226
Chapter 9: The Atlantic Ocean
at Cape Jubi (27
N), and wherever mesoscale cyclonic eddies form on the landward side
of the main flow of the offshore current (Fedoseev, 1970). The Canary Islands may induce
a counterclockwise meander of the offshore part of the coastal boundary zone around
the archipelago at about the 2000-m isobath, a feature that then influences the surface
temperature field nearer to the coast.
These coasts lie below the seasonally migrating trade wind belt that reaches the Iberian
Peninsula during boreal summer and retreats southward in winter. The wind field over
the Canary Current System is typical of those over eastern boundary currents (Bakun
and Nelson, 1991); zero wind-stress curl coincides with the offshore zone of maximum
equatorward wind velocity and with the main flow of the offshore equatorward current.
Landward, cyclonic curl strengthens upwelling, whereas anticyclonic curl forces a convergent front offshore. The annual mean alongshore component of wind stress off northwest
Africa is −15 dyn cm
−2 , compared to only −05 dyn cm
−2 off Oregon, or −08 dyn cm
−2
off Peru (Barber and Smith, 1981). In addition, the winds are upwelling-favorable for
almost the entire year off Africa, but for only half the year off Oregon. Off northwest
Africa, wind mixing frequently reaches to the bottom and this has consequences for
the accumulation of the organic material formed in the euphotic zone during upwelling
blooms.
The seasonal meridional migration of the trade wind system, and hence of the velocity
and curl maxima in the coastal regime, drives the basic seasonal cycle of upwelling, so
that this is most intense in boreal summer (June–October) off Portugal and in boreal
winter (January–May) off Senegal; in the central region, from 20
N to 25
N, upwelling
is more continuous (Wooster et al., 1976; Mittelstaedt, 1991). Event-scale changes of
wind strength and direction may, for a few days, induce or suppress upwelling (van
Camp et al., 1991; Joint et al., 2001b) and thus modify the surface thermal field within a
short period. Cyclonic curl maxima are associated at the coast with the major upwelling
centers discussed later, whereas some anticyclonic curl in the coastal winds may occur as
a local response to coastal orientation off Portugal. A persistent lobe of anticyclonic curl
also interrupts the coastal field of cyclonic curl between Cape Bojador and Cape Sim in
autumn and winter, although strong upwelling occurs here during other seasons (Bakun
and Nelson, 1991).
The climatic seasonal mean fields of temperature and density (e.g., Mittelstaedt, 1991)
indicate that even where upwelling occurs continuously, there are small topographically
locked cells of greater intensity. Cool filaments, exhibiting strong vorticity, develop from
the persistent upwelling cells (Wooster et al., 1976) and may extend far beyond the shelf
edge, especially at 30–35
N and near Cape Blanc at 20–22
N (van Camp et al., 1991).
These cool filaments, although narrow and shallow, may extend for several hundred
kilometers and have attracted much attention recently in this region during the European
CANIGO voyages (Barton et al., 2004), because of their potential for export of organic
material and nutrients to the oligotrophic ocean. An unusually large and persistent
filament occurs south of Cape Blanc; this feature is independent of local winds and
is forced by convergence between the Canary Current and the poleward flow of the
NECC, which loops to the southwest on encountering the Canary Current, thus forming
a quasipermanent cyclonic eddy. This poleward flow is permanent but, during boreal
winter, when the trade-wind belt is at its most southerly, a narrow equatorward current
passes inside it, close along the coast, that continues the flow of the Canary Current from
the north down to Cape Verde and even a little beyond. Upwelling at the coast between
Cape Blanc and Cape Verde occurs only at this season.
The principal foci for upwelling in this province are the following:
Cape Ortegal/Cape Finisterra (42–43
N ): Though upwelling occurs in summer along
the whole Galician coast, and within the mouths of the rias, it is most intense where
the coast turns southerly between these two capes (Fraga, 1981). Here, intensification
Chapter 9: The Atlantic Ocean
at Cape Jubi (27
N), and wherever mesoscale cyclonic eddies form on the landward side
of the main flow of the offshore current (Fedoseev, 1970). The Canary Islands may induce
a counterclockwise meander of the offshore part of the coastal boundary zone around
the archipelago at about the 2000-m isobath, a feature that then influences the surface
temperature field nearer to the coast.
These coasts lie below the seasonally migrating trade wind belt that reaches the Iberian
Peninsula during boreal summer and retreats southward in winter. The wind field over
the Canary Current System is typical of those over eastern boundary currents (Bakun
and Nelson, 1991); zero wind-stress curl coincides with the offshore zone of maximum
equatorward wind velocity and with the main flow of the offshore equatorward current.
Landward, cyclonic curl strengthens upwelling, whereas anticyclonic curl forces a convergent front offshore. The annual mean alongshore component of wind stress off northwest
Africa is −15 dyn cm
−2 , compared to only −05 dyn cm
−2 off Oregon, or −08 dyn cm
−2
off Peru (Barber and Smith, 1981). In addition, the winds are upwelling-favorable for
almost the entire year off Africa, but for only half the year off Oregon. Off northwest
Africa, wind mixing frequently reaches to the bottom and this has consequences for
the accumulation of the organic material formed in the euphotic zone during upwelling
blooms.
The seasonal meridional migration of the trade wind system, and hence of the velocity
and curl maxima in the coastal regime, drives the basic seasonal cycle of upwelling, so
that this is most intense in boreal summer (June–October) off Portugal and in boreal
winter (January–May) off Senegal; in the central region, from 20
N to 25
N, upwelling
is more continuous (Wooster et al., 1976; Mittelstaedt, 1991). Event-scale changes of
wind strength and direction may, for a few days, induce or suppress upwelling (van
Camp et al., 1991; Joint et al., 2001b) and thus modify the surface thermal field within a
short period. Cyclonic curl maxima are associated at the coast with the major upwelling
centers discussed later, whereas some anticyclonic curl in the coastal winds may occur as
a local response to coastal orientation off Portugal. A persistent lobe of anticyclonic curl
also interrupts the coastal field of cyclonic curl between Cape Bojador and Cape Sim in
autumn and winter, although strong upwelling occurs here during other seasons (Bakun
and Nelson, 1991).
The climatic seasonal mean fields of temperature and density (e.g., Mittelstaedt, 1991)
indicate that even where upwelling occurs continuously, there are small topographically
locked cells of greater intensity. Cool filaments, exhibiting strong vorticity, develop from
the persistent upwelling cells (Wooster et al., 1976) and may extend far beyond the shelf
edge, especially at 30–35
N and near Cape Blanc at 20–22
N (van Camp et al., 1991).
These cool filaments, although narrow and shallow, may extend for several hundred
kilometers and have attracted much attention recently in this region during the European
CANIGO voyages (Barton et al., 2004), because of their potential for export of organic
material and nutrients to the oligotrophic ocean. An unusually large and persistent
filament occurs south of Cape Blanc; this feature is independent of local winds and
is forced by convergence between the Canary Current and the poleward flow of the
NECC, which loops to the southwest on encountering the Canary Current, thus forming
a quasipermanent cyclonic eddy. This poleward flow is permanent but, during boreal
winter, when the trade-wind belt is at its most southerly, a narrow equatorward current
passes inside it, close along the coast, that continues the flow of the Canary Current from
the north down to Cape Verde and even a little beyond. Upwelling at the coast between
Cape Blanc and Cape Verde occurs only at this season.
The principal foci for upwelling in this province are the following:
Cape Ortegal/Cape Finisterra (42–43
N ): Though upwelling occurs in summer along
the whole Galician coast, and within the mouths of the rias, it is most intense where
the coast turns southerly between these two capes (Fraga, 1981). Here, intensification