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POLARPROG-Polarforskningsprogram

Ocean-ice shelf Interaction and channelized Melting in Dronning Maud Land

Alternative title: Havsirkulasjon og marin smelting av isbremmer i Dronning Maud Land

Awarded: NOK 9.9 mill.

Project Manager:

Project Number:

295075

Application Type:

Project Period:

2019 - 2024

Funding received from:

Partner countries:

The Antarctic ice sheet and the floating ice shelves fringing the Antarctic continent are losing mass at an accelerating rate, causing the global mean sea level to rise. This is bad news in a time when the majority of the world's population lives along the coast. The Antarctic ice shelves thin mainly because ocean currents bring 'warm' water into their cavities causing them to melt from below. As the ice shelves thin, they offer less support to the ice sheet upstream which then also thins and accelerates. Ice shelf thinning is most prominent in Western Antarctica but changes are also observed in East Antarctica. Here, in Dronning Maud Land, two relatively small ice shelves Fimbulisen and Nivlisen are situated. Offshore of these ice shelves, the Antarctic Slope Front (ASF) runs parallel to the Antarctic continental slope. The ASF divides cold and fresh water on the southern side from warm and salty water on the northern side and prevents a continuous inflow of warm water into the ice-shelf cavities in East Antarctica, ensuring a stable ice mass balance. However, episodic warm water events have been observed under Fimbulisen. The main objectives of this project were to determine how these warm events relate to the regional and large-scale atmospheric forcing and the dynamics of the ASF, if they have changed over time and how that impacts the cavity circulation, and if so, when and how the ocean variations impact basal melting of the ice shelves. Continuous observations of ocean temperature and velocity from under Fimbulisen were initiated in 2009 and have now been extended to 2021, creating the longest record of its kind. While this ice shelf is typically exposed to cold temperatures and only sporadic warm events, the under-ice shelf temperature records showed a sustained temperature increase in 2016. While the short-term warm episodes occur during reduced easterly winds, the change in 2016 was associated with a larger-scale pattern of increased subpolar westerlies and much reduced sea ice conditions. The change from a colder to a warmer period in 2016 was also concurrent with an increase in basal melt under Fimbulisen derived from satellite data. On seasonal time scales, the hydrography close to the ice base is dominated by seasonal variability of Antarctic Surface Water, while at depth, intrusions of Warm Deep Water showed only a weak seasonal variability. Temperature and current meter data from two open ocean moorings offshore of Dronning Maud Land recovered in January 2021, demonstrate a large seasonal cycle in the thermocline depth. The thermocline near the coast was at its deepest in July, leading to a steep ASF and strong currents. Surprisingly, the seasonality of the ASF did not appear to influence the ocean temperature under the ice shelf strongly, but velocity data showed that strongest flow into the cavity occurred in spring-summer when the slope current is at its weakest. Detailed maps of ice shelf freeboard, thickness and draught in Dronning Maud Land were developed from high-resolution satellite data as a baseline for fine-scale studies of basal melting. Melt rates were measured directly at the M2 mooring site on Fimbulisen with an autonomous phase-sensitive radar. The results for 2017-2019 show a relatively low average melt rate (~1.2 m/year), but the melt rate still varies a lot throughout the year. This was found to be closely related with the under ice shelf ocean velocities measured by the M2 mooring. Periods with high velocity give large melting, and the ice shelf is therefore sensitive to potential future changes in ocean current conditions under the ice shelf. Satellite data can capture such changes at larger scales over multiple years, but improved corrections are needed to relate in-situ and satellite observations more directly. The high-resolution Finite Volume Community Ocean Model (FVCOM) was developed for Fimbulisen to investigate the effect of small-scale basal features in the cavity. The model's grid resolution was determined by the high-resolution ice draught map, ranging from 50 m in basal channels under the ice shelf, to 2000 m in the open ocean. The model shows that channels at the base of Fimbulisen entrain warm water from under the ice shelf which locally increases the melting and the size of the channels, further enhancing this mechanism over time. To further verify the feedback mechanism, a coupled model using Elmer/ICE-FVCOM-FISOC coupled model system was set up for Fimbulisen. For that purpose, an innovative accelerated forcing approach was developed that can effectively bridge the timescale discrepancy between the ice sheet and ocean models and improve computational efficiency in the coupled model system. This approach is a powerful tool in coupled ice sheet-ocean modeling, thereby contributing to improve projections of ice-sheet mass balance and sea level rise.

The iMelt project has identified both local and remote oceanic, sea ice and atmospheric drivers of both episodic and more sustained warm water presence in the ice-shelf cavity under Fimbulisen in Dronning Maud Land. Given that this ice shelf is in a so-called cold ice shelf regime and is currently not loosing mass, identifying these processes are key to validate coupled ocean-ice shelf and climate models. This will allow further improvements of model processes and selection of models for future projections of ice shelf melt and potential destabilization in east Antarctica. The project has taken a first step in addressing the importance of small-scale features and related processes under a cold-cavity ice shelf. The newly developed fine-scale maps of surface and basal topography of ice shelves in Dronning Maud Land open new possibilities for detailed process studies like the one with the FVCOM model which has given us new insights to be further explored. From concurrent ocean and melt observations, we found that on sub-monthly timescales, basal melt is primarily driven by ocean velocity variations under the ice shelf while on seasonal timescales, ocean temperature plays a role. Further use of satellite data to derive local patterns of melt will need to rely on improved corrections for ice dynamics and near-surface snow processes. As a consequence of that, we are installing various snow sensors (thermistor, surface ranger and weather station) and a GNSS positioning instrument on the M2 mooring/radar site as a part of the NFR-funded infrastructure project TONe. This will allow us to address the identified discrepancy between our in-situ record of basal melt and that of current satellite-based techniques, which are the only way to quantify basal melt at larger scales. Finally, the new approach developed in WP3 to overcome the differences in time scales related with ocean and ice-shelf processes respectively, and to couple ocean and ice sheet models effectively is a powerful and useful tool in coupled ice sheet-ocean modeling, thereby contributing to improve sea level rise projections.

The recent increase in the Antarctic contribution to global sea-level rise is a major concern given that the majority of the world’s population lives along the coastlines. This increase, which is now thought to be irreversible in West Antarctica, is triggered by ocean-induced melting beneath the floating parts of the ice sheet known as ice shelves. Most basal melting occur near the ice-sheet grounding lines and the ice-shelf fronts, as well as within basal channels underneath the ice shelves. This project will quantify the processes and importance of ocean-ice shelf interactions and channelized basal melting in Dronning Maud Land, East Antarctica. The main focus will be on Fimbulisen ice shelf which has a complex network of basal channels in the central part of the ice shelf and a tongue that extends seaward of the continental shelf. Under-ice shelf data has been collected at Fimbulisen since 2010 and new, planned infrastructure along the coast of Dronning Maud Land will allow us to investigate ocean processes outside the ice shelf. Three autonomous radars are also deployed on Fimbulisen and Nivlisen ice shelves to monitor ice-shelf basal melting directly. The project will quantify the relationship between far-field ocean dynamics, ocean-ice interactions and basal melt rates through these concurrent oceanographic and under-ice shelf measurements. This interdisciplinary research combines in-situ measurements, satellite remote sensing, and high-resolution modeling of ice-ocean interaction in Dronning Maud Land and will provide fundamental new knowledge on processes related to basal melting, essential for a better understanding of the stability of the Antarctic ice sheet.

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Funding scheme:

POLARPROG-Polarforskningsprogram