457
2 Vertical transport in mesoscale structures and impact on biomass
The importance of the three-dimensional ageostrophic motion associated with mesoscale
oceanic features is now well established. More specifically, the understanding of vertical
motions -which are purely ageostrophic- and their biological implications has motivated
several recent studies (Woods et al., 1986; Leach, 1987; Tinton\ et al., 1988, 1991; Pollard
and Regier, 1992). The relevance of the ageostrophic motion comes from the fact that
vertical velocities in mesoscale fronts and eddies have been found to be of the order of
several tens of meters per day, this is an order of magnitude higher than the largest vertical velocities usually observed in upwelling areas. In the CTZ off the California coast,
mesoscale filaments were found to be characterized by vertical motions up to 40 m/day
(Dewey et al., 1991; Haidvogel et al., 1991). In Gulf Stream meanders, several authors
have also found vertical velocities of several tens of meters per day, from both experimental and numerical studies (Bower and Rossby, 1989; Onken, 1992; Lindstrom and Watts,
1994). The existence of significant vertical velocities has important implications on the
exchanges, between the surface layer and the ocean interior. Vertical transport is a key
factor for distributions of biogeochemical tracers (temperature, salinity, nutrients, plankton, suspended matter, pollutants, C02" etc) in the ocean. These exchanges control the
ocean's ability to regulate the climate and the biological production. By bringing the deep
nutrients into upper levels, vertical transport is also particularly relevant for the marine
production in the euphotic layer (Woods, 1986; Strass, 1992). An accurate diagnostic of
the vertical velocity is thus crucial for understanding the interactions between the physical, chemical and biological systems and predicting the ways that the ocean responds to
environmental change (e.g., Nival, 1990; Watson et al., 1991).
2.1 Computation of vertical motion
a - Data quality
Vertical motion is the result of divergent horizontal circulation in the ocean. Therefore,
the main error in estimating vertical velocities arises from inaccuracies in the measurement
or determination of the horizontal velocity field. From an experimental point of view, recent developments in the processing of shipboard ADCP data have improved the accuracy
of measurements of absolute horizontal velocities (King and Cooper, 1993). Multivariate analyses of CTD and ADCP data collected in the Alboran Sea (Gomis and Pedder,
1995) and in the Balearic Sea (Pinot et al., 1995a), have also shown that direct current
measurements lead to a much better resolution of mesoscale features than analyses based
on hydrographic observations alone. However, processed ADCP dat.a are cont.aminated
by tides, near inertial waves, etc., which can induce errors in vertical velocity estimates.
Recently, several methods have been suggested to remove tidal currents from ADCP data
2 Vertical transport in mesoscale structures and impact on biomass
The importance of the three-dimensional ageostrophic motion associated with mesoscale
oceanic features is now well established. More specifically, the understanding of vertical
motions -which are purely ageostrophic- and their biological implications has motivated
several recent studies (Woods et al., 1986; Leach, 1987; Tinton\ et al., 1988, 1991; Pollard
and Regier, 1992). The relevance of the ageostrophic motion comes from the fact that
vertical velocities in mesoscale fronts and eddies have been found to be of the order of
several tens of meters per day, this is an order of magnitude higher than the largest vertical velocities usually observed in upwelling areas. In the CTZ off the California coast,
mesoscale filaments were found to be characterized by vertical motions up to 40 m/day
(Dewey et al., 1991; Haidvogel et al., 1991). In Gulf Stream meanders, several authors
have also found vertical velocities of several tens of meters per day, from both experimental and numerical studies (Bower and Rossby, 1989; Onken, 1992; Lindstrom and Watts,
1994). The existence of significant vertical velocities has important implications on the
exchanges, between the surface layer and the ocean interior. Vertical transport is a key
factor for distributions of biogeochemical tracers (temperature, salinity, nutrients, plankton, suspended matter, pollutants, C02" etc) in the ocean. These exchanges control the
ocean's ability to regulate the climate and the biological production. By bringing the deep
nutrients into upper levels, vertical transport is also particularly relevant for the marine
production in the euphotic layer (Woods, 1986; Strass, 1992). An accurate diagnostic of
the vertical velocity is thus crucial for understanding the interactions between the physical, chemical and biological systems and predicting the ways that the ocean responds to
environmental change (e.g., Nival, 1990; Watson et al., 1991).
2.1 Computation of vertical motion
a - Data quality
Vertical motion is the result of divergent horizontal circulation in the ocean. Therefore,
the main error in estimating vertical velocities arises from inaccuracies in the measurement
or determination of the horizontal velocity field. From an experimental point of view, recent developments in the processing of shipboard ADCP data have improved the accuracy
of measurements of absolute horizontal velocities (King and Cooper, 1993). Multivariate analyses of CTD and ADCP data collected in the Alboran Sea (Gomis and Pedder,
1995) and in the Balearic Sea (Pinot et al., 1995a), have also shown that direct current
measurements lead to a much better resolution of mesoscale features than analyses based
on hydrographic observations alone. However, processed ADCP dat.a are cont.aminated
by tides, near inertial waves, etc., which can induce errors in vertical velocity estimates.
Recently, several methods have been suggested to remove tidal currents from ADCP data