Suchergebnisse

We analysed historical data (Fraga and Manríquez, 1974; Manríquez and Fraga, 1978) to investigate the relation between the position of the Cape Verde frontal region and the geostrophic velocity field in summer and winter. The position of the front is well determined in the plotting of the salinity and nutrient distributions in isopycnic coordinates. The geostrophic velocity shows sharp gradients as a function of density and distance on sections normal to the coast. These gradients are reasonable well correlated with the frontal position and indicate the existence of strong meridional interleaving. The flow field relative to 400 m illustrates this interleaving as fluctuations on a westward mean flow. The surface temperature distribution undergoes very large latitudinal displacements but may help to locate the presence of north and south surface water ; This work was supported by the Spanish government through CICYT´s project MAR96-1893. The principal author, PPR, wishes to thank the Ministerio de Educación y Ciencia of the Spanish government for funding during this work ; Peer reviewed

The flow pattern of the North Atlantic Subtropical Gyre (NASG) is simulated using a highly idealised one-layer quasi-geostrophic wind-driven model. The novel feature of the model is the specification of the eastern boundary conditions. This is an upwelling favourable region with a quasi-permanent southward flowing coastal jet, which is fed by the eastern branch of the Canary Current. The corresponding boundary conditions are non-zero normal flux and constant potential vorticity, the latter being consistent with the generation of anticyclonic vorticity by the coastal jet. We examine the sensitivity of the model to the eastern boundary conditions and compare the results with recent observations for the region ; This work was supported by the European Union through project CANIGO (MAS3-CT96-1893) and the Spanish government trhough projects FRENTES (AMB95-0731) and TALUD (MAR96-1893) ; Peer reviewed

[EN] We study the dependence of salinity on temperature in two by two degrees latitude-longitude boxes, for surface and upper-thermocline waters of the eastern North Atlantic subtropical gyre. The initial data set, from historical databases as well as from recent hydrographic cruises in the region, is carefully scrutinized to reject dubious measurements. We search for polynomial relations of variable degree between salinity and temperature, the optimal fit is selected as the polynomial with the lowest degree that satisfies several statistical criteria. An independent hydrographic cruise is used to confirm that the method performs substantially better than estimates from climatological data, and leads to relatively low deviations in geopotential anomaly and other derived quantities. An error propagation analysis using the Monte Carlo method shows equally good results ; [ES] En este estudio caracterizamos la dependencia de la salinidad con la temperatura en cajas de dos por dos grados de latitud y longitud para aguas superficiales y de la termoclina superior en el margen oriental del giro subtropical del Atlántico Norte. El conjunto inicial de datos, procedente de bases de datos históricos, así como los procedentes de recientes campañas hidrográficas en la región, son cuidadosamente analizados para eliminar las medidas de calidad dudosa. Se analizan varias relaciones polinómicas de grado variable para la salinidad en función de la temperatura, eligiendo como ajuste óptimo aquel polinomio de menor grado que satisface diversos criterios estadísticos. Se utiliza una campaña hidrográfica independiente para confirmar que el método da resultados substancialmente mejores que estimaciones a partir de datos climatológicos, y que produce desviaciones relativamente bajas en la anomalía geopotencial y en otras magnitudes derivadas. Un análisis de la propagación del error usando el Método de Montecarlo muestra resultados igualmente buenos ; We are grateful to the Institut für Meereskunde and IFREMER for making their data sets available. The Spanish cruises that gathered some of the data analyzed here were carried out through projects from the Spanish government (program CYTMAR) and the European Union (program MAST). This work has been carried out with the support of the Spanish Government through project FRENTES (AMB95-0731) and the European Union through projects CANIGO (MAS3-CT96-0060) and OASIS (EVK3-CT-2002-00073). Part of this work was written while JLP was at the University of Wisconsin-Madison with funding from the Secretaría de Estado de Educación y Universidades of the Spanish government ; Peer reviewed

The high cost of oceanographic cruises often makes it advisable to use opportunity vessels for simple measurements, such as determining the temperature of the water column with expandable bathythermographs (XBT). In this work we examine the goodness and reliability of a method aimed at obtaining the maximum possible information from XBT data, and we apply it to the Canary Islands region. It consists in calculating analytic relations between temperature and salinity from historical conductivity-temperature-depth (CTD) data for the region, which are then used to hindcast/forecast the salinity and density distribution, as well as the distribution of other inferred quantities such as velocity. A hindcasting is carried out using direct independent temperature measurements obtained from a hydrographic cruise south of the island of Gran Canaria. At depths greater than 100-150 m the results show good agreement with the calculations obtained from CTD in situ data ; This work was supported by the Spanish government (CICYT projects AMB95-0731 MAR 96-1893)and the European Union (MAST project MAS3-CT96-0060) ; Peer reviewed

This work examines the possible importance of shear-induced diapycnal mixing in controlling the evolution and stability of meanders in oceanic frontal jets. We first review the conditions necessary for vortex stability and investigate how these may be modified in the presence of diapycnal mixing. The procedure used is rather crude but provides a measure of the relative importance of diapycnal mixing. It consists in constructing a simplified equation for the radial velocity that retains the density tendency and examining under what circumstances this velocity may grow in time. Next, we use a simple two-dimensional isopycnic model to examine the intensity of diapycnal mixing in meanders. In the model the along-front velocity is in geostrophic balance and the ageostrophic contributions are an oscillating deformation field and diapycnal mass exchange. The horizontal deformation field increases the slope of the isopycnals in temporal scales typical of Gulf Stream meanders, causing a reduction of the gradient Richardson number, Ri. The diapycnal flux is calculated as the divergence of the density Reynolds flux, which is parameterized in terms of Ri. The results of the model show that diapycnal mixing increases during the frontogenetical stages, reaching density tendency values of the order of 10-4 kg m-3s-1 and convergence/divergence values of the order of 10-3 s-1. It turns out that diapycnal mixing in meanders may be intense enough to control the separation and slope of the isopycnals and to condition the possibility of barotropic instability ; This work was supported by the Spanish government trhough projects FRENTES (CICYT grant AMB95-0731) and TALUD (CICYT MAR96-1893) and the European Union through project CANIGO (MAST-CT96-0060) ; Peer reviewed

Two years of periodic XBT data, along seven transects covering the Canary Basin, were examined to analyse the seasonal cycle of the upper layers of the warmwatersphere. This is characterised by seasonal storage/release of heat in the surface mixed layer and by the winter formation of a subsurface positive temperature anomaly. Its maximum value takes place in early winter and deepens to about 150 m in late spring, disappearing afterwards. We present a very simple argument, which illustrates how the evolution of the temperature anomaly is controlled both by the ocean-atmosphere heat exchange and by the shape of the North-Atlantic Central Water T-S relationship ; This work was supported by the European Union through project CANIGO (MAST III grant number MAS3-CT96-0060) and the Spanish government trhough the FRENTES project (CICYT grant number AMB95-0731) ; Peer reviewed

19 pages, 11 figures, 1 table, supporting information https://dx.doi.org/10.1029/2004JC002526 ; We use the trajectory of three buoys dragged below the surface mixed layer, together with sea surface temperature imagery, to examine the evolution of an anticyclonic warm-core eddy since its generation by the Canary Islands. Two buoys remain within the eddy during some 100 days, and the third one remains almost 200 days, while drifting southwestward up to 500 km with the mean Canary Current. The eddy merges with several younger anticyclonic and cyclonic eddies, in each occasion, suffering substantial changes. The eddy core, defined as a region with near-solid-body-type rotation and radial convergence, initially occupies the whole eddy. After interacting with another vortex the inner core markedly slows down, although it continues displaying radial convergence and relatively small radial oscillations, and an uncoupled outer ring is formed or enhanced, which revolves even more slowly and displays large radial fluctuations. The vortex extensive life is consistent with its inertially stable character and observations of radial convergence. A very simple model of vortex merging, where cylinders fuse conserving mass and angular momentum, gives fair results. The observations suggest that the eddy changes, as the result of its own slow evolution and sporadic mixing events, from a young stage, where the core retains its vorticity and occupies most of the eddy, through a mature stage, where the eddy has a reduced inner core and a slowly revolving outer ring, to a decay stage, where the vorticity maximum is substantially reduced ; This work has been supported by the Spanish government through projects FRENTES (AMB95-0731), COCA (REN2000-1471-C02-02MAR), and BREDDIES (REN2001-2650/ANT) and the special action MAR1999-1489-E and by the European Union through projects CANIGO (MAS3-CT96-0060) and OASIS (EVK3-CT-2002-00073-OASIS) ; Peer Reviewed