TURBULENCE DISSIPATION RATES AND NITRATE SUPPLY IN THE UPPER WATER COLUMN ON GEORGES BANK

Measurements of velocity microstructure in the upper water column on G eorges Bank are used to contrast the summertime structure of turbulent kinetic energy dissipation rate between the central mixed area and th e tidal-mixing front, and to compare vertical nitrate fluxes with fron tal-zone primary production demands. In the mixed area during weak win ds, the dissipation rate varies strongly over the semidiurnal tidal pe riod in close relation to the tidal current strength, varies with the monthly/fortnightly tidal modulation, and generally increases with dis tance below the sea surface. Collectively, these features provide stro ng support for the elevated vertical mixing rates on Georges Bank bein g primarily due to the tides, although wind forcing also contributes s ignificantly. 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 tid al period, and also vary with the monthly/fortnightly tidal modulation . The vertical eddy flux of nitrate into the frontal euphotic zone var ies over the tidal period and with the tidal modulation. Averaged over the tidal period, the estimated fluxes are about one-third of the nit rogen 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 proce sses and/or inadequate data coverage of this complex zone. The measure d dissipation rates at both the mixed and frontal sites are in approxi mate agreement with the turbulence levels in two 3-D numerical models for summertime tidal and mean circulation on the Bank, one with an edd y-viscosity and the other an advanced turbulence closure. The latter m odel has more realistic vertical turbulence distributions and indicate s strong sensitivity of the turbulence levels to horizontal position i n the frontal zone. Copyright (C) 1996 Elsevier Science Ltd.