Building the next generation of South African scientists

A recent issue carried a most interesting suite of articles, demonstrating the success of the Royal Society/National Research Foundation programme in helping to build a new generation of South African scientists, ‘as ageing researchers retire and retention and new blood become increasingly important’. The NRF’s overview of the programme (‘Reaching out to the world’1 ) correctly points out that ‘South Africa needs excellent science and far more scientists, drawing on the research potential of the entire population.

Decay of eddies at the South-West Indian Ridge

The South-West Indian Ridge in the Indian sector of the Southern Ocean is a region recognised for the creation of particularly intense eddy disturbances in the mean flow of the Antarctic Circumpolar Current. Eddies formed at this ridge have been extensively studied over the past decade using hydrographic, satellite, drifter and float data and it is hypothesised that they could provide a vehicle for localised meridional heat and salt exchange. The effectiveness of this process is dependent on the rate of decay of the eddies. However, in order to investigate eddy decay, logistically difficult hydrographic monitoring is required. This study presents the decay of cold eddies at the South-West Indian Ridge, using outputs from a highresolution ocean model. The model's representation of the dynamic nature of this region is fully characteristic of observations. On average, 3-4 intense and well-defined cold eddies are generated per year; these eddies have mean longevities of 5.0±2.2 months with average advection speeds of 5±2 km/day. Most simulated eddies reach their peak intensity within 1.5-2.5 months after genesis and have depths of 2000 m - 3000 m. Thereafter they dissipate within approximately 3 months. The decay of eddies is generally characterised by a decrease in their sea surface height signature, a weakening in their rotation rates and a modification in their temperature-salinity characteristics. Subantarctic top predators are suspected to forage preferentially along the edges of eddies. The process of eddy dissipation may thus influence their feeding behaviour

On the discontinuous nature of the Mozambique Current

The concept of a spatially continuous western boundary current in the Mozambique Channel has historically been based on erroneous interpretations of ships' drift. Recent observations have demonstrated that the circulation in the Channel is instead dominated by anti-cyclonic eddies drifting poleward. It has therefore been suggested that no coherent Mozambique Current exists at any time. However, satellite and other observations indicate that a continuous current - not necessarily an inherent part of Mozambique Eddies - may at times be found along the full Mozambican shelf break. Using a high-resolution, numerical model we have demonstrated how such a feature may come about. In the model, a continuous current is a highly irregularly occurring event, occurring about once per year, with an average duration of only 9 days and with a vertical extent of about 800 m. Surface speeds may vary from 0.5 m/s to 1.5 m/s and the volume flux involved is about 10 Sv. The continuous current may occasionally be important for the transport of biota along the continental shelf and slope

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

A descriptive physical analysis of water movement in the South West African Indian Ocean during the Northeast monsoon season

The Agulhas Current is unique as a western boundary current in having as its source of surface water currents that are linked to the variable current regions of the North Indian Ocean. In which way the Agulhas Current derives its supply from these currents and how these currents interrelate, is very poorly understood. In an attempt to make a contribution to the understanding of this flow an isentropic analysis in depth on seven σt-surfaces for the whole South West Indian Ocean during the Northeast Monsoon season was carried out. This analysis was augmented by the calculation of the velocities according to the Witte-Margules equation on each σt-surface at ten selected vertical sections. In addition the detailed volume transport and the velocity structure for each vertical section were calculated. The results pointed to a significant variation in transport and velocity structure from year to year. The large scale circulation in depth of the ocean is described. It is found that the Agulhas Current derives its supply of water from different sources at different depths and that at depth a large measure of recirculation of Agulhas Current Water takes place. It is not thought that this has been reported before and these findings might be of importance to the understanding of the current system as a whole. A few innovations in the standard oceanographic methods are described and two computer programs to aid analyses are presented

Evidence that the Natal Pulse involves the Agulhas Current to its full depth

Natal Pulses are intermittent, solitary meanders on the trajectory of the otherwise remarkably stable northern Agulhas Current. They play disparate roles in the process of inter-ocean exchange. They have been thought to trigger the spawning of Agulhas Rings at the Agulhas Retroflection, but also to generate an upstream retroflection that prevents Agulhas water from reaching the inter-ocean boundary. For the Natal Pulse to be such a control it has to extend to considerable depths. We present the first hard evidence that demonstrates that the Natal Pulse is indeed an inherent property of the Agulhas Current throughout its full depth. Our data comprise Eulerian current meter observations and Lagrangian float trajectories in combination with sea-surface height and sea-surface temperature data. The results reveal the trapping of water within the Natal Pulse, its southward advection at a phase speed of about 11-12 cm s - 1, and rotation periods of 6 days

The disparate evolution of three Agulhas rings in the South Atlantic Ocean.

International audienceHydrographic sections carried out in January–March 1995 across the pathway of Agulhas rings in the Cape Basin are used for a brief description of the mesoscale thermohaline variability in this region and a detailed study of three rings that were identified in the data. The three eddies exhibited remarkably diverse dimensions, vertical structures, and water mass characteristics. One of them, R1, was located near the Agulhas retroflection, had a diameter of 200 km, a maximum azimuthal speed of 0.40 m s−1, core oxygen values in excess of 260 μmol kg−1 and was characterized by a well-developed thermostad of 11.6°C. A second ring, R2, at 31°30′S, 9°W, by contrast, had a diameter of about 500 km, a core temperature of 17.1°C, and azimuthal speeds of 0.50 m s−1, suggesting a very different history. A third ring, R3, at 26°S, 9°W, although farthest north of the three, had characteristics similar to ring R1, but with a deeper thermostad. Satellite altimetric data allow one to infer the natural histories of these vortices. Ring Rl detached from the retroflection at the beginning of March 1994 and spent the whole of the subsequent winter south of 42°S. This could explain the estimated heat loss of 620 W m−2. The two rings observed at 9°W were spawned as one feature in April 1993, but interaction with the Erica seamount split it into two eddies; R3 being stalled in the retroflection region for the winter, while R2 moved off rapidly into the South Atlantic, by contrast, retaining most of its heat. These histories account for the observed differences between the rings. They also demonstrate that the interaction of Agulhas rings with their environment, including ambient water masses, the overlying atmosphere, and the bottom topography, is critical to their eventual hydrographic characteristics and the manner in which they contribute to the transport of heat and salt from the Indian to the South Atlantic Ocean