Agulhas System Climate Array: 'Sihlol uLwandle' - Investigating the oceans

The Microcat (an instrument measuring conductivity, temperature and pressure) is fastened to the mooring cable.

The flotation buoys containing Acoustic Doppler Current Profilers are prepared for deployment. The large cement blocks weigh up to 1 000 kg each and are used as anchors to keep the moorings in place.

Technicians from NIOZ (Royal Dutch Institute for Sea Research), DEA and the Bayworld Centre for Research and Education deploy the first of the tall moorings. This flotation buoy, which contains an Acoustic Doppler Current Profiler (ADCP), is the top instrument in the long mooring string. The ADCP measures the current speed in the water column above it.

There she goes!! The flotation buoy hits the water. A steel cable connects all the mooring instruments in a long string, up to 3 000 m long. This string will stay in place for a year, until it is retrieved at the next cruise and the data is downloaded from the instruments.

On the edge - technicians are fastening the mooring equipment to the steel cable. This is a precarious task requiring skill, experience, and yes, courage!

By Beate Hölscher, Research Administrator, SAEON

The Agulhas System Climate Array (ASCA) is an international oceanographic project with partners from South Africa, the US, and the Netherlands, and funding support from the South African Departments of Science and Technology (DST) and Environmental Affairs (DEA), the US National Science Foundation (NSF) and the Royal Dutch Institute for Sea Research (NIOZ).

ASCA is designed to provide long-term observations of Agulhas Current volume, heat and salt transport and its variability from mesoscale (eddies), through seasonal to inter-annual timescales. The ASCA shelf and tall moorings will extend 200 km offshore through the core of the Agulhas Current, with Current and Pressure Recording Inverted Echo Sounder measurements extending the array to 300 km offshore.

The Agulhas Current System plays a vital role in regional weather, with mean summer rainfall along the east coast related to the distance to the Agulhas Current core, as well as impacting the local biodiversity. It also critically contributes heat and salt to the Thermohaline Circulation, and thus impacts on climate variability and climate change.

First mooring deployment cruise

The first mooring deployment cruise of the ASCA project that took place on board the South African Research Vessel Algoa in April 2015, saw the two shelf moorings and the first four tall moorings successfully deployed.

These activities were complemented by a CTD (Conductivity, Temperature and Depth) survey along the full ASCA transect, with underway chemistry and ship-borne ADCP (Acoustic Doppler Current Profiler) surveys, and SVP (Surface Velocity Program) Drifter and Slocum Glider deployments.

Operation Phakisa

The baseline study that the ASCA project provides will contribute to Operation Phakisa (a Sesotho word meaning “Hurry Up”), which assesses how the ocean can be used to promote the South African economy.

ASCA will contribute to the initiative by providing knowledge as to how the Agulhas Current impacts marine transport, as it dominates the East coast of South Africa, and the marine protection services and governance in terms of providing data on current and temperature variability on the impacts downstream of marine protected areas and critically endangered ecosystems.

The secret of a successful mooring

Due to very favourable sea conditions, the mooring deployment cruise was smooth sailing. The four tall moorings and two shelf moorings were successfully parked in their rightful place on the sea floor, where they will diligently start logging data. The combination of technicians, students, vessel crew and scientific supervision made for a very effective team, who made this daunting task actually look easy! The moorings will again be visited and retrieved in a year’s time, for downloading of data and replacing of batteries.

Putting in moorings in the Agulhas is a very tricky business, because of the speed and the strength of the current. Being among the fastest ocean currents globally, it flows at a speed of about 2 m/s, which translates to around 7 km/hour. For a massive body of water to move consistently at this speed, this is quite impressive and is equivalent to around 60 million bathtubs of water flowing down the East coast.

To place the moorings is an art of co-ordination between the wind, the swells, the current direction and the ship’s performance. In the case of the first deep moorings, the crew worked in the opposite direction of the flow of the current.

The deployment actually starts well before the designated anchor location, so that there is enough distance for the whole cable (3000 m in the case of mooring D) to be drawn out behind the ship, which is steaming on slowly. Then there is the critical moment when the anchor needs be released, to sink down and settle exactly on the spot marked X. The secret of successful mooring deployment lies in good preparation, good teamwork and, most of all, suitable sea conditions. We were very fortunate to have them all!

In addition, seven surface drifters were deployed. These drifters were donated to SAEON by the South African Weather Service. The drifters consist of a floating ring, to which a “sock” is attached that catches the current. A transponder in the drifter sends a position update every half hour to NOAA (the National Oceanic and Atmospheric Administration [USA]), where the positions are logged.

As they are passively carried by the Agulhas Current, these drifters follow the course of the current, helping to understand the way the current is moving. The drifters are deployed under the Sentinel-1 satellite track, and will assist in ground-truthing the data, in essence ‘checking’ and correlating the accuracy of the satellite data.

SAEON would like to thank all participants and collaborators that made the first leg of this cruise a success. We look forward to an education outreach programme involving the RV Algoa, as well as fruitful collaborations going forwards with the ASCA project.

More information can be found at http://asca.dirisa.org

Official launch of the ASCA project

The ASCA project was officially launched on April 24 to coincide with the closure of the first deployment cruise. SAEON, through the Department of Science and Technology, and in partnership with the Department of Environmental Affairs’ Ocean and Coasts, launched the ASCA project to the top management of the two departments, the Eastern Cape Provincial Government and mayors from Port Elizabeth and East London, collaborating science teams and universities, students, the media and members of the public interested in the project.

The aim of the launch was to showcase to the greater community the benefits of this very large monitoring array in terms of climate research and long-term monitoring, as well as the value of scientific collaboration at the inter-governmental department level, between universities and science groups and at the international marine science community level.