On the influence of submarine ridges on translation and stability of Agulhas rings

A series of experiments with a quasi‐geostrophic model have been carried out to investigate the influence of topographic obstacles on the translatory movement of Agulhas rings. The rings were initialized as Gaussian‐shaped anomalies in the stream function field of a two‐layer ocean at rest. Bottom topography consisted of a meridional ridge of constant height in the middle of the quadratic model domain. The vertical ring structure, the initial ring position, and the height of the ridge were varied. The general northwestward movement of the model eddies has been shown to be modified toward a more equatorward direction by encountering the upslope of the ridge. Sufficient topographic heights and strong slopes can even block the eddies and force them toward a pure meridional movement. During their translation the eddies lose their vertical coherence. After about 150 days the eddy can only be detected by the surface signal, while the lower layer eddy is dispersed by the radiation of Rossby waves. The passage of “young” (regarding the time between their initialization and their contact with the ridge) and energetic eddies is accompanied by the observation of along‐slope currents of significant strength. These may be due to the rectification of radiated Rossby waves at the topographic slope. Only eddies with a significant dynamic signal in the lower layer are influenced by the bottom topography. Strong, shallow eddies over deep lower layers can cross the ridge without strong modification of their translatory movement

Structure and variability of the Denmark Strait Overflow: Model and observations

We report on a combined modeling and observational effort to understand the Denmark Strait Overflow (DSO). Four cruises over the course of 3 years mapped hydrographic properties and velocity fields with high spatial resolution. The observations reveal the mean path of the dense water, as well as the presence of strong barotropic flows, energetic variability, and strong bottom friction and entrainment. A regional sigma coordinate numerical model of interbasin exchange using realistic bottom topography and an overflow forced only by an upstream reservoir of dense fluid is compared with the observations and used to further investigate these processes. The model successfully reproduces the volume transport of dense water at the sill, as well as the 1000-m descent of the dense water in the first 200 km from the sill and the intense eddies generated at 1–3 day intervals. Hydraulic control of the mean flow is indicated by a region supercritical to long gravity waves in the dense layer located approximately 100 km downstream of the sill in both model and observations. In addition, despite the differences in surface forcing, both model and observations exhibit similar transitions from mostly barotropic flow at the sill to a bottom-trapped baroclinic flow downstream, indicating the dominant role of the overflow in determining the full water column dynamics

Meandering of the subtropical front south-east of the Azores

A subtropical front was observed in the area south and southeast of the Azores during cruises of FS Meteor and FS Poseidon in early 1982. The front has a basically west–east extension, with considerable meandering observed. Meso-scale eddies are found on both sides. The overall flow pattern corresponds to earlier results on geopotential differences in the upper northeast Atlantic, but the baroclinic transport of the order of 107 m3 s−1 is found to be concentrated in a 60-km wide jet. We suggest here that the current band is part of the gyre circulation, resulting from a branching of the North Atlantic Current

New direct Overflow Observations at the Denmark Strait Sill

As a key region of the global thermohaline circulation, the Denmark Strait between Iceland and Greenland is the pathway for the densest component of the North Atlantic Deep Water, the Denmark Strait Overflow Water (DSOW).Since 1999, it has been probed with acoustic instruments in the SFB 460 program lead by the Institut für Meereskunde Kiel. The data from bottom Pressure/ Inverted Echo Sounders (PIES) and ADCP current profilers, deployed in a model-optimized mooring array at the Denmark Strait sill, resolve both spatial and temporal variability of the overflow.After recovery of moorings in August 2002, the time series available have a total length of 27 months. The data allow for transport estimates based on direct ADCP measurements and integrating geostrophic PIES observations. Including model comparisons, a mean DSOW transport of 3.0 Sv is calculated with an accuracy of ±0.1 Sv. A statistical analysis with autoregressive moving average models has improved the information about integral time scales and dominant frequencies of the overflow variability.Until now, estimates of the mean DSOW throughflow have been remarkably stable. The PIES observations show a close correlation between sea surface height anomalies and the DSOW plume thickness. This opens a perspective for long term monitoring by remote sensing with relevance for climate change. First comparisons of in-situ measurements and satellite altimetry are presented

Possible Interannual Variations of the Denmark Strait Overflow

A major part of the North Atlantic Deep Water (NADW) enters the Atlantic Ocean across the Denmark Strait Sill between Iceland and Greenland. For monitoring the Global Thermohaline Circulation, the area is predestinated for long term observation of the densest NADW component, the Denmark Strait Overflow Water (DSOW). The goal of the SFB 460 program lead by the Institut f\"{u}r Meereskunde Kiel was to detect possible variations in the deep overflow across Denmark Strait sill by means of acoustic observations, both direct ADCP current measurements and bottom Pressure/ Inverted Echo Sounders (PIES). The observation array has been deployed at positions resulting from multilinear regression of simulated measurements in a high resolution overflow process model. Currently, 39 months of time series of the field experiment from 1999 to 2003 are available. During this period, the overflow, previously regarded as remarkably stable on time scale longer than a few weeks, has exhibited some interannual fluctuations concerning plume thickness, volume transport and temperature. Significance and possible correlations of these variations to external sources are investigated