Atlantic Westerly Winds Biome
165
MODIS images will usually demonstrate some chlorophyll enhancement within the eddy
field of the Gulf Stream as far to the east as the Mid-Atlantic Ridge almost continuously
from spring to autumn, and this is especially noticeable when compared with the more
transient enhancement in the eastern Atlantic at similar latitudes, where mesoscale eddies
are less numerous and less energetic. It had commonly been thought that the passage
of the Gulf Stream across the New England seamount chain induces the formation of
particularly energetic meanders and eddies, but thermal imagery now shows that this is
not the case (Cornillon, 1986).
The ecology of cold-core eddies was well studied in the late 1970s by the Ring Group
(1981), mostly out of Woods Hole. Young rings contain water having the ecological
characteristics and biota of slope water, and the central doming brings the 15
C isotherm
close to the surface. The rate of primary production within a cold-core eddy is, at least
initially, higher than the surrounding oceanic water by a factor of about 1.7; this is similar
to the general ratio between slope water and the open Sargasso Sea. If it is shed at the
end of winter, a spring bloom may occur within the young ring. The deep chlorophyll
maximum layer thus formed weakens (by a factor of 5–10 in maximum chlorophyll
values) and deepens to about 100 m as the summer progresses, coming to lie on the upper
part of the nutricline within the ring. This evolution is due to both seasonal processes
and, as the ring ages, progressive replacement of the slope water flora and fauna by
species typical of the Sargasso Sea.
John Woods (1988) has explored and modeled the biological consequences of
the dynamic changes in pycnocline topography that are associated with instability of
mesoscale jets, such as the Gulf Stream. He points out that mesoscale jets are inherently
unstable, with high isopycnal potential vorticity, and soon develop meanders with alongstream wavelengths of 10–100 km. Vortex contraction on the flanks of the anticyclonic
warm-core meanders should cause upwelling to occur, with downwelling within the
cyclonic meanders. This vertical motion is reflected in the sea-surface temperature field,
and primary production rates are higher in the anticyclonic meanders where both pigment
maximum and the nitracline rise along the upward-sloping isopycnals; a patchy distribution of mixed-layer chlorophyll develops at the same scale as the meanders (Lohrenz
et al., 1993). According to Rossby’s theory of the Gulf Stream, a general cross-stream
effect on production rate and standing stock was proposed by Yentsch (1974), by which
water is drawn into the right side of the jet and discharged toward the left along the
upward-sloping isopycnals. Together with cross-frontal mixing of relatively pigment-rich
slope water, this should result in the cold north wall of the Gulf Stream being observable
at the surface as a chlorophyll front.
However, more recently, Anderson and Robinson (2001) have assimilated data from
two surveys in autumn 1988 (BIOSYNOP 21 and GULFCAST) into a 4D simulation
of physical processes and biological effects, and some of the earlier suggestions are not
confirmed. Primary production and chlorophyll concentration are not enhanced at the
front over values in the adjacent Slope Water; high chlorophyll concentrations observed
at sea on BIOSYNOP appear to be caused primarily by advection and convergence, rather
than by in situ biological growth. The result from the Anderson/Robinson model appears
to be confirmed by the biweekly high-resolution images for chlorophyll and sea surface
temperature produced routinely at Bedford Institute of Oceanography.
Within the cold-core rings that become embedded in the Gulf Stream in winter or
spring, the response of the entrained slope water mesozooplankton species, enclosed
within the warming ring, is to descend progressively into cooler water as the summer
advances. In this way, mature cold-core rings, late in the year, may come to have warmwater species above and shelf species below. Vertically integrated biomass of cold-core
rings may thus exceed that of surrounding water because of the addition in such data of a
165
MODIS images will usually demonstrate some chlorophyll enhancement within the eddy
field of the Gulf Stream as far to the east as the Mid-Atlantic Ridge almost continuously
from spring to autumn, and this is especially noticeable when compared with the more
transient enhancement in the eastern Atlantic at similar latitudes, where mesoscale eddies
are less numerous and less energetic. It had commonly been thought that the passage
of the Gulf Stream across the New England seamount chain induces the formation of
particularly energetic meanders and eddies, but thermal imagery now shows that this is
not the case (Cornillon, 1986).
The ecology of cold-core eddies was well studied in the late 1970s by the Ring Group
(1981), mostly out of Woods Hole. Young rings contain water having the ecological
characteristics and biota of slope water, and the central doming brings the 15
C isotherm
close to the surface. The rate of primary production within a cold-core eddy is, at least
initially, higher than the surrounding oceanic water by a factor of about 1.7; this is similar
to the general ratio between slope water and the open Sargasso Sea. If it is shed at the
end of winter, a spring bloom may occur within the young ring. The deep chlorophyll
maximum layer thus formed weakens (by a factor of 5–10 in maximum chlorophyll
values) and deepens to about 100 m as the summer progresses, coming to lie on the upper
part of the nutricline within the ring. This evolution is due to both seasonal processes
and, as the ring ages, progressive replacement of the slope water flora and fauna by
species typical of the Sargasso Sea.
John Woods (1988) has explored and modeled the biological consequences of
the dynamic changes in pycnocline topography that are associated with instability of
mesoscale jets, such as the Gulf Stream. He points out that mesoscale jets are inherently
unstable, with high isopycnal potential vorticity, and soon develop meanders with alongstream wavelengths of 10–100 km. Vortex contraction on the flanks of the anticyclonic
warm-core meanders should cause upwelling to occur, with downwelling within the
cyclonic meanders. This vertical motion is reflected in the sea-surface temperature field,
and primary production rates are higher in the anticyclonic meanders where both pigment
maximum and the nitracline rise along the upward-sloping isopycnals; a patchy distribution of mixed-layer chlorophyll develops at the same scale as the meanders (Lohrenz
et al., 1993). According to Rossby’s theory of the Gulf Stream, a general cross-stream
effect on production rate and standing stock was proposed by Yentsch (1974), by which
water is drawn into the right side of the jet and discharged toward the left along the
upward-sloping isopycnals. Together with cross-frontal mixing of relatively pigment-rich
slope water, this should result in the cold north wall of the Gulf Stream being observable
at the surface as a chlorophyll front.
However, more recently, Anderson and Robinson (2001) have assimilated data from
two surveys in autumn 1988 (BIOSYNOP 21 and GULFCAST) into a 4D simulation
of physical processes and biological effects, and some of the earlier suggestions are not
confirmed. Primary production and chlorophyll concentration are not enhanced at the
front over values in the adjacent Slope Water; high chlorophyll concentrations observed
at sea on BIOSYNOP appear to be caused primarily by advection and convergence, rather
than by in situ biological growth. The result from the Anderson/Robinson model appears
to be confirmed by the biweekly high-resolution images for chlorophyll and sea surface
temperature produced routinely at Bedford Institute of Oceanography.
Within the cold-core rings that become embedded in the Gulf Stream in winter or
spring, the response of the entrained slope water mesozooplankton species, enclosed
within the warming ring, is to descend progressively into cooler water as the summer
advances. In this way, mature cold-core rings, late in the year, may come to have warmwater species above and shelf species below. Vertically integrated biomass of cold-core
rings may thus exceed that of surrounding water because of the addition in such data of a