465
help clarifying the dependence of the vertical velocity on the horizontal scale considered.
Barth (1994) found that rapidly growing frontal instabilities were associated with vertical
motions much larger than those associated with long-wavelength modes. Strass (1994)
carried out numerical simulations of mesoscale instabilities at fronts and also found the
largest vertical velocities at highest wavenumbers.
However, it is known that primitive equation models may produce noisy vertical velocities, especially when the internal radius of deformation is poorly resolved. Furthermore,
vertical velocities are the result of divergence in the ageostrophic, generally weak, component of the horizontal velocity field and are therefore sensitive to small changes in balance
of the flow, as well as model boundary conditions and numerical resolution. Care must
be taken that the vertical velocities predicted by a primitive equation model are accurate
if they are to be used to drive a biological model.
2.2 Impact of vertical transport on biomass
a - General perspective
Physical forcing at the mesoscale has clear biological implications such as enhancement
of primary production. The transport of phytoplankton across the thermocline and the
phytoplankton growth in the euphotic zone have been studied by a number of authors,
among others and initially by Woods et al. (1986) and Leach (1987).
Recent studies have also demonstrated that biological patchiness of nutrient distributions (Strass, 1992) and zooplankton may be closely coupled with hydrographic variability
and that distributions that are seemingly inexplicable on behavioural grounds are related
to vertical circulation in the upper ocean. Thus, maxima of nutrient concentrations can
be associated with frontal mesoscale structures such as filaments and eddies. This is the
case for the cyclonic eddies of the tide-induced slope front over the Armorican continental
shelf-break (Pingree et al., 1979) but also for the mesoscale filaments of the wind-induced
upwelling front in the California Current system (Brink and Cowles, 1991). The upward
transport of nutrient-rich subsurface waters into the surface layer is one possible mechanism for enhancing primary production. Conversely streamers of phytoplankton have
been observed being drawn down to depths of 350 m in areas of high vertical velocities
(Pollard et al., 1991). A question that remains to be solved is what is the net effect of the
vertical ageostrophic circulation integrated over the spectrum of scales on the exchanges
between the ocean interior and the surface layer?
b - Results in the Western Mediterranean
Recent experiments have shown that the interdisciplinary study of fronts is particularly
relevant in the Western Mediterranean since they provide privileged biologically active
places in the general oligotrophic context of the Mediterranean Sea (Boucher et al., 1987;
help clarifying the dependence of the vertical velocity on the horizontal scale considered.
Barth (1994) found that rapidly growing frontal instabilities were associated with vertical
motions much larger than those associated with long-wavelength modes. Strass (1994)
carried out numerical simulations of mesoscale instabilities at fronts and also found the
largest vertical velocities at highest wavenumbers.
However, it is known that primitive equation models may produce noisy vertical velocities, especially when the internal radius of deformation is poorly resolved. Furthermore,
vertical velocities are the result of divergence in the ageostrophic, generally weak, component of the horizontal velocity field and are therefore sensitive to small changes in balance
of the flow, as well as model boundary conditions and numerical resolution. Care must
be taken that the vertical velocities predicted by a primitive equation model are accurate
if they are to be used to drive a biological model.
2.2 Impact of vertical transport on biomass
a - General perspective
Physical forcing at the mesoscale has clear biological implications such as enhancement
of primary production. The transport of phytoplankton across the thermocline and the
phytoplankton growth in the euphotic zone have been studied by a number of authors,
among others and initially by Woods et al. (1986) and Leach (1987).
Recent studies have also demonstrated that biological patchiness of nutrient distributions (Strass, 1992) and zooplankton may be closely coupled with hydrographic variability
and that distributions that are seemingly inexplicable on behavioural grounds are related
to vertical circulation in the upper ocean. Thus, maxima of nutrient concentrations can
be associated with frontal mesoscale structures such as filaments and eddies. This is the
case for the cyclonic eddies of the tide-induced slope front over the Armorican continental
shelf-break (Pingree et al., 1979) but also for the mesoscale filaments of the wind-induced
upwelling front in the California Current system (Brink and Cowles, 1991). The upward
transport of nutrient-rich subsurface waters into the surface layer is one possible mechanism for enhancing primary production. Conversely streamers of phytoplankton have
been observed being drawn down to depths of 350 m in areas of high vertical velocities
(Pollard et al., 1991). A question that remains to be solved is what is the net effect of the
vertical ageostrophic circulation integrated over the spectrum of scales on the exchanges
between the ocean interior and the surface layer?
b - Results in the Western Mediterranean
Recent experiments have shown that the interdisciplinary study of fronts is particularly
relevant in the Western Mediterranean since they provide privileged biologically active
places in the general oligotrophic context of the Mediterranean Sea (Boucher et al., 1987;