Genesis of the Antarctic Slope Current in West Antarctica

The stability of the West Antarctic Ice Sheet (WAIS) depends on ocean heat transport toward its base and remains a source of uncertainty in sea level rise prediction. The Antarctic Slope Current (ASC), a major boundary current of the ocean's global circulation, serves as a dynamic gateway for heat transport toward Antarctica. Here, we use observations collected from the Bellingshausen Sea to propose a mechanistic explanation for the initiation of the westward‐flowing ASC. Waters modified throughout the Bellingshausen Sea by ocean‐sea‐ice and ocean‐ice‐shelf interactions are exported to the continental slope in a narrow, topographically steered western boundary current. This focused outflow produces a localized front at the shelf break that supports the emerging ASC. This mechanism emphasizes the importance of buoyancy forcing, integrated over the continental shelf, as opposed to local wind forcing, in the generation mechanism and suggests the potential for remote control of melt rates of WAIS' largest ice shelves

Wind-driven transport of fresh shelf water into the upper 30m of the Labrador Sea

The Labrador Sea is one of a small number of deep convection sites in the North Atlantic that contribute to the meridional overturning circulation. Buoyancy is lost from surface waters during winter, allowing the formation of dense deep water. During the last few decades, mass loss from the Greenland ice sheet has accelerated, releasing freshwater into the high-latitude North Atlantic. This and the enhanced Arctic freshwater export in recent years have the potential to add buoyancy to surface waters, slowing or suppressing convection in the Labrador Sea. However, the impact of freshwater on convection is dependent on whether or not it can escape the shallow, topographically trapped boundary currents encircling the Labrador Sea. Previous studies have estimated the transport of freshwater into the central Labrador Sea by focusing on the role of eddies. Here, we use a Lagrangian approach by tracking particles in a global, eddy-permitting (1∕12°) ocean model to examine where and when freshwater in the surface 30m enters the Labrador Sea basin. We find that 60% of the total freshwater in the top 100m enters the basin in the top 30m along the eastern side. The year-to-year variability in freshwater transport from the shelves to the central Labrador Sea, as found by the model trajectories in the top 30m, is dominated by wind-driven Ekman transport rather than eddies transporting freshwater into the basin along the northeast

Ice‐shelf meltwater overturning in the Bellingshausen Sea

Hydrographic data are analyzed for the broad continental shelf of the Bellingshausen Sea, which is host to a number of rapidly‐thinning ice shelves. The flow of warm Circumpolar Deep Water (CDW) onto the continental shelf is observed in the two major glacially‐carved troughs, the Belgica and Latady troughs. Using ship‐based measurements of potential temperature, salinity and dissolved oxygen, collected across several coast‐to‐coast transects over the Bellingshausen shelf in 2007, the velocity and circulation patterns are inferred based on geostrophic balance and further constrained by the tracer and mass budgets. Meltwater was observed at the surface and at intermediate depth toward the western side of the continental shelf, collocated with inferred outflows. The maximum conversion rate from the dense CDW to lighter water masses by mixing with glacial meltwater is estimated to be 0.37±0.1 Sv in both depth and potential density spaces. This diapycnal overturning is comparable to previous estimates made in the neighboring Amundsen Sea, highlighting the overlooked importance of water mass modification and meltwater production associated with glacial melting in the Bellingshausen Sea

Variability in the meridional overturning circulation at 32°S in the Pacific Ocean diagnosed by inverse box models

The meridional circulation and transport at 32°S in the Pacific Ocean in 1992 and 2017 are compared with analogous data from 2003 and 2009 computed by Hernández-Guerra and Talley (2016). The hydrographic data come from the GO-SHIP database and an inverse box model has been applied with similar constraints as in Hernández-Guerra and Talley (2016). In 1992, 2003 and 2017 the pattern of the overturning streamfunction and circulation are similar, but in 2009 the pattern of the circulation changes in the whole water column. The horizontal distribution of mass transports at all depths in 1992 and 2017 resembles the familiar shape of the “classical gyre” also observed in 2003 and is notably different to the “bowed gyre” found in 2009. The hydrographic data have been compared with data obtained from the numerical modelling outputs of ECCO, SOSE, GLORYS, and MOM. Results show that none of these models properly represents the “bowed gyre” circulation in 2009, and this change in circulation pattern was not observed during the entire length of model simulations. Additionally, the East Australian Current in the western boundary presents higher mass transport in the hydrographic data than in any numerical modelling output. Its poleward mass transport ranges from −35.1 ± 2.0 Sv in 1992 to −54.3 ± 2.6 Sv in 2003. Conversely, the Peru-Chile Current is well represented in models and presents an equatorward mass transport from 2.3 ± 0.8 Sv in 2009 to 4.4 ± 1.0 Sv in 1992. Furthermore, the Peru-Chile Undercurrent presents a more intense poleward mass transport in 2009 (−3.8 ± 1.2 Sv). In addition, the temperature and freshwater transports in 1992 (0.42 ± 0.12 PW and 0.26 ± 0.08 Sv), 2003 (0.38 ± 0.12 PW and 0.25 ± 0.02 Sv), and 2017 (0.42 ± 0.12 PW and 0.34 ± 0.08 Sv) are similar, but significantly different from those in 2009 (0.16 ± 0.12 PW and 0.50 ± 0.03 Sv, respectively). To clarify the causes of these different circulation schemes, a linear Rossby wave model is adopted, which includes the wind-stress curl variability as remote forcing and the response to sea surface height changes along 30°S.This study was supported by the SAGA project (RTI2018-100844-B-C31) funded by the Ministerio de Ciencia, Innovación y Universidades of the Spanish Government. This article is a publication of the Unidad Océano y Clima from Universidad de Las Palmas de Gran Canaria, an R&D&I CSIC-associate unit. The wind data were collected from NCEP Reanalysis Derived data (http://www.eslr.noaa.gov/psd/). Hydrographic data were collected from the CCHDO website in the frame of International WOCE and GO-SHIP projects (https://cchdo.ucsd.edu/). We gratefully acknowledge the major efforts of the WOCE/GO-SHIP program’s chief scientists that collected these transect data: H. L. Bryden, M. McCartney, J. Toole M. Fukasawa, S. Watanabe, Y. Yoshikawa, A. Macdonald, R. Curry, S. Mecking, and K. Speer. ECCO data are available for download at https://ecco.jpl.nasa.gov/. MOM data are available at https://www.gfdl.noaa.gov/mom-ocean-model/. SOSE data are available at http://sose.ucsd.edu. GLORYS data are available for download at https://resources.marine.copernicus.eu/. The SSHA data were collected from the Aviso database (http://las.aviso.oceanobs.com). The authors declare no competing interests. This work has been completed as part of C. Arumí-Planas work at IOCAG, in the doctoral program in Oceanography and Global Change. C. Arumí-Planas acknowledges the Agencia Canaria de Investigación, Innovación y Sociedad de la Información (ACIISI) grant program of “Apoyo al personal investigador en formación” TESIS2021010028.Peer reviewe