Description: The Bass Strait cascade is a wintertime downwelling caused by cooling of the shallow waters of Bass Strait. During winter, a front separates the cold shelf water from the waters of the Tasman Sea. Continuous horizontal bands of downwelled water leading oceanward beneath the front imply that it can be transgressed near the bottom anywhere along its length. However, by far the greatest volume crosses at a breach at the northern end. Measured currents in eastern Bass Strait fit a predictable pattern: eastward toward the front, then as the front is approached, swinging north towards the breach. Flow northwards along the slope after downwelling is quantified using a simple analytic model. Cascade water found in the “far-field” was found only in small patches. One such patch was found to possess motion independent from the mean flow in which it was embedded.

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: 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: 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: 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: The Denmark Strait plays an important role as a dense water gateway between the Arctic and the subpolar North Atlantic. Previous studies have shown that the volume transport over the sill is limited by hydraulic constraints. A regional ocean-circulation model (ROMS) with a horizontal resolution of ≈1/20° degree and 30 sigma layers in the vertical is applied to study the through flow characteristics for Last Glacial Maximum to Holocene conditions. The bathymetry of the gateway region is obtained from a geodynamic model that takes into account the differential ice loading of the adjacent continents. First, the upstream reservoir conditions are systematically changed to test hydraulic limitations for altered bathymetry. Generally, the through flow is less than the predicted maximal value from hydraulic theory by almost 50%. The results indicate that the reduction in gateway depth and aperture owing to glacial-isostatic processes alone lead to a considerable further reduction of the overflow, by approximately 33%, compared to the present day. Second, the through flow is modeled using average density profiles and wind stress from global model data. The reduction in the density-driven part of the overflow is partly compensated by an enhanced wind stress but is still reduced by a factor of 5. Owing to the narrowing of the strait during the glacial and the increased northerly wind, the North Icelandic Irminger Current was strongly reduced but still existent.

Description: We report on a rapid high-resolution survey of the Denmark Strait overflow (DSO) as it crosses the sill, the first such program to incorporate full-water-column velocity profiles in addition to conventional hydrographic measurements. Seven transects with expendable profilers over the course of one week are used to estimate volume transport as a function of density. Our observations reveal the presence of a strongly barotropic flow associated with the nearly-vertical front dividing the Arctic and Atlantic waters. The seven-section mean transport of water denser than σθ=27.8 is 2.7±0.6Sv, while the mean transport of water colder than 2.0°C is 3.8±0.8 Sv. Although this is larger than the 2.9 Sv of θ 〈 2°C water measured by a 1973 current meter array, we find that a sampling of our sections equivalent to the extent of that array also measures 2.9Sv of cold water. Both the structure and magnitude of the measured flow are reproduced well by a high-resolution numerical model of buoyancy-driven exchange with realistic topography.