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

Reporting period: 2019-11-01 to 2021-04-30

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. Now, 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 project proposes to take this opportunity and to tackle this glaring gap in our understanding of the global climate : (i) 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. (ii) it proposes to exploit recent developments in cutting-edge observation technology to fill major holes in the current observation network. (iii) 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 UK and in Europe (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.

The project developed novel approach in terms of numerical models by developing regional configurations of the Weddell Sea including a representation of the interaction between ocean and the ice shelf implemented with the ocean model used in many climates modeling group in Europe (Nemo model) (e.g. Haussmann et al., 2020; Bull et al., 2021). It also used unconventional observing method specifically adapted for remote and complex usage of the sea-ice covered Southern Ocean (Akhoudas et al., 2020, 2021; Vignes et al., 2021).
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. 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 built 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). 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).

A major aspect of the previous 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. It 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. After few years of experimentations, most of the instruments have been recovered. Only one mooring gear and the associated instruments have been lost. We were able to recovered most of the data. The former Master student has been working as a PhD to continue investigate the dataset 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 joined the cruise to make these observations. 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). 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”. All purchases and technical choices have been discussed and made.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 developed a new state of the art configuration of numerical model of the Weddell Sea. Task 4 was challenging, because it involved coupling ice-shelves, sea-ice and ocean, but was 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.

The work tackled during the last reporting period concentrated on Task 3, 4, 5; finishing these tasks started during the previous reporting period. The objectives of Task 4 were finished and overpassed our expectations before. We pushed further this Task, in particular through collaboration with colleagues at BAS, which lead to further investigation of the role of climate anomaly on ice-shelf melt (Haussmann et al., 2020; Bull et al., 2021), as well as better understanding limitations and/or strengths of the current generation of climate models to represent the Southern Ocean (Beadling et al., 2020, Silvy et al., 2021, Hobbs et al., 2021). Similarly, we pushed further our work on task 3 to investigate processes involved in the preconditioning of bottom water production on the Weddell Sea continental shelf, as well as continental shelf processes associated with warm water pathways (Akhoudas et al., 2021, Hutchison et al., 2021, Vignes et al., 2021, Labrousse et al., 2021). Task 5 finished with a nice analysis of the processes involved in bottom water production, including an assessment of the importance of diapycnal mixing on the continental slope that was published in Nature Scientific Report (Akhoudas et al., 2021). Finally, we also pushed further Task 2 by an analysis of the Antarctic continental slope current at circumpolar scale (Pauthenet et al., 2021). And as a cherry on the cake, we finished Task 2 with a group study on long term changes of the upper ocean, which we started with a focus on Antarctic subpolar oceans (in the spirit of Task 2), but which ended up as a global analysis that was published in Nature in March 2021. Over the reporting period, 19 peer-review scientific papers have been submitted (17 accepted or published, 2 still under review), including 6 papers in journals of the Nature group of journals, and 1 paper in Nature.
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.

The main research achievement of the project lies in major advances in our understanding of the three themes that link to the three main objectives of the projet: (i) the dynamical forcing of the Southern Ocean and Weddell gyre, and the response of the intensity of the gyre to atmospheric variability (Pellichero et al., 2017; Chapman and Sallée, 2017a,b; Pellichero et al., 2018; Speer et al., 2018; Jones et al., 2019, 2020; Chapman et al., 2020; Auger et al., 2021; Pauthenet et al., 2021; Sallée et al., 2021) ; (ii) ocean dynamics and forcing on the Antarctic continental shelf, and role of the ocean in ice-shelf melting (Darelius and Sallée, 2018; Labrousse et al., 2018, 2020; Haussmann et al., 2020; Akhoudas et al., 2020; Hutchinson et al., 2021; Bull et al., 2021, Vignes et al., 2021) ; (iii) The primary mechanisms involved in the production of Antarctic Bottom Water and links of the Southern Ocean to the global ocean (Sallée et al., 2018; Palmer et al., 2019; Abrahamsen et al., 2019; Pauthenet et al., 2019; Beadling et al., 2020 ; Silvy et al., 2020 ; Hobbs et al., 2021; Akhoudas et al., 2021).
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