Dynamics of the Orkney Passage Outflow (DynOPO)

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.

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 and
presentations 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.