ALBERT

Description: Current and wind stress time series obtained from the F1-mooring are analysed with the aim of examining linear correspondences and testing the adequacy of linear coupling models at near-inertial frequencies. Significant linear correlations are found in the data set which are consistent with a linear winddriven model of the current system. The current in the mixed layer can be described by inertial oscillations directly forced by the local wind stress. A wind-driven simulation model of the mixed layer currents yields an energy input of 3.10-3 W/m2. The current in the thermocline can be described by a linear internal wave field of downward propagating wave groups driven via Ekman suction by the wind stress field. Internal waves are generated at a rate of 10-3 W/m2, consistently estimated from both kinematic and dynamic considerations.

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.

Description: Hydrographic observations from the Iberian Basin demonstrate the variability of water masses in upper and intermediate layers. The surveyed area embraces the internal front between water masses from higher latitudes and the Mediterranean outflow, exhibits several isolated Mediterranean eddy (meddy) structures at middepth, and displays the virtual source region for the Mediterranean Water (MW) tongue off the Portuguese continental slope. The description is enhanced by additional chlorofluoromethane measurements, which show anomalously high concentrations at middepth, due to mixing of MW with the overlying Atlantic waters in the Gulf of Cadiz. The geostrophic stream function shows several meddylike features that not only are remarkably extended in the depth range of the MW, but are also correlated with surface height anomalies.

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: During a multi-institutional air-sea interaction experiment (GATE) in the central Atlantic North Equatorial Countercurrent in September 1974, vector-averaging current meter (VACM) measurements were made within the 30-m thick mixed layer from three different types of surface moorings. The moorings consisted of a single-point taut-line flexible mooring (E3), a spar-buoy (El), and a 2-legged mooring (Fl). Although the kinetic energy density spectral estimates of the E3, El, and Fl records in the low frequency range were equivalent with 95% confidence, the mean progressive vector diagrams differed by 6 % in length and 4 in direction. At frequencies above 1 cph the variances of the 7.2 m Fl current vectors were about 1.5 times larger than the 7.6 m E3 data and the spectral levels of the 20 m El and 21.4 m E3 record were equivalent, suggesting that VACM current vectors recorded near the surface beneath a surface-following buoy do not contain detectable amounts of aliased high-frequency mooring motion.

Description: Progress in modeling the oceanic circulation has been achieved in the last few years by increasing the speed of computers and by refining modeling techniques. The dynamics of major current systems such as the Gulfstream-North Atlantic Current and their corresponding eddy variability is reasonably well understood [58, 32]. Climate models predict global warming as a result of increasing CO2 in the atmosphere and forecast El Nino events in the equatorial Pacific [50]. Freshwater imbalances in the deep convection regions of the polar and subpolar regions of the North Atlantic result in alternating multiple equilibrium states of the global thermohaline vertical circulation - the ”conveyor belt” [53]. On the other hand, large scale modeling relies heavily on the parametrization of ”subgrid” processes. This is especially true for the oceanic boundary layer. Here the modeling suffers from inappropriate information on the fluxes at the air-sea interface. Most coupled models with simplified fluxes do not represent the surface temperature well enough and water mass characteristics drift away from the initial state. Restoring conditions at the sea surface are needed to force the model back to the observations. The fluxes analyzed from runs with restoring conditions show substantial errors. It is evident that progress in the reliability of long-term predictions of climate variations can only be made with a better representation of mixed layer dynamics.

Description: The authors derive a string function that describes the propagation of large-scale, potentially large amplitude, baroclinic energy anomalies in a two-layer ocean with variable topography and rotation parameter. The generality of the two-layer results allows results for the 1-layer, 1.5-layer, inverted 1.5-layer, lens, and dome models to be produced as limiting-cases. The string function is a scalar field that acts as a streamfunction for the propagation velocity. In the linear case the string function is simply c2o/f, where co is the background baroclinic shallow water wave speed, and typically describes propagation poleward on the eastern boundaries, westward (with some topographic steering) over the middle ocean, and equatorward on the western boundaries. In the more general nonlinear case, the string function is locally distorted by the anomaly. In the fully nonlinear examples of a lens or dome, there is no rest or background string function; the string function is generated entirely by the disturbance and propagation is due to asymmetric distribution of the anomalous mass over the string function contours. It is shown that conventional beta/topographic propagation results (e.g., beta drift of eddies, the Nof speed of cold domes) can be obtained as limiting cases of the string function. The string function provides, however, more general propagation velocities that are also usually simpler to derive. The first baroclinic mode string function for the global oceans is calculated from hydrographic data. The westward propagation speeds in the ocean basins as derived from the meridional gradient of the string function are typically two to five times faster than those expected from standard theory and agree well with the propagation speeds observed for long baroclinic Rossby waves in the TOPEX/Poseidon data.

Description: Fahrtgebiet: Irmingersee und Ausgang Dänemarkstraße Zweck: Hydrographische Aufnahme im Ausbreitungsgebiet des Overflow-Wassers, Auslegung einer Inverted Echo Sounder Verankerung (I.E.S), Auslegung von satelliten-georteten Driftbojen, Schleppen eines neuen geomagnetischen Elektrokinetographs (TTM3), Erprobung eines GPS-Arrays zur hochgenauen Ortsbestimmung Wiss. Ziele: Ermittlung mesoskaliger Strukturen im Overflow als Eingangsdaten für Simulationsrechnungen