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

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

The ocean environment off southeastern Africa: A review

The ocean environment off southeastern Africa consists of a continental shelf region of diverse widths and an off-shelf circulation that is similarly multifaceted. These factors lead to a complex and as yet imperfectly understood coastal and shelf habitat for living organisms. To complicate this further, the coverage of the region by hydrographic and other investigations differs markedly. Some regions have been relatively thoroughly studied whereas in others no hydrographic or current observations of any kind have been made to date. The only attribute that connects the coastal oceans of this region is that they all belong to what may be considered to be the greater Agulhas Current system. This consists of the East Madagascar Current, the Mozambique Channel eddies and the Agulhas Current proper. In this paper I review current knowledge on the region from both a physical as well as a biological oceanographic point of view. Some of the most glaring gaps in our knowledge of the region are identified

Ocean currents south of Africa from drifters

The ocean currents and their variability south of Africa are known to cover a wide spectrum, spatially and possibly temporally. Hydrographic observations in this vast ocean region are logistically demanding and expensive. In recent years the prevalence of drifting buoys has allowed one to infer certain current characteristics for the region that otherwise would be difficult. Observations from satellite-tracked drifters drogued at a depth of 15 m, collected between 1988 and 2005, were used to infer the mean surface circulation and kinetic energy distributions of the surface flow in the African sector of the Southern Ocean between 30 and 60°S. Regions of intensified flow and of higher levels of eddy kinetic energy were identified and agree fairly well with those established from remote sensing products. These results confirm the value of these observations and indicate the increasing usefulness of this data set as the number of drifter tracks increases

Madagascar: A pacemaker for the Agulhas Current system?

[1] Western boundary currents are driven by zonally integrated wind-stress curl over the width of subtropical basins. This cross-basin integration is interrupted in the South Indian Ocean where Madagascar presents a formidable barrier. Nevertheless, a western boundary current has been thought to exist in the Mozambique Channel, the Mozambique Current. Recent observations have however shown that no such current exists and that the flow in the channel instead consists of a train of eddies. Is this western boundary anomaly due to the presence of Madagascar? We have used a primitive equations model to investigate the flow in the South West Indian Ocean as if there were no Madagascar. We show that a normal, continuous western boundary current is then formed that constitutes a continuum with the Agulhas Current. The presence of Madagascar is shown to affect the frequency of inter-ocean exchange events south of Africa

Surface drift at the western edge of the Agulhas Bank

The Agulhas Bank is a wide continental shelf that forms the southern tip of the African continent. On the eastern side of this shelf the flow of water is dominated by the adjacent Agulhas Current. On its western border, the movement is more complex. It is influenced by the Benguela Current, by the occasional presence of products from the Agulhas Current such as Agulhas rings, Agulhas filaments and by lee eddies. Understanding the flow on this western side of the Agulhas Bank is of considerable ecological importance because it has been assumed that a shelf edge jet carries immotile or weakly motile fish larvae and eggs from the spawning region on the bank to the biologically productive regions of the Benguela upwelling regime. We have used the tracks of a set of surface drifters to study the movement at the western edge of the bank, and show that on average the movement is indeed equatorward along this shelf edge, but that this movement is not persistent in direction or magnitude. Instead, this movement appears to be driven entirely by mesoscale turbulence created at the termination of the Agulhas Current

On the surface drift of the Southern Ocean

Drift rates of the sea surface have been calculated for the South Atlantic and South Indian Ocean sectors of the Southern Ocean using drift cards and FGGE buoys. Drift patterns and drift rates, based on results from 40,000 plastic drift cards placed from 1978 to 1981, indicate significant equatorward surface exchange between the Southern Ocean and subtropical ocean gyres. Card drift rates increase with latitude up to the 40-45S zone. Average zonal drift rates lie between 10.3 cm/s and 16.4 cm/s. Zonally averaged drift rates of FGGE buoys are also at a maximum between 40 and 45S but are 15% higher; lowest rates are 12.2 cm/s. Significant differences in the drift rates between sectors of the same zone reflect the influence of bottom topography

Sea-surface temperatures at the sub-Antarctic islands Marion and Gough during the past 50 years

Sea-surface temperatures (SSTs) have been measured at Marion and Gough islands for nearly 50 years. These are some of the longest records of their kind in the sub-Antarctic. We present the trend, the seasonal cycle, and the time-frequency characteristics of the SST for both islands, which rose by 1.4°C at Marion Island and by 0.5°C at Gough Island over the 50-year period. Intermittent temperature oscillations, with periods of between 1 and 5 years, were observed throughout the record. A 5-year periodicity, compatible with an Antarctic CircumpolarWave signature, dominated after 1990 in both records. We also observed a strong low-frequency component with a period varying from 9.3 to 11.4 years at Marion Island, and a much weaker component with a period varying from 9.9 to 11.8 years at Gough Island

Intrusions of sub-Antarctic water across the Subtropical Convergence south of Africa

The contents of the Cape Basin of the South Atlantic Ocean, southwest of Cape Town, is a melange of water types from a number of different sources. One of the least studied of these water types comes from intrusions of sub-Antarctic water that are associated with the spawning of Agulhas rings. An analysis of a variety of data on the region shows that these intrusions originate along a latitude of 40°S, but only between longitudes of 8° and 22°E. In extreme cases they can extend equatorward beyond the southern tip of Africa. Intrusions take place at least five times per year. Their distinct surface expressions are shown to be but outcrops of water masses that usually are found at greater depths. These vertical perturbations may extend to depths exceeding 1500 m

Physical and biological coupling in eddies in the lee of the South-West Indian Ridge

Eddies have some decisive functions in the dynamics of the Southern Ocean ecosystems. This is particularly true in the Indian sector of the Southern Ocean, where a region of unusually high-mesoscale variability has been observed in the vicinity of the South-West Indian Ridge. In April 2003, three eddies were studied: eddy A, a recently spawned anticyclone south of the Antarctic Polar Front (APF),; eddy B, an anticyclone north of lying between the Subantarctic Front and the APF; and eddy C, a cyclone north of the APF west of the ridge. Elevated concentrations of total Chl-a coincided with the edges of the cyclonic eddy, whereas both anticyclonic eddies A and B were characterised by low total Chl-a concentrations. Biologically, the two anticyclonic eddies A and B were distinctly different in their biogeographic origin. The zooplankton community in the larger anticyclonic eddy A was similar in composition to the Antarctic Polar Frontal Zone (APFZ) community with an addition of some Antarctic species suggesting an origin just north of the APF. In contrast, the species composition within the second anticyclonic eddy B appeared to be more typical of the transitional nature of the APFZ, comprising species of both subantarctic and subtropical origin and thus influenced by intrusions of water masses from both north and south of the Subantarctic Front. Back-tracking of these features shows that the biological composition clearly demarcates the hydrographic origin of these features