What does the future hold for the Agulhas System Climate Array?

By Whitney Samuels, Jordan Van Stavel and Tamaryn Morris

A depth slice along the line of the ASCA mooring array; the coloured shading represents the velocity in m/s, with reds and yellows indicating the velocity coming out of the page (southwards). Overlaid are the moorings envisaged at the start of the project. The smaller inlay image is the potential temperature, both averaged over the three-year period of the ACT (Agulhas Current Timeseries) array. Figure produced by Prof. Lisa Beal of the Rosenstiel School of Marine and Atmospheric Science, University of Miami

On 28 November 2018, SAEON’s Egagasini Node hosted the first Agulhas System Climate Array (ASCA) Symposium at the Department of Agriculture, Forestry and Fisheries’ Sea Point Research Aquarium in Cape Town.

Students and researchers from various institutions presented their research undertaken through ASCA or similarly related projects, ranging across disciplines within marine science.

The symposium provided an informal platform for delegates to share their work, highlight activities, form partnerships and discuss the future of ASCA.

Why has it been important to study the Agulhas Current?

The Agulhas Current is an extraordinary South African feature which has a strong impact on society given its influence on local and regional weather and climate, as well as biodiversity and fisheries. Globally, the Agulhas Current provides a key pathway of heat and salt from the Indian Ocean into the South Atlantic, which is then transported equatorward.

This distribution of heat and salt in the oceans, the Thermohaline Circulation, is what regulates our global climate.

But it is important at home too – our fisheries depend on it. Friction between the Agulhas Current and the continental shelf edge, which extends to about 50 km south of Port Elizabeth, draws nutrient-rich bottom waters towards the surface.

Surface water usually has few nutrients in it because tiny ocean plants, known as phytoplankton, have already absorbed them. But when the bottom water is brought closer to the surface, phytoplankton now have plenty of light and nutrients, and their blooms sustain the aquatic food web of the east coast of South Africa.

Attendees of the ASCA Symposium are all smiles despite the somewhat challenging logistics involved in capturing this group photo

Regionally the Agulhas Current has impacts on the mesoscale features influencing biodiversity around eastern African countries and islands. The Agulhas Current also has a strong impact on the southern Benguela and the Agulhas Return Current forms an important front to the Southern Ocean.

As climates around the world change, this piece of knowledge helps us to better understand how the southern Indian Ocean responds to seasonally changing winds and highlights how this response may be sped up with a warming of the ocean due to climate change.

The next chapter for ASCA

Subsequent to the project presentations, the floor was opened to all participants to discuss what the next chapter would be for ASCA. This allowed researchers from different institutional backgrounds, as well as visitors from the National Research Foundation, the opportunity to discuss what ideas they had for the future of the project, along with access to resources and support in the event thereof.

Students and researchers listen to a presentation

The much-anticipated topic of access to the ASCA data and metadata was also discussed as well as integration of ASCA with adjacent mooring systems. A key concern raised during this discussion was whether we as the marine science community can afford not to monitor the Agulhas Current given our warming climate. This led to positive discussions which highlighted the ultimate goal – to continue monitoring in terms of CTD surveys twice or three times per year, with a look to redeploy moorings if funding for new instrumentation can be found.

Student participation

Students were acknowledged for their contribution to the symposium and to ASCA research. Michael Hart-Davis won the Best Poster award for his poster titled, “Using ocean particle trajectory modelling in applications in the Greater Agulhas System”. James Maitland’s presentation, “Analysis of the first winter occupations along the Agulhas System Climate Array", scooped the Best Presentation award.

It is vital that the value added by the ASCA project thus far and its contribution to reducing a knowledge gap across a variety of disciplines be recognised, as many anticipate the continuation of this project in the near future.

What is the Agulhas System Climate Array?

ASCA consisted of two shelf moorings deployed at 80 m (C2) and 120 m (C3) respectively, seven tall moorings deployed from 300 to 4500 m (Mooring A to G), and five Current and Pressure Recording Inverted Echo Sounders (CPIES) to augment the tall mooring data and extend the array 300 km offshore. Instruments deployed on the array included 300 kHz, 150 kHz and 75 kHz Acoustic Doppler Current Profilers (ADCP) from Teledyne RD Instruments, Aanderaa and Nortek single point current meters, Sea-Bird Electronics MicroCAT sensors and the CPIES themselves.

Additionally, Conductivity, Temperature and Depth (CTD) cast surveys were undertaken at every opportunity across the ASCA transect to collect high-resolution physical and biogeochemical data across the Agulhas Current, along with phyto- and zooplankton tows for primary and secondary trophic level analysis. Recently this extended to parasite work, micro-plastics and benthic organism profiling.

Seven surveys, not all of them complete across the ASCA transect, were undertaken between 2015 and 2018. This provided the basis of the data presented at the conference.

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