Epw. Horne et al., TURBULENCE DISSIPATION RATES AND NITRATE SUPPLY IN THE UPPER WATER COLUMN ON GEORGES BANK, Deep-sea research. Part 2. Topical studies in oceanography, 43(7-8), 1996, pp. 1683-1712
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.