated via observations and expands our understanding of circulation in this region and how it evolves seasonally and with depth. This work has implications for calculating future regional heat budgets at depth and by season. More accurate heat budgets derived from observations will help produce more accurate models that will increase our understanding of the magnitude and variability (seasonal or inter-annual) of glacial melt and hence sea level rise.