The influence of the Antarctic Circumpolar Current on the oceanographic setting of a sub-Antarctic island

The circulation in the Southern Ocean is dominated by the Antarctic Circumpolar Current (ACe), which is the only current that flows completely around the globe. The ACC is a complex system consisting of narrow regions of sharp horizontal density gradients, which extend through the entire water column as frontal bands separated by broad zones with less intense gradients. Meridional excursions from the otherwise zonaillow in the form of meanders or mesoscale eddies occur in regions of prominent bathymetric features, suggesting a close relationship between the level of mesoscale sea surface variability and bottom topography, The Prince Edward Islands (Marion Island and Prince Edward Island) lying within the Polar Frontal Zone (PFZ) at approximately 38 DE, 47 os form a very special terrestrial habitat and have, for this reason, been designated a declared nature reserve. The ecology of the islands is directly dependent on its ocean environment, With the advent of satellite altimetry it has become apparent that the Prince Edward Islands are located on the northern border of a region of remarkably high mesoscale variability. The source of this variability lies where the current jet associated with the ACC intersects the South-West Indian Ridge. From here newly generated eddies are steered by the bottom topography past the Prince Edward Islands. '[his discovery probably explains the high physical and biological variability found at the islands, and warrants further analysis

Three decades of research on the greater Agulhas Current

The greater Agulhas Current has been shown to be a key link in the global thermohaline circulation and an increased understanding of this current system is therefore of more than just local interest. Knowledge on the Agulhas Current system has in fact increased enormously over the past 30 years. This review covers some aspects of what has been learnt on the northern and the southern parts of the Agulhas Current proper and their influence on the waters and circulation of the adjoining continental shelf. It also discusses the Natal Pulse and new information that has been gained on how it is triggered and what influence it has. It deals with the Agulhas retroflection, the shedding of Agulhas rings and the movement and characteristics of these rings that contributes to the meridional overturning circulation of the global ocean. The Agulhas Return Current forms part of the final outflow of the system and current knowledge on that current is appraised. The sources of the Agulhas Current have been a controversial subject for many years and this dispute continues. This is described and discussed, based on what information has been gained from research over the past three decades. Building on what is currently known, some suggestions are given on the most important remaining knowledge gaps and how these could most efficaciously be filled

Mesoscale perturbations control inter-ocean exchange south of Africa

The quantification of inter-ocean leakage from the South Indian to the South Atlantic Ocean is an important measure for the role of the Agulhas system in the global thermohaline circulation. To explore the specific role of mesoscale variability (such as Agulhas rings and Mozambique eddies) in this process a high-resolution model (based on NEMO-ORCA) for the Agulhas region has been set up. It is nested into a global coarse-resolution model. The high-resolution nest captures all salient features of the greater Agulhas region, including the upstream perturbations of the Agulhas Current and Natal Pulses along the African coast. A comparison of the inter-ocean exchange in the high-resolution nest with its coarse resolution counterpart reveals that the latter significantly over-estimates the amount of water flowing into the Atlantic Ocean, demonstrating the need to explicitly simulate the mesoscale features. A sensitivity experiment that excludes the upstream perturbations revealed no difference in the amount of inter-ocean exchange. However, the realistic representation of Agulhas rings and their drift path into the South Atlantic depends on the simulation of those upstream perturbations

Oceanographic observations of eddies impacting the Prince Edward Islands, South Africa

The ecosystem of the isolated Prince Edward Islands, south of the African continent, is strongly impacted by ocean eddies that are associated with the eastward flowing Antarctic Circumpolar Current. Satellite altimetry has revealed that the archipelago lies in a region of enhanced eddy kinetic energy. In the late 1990s it became apparent that in order to understand the influence of these eddies on the islands' ecosystem, the source, trajectory and nature of these eddies needed to be studied and understood. To this end a special research project with a strong ocean-going component was designed, the DEIMEC (Dynamics of Eddy Impact on Marion's ECosystem) programme. In this review we focus on the physical oceanography and summarize the aims, the results and the successes of this South African research initiative. In the vicinity of the Prince Edward Islands, an average of three intense well-defined eddies is observed per year. Their advection speeds are of the order of a few kilometres per day and longevities of 7-11 months. These features, of c. 100 km in diameter and reaching depths of at least 1000 m, transport anomalous water masses across the Polar Frontal Zone

Variability and coherence of the Agulhas Undercurrent in a High-resolution Ocean General Circulation Model

The Agulhas Current system has been analyzed in a nested high-resolution ocean model and compared to observations. The model shows good performance in the western boundary current structure and the transports off the South African coast. This includes the simulation of the northward-flowing Agulhas Undercurrent. It is demonstrated that fluctuations of the Agulhas Current and Undercurrent around 50–70 days are due to Natal pulses and Mozambique eddies propagating downstream. A sensitivity experiment that excludes those upstream perturbations significantly reduces the variability as well as the mean transport of the undercurrent. Although the model simulates undercurrents in the Mozambique Channel and east of Madagascar, there is no direct connection between those and the Agulhas Undercurrent. Virtual float releases demonstrate that topography is effectively blocking the flow toward the north

Estimation of sea-surface temperature around southern Africa from satellite-derived microwave observations

Sea-surface temperatures may give strong indications of the location of fronts, currents, eddies and other components of ocean circulation. This has been recognized in particular for the seas around southern Africa. Almost all studies using this property have employed measurements of thermal infrared radiation from orbiting satellites. This has distinct disadvantages due to the shading effect of persistent cloud cover. Another option is to use microwave observations, which are not affected by cloud cover. Until recently, however, the spatial resolution of microwave data was far too coarse for the purpose of studying ocean circulation in detail. We describe here a new set of microwave data that does not have this disadvantage, and show how useful it is by describing examples of local applications

Modes of the southern extension of the East Madagascar Current

Data sets from satellite observations and a nested high-resolution model are used to study a source region of the Agulhas Current. Altimeter-derived geostrophic surface currents are averaged over varying periods, providing evidence of the persistence of flow patterns in the extension of the southern branch of the East Madagascar Current (SEMC). South of Madagascar, the SEMC separates into one branch toward the Agulhas Current and into a second branch retroflecting and connecting to the Subtropical Indian Ocean Countercurrent (SICC). Good agreement is found between long-term mean patterns of observational and model dynamic heights. Two basic modes are identified in the SEMC extension, with anticyclonic motion favoring retroflection in the northern Mozambique Basin when the extension is in a southwestward direction and cyclonic motion occurring in the case of the SEMC flowing westward along the southern Madagascar slope. A cross-correlation sequence between model SEMC transports and the modal changes in the extension region displays a correlation at about 1-month lag which agrees with eddy propagation time from the SEMC to the outflow region. Mean model SEMC transports are determined using floats released at 21 degrees S, and the contribution of the SEMC to the SICC is obtained using floats injected at 55 degrees E with the model running backward. Almost half of the SEMC volume transport contributes to the Agulhas system, and about 40% of SICC water originates from the SEMC

Surface Drift in the South-East Atlantic Ocean

Surface drift in the South-East Atlantic Ocean is described using historical shipdrift data. The Benguela Current has a width of 200 km in the south and 750 km in the north. The mean speeds of the current vary fro

KAPEX: an international experiment to study deep water movement around southern Africa

Over the past decade it has been demonstrated unequivocally that the exchange of water masses between the Indian and the Atlantic oceans, south of Africa, is a key component of the global thermohaline circulation cell. This thermohaline circulation plays a large part in controlling global climate. Numerous South African and international studies have now been completed on these water exchanges, but most have concentrated their efforts on the upper water masses. We descriptionbe here an extensive and exciting new venture, called KAPEX, to investigate the movement of water at intermediate depths using sophisticated Lagrangian floats. This research programme is a collaborative effort between research groups from three countries and constitutes the geographically largest oceanographic experiment ever carried out in South African waters. Detailed information on the aims as well as the progress of KAPEX is available on the Internet on the home page http://triton.sea.uct.ac.z