The region of Maud Rise, a seamount in the Weddell Sea, is known for the occurrence of irregular polynya openings during the winter months. Hydrographic observations have shown the presence of a warmer water mass below the mixed layer along the seamount's flanks, commonly termed the warm‐water Halo, surrounding a colder region above the rise, the Taylor Cap. Here we use two observational data sets, an eddy‐permitting reanalysis product and regional high‐resolution simulations, to investigate the interannual variability of the Halo and Taylor Cap for the period 2007–2022. Observations include novel hydrographic profiles obtained in the Maud Rise area in January 2022, during the first SO‐CHIC cruise. It is demonstrated that the temperature of deep waters around Maud Rise exhibits strong interannual variability within the Halo and Taylor Cap, occasionally to such an extent that the two features become indistinguishable. A warming of deep waters by as much as 0.8°C is observed in the Taylor Cap during the years preceding the opening of a polynya in 2016 and 2017, starting in 2011. By analyzing regional simulations, we show that most of the observed variability in the Halo is forced remotely by advection of deep waters from the Weddell Gyre into the region surrounding Maud Rise. Our highest‐resolution simulation indicates that mesoscale eddies subsequently transfer the properties of the Halo's deep waters onto the Taylor Cap. The eddies responsible for such transfer originate in an abrupt retroflection along the inner flank of the Halo.

The Southern Ocean greatly contributes to the regulation of the global climate by controlling important heat and carbon exchanges between the atmosphere and the ocean. Rates of climate change on decadal timescales are therefore impacted by oceanic processes taking place in the Southern Ocean, yet too little is known about these processes. Limitations come both from the lack of observations in this extreme environment and its inherent sensitivity to intermittent processes at scales that are not well captured in current Earth system models. The Southern Ocean Carbon and Heat Impact on Climate programme was launched to address this knowledge gap, with the overall objective to understand and quantify variability of heat and carbon budgets in the Southern Ocean through an investigation of the key physical processes controlling exchanges between the atmosphere, ocean and sea ice using a combination of observational and modelling approaches. Here, we provide a brief overview of the programme, as well as a summary of some of the scientific progress achieved during its first half. Advances range from new evidence of the importance of specific processes in Southern Ocean ventilation rate (e.g. storm-induced turbulence, sea–ice meltwater fronts, wind-induced gyre circulation, dense shelf water formation and abyssal mixing) to refined descriptions of the physical changes currently ongoing in the Southern Ocean and of their link with global climate. This article is part of a discussion meeting issue ‘Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities’.

<p>Maud Rise polynyas are rare events in the Weddell Sea (Atlantic sector of the Southern Ocean) that cause deep vertical mixing within the ocean column and large surface fluxes of heat with large impacts on  the local Weddell gyre circulation and on the Antarctic bottom water properties. Here we use a 1/12<sup>o</sup> ocean reanalysis product to assess the dominant drivers of ocean stratification leading up to the polynya event of 2016 and 2017 in Maud Rise, Weddell Sea. We carry out a potential vorticity (PV) budget to identify the dynamical components of the regional circulation responsible for changes in ocean stratification that culminated in the formation of the 2017 polynya. During 2015, an exceptionally strong (about 2x that of the previous three years) buoyancy-driven destratification led to a shoaling of the pycnocline, and the restratification at the end of 2015 remained weak. During 2016 and 2017, the buoyancy-driven destratification decreased in strength, becoming weakest during the polynya of 2017. The destratification was once again strong in 2018, but this was balanced by a stratifying forcing from the surface stress and advective components, the latter of which was associated with a transport of denser (more saline and cooler) subsurface waters from the flanks of Maud Rise. These denser subsurface waters maintained a strong stratification through 2018. These results show how interannual anomalies in local sea ice production and regional circulation can promote or inhibit the formation of polynyas in the region. Furthermore, it suggests that the Maud Rise polynya opened in 2017 following a chain of perturbations that started at least back in 2015, contrary to the common view that the polynya was initiated solely  by a series of short-lived storms in 2017.</p>