Turbulence dissipation rates and nitrate supply in the upper water column on Georges Bank

Abstract

Measurements of velocity microstructure in the upper water column on Georges Bank are used to contrast the summertime structure of turbulent kinetic energy dissipation rate between the central mixed area and the tidal-mixing front, and to compare vertical nitrate fluxes with frontal-zone primary production demands. In the mixed area during weak winds, the dissipation rate varies strongly over the semidiurnal tidal period in close relation to the tidal current strength, varies with the monthly/fortnightly tidal modulation, and generally increases with distance below the sea surface. Collectively, these features provide strong support for the elevated vertical mixing rates on Georges Bank being primarily due to the tides, although wind forcing also contributes significantly. In the frontal zone on northern Georges Bank, the upper-ocean dissipation rates are about an order of magnitude weaker than in the mixed area, have a more complex temporal variation during the tidal period, and also vary with the monthly/fortnightly tidal modulation. The vertical eddy flux of nitrate into the frontal euphotic zone varies over the tidal period and with the tidal modulation. Averaged over the tidal period, the estimated fluxes are about one-third of the nitrogen demand estimated from concurrent primary production measurements, supportive of an important contribution from turbulent mixing to new production in the frontal zone, but also pointing to additional processes and/ or inadequate data coverage of this complex zone. The measured dissipation rates at both the mixed and frontal sites are in approximate agreement with the turbulence levels in two 3-D numerical models for summertime tidal and mean circulation on the Bank, one with and eddy-viscosity and the other an advanced turbulence closure. The latter model has more realistic vertical turbulence distributions and indicates strong sensitivity of the turbulence levels to horizontal position in the frontal zone.