Abrahamsen E. Povl

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Abrahamsen
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E. Povl
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  • Article
    Rapid mixing and exchange of deep-ocean waters in an abyssal boundary current.
    (National Academy of Sciences, 2019-07-02) Naveira Garabato, Alberto C. ; Frajka-Williams, Eleanor E. ; Spingys, Carl P. ; Legg, Sonya ; Polzin, Kurt L. ; Forryan, Alexander ; Abrahamsen, E. Povl ; Buckingham, Christian E. ; Griffies, Stephen M. ; McPhail, Stephen D. ; Nicholls, Keith W. ; Thomas, Leif N. ; Meredith, Michael P.
    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.
  • Article
    Mixing and transformation in a deep western boundary current: a case study
    (American Meteorological Society, 2021-03-29) Spingys, Carl P. ; Naveira Garabato, Alberto C. ; Legg, Sonya ; Polzin, Kurt L. ; Abrahamsen, E. Povl ; Buckingham, Christian E. ; Forryan, Alexander ; Frajka-Williams, Eleanor E.
    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.
  • Article
    ASPIRE : the Amundsen Sea Polynya International Research Expedition
    (The Oceanography Society, 2012-09) Yager, Patricia L. ; Sherrell, Robert M. ; Stammerjohn, Sharon E. ; Alderkamp, Anne-Carlijn ; Schofield, Oscar M. E. ; Abrahamsen, E. Povl ; Arrigo, Kevin R. ; Bertilsson, Stefan ; Garay, D. Lollie ; Guerrero, Raul ; Lowry, Kate E. ; Moksnes, Per-Olav ; Ndungu, Kuria ; Post, Anton F. ; Randall-Goodwin, Evan ; Riemann, Lasse ; Severmann, Silke ; Thatje, Sven ; van Dijken, Gert L. ; Wilson, Stephanie
    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.
  • Article
    Boundary mixing in Orkney Passage outflow
    (John Wiley & Sons, 2014-12-16) Polzin, Kurt L. ; Naveira Garabato, Alberto C. ; Abrahamsen, E. Povl ; Jullion, Loic ; Meredith, Michael P.
    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.