The Agulhas Current - a fundamental piece of the climate puzzle

By Dr Juliet Hermes & Tamaryn Morris, SAEON Egagasini Node

"Understanding how and why and to what effect the Agulhas flows, will reveal an elegant and beautiful natural system that, by the sheer chance of physics, makes our world a more comfortable place to live than it would otherwise be." - Dallas Murphy

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, thermohaline circulation, is what regulates our climate.

Figure 1. SST over the Agulhas region current observational programs are marked. Modified from Beal et al., Nature 2011

The Agulhas System Climate Array (ASCA) is a multi-institutional, international collaboration. It is designed to provide the first long-term observations of Agulhas Current volume, heat and salt transport and its variability from mesoscale (eddies), through seasonal to interannual timescales, and critically, its contribution in terms of heat and salt to the Thermohaline Circulation - and thus its impacts on climate variability and climate change.

Only velocity measurements over a protracted period can reveal the true behaviour of the current - and that's where the moorings come in. The moorings have been designed so that the big top float remains 600 metres beneath the surface to protect them from heavy seas, fishing trawls, and passing ships (Picture: Tammy Morris)

ASCA's objectives are to determine how the Agulhas Current varies over time, and by including additional instruments such as Sea-Bird Microcats along the mooring lines, to measure the heat, salt and volume flux of the current and better interpret what is being introduced into the Agulhas Return Current and the South Atlantic. Fundamentally this will improve our knowledge of the Thermohaline Circulation and global climate change.

The first cruise, scheduled for April 2015, will have a film crew on board who are making a documentary about the Agulhas Current to showcase our country's incredible oceanographic feature both nationally and internationally.

ASCA array

The ASCA array will consist of two shelf moorings of bottom-mounted Acoustic Doppler Current Profilers (ADCPs), six tall moorings from the shelf edge to 200 km offshore, and five Current and Pressure Recording Inverted Echo Sounders (CPIES), extending the array 300 km offshore (Figure 2) along the descending TOPEX/Jason satellite ground track # 96, through the core of the Agulhas Current. The array looks at the inshore coastal effects, core, and offshore component of the Agulhas Current simultaneously - thus providing a full, long-term picture of this powerful western boundary current.

Figure 2.  A cross-section of the three-year mean velocity (m/s) through the Agulhas Current, off East London. Red denotes southward (out of the page) flow and blue northward (into page), the lines are the mooring positions planned for ASCA. The insert figure is potential temperature (degrees Celsius)

Resources

The Department of Science and Technology (DST), through the National Research Foundation (NRF), have committed five years of funding to ASCA, and, along with the Department of Environmental Affairs (DEA) will procure additional equipment for the project, and jointly cover the associated costs of the deployment and maintenance cruises. In addition to this, the SAEON Egagasini Node will play a coordination role in the project, which includes the recent appointment of the ASCA Coordinator, Ms Tamaryn Morris.

The majority of the instruments - including 75 kHz ADCPs, Aanderaa RCM 11 current meters, Sea-Bird Microcats and CPIES - are being loaned by the Royal Netherlands Institute for Sea Research (NIOZ) and the Rosenstiel School of Marine and Atmospheric Science (RSMAS) of the University of Miami. ASCA builds on the Agulhas Climate Time-Series Experiment (ACT) run by Professor Lisa Beal from RSMAS from 2010 to 2013.

Co-principal investigators Prof. Mike Roberts (DEA), Prof. Herman Ridderinkhof (NIOZ), Prof. Lisa Beal (RSMAS) and Dr Shane Elipot (RSMAS) will contribute their technical and scientific skills to the project in various capacity development initiatives - workshops, cruise preparation and involvement, student supervision and preparation of data for publication.

Benefits

The benefits of a project of this magnitude to the marine science community in South Africa are substantial. The project will also enable disciplines other than physical oceanography to participate in the cruises, post-cruise interpretation of the data collected and various capacity development initiatives created throughout ASCA’s lifespan. These may include work on biogeochemical aspects, phyto- and zooplankton dynamics associated with the Agulhas Current, glider work, Argo floats and Surface Velocity Program (SVP) drifters, and air-sea interactions using marine meteorology instrumentation.

Skills development

An important objective, aside from the scientific goals above, is the transfer of skills from the NIOZ and RSMAS co-principal investigators and technicians to the marine science community in South Africa to ensure the longevity of the array. The future may involve the development of new and perhaps even more complex mooring arrays within South African waters. The ability to deploy and maintain the ASCA array with capabilities developed locally, enhances South Africa’s capability of handling these.

Tamaryn (Tammy) Morris has an MTech in Oceanography from the Cape Peninsula University of Technology. She is currently registered for a PhD in Physical Oceanography at the University of Cape Town, working on mesoscale eddies in the southern Mozambique Channel. Tammy has been involved in a number of international projects, focusing on mesoscale dynamics in the Mozambique Channel and the physical oceanography of the Agulhas Current. She has worked extensively in the offshore marine environment collecting in situ oceanographic data from ship-borne instruments and autonomous platforms. Tammy serves as the representative for South Africa on the Argo Steering Team and is also involved in capacity development projects at both the secondary and tertiary levels of education.