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Water-mass transformation and Pathways In The Weddell Sea: uncovering the dynamics of a global climate chokepoint from In-situ measurements

Periodic Reporting for period 3 - WAPITI (Water-mass transformation and Pathways In The Weddell Sea: uncovering the dynamics of a global climate chokepoint from In-situ measurements)

Reporting period: 2018-05-01 to 2019-10-31

The proposed project has been designed to answer one main scientific objective: what sets the tridimensional water-mass structure and pathways in the Weddell sea, which provide an outlet between the Antarctica ice-shelves and the large-scale world’s ocean circulation?

Dense water formed around the Antarctic continent drives the global ocean circulation. 50-70% of this dense water is formed within only about 10% of the Antarctic circumpolar band: the Weddell Sea area. This makes the Weddell Sea a global climate chokepoint, yet it is very poorly observed and understood. We have now reached a point where ocean instrument technological advances, as well as progresses in ocean modeling, give us the opportunity to make a big step forward in polar physical oceanography. The present project proposes to take this opportunity and to tackle this glaring gap in our understanding of the global climate. The project is timely for a number of reasons. First, it tackles an issue known to be a pivotal limitation of global climate models. This limitation leads to major uncertainty in the predictions of ocean-climate feedback and the global rate of sea-level rise. Second, it proposes to exploit recent developments in cutting-edge observation technology to fill major holes in the current observation network. Third, it builds on recent developments of state of the art global ocean models, which will be used in the next generation of IPCC-class climate simulations in several world-leading modelling groups in France, the United Kingdom and more generally in Europe (namely IPSL, CNRM, Met-Office, and EC-EARTH). In addition, it will provide the understanding of the large-scale ventilation process, which provides the physical context to interpret the results of a number of contemporary European and international observational programs. These programs are delivering striking evidence of the importance, complexity, and climate sensitivity of the world ocean’s ventilation from the Southern Hemisphere. Finally, at a time where pressing questions are being asked by policy-makers concerning the future of global sea-level rise, we crucially need to understand how and where water-masses are pre-conditioned and transported on the Antarctic shelf, before coming into contact with the ice shelves: an highly sensitive process ultimately controlling the rate of melt of the Earth’s largest ice reservoir.
As originally planned the project has been structures around three main objectives:
• (O1) Explore the dynamical forcing of the Weddell gyre, and the response of the intensity of the gyre to atmospheric variability
• (O2) Unveil the ocean dynamics and its forcing on the continental shelf, which provides an efficient pathway between the open ocean gyre and the ice shelf cavities
• (O3) Extract from the first-ever direct Lagrangian observations of the dynamics of the boundary layer in Weddell sea, the primary mechanisms involved in the overflow of water-masses sinking in the abyss

The project started by tackling the first objective “Explore the dynamical forcing of the Weddell gyre, and the response of the intensity of the gyre to atmospheric variability”. Great progress has been achieved on this objective by approaching it through quantifying water-mass formation and transformation (as originally proposed in Task 1). Goal of task 1 have even overpassed by extending the analysis to the entire circumpolar belt, to better grasps the specificity of the Weddell Gyre dynamics within the Southern Ocean. One paper presenting this study is accepted at the Journal of Geophysical Research (Pellichero et al., 2016). In addition, to this study, a parallel study that I lead started to quantify the structure, and seasonal cycle of the large-scale gyre circulation. This second study is building on the same dataset as used in Pellichero et al., 2016, and has been presented in international conferences and meetings, and is currently being written up, in preparation for submission in the Journal of Physical Oceanography (Sallée and Chapman, 2018). A third study has been followed to address objective 1 (task 2) in which we reveal for the first time ever from observation, how sea-ice forcing draws up waters from the deep ocean and transform it in the surface layer (Pellichero et al., 2017). This study is currently in review (after a first round of review) for Nature Communication, and we are confident that it should be accepted soon. The interannual variability (task 2) aspect has been address in a series of annual papers published in Bulletin of American Meteorological Society (Sallée et al., 2016; Mazloff et al., 2017), and a more comprehensive study is currently in preparation. Overall, objective 1 and associated tasks have been met.

A major aspect of this reporting period has been the preparation of objective 2 and 3, and in particular the preparation of the scientific cruise and related instruments. Implementations of the cruise, buying of instrument, transport, etc. took a lot of energy and time; time which was well invested, given the great success that the first cruise of the project was. The cruise was done was a 50-day cruise that I lead as chief scientist on board the RRS James Clark Ross Icebreaker, with a team of 15 scientists, to tackle the objectives of the project. We believe this cruise was an important logistical, administrative, managerial, and scientific achievement and success. We managed to successfully deploy all the floats and sound sources planned as part of task 3. The floats worked well for the first months after deployments, before they had to winter under sea-ice. We are currently waiting next Austral summer (January 2018) to see if the floats survived their wintering and send their unprecedented data. Already, one float found a hole in the ice in November 2017 to send a first thread of winter observations, which is a great indication that the technology is able to resist wintering and we hope that most data will be able to be recovered in the coming months. In order to reduce the risk associated with these floats, a range of originally unplanned (in the proposal) observations have been taken while at sea to be able to partially address task 3 in case of problems with the floats. In addition, a network of moorings has been left on site for two years, and I am currently organising how to recover them (if we successfully do it, it will be unique observation that the project allowed to organised; unplanned in the original proposal, but addressing task 3). A Master student started in 2017 to analyse the dataset got during the first cruise, and she published her thesis addressing some aspects of task 3; in particular fine resolution of the circulation in the Filchner Depression, and water-masses exchanges at the mouth of the ice-shelf (Vignes, 2017). The student is now working as a phD to continue investigate the dataset, with task 3 in mind, and will analyse the data received from the floats. In addition, based on datasets recovered during the cruise, we achieved the first ever study showing the seasonal variability of the exchange between open ocean and sub-ice-shelf seas in the Filchner Depression, which allowed us to investigate forcing of the circulation. This study is submitted to the Geophysical Research Letter (Darelius and Sallée, 2018). As part of task 3, I organise to backup all of these physical observations with geochemical observations to better grasp the forcing of the circulation (unplanned in the original proposal, but addressing task 3). A phD student started on this aspect in September 2016 and join the cruise to make these observations. A paper analysing these observation is currently under preparation. Finally, as part of the analysis of the forcing of the circulation, I managed to organise Fluid Dynamics experiment on the largest fluid dynamics rotating table on Earth (Coriolis platform), to better understand and complement our real ocean observation (unplanned in the original proposal, but addressing task 3). In summary, Objective 2 is in progress; there are still some uncertainty around what we will be able to recover from the floats, but I developed a range of aspects have been developed to make sure Objective 2 can be fully addressed and even overpassed.

Objective 3 has mostly been tackled from Task 5: “Pushing back the limits of observations: the entrainment of the sinking bottom water plume”. As originally noted in the proposal, this task is certainly the riskiest of the proposal, as it involved significant technological developments, and deployment of instrument in hostile environment. Great achievements have been done during the reporting period. All purchases and technical choices have been discussed and made. The development advanced well, with difficulties and problems being discussed and solved along the way. The new prototypes, have been tested in pools, in the Mediterranean, parts of the prototypes have also been tested in the North Atlantic, i.e. in more accessible seas, but with similarities in terms of intense pressure, and cold environment. After a bit less than two years development, we decided to deploy the first two prototypes at seas, in real environment in the Southern Ocean (keeping three prototypes for more development based on the results of the first two instruments). The two instruments did work very well for the first weeks, but one failed after 2 weeks. We managed to organise to reduce the risk well enough to deploy the instruments in a region were other colleagues that I contacted already far in advance, had also a cruise after ours. These colleagues recovered the instrument that failed for us, which was a great managerial achievement for the project, and allow us to better understand the failure for further developments. The other instrument works very well and sends unprecedented observations of the deep ocean. We, however, have some issues with the velocity measurements and we are now starting a second phase of development based on the results of these first two real-world deployments. In addition to these deployments, I organised a collaboration to address part of the scientific goals of task 5, without the instruments (again with the spirit of reducing the risks of the proposal, and make sure all scientific goals can be met, or at least partly met). One very nice study came out of this collaboration (Abrahamson et al., 2017) which is in preparation for Nature Climate Change.

Finally, both Objective 2 and 3 are also attacked with task 4, in which we develop a new state of the art configuration of numerical model of the Weddell Sea. Task 4 is challenging, because it involves coupling ice-sheves, sea-ice and ocean, but is very timely and will serve the next generation of climate models. Great advances have been made on task 4, though these advances are mostly technical. A Postdoc have been working full time for the last year to set-up the configuration, and we are currently at the stage, where we are ready to launch the first simulation, from which, I believe, novel and topical science question (as proposed in task 4), will be addressed. The development of the configuration was however longer than I originally expected due to unexpected technical numerical challenges.
For the first time the seasonal cycle of the ocean under Antarctica Sea-ice has been described regionally based on a novel dataset. It allows us to uncover the large-scale circulation and how sea-ice influences ocean water-masses transformation and pathways. Overall that brings a new vision of the large-scale circulation of the Southern Ocean and its global connexions. In addition, new instruments have been designed and deployed in the difficult environment of the polar seas, allowing to observe circulation in the densest waters of Earth's oceans, and as far south as where ocean directly interacts with Antarctica ice-sheet. A ambitious cruise has been successfully planed to investigate ocean/ice sheet iteractions and data analysis is currently performed.