212
Chapter 9: The Atlantic Ocean
summer SST field reflects the spread of warm conditions eastward across the province,
the winter field the progressive westward cooling from the cold water adjacent to the
Benguela Current.
For the seasonal evolution of surface chlorophyll, the satellite images are very illuminating, and from them we can infer much more than we can observe. There is, in
each year of observation, a very clear seasonal shift in the chlorophyll field. The South
Subtropical Convergence zone (SSTC Province) lies across the ocean as a permanent
zonal high-chlorophyll feature, having its northern boundary at about 40
S in austral
summer. From about April, in austral fall, this boundary becomes increasingly diffuse
and enhanced chlorophyll moves progressively equatorward, reaching almost 25
S in
mid-winter (July–August). At this time, associated with the focus of the gyral circulation southwest of the Rio Grande Rise, diffuse chlorophyll enhancement reaches even
to 15
S, as it also does in the flow of the SEC across the northern limb of the gyre.
This seasonal chlorophyll field with very uniform values of around 03 mg chl m
−3 is
presumably induced by winter mixing, surface values appropriately matching depth of
the baroclinic bowl in the nutrient field. The winter chlorophyll field of this province
becomes continuous with the high chlorophyll of the SSTC zone, which takes similar values at this season. As spring and summer conditions develop, the oceanic South
Atlantic bloom recedes poleward, while the biomass in the SSTC zone increases, to reach
maxima around 1–2 mg chl m
−3 by February. This seasonal sequence has only minor
between-year variability in the SeaWiFS images 1997–2003.
The available images also show an occasional, early fall (January–May) chlorophyll
enhancement of dendritic form apparently associated with incoherence in the circulation
above the Rio Grande Rise at about 45
W 45
S that is seen in Fig. 9.19. This was an
exceptional feature in April 2000, dominating the chlorophyll field of the western ocean.
The location of this feature closely matches the ridge between the double-cell circulation
of Tsuchiya (and of cold anomalies in the SST field) sufficiently well that we may be
confident of the reasons for its formation. In late summer, early fall of 1998, 1999, 2001,
and 2002 much fainter and more obscure linear enhancement is observed in the same
region. This feature would bear investigation for it matches, both in months of occurrence
and location in the ocean basin, a similar dendritic chlorophyll feature in the western
Indian Ocean (see INDW Province). That feature is not associated with any topography,
but it is intriguing that the months of greatest development of the Rio Grande Rise bloom
in 2000 correspond with the formation of a very similar dendritic bloom in the SW
Indian Ocean. It seems very probable that at similar latitudes in both oceans, and near
their western margins, equivalent south subtropical countercurrents take their origins,
being observable at the surface as highly eddying features in the chlorophyll field.
The detailed meridional section run by Agusti and Duarte (1999) down the western
side of the gyral circulation in October–November 1995 confirmed that the chlorophyll
profile has the anticipated relationship with physical structure. The shallow (60–80 m)
DCM of the tropical region persisted until 12
S, although here it was underlain by
a much deeper chlorophyll feature at 100–150 m, just below the depth of maximal
Brunt-Väisälä frequency; this wider DCM deepens progressively to about 20
S and then
shoals toward the south. At 35
S, at the southern end of the section, approaching or
within the SSTC, a near-surface (40-m) DCM is interposed above the remnant deeper
feature. The relationship between depths of DCM and I% surface light is very close along
the entire section, as is that between surface and integrated chlorophyll concentration.
A meridional section reported by Dufour and Stretta (1973) close to the Greenwich
meridian shows a very similar situation, with the phosphatocline coinciding very closely
to the southward-deepening pycnocline.
The AMT sections for October (austral spring) and May (austral fall) confirm these patterns down the western side of the SATL province. However, as Marañon et al. (2000) point
Chapter 9: The Atlantic Ocean
summer SST field reflects the spread of warm conditions eastward across the province,
the winter field the progressive westward cooling from the cold water adjacent to the
Benguela Current.
For the seasonal evolution of surface chlorophyll, the satellite images are very illuminating, and from them we can infer much more than we can observe. There is, in
each year of observation, a very clear seasonal shift in the chlorophyll field. The South
Subtropical Convergence zone (SSTC Province) lies across the ocean as a permanent
zonal high-chlorophyll feature, having its northern boundary at about 40
S in austral
summer. From about April, in austral fall, this boundary becomes increasingly diffuse
and enhanced chlorophyll moves progressively equatorward, reaching almost 25
S in
mid-winter (July–August). At this time, associated with the focus of the gyral circulation southwest of the Rio Grande Rise, diffuse chlorophyll enhancement reaches even
to 15
S, as it also does in the flow of the SEC across the northern limb of the gyre.
This seasonal chlorophyll field with very uniform values of around 03 mg chl m
−3 is
presumably induced by winter mixing, surface values appropriately matching depth of
the baroclinic bowl in the nutrient field. The winter chlorophyll field of this province
becomes continuous with the high chlorophyll of the SSTC zone, which takes similar values at this season. As spring and summer conditions develop, the oceanic South
Atlantic bloom recedes poleward, while the biomass in the SSTC zone increases, to reach
maxima around 1–2 mg chl m
−3 by February. This seasonal sequence has only minor
between-year variability in the SeaWiFS images 1997–2003.
The available images also show an occasional, early fall (January–May) chlorophyll
enhancement of dendritic form apparently associated with incoherence in the circulation
above the Rio Grande Rise at about 45
W 45
S that is seen in Fig. 9.19. This was an
exceptional feature in April 2000, dominating the chlorophyll field of the western ocean.
The location of this feature closely matches the ridge between the double-cell circulation
of Tsuchiya (and of cold anomalies in the SST field) sufficiently well that we may be
confident of the reasons for its formation. In late summer, early fall of 1998, 1999, 2001,
and 2002 much fainter and more obscure linear enhancement is observed in the same
region. This feature would bear investigation for it matches, both in months of occurrence
and location in the ocean basin, a similar dendritic chlorophyll feature in the western
Indian Ocean (see INDW Province). That feature is not associated with any topography,
but it is intriguing that the months of greatest development of the Rio Grande Rise bloom
in 2000 correspond with the formation of a very similar dendritic bloom in the SW
Indian Ocean. It seems very probable that at similar latitudes in both oceans, and near
their western margins, equivalent south subtropical countercurrents take their origins,
being observable at the surface as highly eddying features in the chlorophyll field.
The detailed meridional section run by Agusti and Duarte (1999) down the western
side of the gyral circulation in October–November 1995 confirmed that the chlorophyll
profile has the anticipated relationship with physical structure. The shallow (60–80 m)
DCM of the tropical region persisted until 12
S, although here it was underlain by
a much deeper chlorophyll feature at 100–150 m, just below the depth of maximal
Brunt-Väisälä frequency; this wider DCM deepens progressively to about 20
S and then
shoals toward the south. At 35
S, at the southern end of the section, approaching or
within the SSTC, a near-surface (40-m) DCM is interposed above the remnant deeper
feature. The relationship between depths of DCM and I% surface light is very close along
the entire section, as is that between surface and integrated chlorophyll concentration.
A meridional section reported by Dufour and Stretta (1973) close to the Greenwich
meridian shows a very similar situation, with the phosphatocline coinciding very closely
to the southward-deepening pycnocline.
The AMT sections for October (austral spring) and May (austral fall) confirm these patterns down the western side of the SATL province. However, as Marañon et al. (2000) point