Description: The existence of energetic anticyclonic mid-depth vortices of Mediterranean Water (meddies) questions the validity of a conventional advective–diffusive balance in the eastern Atlantic subtropical gyre. A mesoscale experiment in the Azores–Madeira region reveals a link of these meddies to large-scale subsurface meanders. For the first time it is shown that meddies may have strong surface vorticity, indicative of a generation process involving the Azores Current—a deep reaching near-surface jet.

Description: Hydrographic data from two cruises in the Canary Basin (Meteor 57, July 1981; Poseidon 86, April 1982) are analysed with respect to current distribution and lateral heat flux in the Azores-Madeira region. The first part of the data base consists of long transects of XBT and G.E.K. measurements between Cape Finisterre (North West Spain) and the northern Canary Basin, where several year-long current meter records exist. Further information is obtained by thermosalinograph surface data and by expendable current profilers (XCP). Geostrophic currents are derived from XBT profiles, using the tight temperature-salinity relationship in the depth range of the Warmwassersphäre. The results compare well with the G.E.K. and XCP current observations. The second part consists of CTD data from an eddy resolving, box-shaped CTD survey (500 × 500 km2) centered at the mooring location (33°N, 22°W), The observations are supplemented by satellite-buoy trajectories. Horizontal parameter distribution is shown in terms of objectively contoured maps. Bands of spatially enhanced energetic structures, seen in the long transects are further resolved by the box survey as a deep jet-like current system cross the Canary Basin in a west-east direction. Associated with this Azores Current is a frontal zone with near-surface temperature and salinity steps of order 2 K and 0.3 practical salinity units. The dynamic topography field can be decomposed into a linear background field, a Rossby wave and a mesoscale eddy field. We find that major contributions to the meridional eddy heat flux are confined to the vicinity of the Azores current frontal zone. It is shown that the principal balance in the temperature equation is between heating by the mean horizontal advection terms and cooling by the eddy flux divergence.

Description: In spite of the fundamental role the Atlantic Meridional Overturning Circulation (AMOC) plays for global climate stability, no direct current measurement of the Denmark Strait Overflow, which is the densest part of the AMOC, has been available until recently that resolve the cross-stream structure at the sill for long periods. Since 1999, an array of bottom-mounted acoustic instruments measuring current velocity and bottom-to-surface acoustic travel times was deployed at the sill. Here, the optimization of the array configuration based on a numerical overflow model is discussed. The simulation proves that more than 80% of the dense water transport variability is captured by two to three acoustic current profilers (ADCPs). The results are compared with time series from ADCPs and Inverted Echo Sounders deployed from 1999 to 2003, confirming that the dense overflow plume can be reliably measured by bottom-mounted instruments and that the overflow is largely geostrophically balanced at the sill.

Description: A densely spaced hydrographic survey of the northern Irminger Basin together with satellite-tracked near-surface drifters confirm the intense mesoscale variability within and above the Denmark Strait overflow. In particular, the drifters show distinct cyclonic vortices over the downslope edge of the outflow plume. Growing perturbations such as these can be attributed to the baroclinic instability of a density current. A primitive equation model with periodic boundaries is used to simulate the destabilization of an idealized dense filament on a continental slope that resembles the northeastern Irminger Basin. Unstable waves evolve rapidly if the initial temperature profile is perturbed with a sinusoidal anomaly that exceeds a certain cutoff wavelength. As the waves grow to large amplitudes isolated eddies of both signs develop. Anticyclones form initially within the dense filament and are rich in overflow water. In contrast, cyclones form initially with their center in the ambient water but wrap outflow water around their center, thus containing a mixture of both water types. The nonlinear advection of waters that were originally located within the front between both water masses contributes most significantly to the stronger intensification of the cyclones in comparison with anticyclones. The frontal waters carry positive relative vorticity into the center of the cyclone. The process bears therefore some resemblance to atmospheric frontal cyclogenesis. After saturation there is a bottom jet of overflow water that is confined by counterrotating eddies: anticyclones upslope and cyclones downslope of the overflow core. The parameter dependence of the maximum growth rate is studied, and the implications of eddy-induced mixing for the water mass modification is discussed.

Description: Zeitplan: 9. Juni 1989 Auslaufen Porto, 26. Juni Einlaufen Kiel

Description: Das Teilprojekt C2 des SFB 133 "Warmwassersphäre des Atlantiks" untersucht die mesoskalige Variabilität des Dichte- und Strömungsfeldes im Kanarenbecken. Im Vordergrund der diesjährigen Untersuchungen steht die Frage, in welchem Masse die Zirkulation im Tiefenbereich bis etwa 1500 m durch hydrographische Messungen erfasst werden kann und mit direkten Strömungsmessungen aus verankerten Meßsystemen bzw. aus satelliten-georteten Driftbojen in Einklang zu bringen ist. Da eine quasi-synoptische Aufnahme eines grösseren Areals bisher noch nicht zur Verfügung steht, sollte FS "Poseidon" während der Expedition "KANARENBECKEN 1982" die hydrographische Vermessung eines etwa 200 x 200 sm grossen Gebietes vornehmen, in dem die mesoskalige Aktivität besonders gross war. Meridionale Schnitte während der "METEOR"-Reise 56 (Abschnitt 5, Rückreise Antarktis) hatten gezeigt, daß im Bereich um 33°N, 22°W, im Frühjahr ein System von Temperatur-Salzgehaltsfronten vorhanden ist. Es lag nahe, daß diese Fronten mit starken Oberflächenströmungen verbunden waren. Daher wurde der Einsatz von 4 satellitengeorteten Driftbojen mit Segel in 100 m Tiefe vorgesehen, die bereits 14 Tage vor dem Beginn des Experiments durch FS "METEOR" ausgelegt werden sollten.

Description: Salzreiches Wasser, das durch die Straße von Gibraltar aus dem Mittelmeer in den Atlantik strömt, wird dort verwirbelt und driftet teilweise als rotierende Salzlinse in etwa 1000 Meter Tiefe manchmal mehr als zwei Jahre lang bis zu 1000 Kilometer weit, ehe es sich endgültig mit dem Atlantikwasser vermischt.