Isabel Ambar

Observations of the Mediterranean outflow—I mixing in the Mediterranean outflow

Abstract An identification of the two scores in the Mediterranean water outflow was made near the Straits of Gibraltar at about 7°W. It was estimated that the upper and lower cores constituted 95 and 72%, respectively, of the admixture with North Atlantic Central Water, whereas in proceeding around the Gulf of Cadiz towards Cape St Vincent these fractions were reduced to 50 and 38%. The most likely flow paths were determined by establishing the high coherence in the thermohaline properties associated with these cores. An interpretation of a short set of time series data, to account for the vertical periodicity that was observed in the outflow, was regarded as equally relevant to a possible explanation of a process by which the upper core might be formed within the straits.

Lagrangian Observations of Meddy Formation during A Mediterranean Undercurrent Seeding Experiment

Mediterranean eddies (meddies) play an important role in maintaining the temperature and salinity distributions in the North Atlantic, but relatively little is known about their early life histories, including where, how often, and by what mechanism they form. A major field program, called A Mediterranean Undercurrent Seeding Experiment, has been carried out to directly observe meddy formation and the spreading pathways of Mediterranean Water into the North Atlantic. Between May 1993 and March 1994, 49 RAFOS floats were deployed sequentially in the Mediterranean Undercurrent south of Portugal and tracked acoustically for up to 11 months. The float deployments were accompanied by high-resolution XBT sections across the undercurrent. Nine meddy formation events were observed in the float trajectories, six near Cape St. Vincent, at the southwestern corner of the Iberian Peninsula, and three near the Estremadura Promontory, along the western Portuguese continental slope. Meddy formation thus occurs where the continental slope turns sharply to the right (when facing in the downstream direction of the undercurrent). After conditionally sampling the float dataset to identify floats that were well seeded in the undercurrent, the authors have estimated a meddy formation rate of 15‐20 meddies per year. The timescale for meddy formation at Cape St. Vincent was found to be 3‐7 days, shorter than previous estimates based on the volume of larger meddies. Meddies were observed to form most frequently when the speed of the Mediterranean Undercurrent was relatively fast. The meddy formation process at Cape St. Vincent resembles the conceptual model of E. A. D’Asaro, whereby anticyclonically rotating eddies are formed by separation of a frictional boundary layer (with negative relative vorticity) at a sharp corner. Comparison of the relative vorticity in the anticyclonic shear zone of the undercurrent and that of the newly formed meddies shows that much of the anticyclonic relative vorticity in meddies can be accounted for by the horizontal shear in the undercurrent. This confirms earlier work suggesting that the classical mechanism for the generation of submesoscale coherent vortices, by collapse and geostrophic adjustment of a weakly stratified fluid injected into a stratified ocean, may not be the principle mechanism at work in the formation of meddies at Cape St. Vincent.

Observations of the mediterranean outflow—II the deep circulation in the vicinity of the gulf of cadiz

Abstract An analysis of the field of mass produced a reasonably coherent pattern of circulation for the Mediterranean water outflow and estimates of the velocity of the undercurrent both before and after its subdivision into separate upper and lower cores were in good agreement with several previous direct current meter measurements. Meanders induced in the flow by the influence of the bottom topography are regarded as the main cause of the variability in the thermohaline and dynamic properties of the current system. A downstream increase in the volume of transport is interpreted in terms of an entrainment parameter that compares favourably with a recent streamtube model for bottom boundary currents. The analysis also indicates the presence of a deep countercurrent at about 1500 m beneath the main outflow in a region between 9°30′W and 8°30′W south of Cape St Vincent.

Mediterranean outflow mixing and dynamics

The Mediterranean Sea produces a salty, dense outflow that is strongly modified by entrainment as it first begins to descend the continental slope in the eastern Gulf of Cadiz. The current accelerates to 1.3 meters per second, which raises the internal Froude number above 1, and is intensely turbulent through its full thickness. The outflow loses about half of its density anomaly and roughly doubles its volume transport as it entrains less saline North Atlantic Central water. Within 100 kilometers downstream, the current is turned by the Coriolis force until it flows nearly parallel to topography in a damped geostrophic balance. The mixed Mediterranean outflow continues westward, slowly descending the continental slope until it becomes neutrally buoyant in the thermocline where it becomes an important water mass.

A shallow core of Mediterranean water off western Portugal

Abstract From an analysis of several sets of hydrological data taken in the Gulf of Cadiz area and off the southern half of the western coast of Portugal, it was possible to identify and establish the continuity of a shallow vein of Mediterranean Water, distinct from the already well-established two main cores. Its thermohaline characteristics, in the form of temperature and salinity maxima in the vertical profiles, allowed its identification at density ( σ t ) levels between 27.25 and 27.45 with an equilibrium depth between 400 and 700 m and with a thickness of 50 to 100 m.

Physical, chemical and sedimentological aspects of the Mediterranean outflow off Iberia

A multidisciplinary study of the Mediterranean outflow in the region west of the Strait of Gibraltar and off the southern and southwestern coasts of the Iberian Peninsula was developed in the frame of the Canary Islands Azores Gibraltar Observations Project. Two high-resolution CTD surveys, which included water sampling for chemical (nutrients and dissolved oxygen) and sedimentological analyses, took place in September 1997 (summer cruise) and January 1998 (winter cruise) in the study region. The correspondence between the high-salinity Mediterranean Water (MW) layer and the low-nutrient content and relatively high abundance of particles was a general result. Further details of the thermohaline analysis and of the geostrophic computations, especially for the layer of MW, are compared with the results obtained for the chemical properties and the sedimentological characteristics.

Eddy generation in the Mediterranean undercurrent

Abstract In the framework of the European Union MAST III project Canary Islands Gibraltar Azores Observations, 24 RAFOS floats were deployed in the Mediterranean Water (MW) undercurrent off south Portugal between September 1997 and September 1998. A preliminary analysis of this Lagrangian approach, complemented with XBT and current-meter data, show some of the major aspects of the flow associated with the undercurrent as well as associated eddy activity. Floats that stayed in the undercurrent featured a downstream deceleration and a steering by bottom topography. Three meddy formations at Cape St. Vincent could be isolated from the float data. The dynamical coupling of meddies and cyclones was observed for a considerable period of time. The generation of two dipolar structures in the Portimao Canyon region also was observed with the float data. A major bathymetric relief—Gorringe Bank—was not only an important constraint to the eddy trajectories and of the flow at the MW levels but also a site for meddy formation.

Remotely sensed sea surface thermal patterns in the Gulf of Cadiz and the Strait of Gibraltar: Variability, correlations, and relationships with the surface wind field

Satellite image sequences (covering periods of a few days throughout the annual cycle) of the waters off southern Iberia have been analyzed in conjunction with concurrent surface wind speed data from coastal stations. Qualitative analysis reveals a large degree of temporal and spatial variability in the thermal signature of the sea surface over periods of both a few days and several months. During the summer, a cool seasurface temperature signature extends from the western Iberian coast around Cape St. Vincent and eastward as far as Faro. At the same time, a warm signature originating on the Iberian coast between Faro and Cadiz extends into the Strait of Gibraltar. These two features are shown to sometimes adopt more westerly positions, and the strait experiences regions of cool thermal signature originating at its southern side. During winter, the surface flow into the Mediterranean through the Strait of Gibraltar is anomalously warm and appears to come from the interior of the Gulf of Cadiz. Quantitative measurements show that temporal variability over timescales of a few days at individual sites is maximum in midsummer. Spatial thermal variability over the whole region is found to peak toward the end of the summer. Statistical analyses of the data reveal the coupling between the surface wind field in the Gulf of Cadiz and the surface thermal pattern (especially during the summer). Wind-induced, across-stream upwelling in the Strait of Gibraltar, although dynamically subordinate to tidal and density-driven processes, is shown to occasionally dominate the surface thermal signature.

The Mediterranean outflow splitting—a comparison between theoretical models and CANIGO data

Abstract The Mediterranean outflow in the Atlantic is vertically subdivided into two main cores that are identified by temperature and salinity maxima. Hydrological data collected west of the Strait of Gibraltar and off the south and south-west coasts of Portugal in September 1997 have been used in two different models to shed some light on the mechanisms that cause a splitting of the Mediterranean outflow. These models are (i) a steady, one-dimensional streamtube model, and (ii) a local, time-dependent model that includes cross-stream variations. The streamtube model, applied at the northern, respectively, the southern part of the outflow, appears capable of reproducing the observed density difference between the two cores. The vertical separation, however, was underestimated. The results from the local model suggest that a bump in the bottom topography may be a determining factor for the splitting tendency of the flow

Direct evidence of meddy formation off the southwestern coast of Portugal

Abstract The formation of a Mediterranean Water eddy, or meddy, was observed directly for the first time off the southwestern coast of Portugal near Cape St. Vincent. The formation event is revealed in the 30-day trajectory of a RAFOS float deployed in the lower core of the Mediterranean Undercurrent in the Gulf of Cadiz. For the first several days after deployment, this float was advected westward in the Undercurrent at a speed of about 0.4 m s −1 , generally paralleling the topography. Just after passing Cape St. Vincent, where the coast turns abruptly northward, the float began looping anticyclonically, indicating that it had become trapped in the core of a new meddy. The meddy translated first westward, then southwestward along the southern flank of Gorringe Bank. The float rotated around the meddy center with azimuthal speeds of 0.20–0.25 m s −1 at a radius of about 10 km. The rotation period was on the order of three days, and the average translation speed of the meddy over 25 days was 0.08 m s -1 . Observations of this and four additional 30-day trajectories indicate (1) persistent westward flow of the Undercurrent along the south coast of Portugal, (2) a tendency for the lower core of the Undercurrent to separate from the continental boundary after passing Cape St. Vincent, and (3) evidence of anticyclonic looping west of Cape St. Vincent. These preliminary results confirm the speculation that the region off Cape St. Vincent is one site of meddy generation.