Inflow of Warm Deep Water on the Antarctic Continental Shelves

The observed thinning of the Antarctic ice shelves and the potential collapse of the West Antarctic Ice Sheet (WAIS) and the consequent global sea level rise are of major concern in a world where the majority of the population lives along the coastlines. The recent loss of Antarctic glacial ice is mainly due to basal melting beneath the floating ice shelves caused by oceanic heat supply. WARM aimed at quantifying the inflow warm deep water onto the continental shelves around Antarctica and its contribution to basal melt below the floating ice shelves. The project also intended to improve the understanding of the mechanisms causing the inflow and their relation to large-scale forcing. We used a combination of observations and idealized and regional modeling from three fundamentally different sites around the Antarctic continent: the central Amundsen Sea, the southern and eastern Weddell Sea. During the project we have (1) developed, analyzed and published results from an idealized model representing the southern Weddell Sea; (2) finished the development of a high resolution regional model of the Weddell Sea; (3) analyzed CMIP5 model output; (4) Analyzed and published observational data from the Weddell Sea; (5) Retrieved moorings in the Amundsen Sea and published the first results; (6) been active in outreach, and (7) submitted and defended one PhD-thesis (K. Daae, 2018). 1. Using an idealized model, we have simulated circulation and inflow of warm deep water onto the continental shelf, and into the Filchner Trough. The results are published in JGR (Daae et al, 2017) and suggest that there is a recirculation of the slope current in the Filchner sill area. We have further developed a model describing the seasonality of the thermocline depth within the Weddell gyre (Hattermann, 2018). 2. Development of two model architectures has been pushed forward: ROMS and FVCOM. FVCOM is used to investigate the sensitivity of tides to improved bathymetry measurements beneath the Filchner-Ronne Ice Shelf (Rosier et al. 2018). The setup of ROMS for the southern Weddell Sea region was completed in early 2017, and preliminary results were presented in a talk at the WAIS workshop, 2017. 3. We have analyzed CMIP5 model outputs to assess the dramatic predictions by Hellmer et al. (2012), and the underlying mechanisms described by Hellmer et al. (2017). The outcome of these analysis has been used to force the regional ROMS model to study the melt rate sensitivity beneath the Filchner Ronne Ice Shelf to projected scale forcing changes in the Weddell Sea. A paper describing these results are in preparation. 4. Warm water of open ocean origin was observed at the front of the Filchner Ice Shelf during the autumn, of 2013. The southward flow of warm water is highly dependent on wind, with strong along- coast wind leading to enhanced southward transport of warm water (Darelius et al, 2016). Further analysis of observations from the region show that 1) the flow on the shelf east of the Filchner Through changes seasonally (Ryan et al, 2017 - JGR) , 2) that the inflow of warm water and the outflow of dense water is connected (Daae et al, 2018 - JGR), 3) there is a seasonal outflow of ISW across the ice shelf front (Darelius & Sallée, 2018, GRL) , 4) turbulence levels are much increased in the shelf break region (Fer, Darelius et al, 2016 - GRL), 5) strong mixing at the plume interface is linked to shear spiking (Daae et al, 2009), and 6) the strong diurnal currents on the slope and shelf break region are linked to resonant shelf waves (Semper and Darelius, 2016 - OSD) 5. Moorings in the troughs leading in towards the Getz ice shelf (GIS) have been recovered. The warm water found off the continental shelf reaches the ice shelf front through the Siple trough without being modified, since the advection time scale is very short (Assmann, Darelius et al, 2019). Preliminary results from the APRES, measuring melt rates below GIS, were presented at the FRISP workshop (2018). Oscillations observed in the outflow region from the eastern Getz and Dotson ice shelves are caused by topographic Rossby waves (Wåhlin et al, 2016 - JPO). 6. E. Darelius has written numerous blog posts directed at children and youths (https://skolelab.uib.no/blogg/antarktis, 2016 and 2017), and made presentations in school classes. K. Assmann blogged from the cruise with RV Araon in 2018, and wrote a popular science article about the cruise in Chemistry World. Results from the project haves been featured on several occasions in media (e.g. BT, Forskning.no, Ekko at NRK, Radio Halland, Sweden, SverigesRadio, Sweden, Eos.org).

Results from WARM are published in 17 articles and new data have been added to archives: WARM will influence ongoing and planned research activity in the shelf areas around Antarctica. The advanced numerical modelling tools developed will be used in ongoing and planned projects. The outreach efforts and the media attention related to WARM has hopefully increased the public's awareness about the changes observed in Antarctica and about the importance of Antarctica for e.g. sea level rise. We hope that the activities focused on a young public has increased the interest for research and STEM subjects, potentially inspiring a new generation of polar scientist. Darelius and Hattermann has gained valuable experience in e.g. project management, numerical modelling, high latitude fieldwork and supervision and WARM has allowed them to establish themselves as Antarctic researchers, assuring the continuation of Norwegian research traditions in the Antarctica.

WARM aims at quantifying the inflow warm deep water onto the continental shelves around Antarctica and its contribution to basal melt below the floating ice shelves (objective 1). The project also intends to improve the understanding of the mechanisms cau sing the inflow (objective 2) and their relation to large-scale forcing (objective 3). This knowledge will be applied to the Filchner-Ronne Ice shelf-continental shelf system, and the dramatic - and debated - future scenarios with a twenty-fold increase i n basal melt will be reassessed (objective 4). These objectives will be achieved using a combination of observations - existing data and proposed fieldwork - and high resolution idealized and regional modeling using ROMS. I will study three fundamentally different sites around the Antarctic continent: the central Amundsen Sea (CAS), where the continental shelf is wide and "warm" (above freezing point), the southern Weddell Sea, where it is wide and "cold" (at freezing point) and the eastern Weddell Sea w here it is very narrow or non-existing. Together the tree sites represent a majority of the existing shelf configurations around the Antarctic continent. In WARM, I will make use of national and international expertise and I have gathered around me a res earch team including scientist from Bergen, Tromsø, Sweden, Korea, the US, Canada and Germany. I am granted ship time onboard the Korean icebreaker RVIB Araon for fieldwork in the Amundsen Sea. WARM will expand Norwegian Antarctic oceanographic research beyond the Weddell Sea; it will strengthen Norway as a leading polar nation and integrate the Norwegian research in larger international collaborations. In addition, WARM will give me the opportunity to increase my publication record, to enlarge my nation al and international network and to participate in outreach activities aimed at high-school children, thus promoting my career as a research scientist.