Dynamics of the Orkney Passage Outflow (DynOPO)

Lead Research Organisation: University of Southampton
Department Name: School of Ocean and Earth Science

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 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.

Publications

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Meijers A (2016) Wind-driven export of Weddell Sea slope water WEDDELL SEA SLOPE EXPORT in Journal of Geophysical Research: Oceans

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Naveira Garabato A (2016) The thermodynamic balance of the Weddell Gyre in Geophysical Research Letters

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Naveira Garabato AC (2019) Rapid mixing and exchange of deep-ocean waters in an abyssal boundary current. in Proceedings of the National Academy of Sciences of the United States of America

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Naveira Garabato A (2019) Phased Response of the Subpolar Southern Ocean to Changes in Circumpolar Winds in Geophysical Research Letters

 
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,Other

 
Description Prof Mike Meredith: leadership of Polar Regions chapter in IPCC Special Report on Oceans and Cryosphere
Geographic Reach Multiple continents/international 
Policy Influence Type Gave evidence to a government review
URL https://www.ipcc.ch/report/srocc/
 
Description NERC LTSM NC
Amount £8,400,000 (GBP)
Funding ID NE/N018095/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 04/2016 
End 03/2021
 
Description Joint UK-US partnership in the DynOPO project 
Organisation Woods Hole Oceanographic Institution
Country United States 
Sector Charity/Non Profit 
PI Contribution This NERC award has funded the fieldwork, equipment and scientist/technician time for a major programme investigating the dynamics that control dense water export from the Southern Ocean. The UK participants provide expertise on ocean dynamics, physical oceanography, marine technology (including deep-ocean moorings and an autonomous submarine), and numerical modelling.
Collaborator Contribution NSF have funded a parallel project, led by Prof. Kurt Polzin of the Woods Hole Oceanographic Institution, Massachusetts. This is providing extra ocean equipment, and allowing the participation of Prof. Polzin in the fieldwork elements and analyses. There is also participation from Profs. Sonya Legg and Stephen Griffies of the Geophysical Fluid Dynamics Laboratory, Princeton, who are adding unique capability in ultra-high resolution modelling and climate modelling.
Impact Collaboration has greatly enhanced the planning of the field campaign, which is very soon to commence. Collaboration will strengthen the quality and impact of subsequent outputs, due following completion of the fieldwork.
Start Year 2015
 
Description Academic Consultancy to BBC for series "Frozen Planet 2", Mike Meredith 
Form Of Engagement Activity A broadcast e.g. TV/radio/film/podcast (other than news/press)
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact This series is the successor to BBC's hugely successful Frozen Planet, which reached an audience of 500 million people globally. Prof Mike Meredith has been contracted by the BBC to serve as Academic Consultant for the production of its sequel. This work is underway; the series is due for broadcast in 2021, and will reach an audience similar to its predecessor.
Year(s) Of Engagement Activity 2019
 
Description Videos of first scientific mission of Boaty McBoatface as part of the DynOPO project 
Form Of Engagement Activity A broadcast e.g. TV/radio/film/podcast (other than news/press)
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact As part of the DynOPO project, we produced a series of animations so that the general public could visualize Boaty McBoatface's mission in the Orkney Passage. These are some statistics on the animations: "Boaty McBoatface M44 in Orkney Passage" (vimeo.com/217386697) - 49k views; Animation on SciShow (www.youtube.com/watch?v=xymGM9dbt4o) - 300k views; an early summary of press coverage (including a feature in CBBC) can be found in oceanmixing.github.io/2017/dynopo-boaty-in-the-news.html . Additionally, fragments of these videos featured in the US television show Jeopardy.
Year(s) Of Engagement Activity 2017
URL http://www.youtube.com/watch?v=xymGM9dbt4o