Dynamics of the Orkney Passage Outflow (DynOPO)

Lead Research Organisation: NERC British Antarctic Survey
Department Name: Science Programmes

Abstract

During the last three decades, the Antarctic Botttom Water (AABW) filling the bulk of the global ocean abyss has exhibited a striking warming and contraction in volume over much of the world ocean, particularly in the Atlantic basin. While the causes of these changes are unknown, available evidence suggests that, in the Atlantic Ocean, the warming and contraction of AABW may be caused by changes in winds over the northern Weddell Sea, where much AABW is produced. This hypothesis asserts that those winds regulate the volume and temperature of the AABW exported northward via the Orkney Passage (a major AABW exit route from the Weddell Sea) by altering the intensity of the turbulent mixing between AABW and overlying warmer waters in the passage.
In this proposal, we set out to test and, if necessary, redefine this hypothesis by:
(1) carrying out the first systematic measurements of how AABW flows through the Orkney Passage, how its properties change along the way, and what processes are important in determining the AABW flow and transformation in the passage;
(2) determining how and why the flux and properties of AABW in the Orkney Passage respond to wind forcing on time scales of up to several years.
To address task (1) above, we will measure the velocity and properties of AABW and the intensity of turbulent mixing at several key locations in the passage. The observations will be obtained both with instruments lowered to the seabed from a ship and with a novel autonomous underwater vehicle, which is particularly effective at measuring a range of potentially crucial processes occurring near the ocean floor. To address task (2), we will enhance an array of moorings recently deployed in the Orkney Passage by the British Antarctic Survey to monitor the flux and properties of AABW. We will equip the moorings with sufficient oceanographic instrumentation to identify the processes determining the AABW's response to wind forcing, which are not resolved by the present array.
We will use our findings from tasks (1) and (2) to define how and why the volume and properties of the AABW escaping the Weddell Sea through the Orkney Passage react to changes in winds. Armed with this new understanding, we will revisit the widespread AABW warming and contraction observed over recent decades, and inform the international strategy to monitor future changes in AABW circulation.

Planned Impact

The aim of this proposal is to understand how Antarctic Bottom Water is exported and modified through the Orkney Passage, the overflow's controlling dynamics, and its downstream impact on the Atlantic Ocean abyss. While we will engage with the research community through our publications andpresentations in at least 5 national and international conferences, we will seek to maximise the wider impact of our research as follows.
Who - the following two communities will particularly benefit from our proposed research:
1. Climate modellers, including those at the NOAA Geophysical Fluid Dynamics Laboratory (GFDL);
2. School pupils, their teachers and the general public.
How and what - relevance of our research to these communities and what will be done to ensure that they benefit from our research:
1. The impact of DynOPO on climate modellers will be through providing guidance to their efforts to improve the representation of oceanic overflows in climate models. To maximise this impact, we have developed a close collaboration with one of the leading authorities in the field of overflow parameterization, and entrained her in the planning of the experiment and the exploitation of our results (see Legg Letter of Support);
2. The impact of DynOPO on school pupils, their teachers and the general public will be enabled by a range of outreach activities (development of an interactive website, a teaching package for the "Scientists on the Road" outreach programme, and a rotating-table demonstration of oceanic overflows for showcasing in a range of venues; visits to schools as part of the Hampshire STEMnet ambassadors programme) targeted at cultivating enthusiasm for and understanding of marine and climate science by those communities. The activities for school pupils are also designed to encourage them to take science subjects at school and university, thus contributing to the UK's skillbase.

Publications


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Brown P (2015) Carbon dynamics of the Weddell Gyre, Southern Ocean in Global Biogeochemical Cycles
Jullion L (2014) The contribution of the Weddell Gyre to the lower limb of the Global Overturning Circulation in Journal of Geophysical Research: Oceans
Meijers A (2016) Wind-driven export of Weddell Sea slope water in Journal of Geophysical Research: Oceans
Meredith MP (2014) Dense waters of the Weddell and Scotia Seas: recent changes in properties and circulation. in Philosophical transactions. Series A, Mathematical, physical, and engineering sciences
Naveira Garabato A (2016) The thermodynamic balance of the Weddell Gyre in Geophysical Research Letters
Polzin K (2014) Boundary mixing in Orkney Passage outflow in Journal of Geophysical Research: Oceans
 
Description The Weddell Sea is a key region for the formation and export of Antarctic Bottom Water, the densest water mass in the global ocean. Formation and sinking of this water represents one of the prime mechanisms for the exchange of heat and carbon (including that produced by humans) between the atmosphere and deep ocean interior. The properties of this dense water exported from the Weddell Sea have been observed to change over the last several decades, becoming markedly warmer and fresher, but the mechanisms driving these changes are unclear. A central piece of the puzzle of these observed trends and changes is the South Scotia Ridge, a major topographic obstacle that acts to impede the movement of water between the Weddell Sea and the Scotia Sea downstream, and therefore the wider ocean. Through a coordinated analysis of mooring and ship-based observations at the key export 'choke-points' across the South Scotia Ridge, we have shown that much of the observed interannual variability in export properties may be driven by changes in winds over the Weddell Gyre. Strengthening of eastward winds appears to spin up the Weddell Gyre boundary current on timescales of a few months, and in doing so deepens the interface of density surfaces with the South Scotia Ridge, resulting warmer and saltier export. These mooring-based results over five years agree with the much more irregularly-sampled, but longer (20+ years) time series of ship-based sections across Drake Passage, suggesting that this mechanistic relationship is robust and has wide-reaching consequences over the Scotia Sea and the Atlantic beyond.

We have also shown for the first time that in the eastern Scotia Sea the volume of the densest water masses from the Weddell Sea have been diminishing over the last 20 years. We find that the known freshening trend of this water is insufficient to explain the observed change in water volume, and instead show through analysis of the long term mooring array in the critical Orkney Passage chokepoint that changes in the volume of dense water transport across the South Scotia Ridge may be responsible for this trend. These studies lay the groundwork for ongoing analyses of the processes that may explain the timing and distribution of dense water export trends and ultimately link them back to observed changes in surface forcing around the Antarctic margin and wider climate trends.
Exploitation Route The project is ongoing, and the research conducted to date has laid the groundwork nicely for further investigations to come. In particular, there will be a major research cruise in the key Orkney Passage area in season 2016/17, with deployment of a range of ship-based and autonomous technologies to better understand the dynamical controls on dense water flows and mixing as the deep waters of the Weddell Sea are exported toward the Atlantic. This work is already being used by others, most significantly a related (successful) NSF proposal led by our American collaborators (Woods Hole Oceanographic Institution, and the Geophysical Fluid Dynamics Laboratory at Princeton), which will add further important detail to our understanding of how abyssal ocean processes control and structure dense water properties, and what the consequences of these are for ocean climate on larger scales.
Sectors Environment