NATIONAL PHD COURSE IN EARTH OBSERVATION

Thesis title: Spatio-temporal variability of thermohaline properties and circulation of water masses along the Ross Ice Shelf

The Ross Ice Shelf (RIS) is Earth’s largest ice shelf. Located in the Ross Sea, Antarctica, it acts as a buttress to the Antarctic ice sheet, regulating the flow of inland glaciers. Its stability therefore exerts an indirect but crucial control on the potential contribution of Antarctic ice to global sea level rise. Although currently considered in a stable state, it may be subject to rapid disintegration. It is necessary to understand its long-term stability in a changing climate and its dynamics concerning interacting principal water masses, since its basal melting depends on the ocean circulation along the ice front and underneath the ice shelf. This study presents the most consistent and detailed multi‑decadal hydrographic time series along the Ross Ice Shelf (RIS), spanning from 1995 to 2025, in terms of both spatial and temporal resolution. By integrating CTD profiles, LADCP measurements, and recent year-round autonomous Argo float data, we provide a comprehensive view of the spatio-temporal evolution of key water masses and their circulation. Analysis of the CTD record along the RIS front revealed significant long-term changes in High Salinity Shelf Water (HSSW), a precursor to Antarctic Bottom Water which contributes to the global thermohaline circulation. HSSW freshened between 1995 and 2011, followed by a salinity rebound that persisted through 2025. Ice Shelf Water (ISW), the coldest water mass in the world, which forms as HSSW interacts with the base of the ice shelf, mirrored these trends, indicating that changes in HSSW characteristics and formation directly influence the ISW properties beneath the RIS. We quantified interannual variability of Ocean Heat Content and summer-warmed, freshened Antarctic Surface Water (AASW) layer. Peak Ocean Heat Content values in 2012, linked to elevated surface temperatures and greater surface layer thickness, indicated that AASW may have delivered up to 4.7 × 10¹⁹ J of heat toward the ice shelf base, highlighting its potential role in modulating basal melting. Moreover, an Optimum Multi Parameter mixing analysis was applied to the hydrographic data in order to understand the contribution of key water masses to the composition of the water column along the RIS, and how these contributions changed over time. LADCP data from six summer campaigns between 2006 and 2025 enabled estimates of cross-shelf heat fluxes and revealed also pathways of water mass transport towards the RIS cavity. Water mass circulation analysis confirmed known features, such as the westward flow of AASW, the ISW outflow in the central and eastern RIS sector, and the southward intrusion of HSSW near Ross Island. However, they also unveiled a greater complexity, including intermittent inflow-outflow behaviour, pointing to a more dynamic circulation regime along the RIS than previously assumed. Finally, observations from 13 Argo floats, deployed in key areas along the RIS front between 2020 to 2024, bridged a critical wintertime observational gap. This unique dataset directly captured the full year-round water column evolution, and particularly the HSSW formation in the RIS polynya during winter, a process previously only inferred. An observed AASW intrusion event beneath the RIS in 2021 allowed for the quantification of heat advected into the sub-ice shelf cavity (~1.8 × 10¹⁹ J) and the estimation of a consequent localized basal melt rate of ~0.65 m yr⁻¹. A warm layer of modified Circumpolar Deep Water (mCDW), characterized by its oceanic origin and elevated temperature and salinity, was observed during winter in the eastern Ross Sea in 2024, opening critical questions about its inflow pathways and its potential to drive basal melting if in direct contact with the RIS base and/or grounding line, challenging our current understanding of RIS stability. A focused analysis of the RIS polynya provided quantitative estimates of HSSW production rate for 2021 and 2022 (~0.22 and 0.43 Sv, respectively), derived from weekly Argo float salinity profiles and satellite-derived polynya extent. This production was found to be spatially constrained, with only specific sectors of the RIS polynya actively contributing to dense water production. Furthermore, the analysis directly linked polynya expansion to a specific wind regime, revealing that frequent, rather than strong, synoptic wind events are the primary driver of its efficiency and consequent dense water formation. These observations enhance our understanding of the variability in RIS water mass properties and provide a starting point for understanding the complex processes that govern RIS stability in a changing climate.