| Popis: |
This research describes the position, strength and variability of the mixing front along the East Shetland coast and the geostrophic current associated with the front. Between 2004 and 2010, new current meter measurements have been made at two mooring sites on the East Shetland shelf. By combining the current meter datasets with other surface temperature and salinity observations, such as surface underway data from the ferry MV “Norröna” and existing hydrographic datasets, this research provides new insights into the seasonal variability of the currents on the east Shetland shelf. The results show that in this region of the northern North Sea the balance of heating-stirring results in a tidal mixing front close to a (log10( h/u3)) of 3.4. Along the steeply sloping shelf to the east of Shetland a mixing front is found in water around 100m depth and results in significant geostrophic currents of up to 20 cm/sec between June and August each year. Monthly volume transports associated with broad scale wind forcing are estimated to be between -0.27 and -0.51 Sv during the well mixed periods (December to March). During the summer months (June, July and August) wind driven transport reduced to an average of -0.27 Sv, at the same time the persistent southerly flow associated with the geostrophic transport contributes an equivalent transport (-0.24 Sv). Taking the east Shetland shelf as a whole (coast to 1.5°E) the maximum volume transport is estimated to occur in Autumn (September to October) when density driven currents remain significant and wind speeds begin to increase. In the narrow region close to the coast, however, monthly mean volume transport is observed to be highest during the summer months as a result of the strong jet-like currents that flow along the density front. |
