A controlled volume box model of the western basins of the Nordic Seas for water denser than 1027.8 kg m −3 is constructed, where accumulation in volume ( ) is driven by net imbalances between prescribed net inflow from the northern, eastern and top boundaries ( Q s ) and hydraulically limited outflow through the Denmark Strait. The resulting Riccati equation is solved analytically for filling and flushing experiments with constant Q s and numerically for stochastic forcing Q s ( t ). For small perturbations to Q s with white noise spectrum, the overflow response is red noise with a time scale between 5 and 15 years depending on the mean interface height and area. For Q s proportional to the NAO index, the overflow is positively correlated with the NAO. A 140 years integration reveals variations in the overflow between 2.5 Sv in the 1970s and a maximum of 4 Sv in the 1990s. Hydraulic transport calculations from hydrographic data north of Iceland show good agreement with the model hindcast.
Abstract The results of laboratory experiments and numerical model simulations are described in which the motion of a round, negatively-buoyant, turbulent jet discharged horizontally above a slope into a rotating homogeneous fluid has been investigated. For the laboratory study, flow visualisation data are presented to show the complex three-dimensional flow fields generated by the discharge. Analysis of the experimental data indicates that the spatial and temporal developments of the flow field are controlled primarily by the lateral and vertical discharge position of the jet (with respect to the bounding surfaces of the container of width W) and the specific momentum (M 0) and buoyancy (B 0) fluxes driving the jet. The flow is seen to be characterised by the formation of (i) a primary anticyclonic eddy (PCC) close to the source, (ii) an associated secondary cyclonic eddy (SCE) and (iii) a buoyancy-driven bottom boundary current along the right side boundary wall. For the parameter ranges studied, the size L p, s and spatial location x p, s of the PCC and SCE (and the nose velocity u N of the boundary current) are shown to be only weakly-dependent upon the value of the mixed parameter M 0Ω/B 0, where Ω is the background rotation rate. Both L p and x p are shown to scale with the separation distance y∗/W of the right side wall (y = 0) from the source (y = y∗), both L s and x s scale satisfactorily with the length scale l M (= M 0 3/4/B 0 ½) and u N is determined by the appropriate gravity current speed [(g']0 H]½ and the separation distance y∗/W. Numerical model results show good qualitative agreement with the laboratory data with regard to the generation of the PCC, SCE and boundary current as characteristic features of the flow in question. In addition, extension of the numerical model to diagnose potential vorticity and plume thickness distributions for the laboratory cases allow the differences in momentum-and buoyancy-dominated flows to be clearly delineated. Specifically, the characteristic features of the SCE are shown to be strongly dependent upon the value of M 0Ω/B 0 for the buoyant jet flow; not least, the numerical model data are able to confirm the controlling role played by the boundary walls in the laboratory experiments. Quantitative agreement between the numerical and laboratory model data is fair; most significantly, the success of the former model in simulating the dominant flow features from the latter enables the reliable extension of the numerical model to be made to cases of direct oceanic interest.
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.
Interannual changes in simulated flow fields of the Nordic Seas are analyzed with respect to their dynamic causes and consequences regarding the flow of dense water from the Nordic Seas into the subpolar North Atlantic across the Greenland‐Iceland‐Scotland Ridge. The simple case of pure density‐driven outflow with closed northern boundaries shows that dense water mainly originates in the northern Lofoten Basin and flows southward in three branches, namely along the Norwegian continental slope, along the Mohn and Jan Mayen Ridges, and a weak current along the east Greenland continental slope. Adding variable exchange through Fram Strait shows a strengthening of the most western branch and strong recirculations that may reverse the other two branches. For this case, we find in‐phase modulation of the Denmark Strait overflow (DSO) by a changing Fram Strait supply and a Faroe‐Shetland transport that is in opposite phase. The scaling of this relation provides a potential explanation of recently observed DSO changes. However, details of the changes in the simulated pathways suggest, in accord with the size of the prescribed varying Fram Strait supply, basin‐wide wind stress curl and local convection, which feeds water from different source regions into the outflow pathways, as the primary cause for the upstream flow field reorganizations.
Salzreiches Wasser, das durch die Strase von Gibraltar aus dem Mittelmeer in den Atlantik stromt, 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 endgultig mit dem Atlantikwasser vermischt.
The evolution of the dynamical signal of an isolated Mediterranean Water lens (‘meddy’) is simulated using a quasigeostrophic model on the β‐plane. The initially circular anticyclonic vortex moves south‐westward at a non‐uniform velocity of about 1 cm/s for over two years. Repeated instability processes (both barotropic and baroclinic) modify the horizontal shape and the radius of the meddy as well as its vertical structure. Particle trajectories inside the lens show strong structural similarities with tracks of ‘SOFAR’ floats deployed in a meddy by Armi et al. (1988). A tracer concentration inside the meddy is transported away from the source region and slowly spreads to the surrounding. ‘SOFAR’ floats injected in nearly zonal tracer tongues produced by the quasi‐barotropic far field behind the meddy moved westward in a narrow band as observed by Price et al. (1986).
