The Faroe shelf circulation and its potential impact on the primary production

• We model the Faroe shelf water mass.
• Observed and modeled data compares reasonably well.
• A link between the Chl concentration and the summer mixed layer depth is found.
• Tracer experiment shows that shelf water is exported near the bottom.

The ecosystem on the Faroe shelf has been shown to be tightly controlled by the primary production. It has been suggested that the primary production is governed by the physical processes controlling this water mass. The objective of this study is to identify the physical control mechanisms that control this water mass, link these to the interannual variability of the chlorophyll content on the Faroe shelf and through this discuss the influence on the primary production. In order to achieve this, a 10 year hindcast (2000–2009) with a regional ocean circulation model has been set up for the focus area. Results are compared with measurements on the Faroe shelf.The model reproduces the clockwise residual circulation around the Faroe Islands. The vertical velocity profile is validated using observations at a location west of the Islands. Observations show a logarithmic profile in the entire water column indicating a fully developed boundary layer. The modeled profile matches the observations in the bottom part of the water column, however the thickness of the bottom boundary layer is underestimated, which results in a constant profile in the upper part of the water column. As a consequence, the modeled velocity in the upper part of the water column is up to 20% lower than the observed velocity. The direction of the modeled velocity profile compares well with observations. The model realistically forms the partly isolated unique shelf water mass. Years with anomalously early and persistent modeled spring stratification correspond with years with a high on-shelf chlorophyll concentration.An integration of the exchange across the 120 m isobath shows intense water mass exchange across this depth contour. The major part of this includes tidal shifting of the front between on-shelf and off-shelf waters and is associated with little effective water mass exchange. The result is a shelf water mass that is relatively isolated. The modeled net exchange is constituted by an on-shelf flow near the surface and an off-shelf flow near the bottom associated with the frictional boundary layer. This is confirmed by the tracer experiment. Both the tracer experiment and the exchange across the 120 m isobath indicate that there is spatial variability in the exchange of the shelf water.The renewal rate of the shelf water mass during the spring bloom period is assessed by a set of passive tracer simulations. Using passive tracers, the time scale for the half-life is found to be between 30 and 40 days. This compares favorably with the time-scale based on the net, advected water mass exchange that yields 60–70 days, considering that this estimate neglects turbulent and diffusive exchange.