168
Chapter 9: The Atlantic Ocean
thermocline. The consequent convective mixing deepens Z m to 125–150 m, a process
leading to the formation of the 18
C subtropical mode water (Worthington, 1986);
in winter, this extends from 30
N almost to 40
N, then sinks and spreads, effectively
separating the seasonal from the permanent thermocline. Toward the center of the
Sargasso Sea, in the vicinity of Bermuda, the upper 200 m of the water column may
become thermally uniform from January to April, after which stratification is reimposed
not only by increasing irradiance and reduced wind stress at the surface, but also by
rainfall that induces a shallow, freshened layer that is enriched in atmospheric nitrogen
(Michaels et al., 1993). As the summer progresses, the increasing frequency of tropical
storms progressively modifies this simple shoal mixed layer; cooling occurs rapidly after
early October.
Even though here we are far from the unpredictable higher latitudes, there remains
significant between-year variability in seasonal mixing and stratification: maximum winter
Z m varied over a 9-year period from >300 m in 1993–94 to only 170 m in several later
winters, although in each year the maximum occurred predictably in February. Surface
nitrate in most years briefly exceeds 10 mol kg
−1 at the surface at this time, but this
enrichment is not observed in those years when winter mixing is relatively shallow.
Many of the observations on which we depend for understanding the NAST
province have originated from the Bermuda Atlantic Time Series (BATS) program, but
it must be emphasized that BATS is placed right on the edge of the region of the North
Atlantic where winter mixing penetrates sufficiently deeply to induce a weak spring
bloom; as Nelson et al. (2004) point out, just south of the BATS site there is a strong
meridional gradient in surface properties, beyond which the tropical conditions of the
NATR province obtain. Because the BATS site is marginal to the winter-mixing regime
of the North Atlantic, we should not be surprised that between-year variability here is
strong, as has been observed.
Along the southern limit of the province, the transition between midlatitude westerlies and trade winds from the east drives convergence of surface water along the STC
(Iselin, 1936; Voorhuis, 1969). The thermal fronts comprising the STC thus separate the
anticyclonic gyre into a northern subtropical and a southern tropical portion at about
25–30
N. To the south of Bermuda, such a transition has long been known to occur,
across which subtropical, winter-mixing conditions are replaced by tropical conditions
in which surface water in winter never falls below 20
C. Schroeder (1965) traced the
thermal front of the STC from 70
W eastward to 40
W, always between 20 and 31
N;
at 30
W, to the south of the Azores, Gould (1985) mapped the same front carefully.
The STC is found to be rich in eddy activity at all scales; shear instability and random
superposition of internal waves produce small-scale structure along the front (Voorhuis
and Bruce, 1982; Toole and Schmidt, 1987). Wavelike baroclinic eddies (of scale 800 km,
200 days) induce persistent westward-propagating sea surface temperature anomalies at
the STC (Halliwell et al., 1991), whose movement along the front is spaced 3–5 days
apart (Leetma and Voorhis, 1978). The instantaneous location of the STC can readily be
found in basin-scale TOPEX-Poseidon data.
The topography of the Mid-Atlantic Ridge constrains the main recirculation of western boundary current water within the western basin (e.g., Richardson, 1985), and the
biological properties of the eastern and western basins differ significantly. Therefore, for
some purposes, it may be useful to consider treating the western and eastern basins as
distinct subprovinces: NAST-E and NAST-W. In this case, we should draw the boundary
along the topography of the Mid-Atlantic Ridge, running southwest from the Azores.
Response of the Pelagic Ecosystems
Because the geographical Sargasso Sea corresponds with the subprovince NAST-W, there
is a very appropriate marker to demonstrate that the line between the eastern and the
Chapter 9: The Atlantic Ocean
thermocline. The consequent convective mixing deepens Z m to 125–150 m, a process
leading to the formation of the 18
C subtropical mode water (Worthington, 1986);
in winter, this extends from 30
N almost to 40
N, then sinks and spreads, effectively
separating the seasonal from the permanent thermocline. Toward the center of the
Sargasso Sea, in the vicinity of Bermuda, the upper 200 m of the water column may
become thermally uniform from January to April, after which stratification is reimposed
not only by increasing irradiance and reduced wind stress at the surface, but also by
rainfall that induces a shallow, freshened layer that is enriched in atmospheric nitrogen
(Michaels et al., 1993). As the summer progresses, the increasing frequency of tropical
storms progressively modifies this simple shoal mixed layer; cooling occurs rapidly after
early October.
Even though here we are far from the unpredictable higher latitudes, there remains
significant between-year variability in seasonal mixing and stratification: maximum winter
Z m varied over a 9-year period from >300 m in 1993–94 to only 170 m in several later
winters, although in each year the maximum occurred predictably in February. Surface
nitrate in most years briefly exceeds 10 mol kg
−1 at the surface at this time, but this
enrichment is not observed in those years when winter mixing is relatively shallow.
Many of the observations on which we depend for understanding the NAST
province have originated from the Bermuda Atlantic Time Series (BATS) program, but
it must be emphasized that BATS is placed right on the edge of the region of the North
Atlantic where winter mixing penetrates sufficiently deeply to induce a weak spring
bloom; as Nelson et al. (2004) point out, just south of the BATS site there is a strong
meridional gradient in surface properties, beyond which the tropical conditions of the
NATR province obtain. Because the BATS site is marginal to the winter-mixing regime
of the North Atlantic, we should not be surprised that between-year variability here is
strong, as has been observed.
Along the southern limit of the province, the transition between midlatitude westerlies and trade winds from the east drives convergence of surface water along the STC
(Iselin, 1936; Voorhuis, 1969). The thermal fronts comprising the STC thus separate the
anticyclonic gyre into a northern subtropical and a southern tropical portion at about
25–30
N. To the south of Bermuda, such a transition has long been known to occur,
across which subtropical, winter-mixing conditions are replaced by tropical conditions
in which surface water in winter never falls below 20
C. Schroeder (1965) traced the
thermal front of the STC from 70
W eastward to 40
W, always between 20 and 31
N;
at 30
W, to the south of the Azores, Gould (1985) mapped the same front carefully.
The STC is found to be rich in eddy activity at all scales; shear instability and random
superposition of internal waves produce small-scale structure along the front (Voorhuis
and Bruce, 1982; Toole and Schmidt, 1987). Wavelike baroclinic eddies (of scale 800 km,
200 days) induce persistent westward-propagating sea surface temperature anomalies at
the STC (Halliwell et al., 1991), whose movement along the front is spaced 3–5 days
apart (Leetma and Voorhis, 1978). The instantaneous location of the STC can readily be
found in basin-scale TOPEX-Poseidon data.
The topography of the Mid-Atlantic Ridge constrains the main recirculation of western boundary current water within the western basin (e.g., Richardson, 1985), and the
biological properties of the eastern and western basins differ significantly. Therefore, for
some purposes, it may be useful to consider treating the western and eastern basins as
distinct subprovinces: NAST-E and NAST-W. In this case, we should draw the boundary
along the topography of the Mid-Atlantic Ridge, running southwest from the Azores.
Response of the Pelagic Ecosystems
Because the geographical Sargasso Sea corresponds with the subprovince NAST-W, there
is a very appropriate marker to demonstrate that the line between the eastern and the