Our changing oceans
The world’s oceans are undergoing significant changes – seen in indicators such as temperature and salinity. Isabelle Ansorge and Mike Roberts explain how South Africa is involved in investigations of these changes.
The meridional overturning circulation (MOC) is a system of surface and deep ocean currents that extends across the globe. It is the main way that warm and salty surface waters from the tropics are transported to the poles. In the polar region, the water cools and sinks to great depths. The water then flows southwards as a deep (>2 000 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 large-scale ocean circulation of heat, freshwater and carbon between ocean basins. This circulation also connects the surface ocean and atmosphere with the huge reservoir of the deep sea.
The physical structure of this circulation belt and its efficiency in regulating climate is influenced significantly by the way that water masses are exchanged between ocean basins. A few climate models have predicted that increased levels of greenhouse gases may interfere with the MOC process by disrupting or slowing down the circulation. Warming will cause increased glacier melt, increasing the amount of cold, fresh water moving into the 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.
We know that the MOC provides a vehicle connecting surface and deep ocean currents around the world’s oceans but little is known of the response and degree in which heat is being taken up by the deep oceans. Because of this lack of knowledge the scientific community is slowly realising that we need deep ocean measurements and we need to develop full basin-wide arrays to take these measurements. This is crucial if we want to reduce the uncertainties in model projections of global warming.
Essentially this means that, in order to track the evolving ocean inventory of heat, freshwater and carbon, measurements need to be taken below 2 000 m. Argo floats, gliders and satellite remote sensing have revolutionised our ability to measure the upper layers of the ocean, but the deep ocean is currently beyond their reach. It is becoming more and more obvious that there is a close link between the deep ocean and the climate, so observations need to be taken to areas as deep as the ocean floor. The problem is that the ocean floors are deep and conditions this deep are hostile – particularly in the Southern Ocean.
At the moment there is only one system monitoring the MOC in the North Atlantic Ocean: the RAPID/MOCHA array located at 26.5˚N. Because the MOC covers the world’s oceans it is obvious that changes happening between the South and North Atlantic sectors must be considered.
The importance of the Southern Ocean
The Southern Ocean takes in the southern sectors of the Indian, Atlantic and Pacific basins and provides a link between the upper and lower layers of the global ocean circulation.
Because of this link, the overturning circulation within the Southern Ocean strongly influences climate patterns and the cycling of carbon and nutrients. As a consequence, any changes in the Southern Ocean will affect the rest of the world’s oceans. The observations that have been made so far suggest the Southern Ocean is warming more rapidly than the global ocean average – salinity changes driven by shifts in precipitation and ice melt have been observed and the uptake of carbon has decreased. In response to these changes, Southern Ocean ecosystems are being forced to adapt or are becoming less diverse. There may also be fewer different Southern Ocean ecosystems overall. Unfortunately, the lack of historical observations means that it will take time to understand this region. The scientific oceanographic community really needs an MOC monitoring array across the entire South Atlantic – extending south of Africa because, in spite of the South Atlantic’s climatic importance, there is no observational system in place to monitor the inter-ocean exchanges. Although individual efforts to measure the circulation across natural chokepoints such as the Drake Passage and south of Africa exist, none of these efforts have previously been coordinated, nor have these systems been designed for long-term monitoring. This is where the South Atlantic MOC (SAMOC) comes in. SAMOC-SA has been developed and funded by the National Research Foundation, the Department of Science and Technology and the South African National Antarctic Programme.
How does South Africa fit into SAMOC?
SAMOC is an integrated international observational oceanographic programme extending from south of Africa across the entire South Atlantic (Figure 3). The large meridional gap between the African and Antarctic continents provides a significant crossroad for water mass exchange between the subtropical Indian and Atlantic oceans. Most of the Indian to Atlantic Ocean transfer takes place near the Agulhas Current Retroflection through the generation of large warm rings – known as the Agulhas Leakage. Observations have reported that a highly energetic field of anticyclonic and cyclonic eddies in the Cape Basin interact extensively with each other, resulting in the vigorous stirring of water mass properties and the transport of substantial anomalies of heat from the Indian into the Atlantic Ocean. Recent studies suggest that the Agulhas leakage is critical – through the shedding of Agulhas rings this gateway is one of the major sources of salinity increase in the South Atlantic. Sediment records dating back 15 000 years prove that this leakage correlates to the strength of the North Atlantic MOC. In order to understand the nature of global climate change, the physical understanding and long-term monitoring of the inflow of Indian waters into the Atlantic Ocean is unavoidable. The aim of SAMOC is to improve and deepen this knowledge.
The state of observations and modelling south of Africa is not as developed as it is in other regions of the world’s oceans. This is largely due to limited ship availability, the lack of available technical support and the lack of sufficient funds to establish a mooring array. As a result South Africa’s role in the international framework of SAMOC has been limited to studies that take place during annual relief voyages into the Southern Ocean. SAMOC-South Africa (SAMOC-SA) provides the necessary tool to pull these surveys into a single entity.
This programme provides future opportunities for all young, early career and established researchers working in the South Atlantic and Southern Ocean to interact and strengthen international and national networks through a set of joint cruise plans. It will provide young students with a variety of bursaries as well as many cruise opportunities on the new SA Agulhas II and will form a crucial role in developing skills, knowledge and expertise of the area – a critical component if South Africa is to retain top-class marine scientists.
The new SA Agulhas II: The ship was commissioned by the Department of Environmental Affairs (DEA) and built in Rauma, Finland by STX. Launched in April 2012, she is currently the most state-of-the-art polar research vessel in the world. The Agulhas II will form the backbone of the SAMOC-SA programme providing the SAMOC-SA science team of UCT Oceanography and DEA Oceans and Coast scientists and students with a platform to deploy and service the many moorings and measurements along the transect shown in Figure 3.
The planning of a long term observational platform across the entire South Atlantic (Figure 3) takes time but is currently being coordinated between Brazilian, American, French and South African institutes. The new SA Agulhas II (Figure 4) provides the ideal platform to deploy and service the many moorings and undertake ship-based measurements (temperature, salinity, pH, CO2 etc) along the dedicated transects shown in Figure 3. The SAMOC-SA science team, made up of UCT-Oceanography and DEA-Oceans and Coasts researchers and students, is presently planning Phase 1 of the mooring deployments across the eastern sector of the South Atlantic in mid-October 2013. The recent establishment and endorsement of SAMOC-SA as well as the availability of the SA Agulhas II should mean that these objectives can finally be achieved, so finally placing South Africa in a more pivotal role within the international arena.
The Agulhas current
The Agulhas current flows down the east coast of Africa from 27°S to 40°S. It is a narrow current, fast flowing and strong. The current forms the western boundary of the southwest Indian Ocean.
Agulhas current retroflection
The Agulhas current retroflects – turns back on itself – in what is called the Agulhas retroflection. The water from the retroflection becomes the Agulhas return current, which then rejoins the Indian Ocean gyre. A gyre is any large system of rotating ocean currents – particularly those that are involved in large wind movements. The remaining water is transported into the South Atlantic gyre in the Agulhas leakage – through surface water filaments and Agulhas eddies.
A large amount of Indian Ocean water is leaked directly into the South Atlantic. Most of this is warm, salty water – the rest is cold, low salinity South Atlantic water. The Indian Ocean water is significantly warmer and saltier than South Atlantic water, which means that the Agulhas leakage is a significant source of heat and salt for the South Atlantic gyre. The heat exchange probably contributes to the high rate of evaporation in the South Atlantic, which is a key mechanism in the MOC.
Eddies are released into the South Atlantic gyre when the Agulhas turns back on itself. Eddies are formed when swirling fluid reverses either in an anticyclonic or cyclonicdirection when the fluid flows past an object. Eddies are common in the ocean and can range in diameter from centimetres to hundreds of kilometres.
The Agulhas rings
In the Agulhas current the eddies form the Agulhas rings, which are warm cores of water that are less biologically productive than the cold water that these rings move into. This means that they carry water with a low concentration of chlorophyll into the South Atlantic –chlorophyll is the pigment that allows photosynthesis. So, the water carried by these rings carries fewer phytoplankton, often smaller in size.
Lumpkin R. and Speer K. Global ocean meridional overturning. J. Phys. Oceanogr. 2007; 37:2550–2562.
Dr Isabelle Ansorge is a senior lecturer in the Oceanography Department at the University of Cape Town. She is an observational oceanographer and spends many weeks at sea in the Southern Ocean. Her research interests are in the Subantarctic Belt and in particular the impact changes in ocean circulation have on the ecosystem of the Prince Edward Islands.
Dr Mike Roberts is an observational oceanographer and is employed at the Department of Environmental Affairs – Oceans and Coasts Division. Mike is one of South Africa’s leading experts in deep-sea oceanographic mooring deployments and has been involved in technical training with the Cape Peninsula Techikon (CPUT) since the early 1990s.