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COMPASS: Climate-relevant Ocean Measurements and Processes on the Antarctic continental Shelf and Slope

Periodic Reporting for period 3 - COMPASS (COMPASS: Climate-relevant Ocean Measurements and Processes on the Antarctic continental Shelf and Slope)

Reporting period: 2020-09-01 to 2022-02-28

Physical processes on the continental shelf and slope all around Antarctica are crucially important for determining future sea level rise, for setting the properties and volume of dense bottom water exported globally, and for regulating the carbon cycle. Yet our ability to model and predict these processes over future decades is still rudimentary. This deficiency in understanding originates in a lack of observations in this inaccessible region. COMPASS is advancing understanding of the most critical mechanisms by which the ocean interacts with the atmosphere, sea-ice and ice shelves. It is vital that we improve our understanding of these processes, so that we can increase our confidence in future climate predictions.

The overall goal of COMPASS is to advance our understanding of the climate-relevant oceanographic processes on the continental shelf and slope of Antarctica. Specifically the objectives are:
1. To determine what enables or prevents exchange of water masses, heat and freshwater between the Antarctic continental shelf and the open ocean;
2. To elucidate the role of the Antarctic Slope Undercurrent in along-slope and across-slope processes;
3. To quantify the water mass conversion processes taking place in coastal polynyas;
4. To assess the mechanisms and variability of the ocean heat and freshwater fluxes entering beneath, and exiting from, ice shelf cavities.
To achieve these objectives we will harness new technology to make measurements in parts of the ocean that are data-sparse and poorly understood, use novel approaches to data analysis to tease out the mechanisms driving the phenomena we observe, assess to what extent these processes occur in numerical models (both high-resolution ocean models, and coupled climate models), and deduce what steps are needed for the future (both for Antarctic marine observing strategies, and for numerical modelling development).
The major task achieved was the planning, design and purchase of an autonomous ocean surface vehicle with the capability for deploying a profiling glider. This entailed tendering for the surface vehicle purchase, and working with both manufacturers (profiling glider manufacturer Kongsberg and surface vehicle manufacturer AutoNaut) to devise the best way to carry and release the glider beneath the surface vehicle. The wave-powered vehicle, an AutoNaut named Caravela, was built and tested, and officially handed over to COMPASS in July 2019 following trials of the vessel and the release mechanism. Desk-based training of the UEA team in operating the vehicle was carried out in August 2019.

In September 2019, we undertook our first deployment of Caravela on the west coast of Scotland. This allowed the UEA team to become familiar with procedures and techniques for build and piloting of the vessel. We practised towing her out, piloting using the joystick and local comms, and piloting remotely, with Autonaut personnel providing further training. We successfully tested the mechanism to release the Seaglider.

Although the goal of COMPASS is eventually to deploy Caravela in the Antarctic, we required a more benign environment for our first major science campaign where we would be gaining experience in operating and piloting the vehicle ourselves. We were fortunate to be invited to collaborate with another ERC project, Eurec4a, in a large ocean-atmosphere field campaign off Barbados in early 2020. We installed meteorological sensors on Caravela so that we could calculate air-sea fluxes of heat and momentum. The team travelled to Barbados where we deployed Caravela, carrying a Seaglider named Humpback, in January 2020. Caravela carried the Seaglider to the study site where the glider was successfully released. Simultaneous measurements of the upper ocean and sea surface were made for several weeks using a combination of Caravela, Humpback and two additional Seagliders carring sensors for calibration of Caravela's sensors. The three gliders were recovered from research vessel Meteor, and Caravela returned under her own wave-propulsion to Barbados where she was recovered. One glider carried a microstructure shear sensor for development of techniques for estimating turbulent mixing from a glider, one of the aims of COMPASS. Overall the campaign was much more successful than we had dared to hope, with all objectives achieved.

In parallel with the development of Caravela, we undertook experiments in the Roland von Glasow sea ice chamber to test various different anti-icing coatings. This included spraying samples of Caravela parts (of different materials) with both salt water and fresh water, to simulate rain and seawater freezing onto Caravela's mast, hull, foils or struts. A novel experimental test kit was designed and built to simulate the vehicle dipping into and out of the seawater. The results are too late to influence the build of Caravela which is now complete, but may influence future developments, and the best-performing coatings could be applied retrospectively.

COMPASS has undertaken a series of deployments of ocean gliders around Antarctica in two Antarctic seasons 2018-2019 and 2019-2020. This has included deployments in the Amundsen Sea in 2019, Bellingshausen Sea in 2020, and Weddell Sea in 2019 and 2020. Analysis of the data from these glider deployments is underway. A review paper on Antarctic slope front processes has been published. An assessment of the contributions of gliders to ocean observing systems was published. Two papers were written developing techniques and approaches to apply glider data, using data sets obtained prior to COMPASS (one published, one in press). Significant effort has been devoted in COMPASS into developing analysis and data quality approaches to glider microstructure data.

Analysis of ocean and climate model output is being pursued through COMPASS, in particular an assessment of the seasonal cycle in the most recent generation of climate models. High resolution numerical ocean models are being assessed against observations in the Amundsen and Bellingshausen Seas, and used to study the spatial and temporal context of the glider campaigns.
The most exciting and novel aspect of COMPASS has been the development and successful field campaign of Caravela - using one robot to deploy another. This has not been done before, and has attracted a good deal of attention. There are many potential applications of such a technique - for example enabling a timely deployment of a glider at a particular time and place (e.g. immediately prior to a hurricane, or after a volcanic eruption).

Scientifically, we anticipate the following results:
(i) an analysis of the heat budget in the upper ocean combining Caravela and glider measurements.
(ii) a "best practices" review of the calculation of ocean turbulent mixing from ocean gliders
(iii) measurement of ocean mixing on the Antarctic continental slope and adjacent to ice shelf cavities
(iv) understanding of the exchanges of water masses across the Antarctic continental shelf and slope.
(v) identification of the strengths and weaknesses of coupled climate models in the region.

At the time of writing, deployments in the Antarctic season 2020-2021 appear to be unlikely in view of the COVID-19 situation. We have also had delays of ship time allocated to Antarctic deployments in the Amundsen Sea (from 2021 to 2022) and the Weddell Sea (from 2019 to 2023) so future glider deployments are unclear.
Recovery of a COMPASS glider in the sea ice in the Amundsen Sea, February 2019
Caravela prior to first science campaign, deploying from Barbados