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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
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.