Atlantic Westerly Winds Biome
157
North Atlantic Drift Province (NADR)
Extent of the Province
NADR comprises part of the west wind drift region of the North Atlantic in the sense of
Dietrich (1964), including the jet current along the OPF and the broader, slower flow in
the more eastern part of the ocean. Thus, the poleward boundary is the OPF itself along
the south of the ARCT, that extends from 55
N at the Grand Banks of Newfoundland
to 60
N, south of Iceland: from here, an arbitrary line is taken zonally to the edge of the
Shetlands shelf, separating NADR and SARC provinces.
To the south, the boundary of NADR separates the northeasterly North Atlantic Current from the flow of Gulf Stream water into the southeasterly Azores Current (previously
known as the “southern branch” of the Gulf Stream). This line of separation is poorly
defined, but lies across the ocean at about 42–44
N, the latitude of NW Spain. In the
east, the edge of the European continental shelf is taken as the boundary of the province.
Defining Characteristics of Regional Oceanography
Bifurcation of the Gulf Stream extension occurs in the western part of the ocean; prior
to reaching the Mid-Atlantic Ridge (30
W) the flow is clearly in two streams, the North
Atlantic Current toward the northeast and the Azores Current toward the southeast
around the northern limb of the subtropical gyre (Krause, 1986). NADR thus comprises the slow northeastward drift of surface water toward the Iceland-Faeroes Channel,
together with the North Atlantic Current (NAC), the frontal jet of the OPF along the
northern boundary of the province.
The province coincides very closely with that part of the North Atlantic where seasonal
surface temperature differences are greatest (>5
C), exceeding those in the provinces
to north and south (Louanchi and Najjar, 2001). It is also characterized by significant
presence of mesoscale eddies originating in the very active vorticity of Gulf Stream and
in the NAC that are even more important in the GFST province (see later discussion);
however, the consequences in NADR are sufficient that, at any location, conditions
tend to have significant variability at all scales. The strength, location, and longevity of
mesoscale eddies are now readily monitored as SLA surfaces obtained from the TOPEXPOSEIDON data; along meridional sections obtained with undulating, towed instrument
instrumentation (such as the “Sea Rover”) it is easy to demonstrate that the axes of both
the Azores Front (see NAST) and the OPF are each associated with strongly increased
vorticity (Strass and Woods, 1988).
As noted previously, wind speed in autumn and winter is greater in this province than
anywhere else except in the Southern Ocean, and the consequent stress at the sea surface,
combined with rapid seasonal heat loss, induces anomalous deepening of the mixed
layer, so that by midwinter the pycnocline slopes down from about 300 m in the central
ocean to more than 500 m along the European continental edge. It should be noted that
the ratio Z cr /Z m in this province takes relatively low values of about 0.3–0.4 compared
with 0.8–0.9 in the subtropical gyre to the south (Dutkiewicz et al., 2001). The great
depth of mixing in winter ensures a sufficiently deep and sufficiently permanent surface
stratification, such that Z m < Z cr shall be a necessary condition to initiate growth of the
phytoplankton. That this condition is forced by complex, and hence highly variable, local
meteorological conditions is clear from the analysis by Taylor and Stephens (1993) of the
consequences of the difference between depths of diurnal and nocturnal mixing due to
the effect of increased buoyancy by solar heating during daytime. The Sverdrup criterion
can only be satisfied if and when sufficient buoyancy is induced during the daytime to
overcome the loss of cells below Z cr by mixing at night. Thus, development of a vernal
shoal pycnocline is associated with increasing sun angles and relaxation of wind stress.
157
North Atlantic Drift Province (NADR)
Extent of the Province
NADR comprises part of the west wind drift region of the North Atlantic in the sense of
Dietrich (1964), including the jet current along the OPF and the broader, slower flow in
the more eastern part of the ocean. Thus, the poleward boundary is the OPF itself along
the south of the ARCT, that extends from 55
N at the Grand Banks of Newfoundland
to 60
N, south of Iceland: from here, an arbitrary line is taken zonally to the edge of the
Shetlands shelf, separating NADR and SARC provinces.
To the south, the boundary of NADR separates the northeasterly North Atlantic Current from the flow of Gulf Stream water into the southeasterly Azores Current (previously
known as the “southern branch” of the Gulf Stream). This line of separation is poorly
defined, but lies across the ocean at about 42–44
N, the latitude of NW Spain. In the
east, the edge of the European continental shelf is taken as the boundary of the province.
Defining Characteristics of Regional Oceanography
Bifurcation of the Gulf Stream extension occurs in the western part of the ocean; prior
to reaching the Mid-Atlantic Ridge (30
W) the flow is clearly in two streams, the North
Atlantic Current toward the northeast and the Azores Current toward the southeast
around the northern limb of the subtropical gyre (Krause, 1986). NADR thus comprises the slow northeastward drift of surface water toward the Iceland-Faeroes Channel,
together with the North Atlantic Current (NAC), the frontal jet of the OPF along the
northern boundary of the province.
The province coincides very closely with that part of the North Atlantic where seasonal
surface temperature differences are greatest (>5
C), exceeding those in the provinces
to north and south (Louanchi and Najjar, 2001). It is also characterized by significant
presence of mesoscale eddies originating in the very active vorticity of Gulf Stream and
in the NAC that are even more important in the GFST province (see later discussion);
however, the consequences in NADR are sufficient that, at any location, conditions
tend to have significant variability at all scales. The strength, location, and longevity of
mesoscale eddies are now readily monitored as SLA surfaces obtained from the TOPEXPOSEIDON data; along meridional sections obtained with undulating, towed instrument
instrumentation (such as the “Sea Rover”) it is easy to demonstrate that the axes of both
the Azores Front (see NAST) and the OPF are each associated with strongly increased
vorticity (Strass and Woods, 1988).
As noted previously, wind speed in autumn and winter is greater in this province than
anywhere else except in the Southern Ocean, and the consequent stress at the sea surface,
combined with rapid seasonal heat loss, induces anomalous deepening of the mixed
layer, so that by midwinter the pycnocline slopes down from about 300 m in the central
ocean to more than 500 m along the European continental edge. It should be noted that
the ratio Z cr /Z m in this province takes relatively low values of about 0.3–0.4 compared
with 0.8–0.9 in the subtropical gyre to the south (Dutkiewicz et al., 2001). The great
depth of mixing in winter ensures a sufficiently deep and sufficiently permanent surface
stratification, such that Z m < Z cr shall be a necessary condition to initiate growth of the
phytoplankton. That this condition is forced by complex, and hence highly variable, local
meteorological conditions is clear from the analysis by Taylor and Stephens (1993) of the
consequences of the difference between depths of diurnal and nocturnal mixing due to
the effect of increased buoyancy by solar heating during daytime. The Sverdrup criterion
can only be satisfied if and when sufficient buoyancy is induced during the daytime to
overcome the loss of cells below Z cr by mixing at night. Thus, development of a vernal
shoal pycnocline is associated with increasing sun angles and relaxation of wind stress.