Observations of the ocean beneath Fimbul Ice Shelf (Deliverable D2.4)

Ocean Cryosphere Exchanges in Antarctica: Impacts on Climate and the Earth system Observations of the ocean beneath Fimbul Ice Shelf Deliverable D2.4 UK Partners are funded by UK Research and Innovation (UKRI) under the UK government’s Horizon Europe funding Guarantee. https://ocean-ice.eu/ OCEAN ICE – GA 101060452 Deliverable D2.4 2 About this document Deliverable: D2.4 Observations of the ocean beneath Fimbul Ice Shelf Work Package: WP2 Cryosphere-ocean interaction, processes and feedbacks Delivery date: 24 October 2025 Type of document: Report Dissemination level: Public Lead beneficiary and author: PP13: United Kingdom Research and Innovation - British Antarctic Survey (UKRI-BAS), Yixi Zheng Contributors: PP4: Centre National De La Recherche Scientifique (CNRS), Roberto Grilli PP11: Norwegian Polar Institute (NPI), Tore Hattermann PP13: United Kingdom Research and Innovation - British Antarctic Survey (UKRI-BAS), Peter Davis, Keith Nicholls Review: PP1: Danish Meteorological Institute (DMI), Chiara Bearzotti ([email protected]) Cover sheet: A photo of the turbulence instrument cluster for measuring water temperature, salinity, depth, and fine-scale velocity, taken prior to its deployment 1 m beneath Fimbul Ice Shelf. Peter Davies BAS. Disclaimer: Funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or European Research Executive Agency (REA). Neither the European Union nor the granting authority can be held responsible for them. OCEAN ICE – GA 101060452 Deliverable D2.4 3 Table of contents 1. Publishable summary .................................................................................................................................... 4 2. Work performed and main achievements .................................................................................................... 4 2.1 Description of the work performed ..................................................................................................... 4 2.1.1 Observations from the Turbulence Instrument Cluster ................................................... 4 2.1.2 Data quality control .......................................................................................................... 5 2.1.3 Deviations from the original plan ..................................................................................... 6 2.2 References ........................................................................................................................................... 7 2.3 Open Science ....................................................................................................................................... 7 3. Impact ............................................................................................................................................................ 7 OCEAN ICE – GA 101060452 Deliverable D2.4 4 1. Publishable summary In this deliverable, we present year-long observations collected by a turbulence instrument cluster (TIC) deployed within the ice-ocean boundary layer beneath the Fimbul Ice Shelf (January 2024 - January 2025). The deployment site, located beneath an ice shelf basal channel (70.37°S, 0.10°W), is an area where periodic warm-water intrusion events have previously been reported (Hattermann et al., 2012). This site provides a wide range of environmental conditions in which the relationship between oceanic turbulence and melt rate can be investigated. The TIC recorded water temperature and flow speed at 5 Hz for 15 minutes every two hours. Temperatures were calibrated against an RBR sensor deployed at the same site. These observations provide the basis for estimating the turbulent kinetic energy and turbulent heat flux at the ice-ocean boundary layer and their relationship to basal melting. 2. Work performed and main achievements 2.1 Description of the work performed 2.1.1 Observations from the Turbulence Instrument Cluster We deployed the Turbulence Instrument Cluster (TIC) in the 2023/2024 field season on Fimbul Ice Shelf, at a depth of 1 m beneath an ice-shelf basal channel where warm water intrusions have been reported (Hattermann et al., 2012). Similar to a traditional ocean mooring, the TIC was deployed for one year to monitor the ocean conditions and turbulence year-round. The TIC measured the water temperature and flow speed at 5 Hz for 15 minutes every two hours. We revisited the deployment site in the 2024/2025 field season and successfully recovered the SD card storing the year-long TIC measurements. In total, we retrieved 4,273 bursts (each representing 15 mins of observations) between 9/Jan/2024 to 10/Jan/2025. (Fig. 1) Fig. 1: The burst-averaged (thin lines) and daily averaged (thick lines) in-situ temperature and flow speed measured by the TIC. Oceanic turbulence can be derived from the deviations of temperature and velocity relative to their burstaveraged values. Figure 2 presents two example bursts, illustrating weak (green) and strong (blue) turbulence events, shown through the turbulence-related deviations of water temperature (Fig. 2a) and flow speed (Fig. 2b). OCEAN ICE – GA 101060452 Deliverable D2.4 5 Fig. 2: Turbulence-related deviations of a. temperature and b. streamwise flow speed from their bust-averaged values, from two example bursts collected during weak (green lines; 20 May 2024) and strong (blue lines; 12 Aug 2024) turbulence events. 2.1.2 Data quality control We conducted a data quality control for the TIC-measured temperature against that recorded by an RBR temperature sensor deployed at the same location, but 28 cm higher in the water column. A slight decreasing trend with the time was removed and an offset of about 0.05 ºC was applied to the TIC-measured temperature (Fig. 3). OCEAN ICE – GA 101060452 Deliverable D2.4 6 Fig. 3: The difference between the TIC-measured temperature and the RBR temperature sensor-measured temperature, before (bright pink) and after (dark pink) the calibration. 2.1.3 Deviations from the original plan Unfortunately, we were only able to deploy one TIC, rather than the planned three, during the 2023/2024 field season. This was due to the high time and logistical cost of drilling extra access holes. We therefore have a reduced spatial coverage of turbulence measurements, which may limit our ability to fully capture variability across the basal channel. However, the year-long dataset from the single TIC, together with supporting mooring measurements, still enables us to address the core project objectives. As corrective action, we will integrate these observations with model simulations (NIP and collaborators from UKRI-BAS and Cambridge University) and plan for the deployment of additional TICs in future field seasons beneath other ice shelves. During this field campaign, we deployed for the first time a new optical probe for high-resolution in situ measurements of water isotopes. The sensor is based on the optical feedback cavity enhanced absorption spectroscopy (OFCEAS) technique combined with a membrane pervaporation method (Wohleber et al. 2024). This technique increases the number of observations under the ice shelf, for better spatial and temporal resolution, without having to collect and analyse in the laboratory a large number of water samples. The sensor was deployed twice through the borehole; however, on both occasions, the signal did not meet the required quality to accurately indicate the isotopic composition of the water. Fig. 4: The SWIS instrument (Subsea Water Isotope Sensor) in the Fimbul field campaign during its test prior deployment. As part of the corrective actions during the campaign, water samples for oxygen isotope analysis were collected. Moreover, once the instrument had returned from the field campaign, several improvements were OCEAN ICE – GA 101060452 Deliverable D2.4 7 made: 1) the mechanics of the instrument were redesigned, and wire-rope isolators were added to protect the optical assembly from shock and vibration; 2) a passive vacuum system was developed, making it possible to dispense with the vacuum pump, which often malfunctioned due to the presence of moisture. Instead, the entire instrument is put under vacuum, and a silica gel dryer is added inside the instrument to trap the water vapor being analysed, therefore maintaining the low pressure inside the instrument; 3) the pressure regulator has been replaced by a commercial system enabling faster and higher-quality stabilization of the pressure inside the optical cavity. This new design is better suited for deployments in harsh field conditions. The IGE is in the process of purchasing, through another funding source, a vibration table to submit instruments to vibrations before they are sent out and deployed in the field. Fig. 5: Last version of the SWIS instrument, with the integration of wire-rope isolators and an improved mechanical design, better adapted for field deployment. 2.2 References T. Hattermann, O.A. Nøst, J.M. Lilly & L.H. Smedsrud, Two years of oceanic observations below the Fimbul Ice Shelf, Antarctica, Volume 39, 2012, L12605, https://doi.org/10.1029/2012GL051012 2.3 Open Science The data underlying this deliverable will be deposited on the UK Polar Data Centre, with metatags pointing to OCEAN ICE. The dataset is embargoed until 30 September 2026 due to a publication currently in preparation using the data. Data will be made available in open access as soon as the publication is accepted. This deliverable is uploaded to Zenodo, and the data will be made available in the description of the Zenodo record. 3. Impact O1: Reduce the spatial and knowledge gaps in ocean observations around Antarctica, particularly relating to ice sheet-ocean interaction and deep water formation and export. Observations of turbulence beneath Antarctic ice shelves are extremely scarce. This dataset, collected 1 m beneath Fimbul Ice Shelf (a cold-cavity system), provides rare year-long time series of sub-ice-shelf turbulence and helps to close critical spatial and knowledge gaps in Antarctic ice-ocean observations. Comparing these data with our previous observations from warm-cavity systems such as Thwaites will reveal how cavity conditions influence turbulence structure and heat exchange. This, in turn, will offer new insights into the processes regulating ice shelf melt rates in contrasting regimes, and their role in water mass formation and export. O2: Improve critical ice sheet-ocean processes in numerical models, using historical observations and new data sets obtained in the project. OCEAN ICE – GA 101060452 Deliverable D2.4 8 Our year-long turbulence dataset, alongside supporting measurements, provides a valuable resource for validating ice sheet-ocean models. We are actively using these observations to test the commonly used three-equation parameterisation in ice–ocean models. In addition, we are working closely with modellingfocused researchers from NIP (WP2), UKRI-BAS, and Cambridge University to support their set up of largeeddy simulations and multiple idealised models. Together, these efforts will contribute to the development of improved parameterisation schemes for future ice sheet–ocean models. O7: Deliver free and open access to all data obtained in the project and contribute to international assessments (e.g. IPCC), climate model development, multi-national ocean observing initiatives (e.g. SOOS, All-Atlantic Ocean Research Alliance) and policymakers. Our year-long turbulence dataset will be made available online and open access under FAIR principles and be INSPIRE compliant. The data will underpin the development of cutting-edge melt rate parameterisations for use in climate models, which will lead to improved sea level projections for policy makers. The dataset will provide a benchmark against which future observational campaigns can be planned and delivered by multi-national ocean observing initiatives (e.g. SOOS).