Description: Extremely low summer sea-ice coverage in the Arctic Ocean in 2007 allowed extensive sampling and a wide quasi-synoptic hydrographic and δ18O dataset could be collected in the Eurasian Basin and the Makarov Basin up to the Alpha Ridge and the East Siberian continental margin. With the aim of determining the origin of freshwater in the halocline, fractions of river water and sea-ice meltwater in the upper 150 m were quantified by a combination of salinity and δ18O in the Eurasian Basin. Two methods, applying the preformed phosphate concentration (PO*) and the nitrate-to-phosphate ratio (N/P), were compared to further differentiate the marine fraction into Atlantic and Pacific-derived contributions. While PO*-based assessments systematically underestimate the contribution of Pacific-derived waters, N/P-based calculations overestimate Pacific-derived waters within the Transpolar Drift due to denitrification in bottom sediments at the Laptev Sea continental margin. Within the Eurasian Basin a west to east oriented front between net melting and production of sea-ice is observed. Outside the Atlantic regime dominated by net sea-ice melting, a pronounced layer influenced by brines released during sea-ice formation is present at about 30 to 50 m water depth with a maximum over the Lomonosov Ridge. The geographically distinct definition of this maximum demonstrates the rapid release and transport of signals from the shelf regions in discrete pulses within the Transpolar Drift. The ratio of sea-ice derived brine influence and river water is roughly constant within each layer of the Arctic Ocean halocline. The correlation between brine influence and river water reveals two clusters that can be assigned to the two main mechanisms of sea-ice formation within the Arctic Ocean. Over the open ocean or in polynyas at the continental slope where relatively small amounts of river water are found, sea-ice formation results in a linear correlation between brine influence and river water at salinities of about 32 to 34. In coastal polynyas in the shallow regions of the Laptev Sea and southern Kara Sea, sea-ice formation transports river water into the shelf’s bottom layer due to the close proximity to the river mouths. This process therefore results in waters that form a second linear correlation between brine influence and river water at salinities of about 30 to 32. Our study indicates which layers of the Arctic Ocean halocline are primarily influenced by sea-ice formation in coastal polynyas and which layers are primarily influenced by sea-ice formation over the open ocean. Accordingly we use the ratio of sea-ice derived brine influence and river water to link the maximum in brine influence within the Transpolar Drift with a pulse of shelf waters from the Laptev Sea that was likely released in summer 2005.

Description: With the aim of determining the origin of freshwater in the halocline, fractions of river water and sea-ice meltwater (or brine influence from sea-ice formation) in the upper 150 m were quantified by a combination of salinity and _18O and nutrients in the Eurasian basins and the Makarov Basin. Our study indicates which layers of the Arctic Ocean halocline are primarily influenced by sea-ice formation in coastal polynyas and which are primarily influenced by sea-ice formation over the open ocean. With the ongoing changes in sea-ice coverage in the Arctic Ocean it can be expected that these processes will change in the immediate future and that the relative contributions to the halocline will change accordingly. Within the Eurasian Basin a west to east oriented front between net melting and production of sea-ice is observed. Outside the Atlantic regime dominated by net sea-ice melting, a pronounced layer influenced by brines released during sea-ice formation is present at about 30 to 50 m water depth with a maximum over the Lomonosov Ridge. The geographically distinct definition of this maximum demonstrates the rapid release and transport of signals from the shelf regions in discrete pulses within the Transpolar Drift. We use the ratio of sea-ice derived brine influence and river water to link the maximum in brine influence within the Transpolar Drift with a pulse of shelf waters from the Laptev Sea likely released in summer 2005. For a distinction of Atlantic and Pacific-derived contributions the initial phosphate corrected for mineralization with oxygen (PO*) and alternatively the nitrate to phosphate ratio (N/P) in each sample were used. While PO*-based assessments systematically underestimate the contribution of Pacific-derived waters, N/P-based calculations overestimate Pacific-derived waters within the Transpolar Drift due to denitrification in bottom sediments of the Laptev Sea. The extent of Pacific-derived water in the Arctic Ocean was approximately limited by the position of the Lomonosov Ridge in 2007. The ratio of sea-ice derived brine influence and river water is roughly constant within each layer of the Arctic Ocean halocline. The correlation between brine influence and river water reveals two clusters that can be assigned to the two main mechanisms of sea-ice formation within the Arctic Ocean. Over the open ocean or in polynyas at the continental slope sea-ice formation results in a linear correlation between brine influence and river water at salinities of ~ 32 to 34. In coastal polynyas in the shallow regions of the Laptev Sea and southern Kara Sea, sea-ice formation transports river water into the shelf’s bottom layer due to the close proximity to the river mouths. This process results in a second linear correlation between brine influence and river water at salinities of ~ 30 to 32.

Description: Pine Island Glacier has thinned and accelerated over recent decades, significantly contributing to global sea level rise. Increased oceanic melting of its ice shelf is thought to have triggered those changes. Observations and numerical modeling reveal large fluctuations in the ocean heat available in the adjacent bay and enhanced sensitivity of ice shelf melting to water temperatures at intermediate depth, as a seabed ridge blocks the deepest and warmest waters from reaching the thickest ice. Oceanic melting decreased by 50% between January 2010 and 2012, with ocean conditions in 2012 partly attributable to atmospheric forcing associated with a strong La Niña event. Both atmospheric variability and local ice shelf and seabed geometry play fundamental roles in determining the response of the Antarctic Ice Sheet to climate.

Description: The West Antarctic Ice Sheet (WAIS) is one of the largest potential sources of future sea-level rise, with glaciers draining the WAIS thinning at an accelerating rate over the past 40 years. Due to complexities in calibrating palaeoceanographic proxies for the Southern Ocean, it remains difficult to assess whether similar changes have occurred earlier during the Holocene or whether there is underlying centennial- to millennial-scale forcing in oceanic variability. Archaeal lipid-based proxies, specifically glycerol dialkyl glycerol tetraether (GDGT; e.g. TEX86 and TEXL86), are powerful tools for reconstructing ocean temperature, but these proxies have been shown previously to be difficult to apply to the Southern Ocean. A greater understanding of the parameters that control Southern Ocean GDGT distributions would improve the application of these biomarker proxies and thus help provide a longer-term perspective on ocean forcing of Antarctic ice sheet changes. In this study, we characterised intact polar lipid (IPL)-GDGTs, representing (recently) living archaeal populations in suspended particulate matter (SPM) from the Amundsen Sea and the Scotia Sea. SPM samples from the Amundsen Sea were collected from up to four water column depths representing the surface waters through to Circumpolar Deep Water (CDW), whereas the Scotia Sea samples were collected along a transect encompassing the sub-Antarctic front through to the southern boundary of the Antarctic Circumpolar Current. IPL-GDGTs with low cyclic diversity were detected throughout the water column with high relative abundances of hydroxylated IPL-GDGTs identified in both the Amundsen and Scotia seas. Results from the Scotia Sea show shifts in IPL-GDGT signatures across well-defined fronts of the Southern Ocean. Indicating that the physicochemical parameters of these water masses determine changes in IPL-GDGT distributions. The Amundsen Sea results identified GDGTs with hexose-phosphohexose head groups in the CDW, suggesting active GDGT synthesis at these depths. These results suggest that GDGTs synthesised at CDW depths may be a significant source of GDGTs exported to the sedimentary record and that temperature reconstructions based on TEX86 or TEXL86 proxies may be significantly influenced by the warmer waters of the CDW.

Description: Author Posting. © The Oceanography Society, 2012. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 25, no. 3 (2012): 40-53, doi:10.5670/oceanog.2012.73.

Description: In search of an explanation for some of the greenest waters ever seen in coastal Antarctica and their possible link to some of the fastest melting glaciers and declining summer sea ice, the Amundsen Sea Polynya International Research Expedition (ASPIRE) challenged the capabilities of the US Antarctic Program and RVIB Nathaniel B. Palmer during Austral summer 2010–2011. We were well rewarded by both an extraordinary research platform and a truly remarkable oceanic setting. Here we provide further insights into the key questions that motivated our sampling approach during ASPIRE and present some preliminary findings, while highlighting the value of the Palmer for accomplishing complex, multifaceted oceanographic research in such a challenging environment.

Description: This project was funded by the National Science Foundation Office of Polar Programs, Antarctic Organisms and Ecosystems (ANT-0839069 to PY, ANT-0838995 to RS, ANT-0838975 to SS, ANT-0838995 to OS, ANT- 0944727 to KA, and ANT-0839012 to Hugh Ducklow), and the Swedish Research Council (Grant 2008-6430 to SB and LR), with logistic support from the Swedish Polar Research Secretariat and Raytheon Polar Services.

Description: Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 119 (2014): 8627–8645, doi:10.1002/2014JC010099.

Description: One of the most remarkable features of contemporary oceanic climate change is the warming and contraction of Antarctic Bottom Water over much of global ocean abyss. These signatures represent changes in ventilation mediated by mixing and entrainment processes that may be location-specific. Here we use available data to document, as best possible, those mixing processes as Weddell Sea Deep and Bottom Waters flow along the South Orkney Plateau, exit the Weddell Sea via Orkney Passage and fill the abyssal Scotia Sea. First, we find that an abrupt transition in topography upstream of Orkney Passage delimits the extent of the coldest waters along the Plateau's flanks and may indicate a region of especially intense mixing. Second, we revisit a control volume budget by Heywood et al. (2002) for waters trapped within the Scotia Sea after entering through Orkney Passage. This budget requires extremely vigorous water mass transformations with a diapycnal transfer coefficient of inline image m2 s−1. Evidence for such intense diapycnal mixing is not found in the abyssal Scotia Sea interior and, while we do find large rates of diapycnal mixing in conjunction with a downwelling Ekman layer on the western side of Orkney Passage, it is insufficient to close the budget. This leads us to hypothesize that the Heywood budget is closed by a boundary mixing process in which the Ekman layer associated with the Weddell Sea Deep Water boundary current experiences relatively large vertical scale overturning associated with tidal forcing along the southern boundary of the Scotia Sea.

Description: KLP gratefully acknowledges salary support from Woods Hole Oceanographic Institution bridge support funds. ACNG acknowledges the support of a Philip Leverhulme Prize. LJ and MPM were supported by the ANDREX project, funded by the U.K. National Environment Research Council (NE/E01366X/1).

Description: 2015-06-16

Description: Author Posting. © National Academy of Sciences, 2019. This article is posted here by permission of National Academy of Sciences for personal use, not for redistribution. The definitive version was published in Proceedings of the National Academy of Sciences 116(27), (2019): 13233-13238, doi: 10.1073/pnas.1904087116.

Description: The overturning circulation of the global ocean is critically shaped by deep-ocean mixing, which transforms cold waters sinking at high latitudes into warmer, shallower waters. The effectiveness of mixing in driving this transformation is jointly set by two factors: the intensity of turbulence near topography and the rate at which well-mixed boundary waters are exchanged with the stratified ocean interior. Here, we use innovative observations of a major branch of the overturning circulation—an abyssal boundary current in the Southern Ocean—to identify a previously undocumented mixing mechanism, by which deep-ocean waters are efficiently laundered through intensified near-boundary turbulence and boundary–interior exchange. The linchpin of the mechanism is the generation of submesoscale dynamical instabilities by the flow of deep-ocean waters along a steep topographic boundary. As the conditions conducive to this mode of mixing are common to many abyssal boundary currents, our findings highlight an imperative for its representation in models of oceanic overturning.

Description: The DynOPO project is supported by the UK Natural Environment Research Council (grants NE/K013181/1 and NE/K012843/1) and the US National Science Foundation (grants OCE-1536453 and OCE-1536779). A.C.N.G. acknowledges the support of the Royal Society and the Wolfson Foundation. S.L. acknowledges the support of award NA14OAR4320106 from the National Oceanic and Atmospheric Administration, US Department of Commerce. The statements, findings, conclusions, and recommendations are those of the authors, and do not necessarily reflect the views of the National Oceanic and Atmospheric Administration, or the US Department of Commerce. We are grateful to the scientific party, crew, and technicians on the RRS James Clark Ross for their hard work during data collection.

Description: 2019-12-18

Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Spingys, C. P., Garabato, A. C. N., Legg, S., Polzin, K. L., Abrahamsen, E. P., Buckingham, C. E., Forryan, A., & Frajka-Williams, E. E. Mixing and transformation in a deep western boundary current: a case study. Journal of Physical Oceanography, 51(4), (2021): 1205-1222, https://doi.org/10.1175/JPO-D-20-0132.1

Description: Water-mass transformation by turbulent mixing is a key part of the deep-ocean overturning, as it drives the upwelling of dense waters formed at high latitudes. Here, we quantify this transformation and its underpinning processes in a small Southern Ocean basin: the Orkney Deep. Observations reveal a focusing of the transport in density space as a deep western boundary current (DWBC) flows through the region, associated with lightening and densification of the current’s denser and lighter layers, respectively. These transformations are driven by vigorous turbulent mixing. Comparing this transformation with measurements of the rate of turbulent kinetic energy dissipation indicates that, within the DWBC, turbulence operates with a high mixing efficiency, characterized by a dissipation ratio of 0.6 to 1 that exceeds the common value of 0.2. This result is corroborated by estimates of the dissipation ratio from microstructure observations. The causes of the transformation are unraveled through a decomposition into contributions dependent on the gradients in density space of the: dianeutral mixing rate, isoneutral area, and stratification. The transformation is found to be primarily driven by strong turbulence acting on an abrupt transition from the weakly stratified bottom boundary layer to well-stratified off-boundary waters. The reduced boundary layer stratification is generated by a downslope Ekman flow associated with the DWBC’s flow along sloping topography, and is further regulated by submesoscale instabilities acting to restratify near-boundary waters. Our results provide observational evidence endorsing the importance of near-boundary mixing processes to deep-ocean overturning, and highlight the role of DWBCs as hot spots of dianeutral upwelling.

Description: CS, ACNG, AF, and EFW were supported by the U.K. Natural Environment Research Council (NERC) Grant NE/K013181/1. ACNG was supported by the Royal Society and Wolfson Foundation. EPA and CEB were supported by NERC Grant NE/K012843/1. CEB was funded by an MSCA grant (No. 798319) from the European Union’s Horizon 2020 program. EPA was supported by NERC Grant NE/N018095/1. SL and KP were supported by U.S. National Science Foundation Grants OCE-1536453 and OCE-1536779. SL acknowledges support of Award NA18OAR4320123 from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce. The statements, findings, conclusions, and recommendations are those of the authors, and do not necessarily reflect the views of the National Oceanic and Atmospheric Administration, or the U.S. Department of Commerce.