Description: 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.

Description: A method to derive salinity data from RAFOS float temperature and pressure measurements is described. It is based on evaluating the float's 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 float's 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 float's 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.

Description: 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.za

Description: Ocean bottom pressure (OBP) variability serves as a proxy of ocean mass variability, the knowledge of which is needed in geophysical applications. The question of how well it can be modeled by the present general ocean circulation models on time scales in excess of 1 day is addressed here by comparing the simulated OBP variability with the observed one. To this end, a new multiyear data set is used, obtained with an array of bottom pressure gauges deployed deeply along a transect across the Southern Ocean. We present a brief description of OBP data and show large‐scale correlations over several thousand kilometers at all time scales using daily and monthly averaged data. Annual and semiannual cycles are weak. Close to the Agulhas Retroflection, signals of up to 30 cm equivalent water height are detected. Further south, signals are mostly intermittent and noisy. It is shown that the models simulate consistent patterns of bottom pressure variability on monthly and longer scales except for areas with high mesoscale eddy activity, where high resolution is needed to capture the variability due to eddies. Furthermore, despite good agreement in the amplitude of variability, the in situ and simulated OBP show only modest correlation.

Description: The subsurface flow within the subantarctic and subtropical regions around the Brazil-Malvinas (Falkland) Confluence Zone is studied, using daily hydrographic and kinematic data from four subsurface floats and a hydrographic section parallel to the South American shelf. The float trajectories are mapped against sea surface flow patterns as visible in concurrent satellite sea surface temperature (SST) images, with focus on the November 1994 and October/November 1995 periods. The unprecedented employment of Lagrangian θ-S diagrams enables us to trace the advection of patches of fresh Antarctic Intermediate Water (AAIW) from the Confluence Zone into the subtropical region. The fresh AAIW consists of a mixture of subtropical AAIW and Malvinas Current core water. Within the subtropical gyre, these patches are discernible for extended periods and drift over long distances, reaching north to 34°S and east to 40°W. The cross-frontal migration of quasi-isobaric floats across the Confluence Zone from the subtropical to the subantarctic environment is observed on three occasions. The reverse process, float migration from a subpolar to a subtropical environment was observed once. These events were located near 40°S, 50°W, the site of a reoccurring cold core feature. Subsurface float and SST data comparison reveals similarities with analogous observations made in the Gulf Stream [Rossby, 1996] where cross-frontal processes were observed close to meander crests. The limited number of floats of this study and the complex structure of the Brazil-Malvinas Confluence Zone, however, restricts the analysis to a description of two events.

Description: Ocean currents’ effect on long-range sound propagation, though considerable in many cases, is difficult to separate from much stronger effects due to sound speed inhomogeneities, as flow velocity is usually much smaller than typical variations in the sound speed. Dramatic improvement can be achieved in reciprocal transmission experiments when sound signals propagate in opposite directions between two transceivers (source–receiver pairs). The presence of a current results in the breaking of the principle of acoustic reciprocity, thus making it possible to use nonreciprocity of acoustic field as an indicator of water movement. In this paper, reciprocal acoustic transmissions through a submesoscale interthermocline lens of Mediterranean Water (meddy) in the Atlantic are considered theoretically as a possible tool for meddies detection. A simple model of acoustic ray-travel-time nonreciprocity due to a meddy is proposed. The analytic estimates obtained from the model show that the influence of rotary flow is more important than that of drift and seems to be measurable. The problem is studied in more detail via computer simulations. The environmental model used in the simulations corresponds to case studies performed in the Iberian Basin in 1989 and 1991. Numerical simulations show that travel times between two transceivers can be gathered into several groups; for the most part, rays in each set have similar geometry for both propagation directions. However, the lens strongly affects the number of rays in each group, their launch angles, and number of surface interactions, making it impossible to identify these arrivals as required for conventional ocean acoustic tomography. In spite of complexity of ray structure, travel-time nonreciprocity predicted by the model proposed is in good agreement with numerical results. This fact suggests that the model could be used to estimate some parameters of a meddy.

Description: 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.