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The Southern Ocean is a critical component of Earth's climate system, but its remoteness makes it challenging to develop a holistic understanding of its processes from the small scale to the large scale. As a result, our knowledge of this vast region remains largely incomplete. The Antarctic Circumnavigation Expedition (ACE, austral summer 2016/2017) surveyed a large number of variables describing the state of the ocean and the atmosphere, the freshwater cycle, atmospheric chemistry, and ocean biogeochemistry and microbiology. This circumpolar cruise included visits to 12 remote islands, the marginal ice zone, and the Antarctic coast. Here, we use 111 of the observed variables to study the latitudinal gradients, seasonality, shorter-term variations, geographic setting of environmental processes, and interactions between them over the duration of 90 d. To reduce the dimensionality and complexity of the dataset and make the relations between variables interpretable we applied an unsupervised machine learning method, the sparse principal component analysis (sPCA), which describes environmental processes through 14 latent variables. To derive a robust statistical perspective on these processes and to estimate the uncertainty in the sPCA decomposition, we have developed a bootstrap approach. Our results provide a proof of concept that sPCA with uncertainty analysis is able to identify temporal patterns from diurnal to seasonal cycles, as well as geographical gradients and "hotspots" of interaction between environmental compartments. While confirming many well known processes, our analysis provides novel insights into the Southern Ocean water cycle (freshwater fluxes), trace gases (interplay between seasonality, sources, and sinks), and microbial communities (nutrient limitation and island mass effects at the largest scale ever reported). More specifically, we identify the important role of the oceanic circulations, frontal zones, and islands in shaping the nutrient availability that controls biological community composition and productivity; the fact that sea ice controls sea water salinity, dampens the wave field, and is associated with increased phytoplankton growth and net community productivity possibly due to iron fertilisation and reduced light limitation; and the clear regional patterns of aerosol characteristics that have emerged, stressing the role of the sea state, atmospheric chemical processing, and source processes near hotspots for the availability of cloud condensation nuclei and hence cloud formation. A set of key variables and their combinations, such as the difference between the air and sea surface temperature, atmospheric pressure, sea surface height, geostrophic currents, upper-ocean layer light intensity, surface wind speed and relative humidity played an important role in our analysis, highlighting the necessity for Earth system models to represent them adequately. In conclusion, our study highlights the use of sPCA to identify key ocean-atmosphere interactions across physical, chemical, and biological processes and their associated spatio-temporal scales. It thereby fills an important gap between simple correlation analyses and complex Earth system models. The sPCA processing code is available as open-access from the following link: https://renkulab.io/gitlab/ACE-ASAID/spca-decomposition (last access: 29 March 2021). As we show here, it can be used for an exploration of environmental data that is less prone to cognitive biases (and confirmation biases in particular) compared to traditional regression analysis that might be affected by the underlying research question., Earth System Dynamics, 12 (4), ISSN:2190-4987, ISSN:2190-4979

The 1st Southern Ocean Regional Workshop to provide Southern Ocean community inputs to the Science Action Plan of the UN Decade of Ocean Science was held on 16 February 2020 in San Diego, CA, USA. The workshop was held prior to the 2020 Ocean Sciences Meeting to engage diverse participation. The 30 participants, who represented 11 countries, included representatives with expertise in ocean sciences, policy, governance and science communication, from all career stages. The workshop was structured around an opening plenary session, three themed sessions with accompanying breakout groups, and a final plenary session focused on cross-cutting themes (see also Appendix I + II). The opening plenary session provided information about the UN Decade of Ocean Science. This session informed the three theme sessions that focused on a Healthy and Resilient Ocean, a Predicted Ocean, and a Sustainable Productive Ocean. These three of the six UN Ocean Decade themes were chosen as the focus of the workshop because of relevance to the Southern Ocean, and these could be addressed in a one-day workshop. The other UN Ocean Decade themes, a Clean Ocean, a Safe Ocean, and a Transparent and Accessible Ocean, were included implicitly in the discussions of these three themes. The closing plenary session addressed cross-cutting themes. Additional information about the workshop organization is provided in Appendix II. Summaries and recommendations from each of the theme-focused breakout groups and cross-cutting session follow.&nbsp

This community white paper describes the motivation and potential ideas for a field campaign focused on understanding salinity variability in the high-latitude oceans. Such an experiment would potentially be of interest to the NASA Physical Oceanography program.

2020 European Polar Science Week, 26 October 2020

Much of the Southern Ocean surface south of 55° S cooled and freshened between at least the early 1980s and the early 2010s. Many processes have been proposed to explain the unexpected cooling, including increased winds or freshwater fluxes. However, these mechanisms so far failed to fully explain the surface trends and the concurrent subsurface warming (100 to 500 m). Here, we argue that these trends are predominantly caused by an increased wind‐driven northward sea‐ice transport, enhancing the extraction of freshwater near Antarctica and releasing it in the open ocean. This conclusion is based on factorial experiments with a regional ocean model. In all experiments with an enhanced northward sea‐ice transport, a strengthened salinity‐dominated stratification cools the open‐ocean surface waters between the Subantarctic Front and the sea‐ice edge. The strengthened stratification reduces the downward mixing of cold surface water and the upward heat loss of the warmer waters below, thus warming the subsurface. This sea‐ice induced subsurface warming mostly occurs around West Antarctica, where it likely enhances ice‐shelf melting. Moreover, the subsurface warming could account for about 8 ± 2% of the global ocean heat content increase between 1982 and 2011. Antarctic sea‐ice changes thereby may have contributed to the slowdown of global surface warming over this period. Our conclusions are robust across all considered sensitivity cases, although the trend magnitude is sensitive to forcing uncertainties and the model's mean state. It remains unclear whether these sea‐ice induced changes are associated with natural variability or reflect a response to anthropogenic forcing., AGU Advances, 1 (2), ISSN:2576-604X

European Geosciences Union (EGU) General Assembly, 7-12 April 2019, Vienna, Austria.-- 1 page, The Southern Ocean (SO), directly connected to the global ocean through the Atlantic, the Indian and the Pacific basins, may be responsible for transporting vast amounts of salt, heat and nutrients across basins, which in turn might have a direct influence in the global climate. According to the Coupled Model Intercomparison Project Phase 5 (CMIP5) predictions, a freshening around the Antarctic coast which can change the ocean dynamics around the Antarctic Peninsula is possible. However these predictions are hampered by the limited number of in situ temperature and salinity observations. the development of reliable satellite observation systems for sea surface salinity (SSS) and sea surface temperature at high southern latitudes can therefore contribute to improve the CMIP5 inter-annual variability and trends, as well as the understanding of the dynamics associated with the SO seasonal and intra-seasonal variability. The Barcelona Expert Centre (BEC) has generated an enhanced SO SSS dataset (2011-2018) from the Soil Moisture and Ocean Salinity (SMOS) mission. This SSS product uses an innovative SSS retrieval approach, known as debiased non-Bayesian scheme [1], which has proven to increase the satellite capabilities to observe SSS dynamics in the Arctic and sub-Arctic regions [2]. The new SMOS SSS product is validated in the SO region against both in situ and an ocean reanalysis (ARMOR), model (GLORYS), and climatology (WOA) data. The in situ database comprises a suite of Fiducial Reference Measurements (FRM) which include ARGO floats, marine mammals observant and ship based observations (e.g., CTD, TSG, etc.) which have been collected by different research vessels (e.g. the Astrolabe, Hesperides, Agulhas, Agulhas II, and Akademik Treshnikov) over their Southern Ocean crossings. We have assessed the SMOS salinity fields in three different bands: Subantarctic, Antarctic and Subpolar bands. ARMOR, GLORYS, WOA and SMOS are in good agreement in the Subantarctic and Antarctic bands (with SMOS discrepancies of +/-0.1 psu). In the Subpolar bands SMOS is in better agreement with GLORYS than with ARMOR and WOA. In this region both, GLORYS and SMOS show fresher salinityfields and larger salinity variations than WOA and ARMOR. Regarding comparison with TSG, SMOS is able to capture fresh and saline plumes in the Weddell Sea, which are not captured by any of the other analysed products (ARMOR, GLORYS and WOA). References: [1] Olmedo, E., Martinez, J., Turiel, A., Ballabrera-Poy, J., Portabella, M. >Debiased non-Bayesian retrieval: A novel approach to SMOS Sea Surface Salinity>, 2017, Remote Sens. Environ.,193, 103-126. [2] Olmedo, E., Gabarró, C., González-Gambau, V., Justino Martínez, J., Ballabrera-Poy, J., Turiel, A., Portabella, M., Fournier, S. and Tong Lee, T. >Seven Years of SMOS Sea Surface Salinity at High Latitudes: Variability in Arctic and Sub-Arctic Regions>, 2018, Remote Sensing 10(11):1772 , DOI: 10.3390/rs10111772

The biogeography of Southern Ocean phytoplankton controls not only the local biogeochemistry, but also the export of macronutrients to lower latitudes and depth. Of particular relevance is the interaction between coccolithophores and diatoms, with the former being prevalent along the "Great Calcite Belt" (40–60°S), while diatoms tend to dominate the regions south of 60°S. To address the factors controlling coccolithophore distribution and the competition between them and diatoms, we use a regional high-resolution model (ROMS-BEC) for the Southern Ocean (24–78°S) that has been extended to include an explicit representation of coccolithophores. We assess the relative importance of bottom-up (temperature, nutrients, light) and top-down (grazing by zooplankton) factors in controlling Southern Ocean coccolithophore biogeography over the course of the growing season. In our simulations, coccolithophores are an important member of the Southern Ocean phytoplankton community, contributing 15% to annually integrated net primary productivity south of 30°S. Coccolithophore biomass is highest north of 50°S in late austral summer, when light levels are high and diatoms become silicate limited. Furthermore, we find top-down factors to be a major control on the relative abundance of diatoms and coccolithophores in the Southern Ocean. Consequently, when assessing potential future changes in Southern Ocean coccolithophore abundance, both abiotic (temperature, light, nutrients, pH) and biotic factors (interaction with diatoms and zooplankton) need to be considered. ISSN:1810-6277 ISSN:1810-6285

Nature, 537 (7618), ISSN:0028-0836, ISSN:1476-4687

In this study, we identify a biomass-burning signal in molecular hydrogen (H2) over the Amazonian tropical rainforest. To quantify this signal, we measure the mixing ratios of H2 and several other species as well as the H2 isotopic composition in air samples that were collected in the BARCA (Balanço Atmosférico Regional de Carbono na Amazônia) aircraft campaign during the dry season. We derive a relative H2 emission ratio with respect to carbon monoxide (CO) of 0.31 ± 0.04 ppb ppb−1 and an isotopic source signature of −280 ± 41‰ in the air masses influenced by tropical biomass burning. In order to retrieve a clear source signal that is not influenced by the soil uptake of H2, we exclude samples from the atmospheric boundary layer. This procedure is supported by data from a global chemistry transport model. The ΔH2 / ΔCO emission ratio is significantly lower than some earlier estimates for the tropical rainforest. In addition, our results confirm the lower values of the previously conflicting estimates of the H2 isotopic source signature from biomass burning. These values for the emission ratio and isotopic source signatures of H2 from tropical biomass burning can be used in future bottom-up and top-down approaches aiming to constrain the strength of the biomass-burning source for H2. Hitherto, these two quantities relied only on combustion experiments or on statistical relations, since no direct signal had been obtained from in-situ observations., Atmospheric Chemistry and Physics, 13 (18), ISSN:1680-7375, ISSN:1680-7367