Collaborative Research: Dynamics of the Orkney Passage Outflow

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

Cold and dense water masses are formed through air-sea interaction near the Antarctic and are funneled through narrow passages as they flow downward into the deep ocean basins. The intense mixing that occurs along the way in these passages sets the properties of the Antarctic Bottom Water, which spreads out to fill the deepest layers over much of the global ocean. One of the most remarkable features of contemporary oceanic climate change is the warming and contraction of Antarctic Bottom Water. This study will make new field measurements and computer simulations to address questions of how long-term variability in the dense waters formed near the Antarctic is translated into downstream variability elsewhere, such as the deep basins of the Atlantic Ocean, and whether the observed warming trends result from diminishing Antarctic contributions to the Meridional Overturning Circulation or from increased mixing. Orkney Passage is a key circulation choke point that governs ocean exchanges between the marginal seas of the Antarctic Continent and the Southern Ocean: approximately 5 Sverdrups (Sv) of newly ventilated Antarctic Bottom Water are funneled through this narrow passage into the Scotia Sea (Naveira Garabato et al., 2002) which represents a significant contribution to the total 15 Sv of Antarctic Bottom Water estimated to pass equatorward of the Antarctic Circumpolar Current's southern boundary (Naveira Garabato et al., 2014). Existing data (LADCP and CTD) reveal the presence of thick bottom boundary layers, 500 meters in vertical extent, with intense thermal wind shear above. Flows within the passage may also exhibit intense horizontal velocity gradients and be hydraulically controlled upstream and/or downstream of these observations: in the highly energetic and variable environment above the bottom boundary layer, overturns exceeding 100 meters extent have been observed. A control volume budget suggests that high levels of mixing must continue downstream of Orkney Passage: the absence of observed mixing in the Scotia Sea interior suggests this enhanced mixing must be located along a boundary. This study will test the hypothesis that enhanced mixing in the downstream boundary current results from overturning generated by tidally-driven cross-slope shear in the Ekman boundary layer. This parameter regime, however, is poorly understood from a theoretical standpoint owing in part to a paucity of direct sampling of flows and diapycnal processes in situations such as this. The dynamics that set turbulent mixing and transports within Orkney Passage and in the boundary current downstream will be investigated using a combination of numerical modeling and field measurements. The fieldwork will complement observations of the diabatic and frictional processes in Orkney Passage being carried out by collaborators in the UK, by employing a mooring downstream of Orkney Passage. Regional numerical simulations using the MITgcm model will be used in planning and interpreting the field measurements, and importantly, in improving methods to accurately represent flows through narrow passages in climate models such as the GFDL MOM6. Ultimately, the observations and simulations will be used to address questions of how long-term variability in the upstream Weddell Sea Deep and Bottom Water properties is translated into downstream variability in the Scotia Sea: whether warming trends of Antarctic Bottom Water in the Scotia Sea and Atlantic Ocean result from diminishing Antarctic contributions to the Atlantic Meridional Overturning Circulation [e.g. Johnson et al. (2008)] or increased diabatic mixing associated with a strengthened Weddell Gyre (Meredith et al., 2011). This study will significantly improve the understanding of continuously stratified, rotating flow dynamics in a sparsely sampled parameter regime with horizontal velocities having strong horizontal and vertical gradients [Rossby number, Froude number ∼ O(1)] and be applicable to many choke points of global deep ocean circulation. The observations will be applied to improve the GFDL ocean general circulation model, among the best of the coupled models contributing to IPCC future climate projections. The project will promote teaching and training, by including 3 undergraduate interns (2 at Princeton, 1 at WHOI), with a particular effort made to recruit under-represented minority students.