Shelf-edge Fronts Research Articles

Anatomy of shelf–deep sea exchanges by a mesoscale eddy in the North West Black Sea as derived from remotely sensed data

Satellite derived full resolution data on chlorophyll, water leaving radiance, sea surface temperature, as well as altimetry data are used to characterise the process of shelf–open sea exchange induced by an individual mesoscale eddy off the North West coast of the Black Sea in the summer 2005. The shelf edge front in the NW part of the Black Sea separates biologically productive and often eutrophicated shelf waters from the open sea waters. A large mesoscale eddy (diameter about 120 km) was formed away from the shelf front southwest of Crimean peninsula in the spring 2005. The eddy moved towards the shelf break and repeatedly attached itself to the shelf edge front causing its disturbance and formation of a cross-shelf jet and a Chl-a rich filament spiralling around the eddy periphery. The jet represented an important cross-shelf transport mechanism sustaining an average cross-frontal flux of shelf waters of 0.3 Sv over period of 40 days. The total transport of water in our case study was equivalent to 40% of the overall volume of water on the shelf. Formation of the filament significantly increased the length of contact between shelf and deep sea waters from 70 km at the base of filament to about 800 km along its sides, thus providing a highly efficient mixing mechanism.

Eddy overturning across a shelf edge front: Kongsfjorden, west Spitsbergen

Eddy overturning across a shelf edge front has been investigated in a simplified model setup comprising an along‐slope uniform shelf and slope with a slope current. Results from three model runs are presented, each initiated with and nudged toward a sharp shelf edge front in temperature, salinity, density, and velocity. Data collected off Kongsfjorden at Spitsbergen were used for initiating the model runs. Three distinctly different density fronts were used to initialize the model runs, one with a negative cross‐frontal density gradient toward the shelf, one with a positive density gradient, and one with a combination of negative density gradient in the upper water column and a positive in the lower water column. The model results indicate that the initial cross‐frontal density gradients determine the eddy overturning direction, acting toward flattened or terrain‐following isopycnals. The steady state modeled fronts resemble quite well the observed front structures, as well as temperature and salinity fields. This indicates that eddy overturning plays an important role in cross‐frontal exchange of water masses along the shelf edge front off west Spitsbergen. The Eulerian mean cross‐slope flow is close to zero, and the corresponding fluxes do not describe the correct vertical distribution.

Estimates of dispersion from clustered-drifter deployments on the southern flank of Georges Bank

Abstract Data from 16 clustered-drifter deployments are used to examine horizontal dispersion on the southern flank of Georges Bank. The spreading rates of all clusters have an average of 1.6 km d −1 with a standard deviation of 1.8 km d −1 . Both “effective” and “apparent” diffusivities are calculated for each cluster. Their ranges (e.g. −54 to+757 m 2 s −1 for effective diffusivity) are related to differences in cluster size and proximity to the tidal mixing front. Cross-bank convergence is documented for nearly 40% of the clusters. This occurs especially for clusters with centroids within 10 km of the tidal mixing front location, as deduced from conductivity, temperature, and depth transects (CTD) conducted concurrently with the cluster deployments. Estimates of turbulent dispersion (distinct from shear effects) are derived by the method of A. Okubo and C.C. Ebbysmeyer (1976. Determination of vorticity, divergence, and deformation rates from analysis of drogue observations. Deep-Sea Research 23, 349–352). The results reveal that the effects of horizontal shear are important in spreading of larger drifter clusters. Often the impact of shear is evidenced by the track of a lone drifter that separates from a cluster as it is entrained into the current of the shelf-edge front or the tidal mixing front. Cluster dispersion is time dependent as evidenced by a significant modulation of cluster size at the M 2 tidal frequency. This modulation is due to the spatial variation of tidal currents over the southern flank of Georges Bank and is closely reproduced by immersing drifter clusters into the flow field of a Georges Bank tidal model.

A multivariate analysis of planktonic molluscan distribution in the Middle Atlantic Bight

Faunal structure is described for the planktonic molluscs of the Middle Atlantic Bight based on two years of seasonal data from the continental shelf. Collection and taxa groups are constructed using numerical classification and reciprocal averaging ordination. Discriminant analysis is used to relate surface collection groups to physical variables, then taxa group distribution among these collection groups is analyzed by nodal fidelity analysis. The areal distribution of dominant species is presented by season, as is their surface temperature-salinity distribution. Four communities are recognized on the continental shelf. A subarctic community, including Limacina retroversa, Paedoclione doliiformis, and Clione limacina, is advected down the central shelf region from the northeast. A Gulf Stream community of weak vertical migrators, including Limacina trochiformis, Cavolinia longirostris, Creseis conica, Atlanta peroni, and A. gaudichaudi, is introduced onto the shelf in occasional intrusions across the shelf-edge front. A depth-limited warm-water community of strong vertical migrators, including Limacina inflata, L. bulimoides, L. lesueuri, and Cavolinia inflexa is generally confined offshore of the 100-m isobath since the extent of their daily vertical migration is greater than the bottom depths on the continental shelf. A coastal community, including the larvae of Loligo pealei and of Ensis directus is found in coastal water of local origin and is generally confined within a coastal boundary layer.

Mid‐level intrusions at the continental shelf edge

Observations across the continental shelf offshore from New Jersey in late summer 1976 show an intrusion of saline water at the mid level of the water column across the shelf edge front, which appears in density only as an offshore thickening of the pycnocline. This internal density field produces horizontal pressure gradient forces within the pycnocline in the onshore direction. These forces, in the linearized equation of motion with a constant eddy viscosity, drive a circulation which resembles a double Ekman spiral for internal pressure vertical distributions which are thin with respect to the Ekman depth. For thick pressure distributions, the circulation is geostrophic. The resulting flow pattern has no net crossshelf transport. For the continental shelf edge in the example, a northward geostrophic mid‐level jet is predicted by the theory. Ekman depth, and thus the vertical coefficient of eddy viscosity, can be determined from hydrographic data describing an intrusion.

Wind effects on surface to bottom fronts

In nearshore regions, water of reduced density is frequently present owing to freshwater influx or spring heating. Under some circumstances, light nearshore water is confined to one side of a density front, extending from surface to bottom, and is called ‘spring thermocline’ or ‘shelf edge front.’ The shape and permanency of this front are affected by wind stress, which may interfere with the momentum balance in a direction parallel to the front and cause geostrophic adjustment motions normal to the front. A simple geostrophic adjustment theory elucidates some of the more important effects of wind on such fronts. Winds opposing the geostrophic flow above the inclined front tend to flatten its shape and eventually destroy the front, sometimes causing the formation of a surface ‘lens’ or ‘bubble.’ Comparison with observations from Lake Ontario and from the New England continental shelves shows that the theory gives a realistic first‐order description of frontal behavior.