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Detecting Ocean Variability under Filchner-Ronne Ice Shelf

Periodic Reporting for period 1 - DOVuFRIS (Detecting Ocean Variability under Filchner-Ronne Ice Shelf)

Berichtszeitraum: 2018-09-30 bis 2020-09-29

Ice shelves, the floating extension of the Antarctic Ice Sheet, help stabilise the ice sheet by hindering the flow of grounded ice into the ocean. At their extensive base, ice shelves are exposed to the heat delivered by the oceans, and as this heat delivery changes, so does the rate at which ice shelves melt. Ultimately, changes in the heat delivery to the ice shelves can lead to changes in ice-sheet flow and in the rate at which ice mass is discharged from the continent into the ocean. Ice-shelf cavities are challenging environments to observe, because of their remote locations and extreme climate conditions. As a result, the knowledge of the temporal variability and trends that the ocean underneath experiences are largely unknown. Exploration of the way oceanic variability expresses itself by its signature in the rate of melting at the ice-shelf base has started with the recent development of the autonomous phase-sensitive radio-echo sounder (ApRES) by the British Antarctic Survey. ApRES is able to detect small changes inside the ice shelf and at its base and the primary outcome from its measurement is a time series of basal melt rates. However, the measurement ApRES provides is indirect and there are a number of difficulties that arise and need to be overcome.

The primary purpose of the MSCA Individual Fellowship DOVuFRIS was to analyze and explore the full potential of the first extensive collection of ApRES data. There are three main objectives that DOVuFRIS addressed. The first was to determine the extent of oceanographic temporal variability under the Filchner-Ronne Ice Shelf. The second aimed to identify and investigate the physical mechanisms governing this oceanic variability and the significance of this variability for the overall state of the ice-shelf cavity. The third objective was to make these ApRES-derived data useful for ocean modellers that attempt to design realistic simulations of sub-ice-shelf cavities.
In the first part of DOVuFRIS I developed a method that achieves the separation of the melt rate from other dynamics originating from internal motions of an ice shelf that are also present in the received radar signal. This method allowed me to derive melt rate variations as frequent as those due to the semidiurnal (12 hour) tide. I was also able to use these extracted tidal melt rates to gain knowledge about the background oceanic flow beneath the ice shelf, showing that observations of short term variations have a utility when attempting to understand the mean state of a system. Further, I was able to take advantage of the internal tidal motions of an ice shelf to better constrain a mean melt rate estimate at sites where the ice-shelf base is complicated. This is an important result, because complicated ice bases often occur near grounding lines of deep ice shelves, where the rate of melting is highest and where changes in melting have the most profound consequences for ice discharge.

In the second part of DOVuFRIS I was able to identify freezing events that occurred intermittently on the timescale of days to weeks. A new approach was developed to quantify the amount of basal freezing and to infer the nature of the accreted material, which was necessary because melting and freezing in a salty ocean are not symmetric and the properties and therefore radar reflections from ice formed by frozen sea water are different to those from ice formed by snow accumulation. The direct implications of this work is that it will allow the monitoring of the motion of cold waters modified by ice shelf melting, which was previously thought to be impossible using the ApRES technique, and this result can be readily exploited in the planning of upcoming ApRES deployments.

In the final part I worked alongside ocean modellers towards exploiting the ApRES melt rate observations in a way that could improve model validation and inform the design of experiments aimed to improve our understanding of the importance of oceanic variability in ice shelf cavities. We have developed a framework for evaluating the performance of coarsely resolved models and sparse but frequent observations. We have integrated ApRES observations and an ice shelf cavity model to understand the dominant modes of variability at Totten Ice Shelf in East Antarctica. And finally, I have also begun to address the role of mesoscale variability in setting the overall melt rate of an ice shelf.

DOVuFRIS will result in a minimum of six peer-reviewed publications. Two articles have been published, two articles are being revised for resubmission, and two more articles will be submitted shortly. The results have been presented at five international conferences and I have given thirteen invited talks just over the duration of the Fellowship. In addition, there were several activities that went beyond dissemination of the scientific results to the specific audience
of ice-ocean interactions. I convened a session at EGU 2020 drawing the attention of the broader polar science community to the importance of small scale ocean processes in ice-shelf cavities. I also arranged and supervised a summer research experience for two undergraduate students and participated in a Cambridge Science Festival event, exposing the general public and especially children to what it is like to do field research in the polar regions and why it is relevant.
The progress beyond the state of the art includes:

1) the capability to accurately estimate melt rates at ice shelves with complicated basal geometries,
2) the capability to detect signatures of fast ocean driven processes in the basal melt rate of an ice shelf, such as
tidal motions,
3) the capability to detect and quantify basal freezing that occurs intermittently, and thus the potential to monitor
the dynamics of ice shelf water, the water mass that carries the signature of integrated basal melting,
4) and the availability of a new resource for ocean and ice-sheet modellers to benchmark their models with melt
rate time series, which can lead to improvement in model biases and representation of unresolved processes.

Throughout DOVuFRIS I have been able to show that the study of short time scale processes can provide a pathway to understanding and observation of longer term mean states and slow changes and dynamics, and this is a key aspect that will help shape future deployments and observational campaigns. I have initiated a working group under SOOS (Southern Ocean Observing System) to coordinate future ApRES deployment efforts and to exchange knowledge and expertise in the joint effort towards exploration of the ice-shelf environment. My findings have been or will soon be published in top journals in the field, making them available to the community. I have initiated several collaborations between different institutions and between researchers with wide ranging expertise in polar science and I hope these to continue in the future as successfully as they have been until now.
Locations of melt rate time series collected at the Filchner-Ronne Ice Shelf