14 Oct 2013 - 12:30

The Meridional Overturning Circulation (MOC) is a system of surface and deep ocean currents that extend across the globe. It is the main pathway in which warm and salty surface waters from the tropics are transported polewards. In the North Atlantic Ocean, the water cools and sinks to great depths, flowing southwards as a deep (>2000 m) ocean current in a process that is called ‘overturning’. In this way, water is returned to lower latitudes. The MOC provides a mechanism for the large-scale ocean circulation of heat, salt and carbon dioxide between ocean basins. Climate models have predicted that increased levels of greenhouse gases may interfere with the MOC process by slowing down its circulation. One impact of continued warming is the threat of increased melting of Greenland’s ice cap, which may result in an influx of cold and fresh surface waters into the North Atlantic Ocean. Some scientists predict that the increase in fresh water may be enough to change the composition and flow of ocean water on time scales from decades to centuries.


Until September, there was only one system in the North Atlantic monitoring the MOC: the RAPID/MOCHA array located at 26.5˚N (Figure 2). Because the MOC extends throughout the world’s oceans, it is obvious that changes happening between the South and North Atlantic sectors must be considered in tandem. South of Africa a large crossroad for water mass exchange between the subtropical Indian and Atlantic Oceans occurs through the generation of large warm and salty rings – known as the Agulhas leakage. Recent studies involving UCT researchers suggest that the Agulhas leakage is a gateway for the salinity increase in the South Atlantic and acts to mitigate changes in freshwater brought on by an increase in ice melt in the North Atlantic.Recognition of the critical importance of the MOC in this region and the need to understand better the inter-ocean exchange of heat and salt led to the creation of an international initiative - the South Atlantic Meridional Overturning Circulation (SAMOC) in which Dr Isabelle Ansorge is a committee member.

The 10th of September 2013 saw the final deployment of a CPIES mooring (Current Pressure Inverted Echo-Sounder) from the South African research vessel SA Agulhas II as part of the SAMOC-SA Initiative led by Dr Isabelle Ansorge from the Oceanography Dept and chief scientist Marcel van den Berg from DEA Oceans and Coasts. This final CPIES instrument (Photo) completes a network of eight bottom-moored CPIES moorings that are able to record vertical acoustic travel time and near-bottom pressure and velocity – from these measurements such as temperature, salinity and north-south velocity throughout the water column can be inferred.