Olaf Boebel

The Cape Cauldron: A regime of turbulent inter-ocean exchange

Combining in-situ Lagrangian intermediate depth velocity measurements from the KAPEX (Cape of Good Hope Experiments) float program with sea-surface height data, this study reviews the inter-ocean exchange mechanisms around southern Africa. In the southeastern Cape Basin, a highly energetic field of coexisting anticyclonic and cyclonic eddies is documented. Agulhas Rings of typically 200 km diameter are observed to merge, split, deform, and to reconnect to the Agulhas Retroflection. Concomitant, slightly smaller cyclones are observed to drift across the northwestward migration path of the Agulhas Rings. These cyclones, with typical diameters of 120 km, are formed within the Cape Basin along the African shelf, inshore of the Agulhas Current, and in the subantarctic region south of Africa. The data suggest the annual formation of 3–6 long-lived Agulhas Rings that eventually cross 5°E longitude, while approximately twice the number of rings occur in the southeastern Cape Basin. Within this region, cyclones outnumber anticyclones by a factor of 3:2. Both cyclones and anticyclones extend through the upper thermocline into the intermediate depth layer. Mean drifts of anticyclones are 3.8±1.2 cm s−1 to the northwest, while cyclones follow a west–southwestward route at 3.6±0.8 cm s−1. Transport estimates suggest that the intermediate depth layer in the southeastern Cape Basin is primarily supplied from the east (approximately 9 Sv), with minor direct inflow from the Atlantic to the west and south. Cyclone/anticyclone interaction is surmised to result in vigorous stirring and mixing processes in the southeastern Cape Basin, which necessitates a review of the traditional concept of Indo-Atlantic inter-ocean exchange. We propose to limit the concept of “isolated Agulhas Rings embedded in a sluggish Benguela Drift” to the northwestern Cape Basin and beyond, while linking this regime to the Agulhas Retroflection proper through a zone of turbulent stirring and mixing in the southeastern Cape Basin, named for the first time the “Cape Cauldron” hereinafter.

Path and variability of the Agulhas Return Current

The combined analysis of hydrographic, kinematic, and dynamic data collected during the 1997–1999 KAPEX (CAPe of Good Hope EXperiments) reveals a quasi-stationary meandering pattern of the Agulhas Retroflection Current east and upstream of the Southwest-Indian Ridge. The current meanders between 38°S and 40°S in a spatially and temporally continuous fashion and has a core width of approximately Full-size image (<1 K) with an associated transport of Full-size image (<1 K) in the upper Full-size image (<1 K). Peak surface velocities decrease from Full-size image (<1 K) near the Agulhas Retroflection to Full-size image (<1 K) around 32°E. Meander troughs (northward extremes) are found predominantly near 26.8°E, 32.6°E and 38.9°E, while crests (southward extremes) are located with high probability near 29.7°E, 35.5°E and 42.9°E, resulting in a typical wavelength of Full-size image (<1 K). Cold eddies are shed along the northern boundary of the current from meander troughs into the recirculation regime between the Agulhas Current proper and the Agulhas Return Current. Strongest cyclonic eddies are preferably shed in austral autumn. The cyclonic eddies so formed propagate westward at an average phase-speed of Full-size image (<1 K), with, however, a variability of at least the same magnitude. Subsequently, the cyclones are absorbed by the next meander trough located upstream and to the west of the shedding trough.

Flow and recirculation of Antarctic Intermediate Water across the Rio Grande Rise

The flow of the low-salinity Antarctic Intermediate Water (AAIW) at 700–1150 m depth across the Rio Grande Rise and the lower Santos Plateau is studied under the auspices of the World Ocean Circulation Experiment (WOCE) in the context of the Deep Basin Experiment. Our data set consists of several hydrographic sections, a collection of 15 RAFOS float trajectories, and records from 14 moored current meters. The data were gathered during different intervals between 1990 and 1994. The inferred flow field strongly supports a basinwide anticyclonic recirculation cell in the subtropical South Atlantic underneath the wind-driven gyre. Its center, which appears to be southeast of the Rio Grande Rise, separates the eastward advection of AAIW below the South Atlantic Current from the westward flowing, recirculating AAIW. The two near-shelf limbs closing the circumference of AAIW flow are formed in the east by the deep Benguela Current, potentially modulated by salty inflow of Indian Ocean Intermediate Water, and in the west by the Brazil Current system. Further important circulation elements are the Brazil-Falkland (Malvinas) Confluence Zone at 40°S and an unnamed divergence at 28°S close to the 1000 m isobath. The resulting broad southward flow of AAIW augments the share of modified, i.e., saltier, intermediate water in the source region of the South Atlantic Current, while the smaller northward flow marks the source of a narrow equatorward Western Intermediate Boundary Current, ultimately leaving the South Atlantic. This shelf-trapped jet is clearly documented in hydrographic data from 19°S and in nearby current meter records. The jet contrasts a sluggish flow across this latitude east of 35°W. A continuous flow of AAIW from its subpolar region in the southwestern Argentine Basin all along the western slope toward the equator appears unlikely between 35°S and 25°S.

New observations of Meddy Movement south of the Tejo Plateau

Mediterranean salt lenses (meddies) are a dominant factor in the salt budget of the Atlantic at middepth. In spite of their important role, their juvenile migration has not yet been directly observed. For the first time, two RAFOS float trajectories show strong evidence of a meddy along the Iberian continental slope off Lisbon. Over six weeks we obtained drift observations from two levels (629, 847 dbar). Both instruments recorded a series of loops with an azimuthal speed O (30 cm s−1) at a radius of about 25 km. Relatively high propagation speeds of several centimeters per second indicate the meddy was probably carried along with the undercurrent of Mediterranean Water. The Tejo Plateau, a prominent feature of the continental slope and a natural obstacle for the spreading Mediterranean Water tongue, appears to act as a deflector for advected meddies possibly formed by interaction of the undercurrent with the canyon-rich topography farther south.

A Profiling Float’s Sense of Ice

Abstract The Argo project intends to continuously monitor temperature and salinity of the upper 2000 m of the global ocean by use of autonomous, vertically profiling floats. They are currently generating the largest oceanographic dataset that ever existed, covering most of the world’s oceans. However, the use of these instruments in the polar oceans is seriously impeded by the presence of sea ice, as floats are hindered from transmitting their profile data or, even more seriously, potentially damaged when ascending to, or being at, the ice-covered sea surface. The authors present a cost neutral ice sensing algorithm (ISA), which alerts for the likely presence of sea ice. In this event, the profile is aborted and no surfacing attempted. To retrospectively track floats that actively remained under the sea ice because of ISA, acoustic tracking via the RAFOS technique was tested in the Weddell Sea. Last but not least, the most recent version of floats features the option of interim storage of profiles that co...

Early evolution of an Agulhas Ring

Abstract Rings shed at the Agulhas retroflection are an integral part of interoceanic exchange south of Africa. There is clear evidence of westward ring translation from the northern Cape Basin across the South Atlantic Ocean. Early ring development and translation from the southern to the northern Cape Basin, however, are obscured by an intensely variable kinematic field close to the spawning site. In this study unique in situ observations, obtained in March to September 1997, are analyzed to improve the understanding of the early development of a juvenile Agulhas Ring. In March the ring was surveyed near 37°S, 16°E, approximately 4 months after its generation. Its strength and size were in the upper range typical for Agulhas Rings, and its trapping depth extended down to at least 1600 dbar according to geostrophic velocities and RAFOS trajectories in the ring. Between March and September the ring propagated in a general northwestward direction; however, RAFOS trajectories and MODAS sea-surface steric height fields revealed a large variability of the translation speed ( 3 cm s −1 to more than 20 cm s −1 ) and direction. In September 1997, the mature ring was examined near 31°S, 9°E. By this time, its available heat and salt anomaly were reduced by about 30% and its available potential energy was reduced by about 70%. This indicates that a significant loss of the ring characteristics occurred on the way from the southern to the northern Cape Basin. One-third of this loss is due to changes at intermediate depth (between 800 and 1600 m ).

Quantification of the interocean exchange of intermediate water masses around southern Africa

Abstract A large collection of historic and recent hydrographic data has been used in a water-mass mixing model to quantify the remote sources of intermediate water that pass through the region around southern Africa. Special emphasis has been given to deriving the fractional transports of water with Indian Ocean origins that enter the South Atlantic and that thus contribute to the overturning circulation of the Atlantic. The model includes five remote sources: Antarctic Intermediate Water (AAIW) from the Drake Passage (dAAIW), a component of AAIW from the South Indian Ocean (siAAIW), intermediate water from the Indonesian Seas (IIW) and from the Red Sea (RSIW), and a transformed end member of the mixture of these sources (aAAIW). The study is performed on five neutral density surfaces chosen to follow the core of AAIW and to span the AAIW layer, encompassing a depth range between about 700 and 1200 m . To resolve the mixing of water masses from the five remote sources, the six physical–chemical tracers, potential temperature, salinity, dissolved oxygen, silicate, initial phosphate, nitrate (NO) and potential vorticity, have been used as input information. The model-derived mixing fraction gives a quantitative description of the remote sources when they are mapped onto neutral density surfaces. Results show that dAAIW is a dominant source, with a large proportion of its water extending to the southwestern Indian Ocean in a continuous distribution, while the Indian Ocean sources spread to the southeastern South Atlantic mostly in a patchy distribution. Given the mixing fractions, the geostrophic velocity fields and the RAFOS velocity fields, the volume transport can be separated into individual contributions from each source. It is found that dAAIW provides most of the transport (63% or 1.9 Sv , 1 Sv =10 6 m 3 s −1 ) crossing the 25°S section (10°W–14°E). The rest of the equatorward transport is contributed by siAAIW (16% or 0.5 Sv ), by IIW (10% or 0.3 Sv ) and by RSIW (12% or 0.4 Sv ), respectively. The net eastward transport of dAAIW passing south of Africa into the Indian Ocean is estimated to be 13.5 Sv at 20°E (50°S–36°S). Its westward return transport from the Indian Ocean is 2.1 Sv , amounting to about one-seventh of its total transport into the Indian Ocean. The Benguela Current transport from RAFOS observations for the AAIW layer at 6°W (34°S–22°S) is 12.9 Sv westward. Of this 80% is contributed by dAAIW, 11% by siAAIW, 3% by IIW, and 6% by RSIW, respectively. This means that of the 12.9 Sv Benguela Current transport, 10.3 Sv is contributed by dAAIW alone, which is composed of 8.8 Sv directly fed from the South Atlantic and 1.5 Sv returned after a loop in the Indian Ocean. Due to a weak baroclinic shear the corresponding geostrophic transport is 6.5 Sv westward, only half of the RAFOS transport. Of this 80% is contributed by dAAIW, 15% by siAAIW, 2% by IIW and 3% by RSIW.

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.

A comparison of in-situ float velocities with altimeter derived geostrophic velocities

Abstract Satellite borne altimetric data are of increasing prominence for assimilation in ocean circulation models and interpretation of localized in-situ measurements. Physically, geo-referenced sea-surface height (SSH) data products are mostly referenced relative to a long-term SSH mean, and consequently called SSH-anomalies. The Modular Ocean Data Assimilation System (MODAS) adds climatological SSH fields to provide space–time interpolated absolute steric SSH fields. This, in theory, should provide realistic geostrophic surface velocities and flow patterns, including quasi-permanent features such as western boundary currents or free jets. This study compares such data for the wider Agulhas Retroflection region with co-located, simultaneous velocity measurements from Cape of Good Hope Experiments (KAPEX). KAPEX used ship-borne Acoustic Doppler Current Profiles (ADCP) and neutrally buoyant RAFOS (Ranging and Fixing of Sound) floats at intermediate depths to obtain in-situ velocity data. Correlation coefficients of MODAS-2D geostrophic and RAFOS subsurface flow directions fall between 0.8 and 0.9 with a typical error less than 0.05. The high correlation suggests that MODAS-2D provides a correct depiction of anticyclonic/cyclonic flow patterns in this region, making it a useful tool to describe the Agulhas Retroflection. Root-mean-square differences between velocities as measured by the various data sources rage between 20 and 30 cm s −1 , lying between the natural variability observed for the intermediate and surface layers. Decreasing slope parameters of linear regressions between MODAS, RAFOS and ADCP velocities reflect the baroclinic velocity shear. Slope equals 1 at surface and decreases to 0.4 at depths below 1000 m . Offsets of linear regression of these fits are not significantly different from zero, except for the zonal component in the Agulhas Return Current (5– 10 cm s −1 ). This discrepancy suggests a missing meridional gradient in this regions climatological signal that is added to the SSH anomaly field within MODAS.

Calculation of salinity from neutrally buoyant RAFOS floats

A method to derive salinity data from RAFOS float temperature and pressure measurements is described. It is based on evaluating the floats in situ density from its mechanical properties and in situ pressure and temperature data. The salinity of the surrounding water may then be determined, assuming that the float has reached equilibrium with its environment. This method, in comparison with the possible use of floatborne salinity cells, has the advantage of being both cost and energy neutral and highly stable in the long term. The effect on the estimated salinity of various parameters used in the determination of the floats in situ density is discussed. Results of seven RAFOS Boats deployed in the Brazil Basin are compared with corresponding CTD data to estimate the magnitude of these errors. At present, an accuracy of 0.3 psu is achieved. The accuracy may be improved to 0.02 psu by referring the floats calculated density to a reference density established by a CTD cast at the time of launch. Results from five floats deployed in the heterogeneous water masses of the Iberian Basin are compared with the corresponding CM casts to demonstrate the variability and interpretation of p-T-S float datasets from different areas.