450
Chapter 12: The Southern Ocean
Ballegooyen, 1984); this retroflection is again associated with an active eddy field. Large
meanders form between the warm Agulhas water and the cooler water to the south; on the
coastal side of the Agulhas Current, eddies are to some extent topographically controlled
by the Agulhas Plateau. To the east of Cape Horn, flow in the ACC turns northward (as
the Falkland Current) across the Argentine Basin, and the STF is re-established between
the eastward-turning subtropical water of the Brazil Current and the colder subantarctic
water at about 40–45
S (Peterson and Whitworth, 1989; Peterson and Stramma, 1991).
The STCZ then proceeds across the South Atlantic, reaching its most northerly position
close to 35
S at the Greenwich meridian.
As noted in the previous chapter, the STCZ conforms to topography in the New
Zealand sector, looping northward along the continental edge east of Tasmania (Harris
et al., 1987) and south and east of New Zealand. It is only across the Tasman Sea and
eastward from the Chatham Rise that the convergence again becomes detached from the
steep topography of a continental edge.
Biological Response and Regional Ecology
Here we have to address, in a local sense, some of the questions discussed generally
in Chapter 5: what exactly are the mechanisms that support phytoplankton growth
and its accumulation in the Southern Ocean? Why should the seasonal progression of
light, temperature, and stratification produce phytoplankton growth preferentially in
this frontal zone? The nutrient distribution, discussed briefly earlier, offers no simple
explanation; but since frontal processes everywhere tend to establish near-surface stability
in mixed water columns, this may provide sufficient stability to initiate and maintain
a bloom (Marra, 1982), especially where, as here, the density surfaces in the front
slope downward. It is not necessary to postulate, as did Banse and English (1997), that
chlorophyll patches in this habitat represent undemonstrated “patchy” dust fallout. The
suggestion that dust deposition induces the high chlorophyll observed in the STCZ east
of Patagonia (Erickson et al., 2003) is based on an illogical argument, as has already been
observed.
Similar situations have been observed elsewhere in the STCZ and have been generalized
by Franks and Walstad (1997). Following their model, chlorophyll accumulation in
the STCZ should be strongest where eddy kinetic energy is highest, downstream of
topographic disturbances to the zonal flow; this, of course, is what is observed. The
mingling of subtropical water (nitrate-depleted, Fe-sufficient) with subantarctic water
(nitrate-sufficient, Fe-depleted) across the STCZ provides an unusually strong nutrient
gradient that must result in local phytoplankton growth where other conditions permit.
Blooms are proximally controlled by the dynamics of eddies and meanders as is clearly
shown by the abundance of arcuate streamers and annular features in appropriate 7-day
chlorophyll images. These may be matched—feature for feature—with appropriate sealevel anomalies (see Color plate 13, for example).
The STCZ is one of the most prominent features routinely seen in global sea-surface
chlorophyll images for which several mechanisms have been invoked (Lutjeharms et al.,
1985; Furuya et al., 1986). Although simple physical aggregation of biota may occur in
the STCZ, as in all convergent fronts, there seems no reason to invoke this mechanism
because the scales are not right nor are there obvious sources of biota to be transported.
Rather, it is more likely that a variety of mechanisms, differing from place to place and
from season to season, are involved. Seasonally, chlorophyll accumulates more rapidly
than it is dissipated, again an unsurprising observation. A simple seasonal cycle occurs
in the STCZ, so that phytoplankton biomass begins to accumulate in austral spring soon
after MLD has started to shoal and takes its regional maximum in midsummer when
MLD is at its shoalest.
Chapter 12: The Southern Ocean
Ballegooyen, 1984); this retroflection is again associated with an active eddy field. Large
meanders form between the warm Agulhas water and the cooler water to the south; on the
coastal side of the Agulhas Current, eddies are to some extent topographically controlled
by the Agulhas Plateau. To the east of Cape Horn, flow in the ACC turns northward (as
the Falkland Current) across the Argentine Basin, and the STF is re-established between
the eastward-turning subtropical water of the Brazil Current and the colder subantarctic
water at about 40–45
S (Peterson and Whitworth, 1989; Peterson and Stramma, 1991).
The STCZ then proceeds across the South Atlantic, reaching its most northerly position
close to 35
S at the Greenwich meridian.
As noted in the previous chapter, the STCZ conforms to topography in the New
Zealand sector, looping northward along the continental edge east of Tasmania (Harris
et al., 1987) and south and east of New Zealand. It is only across the Tasman Sea and
eastward from the Chatham Rise that the convergence again becomes detached from the
steep topography of a continental edge.
Biological Response and Regional Ecology
Here we have to address, in a local sense, some of the questions discussed generally
in Chapter 5: what exactly are the mechanisms that support phytoplankton growth
and its accumulation in the Southern Ocean? Why should the seasonal progression of
light, temperature, and stratification produce phytoplankton growth preferentially in
this frontal zone? The nutrient distribution, discussed briefly earlier, offers no simple
explanation; but since frontal processes everywhere tend to establish near-surface stability
in mixed water columns, this may provide sufficient stability to initiate and maintain
a bloom (Marra, 1982), especially where, as here, the density surfaces in the front
slope downward. It is not necessary to postulate, as did Banse and English (1997), that
chlorophyll patches in this habitat represent undemonstrated “patchy” dust fallout. The
suggestion that dust deposition induces the high chlorophyll observed in the STCZ east
of Patagonia (Erickson et al., 2003) is based on an illogical argument, as has already been
observed.
Similar situations have been observed elsewhere in the STCZ and have been generalized
by Franks and Walstad (1997). Following their model, chlorophyll accumulation in
the STCZ should be strongest where eddy kinetic energy is highest, downstream of
topographic disturbances to the zonal flow; this, of course, is what is observed. The
mingling of subtropical water (nitrate-depleted, Fe-sufficient) with subantarctic water
(nitrate-sufficient, Fe-depleted) across the STCZ provides an unusually strong nutrient
gradient that must result in local phytoplankton growth where other conditions permit.
Blooms are proximally controlled by the dynamics of eddies and meanders as is clearly
shown by the abundance of arcuate streamers and annular features in appropriate 7-day
chlorophyll images. These may be matched—feature for feature—with appropriate sealevel anomalies (see Color plate 13, for example).
The STCZ is one of the most prominent features routinely seen in global sea-surface
chlorophyll images for which several mechanisms have been invoked (Lutjeharms et al.,
1985; Furuya et al., 1986). Although simple physical aggregation of biota may occur in
the STCZ, as in all convergent fronts, there seems no reason to invoke this mechanism
because the scales are not right nor are there obvious sources of biota to be transported.
Rather, it is more likely that a variety of mechanisms, differing from place to place and
from season to season, are involved. Seasonally, chlorophyll accumulates more rapidly
than it is dissipated, again an unsurprising observation. A simple seasonal cycle occurs
in the STCZ, so that phytoplankton biomass begins to accumulate in austral spring soon
after MLD has started to shoal and takes its regional maximum in midsummer when
MLD is at its shoalest.