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: 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: Processes that influence the volume and heat transport across the Greenland–Scotland Ridge system are investigated in a numerical model with ° horizontal resolution. The focus is on the sensitivity of cross-ridge transports and the reaction of the subpolar North Atlantic Ocean circulation to changes in wind stress and buoyancy forcing on seasonal to interannual timescales. A general relation between changes in wind stress or cross-ridge density contrasts and the overturning transport of Greenland–Iceland–Norwegian Seas source water is established from a series of idealized experiments. The relation is used subsequently to interpret changes in an experiment over the years 1992–97 with realistic forcing. On seasonal and interannual timescales there is a clear correlation between heat flux and wind stress curl variability. The realistic model suggests a steady decrease in the strength of the cyclonic subpolar gyre of the North Atlantic with a corresponding decrease in heat transport during the 1990s

Description: Just as a stream function gives both a qualitative and quantitative description of the flow, a 'string function' can be constructed to describe the propagation in a rotating fluid of large-scale energy anomalies such as eddies and Rossby waves. To simply introduce the string function in this paper, we consider only a homogenous 1-layer fluid. In this case, the string function is inversely proportional to large-scale potential vorticity and the contours of both are aligned. The string function contains more information, however, because the propagation speeds are described by the contour spacing. We introduce the string function and use it to derive and reinterpret governing equations for linear and nonlinear shallow-water dynamics. The string function allows for simple evolution equations incorporating both beta and topographic effects.

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