Andrés L. Rivas

Temperature and depth profiles recorded during dives of elephant seals reflect distinct ocean environments

Abstract Foraging adult southern elephant seals, Mirounga leonina , from Peninsula Valdes, Argentina, dive continuously while travelling across the continental shelf towards deep waters of the SW Atlantic. This study attempted to identify distinct ocean environments encountered by these seals during foraging migrations based on bathymetric and water temperature profiles, and to interpret these profiles in terms of mixing and systems of currents. Depth and water temperature were obtained with data loggers carried by 14 diving adult animals during spring (October–December) and summer (February–March) months. Dive depths allowed us to unmistakably differentiate extensive areas of the SW Atlantic: the Patagonian shelf, shelf slope and open waters of the Argentine Basin. Water temperature profiles added further details to the latter general oceanographic areas, and could be related to large-scale oceanographic processes that led to different water column structures. Temperature data reflected the mixing effects of winds and tides in coastal waters, the formation of a thermocline in mid-shelf areas, the northward flow of the sub-antartic Malvinas Current at the edge of the shelf, and the effect of the subtropical Brazil Current further east over deep off-shelf waters. Some of these distinct areas are known for their enhanced primary production associated with frontal systems. The study shows that elephant seals could be useful, low-cost platforms to obtain oceanographic data. Studies that require extensive sampling of physical variables in large areas over long periods of time would benefit from this approach, pending on more precise and frequent locations of animals at sea.

Current-meter observations in the Argentine Continental Shelf

Abstract The first moored current measurements carried out on the Argentine Continental Shelf are presented. In order to explore the mean value and the main temporal scales of the velocity field, current meter data were collected in a cross-shelf section near 43°S between September 1991 and August 1992. The records indicate that the mean flow is parallel to the local topography and that its intensity increases towards the shelf-break. The mean flow values seem to be related to the interaction of tidal currents with sea-surface height in the inner shelf and cross-shelf variations in the mass field in the middle shelf. The high frequency component (periods of less than 30 h) accounts for more than 80% of the observed kinetic energy. Close to the coast the semidiurnal component is dominant (possibly due to a resonance process), while in the middle shelf the inertial and diurnal components become equally important. The very low frequency component (periods longer than 10 days) increases its energy seaward, which suggests that it might be forced by ocean circulation. The relation between currents in the band 0.5-0.1 cycles per day and the wind measured on the coast is not clear.

Vertical stratification at the shelf off northern Patagonia

The upper layer of the continental shelf off Patagonia undergoes large temperature changes primarily in response to the seasonal incoming solar radiation. The present work analyses two temperature time series from near-surface and near-bottom levels gathered from a mooring located near 43°S in the central Argentine Continental Shelf from September 1991 to August 1992. A well-defined annual cycle and the setting of the seasonal thermocline starting in early October are observed. A number of intense episodic mixed layer cooling events are of particular interest. The temperature fluctuations are simulated using a numerical model (Bowers, A two-layer model of the seasonal thermocline and its application to the Celtic Sea. UCES Report U84-4, University College of North Wales, Vol. 65, pp. 1984), forced by the wind velocity, the heat exchanged with the atmosphere through the sea surface and the current velocity. The model reveals that the rapid cooling events are produced, mainly, by sudden changes in the surface heat flux associated to northward penetrations of subpolar air. In order to simulate the annual cycle, the model must be forced with the observed wind. In contrast, the bottom mixing can be parameterized based on the time-averaged current amplitude.

Spatial variation of the annual cycle of temperature in the Patagonian shelf between 40 and 50° of south latitude

Abstract This paper analyses the geographic and seasonal variation of depth-integrated water temperature in the region of the Argentinian shelf extending between 40 and 50°S. The data used were taken from more than 1300 oceanographic stations; they were averaged in a network of 1° × 1° and fitted by least squares to a mean value plus a co-sinusoidal signal. The main characteristics of amplitude and phase of the annual signal are explained with a one-dimensional model which shows that the seasonal cycle depends basically on local conditions (water depth and seasonal variation of surface heat flux), and that horizontal advection and diffusion play only a secondary role. On the other hand, the annual mean value of temperature depends on the mean heat flux through the surface and on horizontal advection, which is what prevents temperature from increasing as a consequence of a net gain of heat in the air-sea interface. From the observed data of mean temperature a geostrophic field of velocities was calculated and then modified so as to satisfy the equation of heat conservation. The obtained circulation coincides acceptably with other estimates and observations carried out in the area. It consists of a flux in the N-NE direction, with a mean velocity of the order of 2 cm s −1 , which enters the area under study with a temperature 4°C lower than when it leaves the northern frontier.

Mass and heat transport in the Argentine Continental Shelf

This paper investigates the barotropic circulation and its seasonal evolution in the area of the Argentine Continental Shelf extending between 40 and 50° S latitude. The study area has been divided into 66 elements of 1 x 1° and depth-integrated equations of mass- and heat-conservation have been set up for each of them. This procedure leads to a system of 124 equations with an easily understandable solution. The results obtained generally show a N-NE flow with a maximum speed of 10 cm s−1 which advects heat excess from the atmosphere. A strong seasonal signal is apparent in the circulation, with stronger (weaker) northeastward (variable) flow during the austral summer and autumn (winter and spring). The derived velocity field is almost independent of the value of the horizontal diffusion coefficient for KH varying between 0 and 103 m2 s−1. Model runs are presented for KH = 0 without diffusion) and KH = 102 m2 s−1 for both the yearly averaged and seasonal cases. Theoretical estimates of the mean annual circulation, and particularly the circulation calculated for summer and autumn, are in good agreement with observations made in the area and theoretical estimates obtained from rather sophisticated numeric models.