Dynamics of the Orkney Passage Outflow (DynOPO)

Dynamics of the Orkney Passage Outflow (DynOPO) is a collaboration between BAS, the University of Southampton and the National Oceanography Centre (NOC). The project aims to investigate the flow of Antarctic Bottom Water (AABW) into the Atlantic Ocean through the Orkney Passage, a submarine valley that connects the Atlantic Ocean to the Weddell Sea and allows the movement of abyssal water masses.

Warming of Antarctic Bottom Water

AABW is among the deepest, coldest abyssal water masses on Earth. It forms in the Southern Ocean as water cools and grows more dense underneath ice shelves and as a result of cold katabatic winds originating on the ice sheet. As they grow more dense, these water masses sink to the seafloor, flow northwards and become part of the global circulation of ocean water.

During the last three decades, the AABW filling the bulk of the global ocean abyss has warmed and contracted in volume, particularly in the Atlantic Ocean. While the causes of these changes are unknown, available evidence suggests that the warming and shrinkage of AABW in the Atlantic may be caused by changes in winds over the northern Weddell Sea, where this water is produced.

Our 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.

Combining stationary and moving data sources

We aim to test 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 answer these questions, we will collect data from both stationary and moving data sources. We deployed moorings in the Orkney Passage in 2015 and will recover these during a research cruise aboard the RRS James Clark Ross in early 2017. Pictures from the 2015 cruise are available on co-investigator Dr Eleanor Frajka-Williams’ website.

During the 2017 cruise we will also deploy an autonomous underwater vehicle (AUV) that will move back and forth in an AABW along the Orkney Passage while measuring the intensity of the turbulence. We will be using Autosub Long Range, a class of AUV developed by NOC and also known to the public as Boaty McBoatface.