Stratification and Irradiance: The Consequences of Latitude
55
Planetary waves present major anomalies in the global field of surface chlorophyll, and
Killworth et al. (2004) recently reviewed the possible causes of the observed mesoscale
chlorophyll anomalies in such eddying flow. Three possible mechanisms were examined:
(i) horizontal advection of phytoplankton cells and their concentration within a frontal
structure, (ii) vertical advection of phytoplankton cells from below to the surface, and
(iii) upgrowth of cells following vertical transport of nutrients into the photic zone. The
latter, the rototiller effect (Siegel, 2001), differs significantly from what occurs ideally
in a cyclonic eddy, which only upwells nutrients when it forms or intensifies; during
their entire westward propagation across the ocean, planetary waves will be expected to
upwell nutrients continuously on their leading edge. Nevertheless, cross-spectral analysis
of TOPEX-POSEIDON SLA and SeaWiFS chlorophyll suggested to Killworth et al. that
this process is less significant than the meridional transport of chlorophyll against the
background field. This is sufficient to account for most of the observed wave propagation
seen in the SeaWiFS chlorophyll field, although other mechanisms must contribute. One
of these is the effect of increased wind stress seasonally when Rossby-wave upwelling
may cause simple vertical entrainment of cells that then dominate the regional surface
chlorophyll pattern; this can only occur when the mixed-layer depth lies close to that of
the chlorophyll maximum (Kawamiya and Oschlies, 2001) and is important in establishing
the surface chlorophyll pattern at low latitudes in the Indian Ocean.
However, an even simpler “cell-lifting mechanism” explanation of mesoscale pattern
within the basin-scale chlorophyll field is proposed both by Kawamiya and Oschlies
(2001) and Charria et al. (2003). If the passage of a Rossby wave induces no significant
input of nutrients into the mixed layer and if the deep chlorophyll maximum (DCM) lies
at or just below the thermocline, it is sufficient for the passage of a Rossby wave to raise
the DCM into contact with the mixed layer; this will erode the DCM and increase the
observed cell count within the mixed layer. Thus, although Rossby waves may have an
appreciable effect on variation in primary and export production, the surface pattern of
chlorophyll observable by satellite sensors “is controlled by the difference between MLD
and the depth of the DCM.” This may be, suggest Kawamiya and Oschlies, yet another
complication for algorithms used in the computation of regional productivity from the
sea surface chlorophyll field. For Charria et al. this mechanism is sufficient explanation
of the observed regional chlorophyll field in the Subtropical Convergence Zone of the
South Atlantic.
Stratification and Irradiance: The
Consequences of Latitude
Any ecological geography of the ocean must be sensitive to the physical processes that
determine when and where phytoplankton growth will occur, and which taxa will participate. From Sverdrup, we may assume that what we need to analyze are seasonal changes
in (i) the light field and (ii) stratification within the water column. Apparently, these are
the consequences of solar irradiance and of wind stress at the sea surface; however, both
are strongly modified by continental geography and the distribution of land masses, and
each also responds characteristically to latitude. It will be useful at this point to review
the consequences of latitude for irradiance at the sea surface and on the processes that
are induced by wind stress at the sea surface. The first are easily stated, the second are
much more complex.
The regional, seasonal characteristics of irradiance at the sea surface are readily encapsulated in two relatively simple propositions: (i) in low latitudes the seasonal cycle has
a smaller amplitude than the diel cycle, whereas the annual cycle exceeds the diel at
55
Planetary waves present major anomalies in the global field of surface chlorophyll, and
Killworth et al. (2004) recently reviewed the possible causes of the observed mesoscale
chlorophyll anomalies in such eddying flow. Three possible mechanisms were examined:
(i) horizontal advection of phytoplankton cells and their concentration within a frontal
structure, (ii) vertical advection of phytoplankton cells from below to the surface, and
(iii) upgrowth of cells following vertical transport of nutrients into the photic zone. The
latter, the rototiller effect (Siegel, 2001), differs significantly from what occurs ideally
in a cyclonic eddy, which only upwells nutrients when it forms or intensifies; during
their entire westward propagation across the ocean, planetary waves will be expected to
upwell nutrients continuously on their leading edge. Nevertheless, cross-spectral analysis
of TOPEX-POSEIDON SLA and SeaWiFS chlorophyll suggested to Killworth et al. that
this process is less significant than the meridional transport of chlorophyll against the
background field. This is sufficient to account for most of the observed wave propagation
seen in the SeaWiFS chlorophyll field, although other mechanisms must contribute. One
of these is the effect of increased wind stress seasonally when Rossby-wave upwelling
may cause simple vertical entrainment of cells that then dominate the regional surface
chlorophyll pattern; this can only occur when the mixed-layer depth lies close to that of
the chlorophyll maximum (Kawamiya and Oschlies, 2001) and is important in establishing
the surface chlorophyll pattern at low latitudes in the Indian Ocean.
However, an even simpler “cell-lifting mechanism” explanation of mesoscale pattern
within the basin-scale chlorophyll field is proposed both by Kawamiya and Oschlies
(2001) and Charria et al. (2003). If the passage of a Rossby wave induces no significant
input of nutrients into the mixed layer and if the deep chlorophyll maximum (DCM) lies
at or just below the thermocline, it is sufficient for the passage of a Rossby wave to raise
the DCM into contact with the mixed layer; this will erode the DCM and increase the
observed cell count within the mixed layer. Thus, although Rossby waves may have an
appreciable effect on variation in primary and export production, the surface pattern of
chlorophyll observable by satellite sensors “is controlled by the difference between MLD
and the depth of the DCM.” This may be, suggest Kawamiya and Oschlies, yet another
complication for algorithms used in the computation of regional productivity from the
sea surface chlorophyll field. For Charria et al. this mechanism is sufficient explanation
of the observed regional chlorophyll field in the Subtropical Convergence Zone of the
South Atlantic.
Stratification and Irradiance: The
Consequences of Latitude
Any ecological geography of the ocean must be sensitive to the physical processes that
determine when and where phytoplankton growth will occur, and which taxa will participate. From Sverdrup, we may assume that what we need to analyze are seasonal changes
in (i) the light field and (ii) stratification within the water column. Apparently, these are
the consequences of solar irradiance and of wind stress at the sea surface; however, both
are strongly modified by continental geography and the distribution of land masses, and
each also responds characteristically to latitude. It will be useful at this point to review
the consequences of latitude for irradiance at the sea surface and on the processes that
are induced by wind stress at the sea surface. The first are easily stated, the second are
much more complex.
The regional, seasonal characteristics of irradiance at the sea surface are readily encapsulated in two relatively simple propositions: (i) in low latitudes the seasonal cycle has
a smaller amplitude than the diel cycle, whereas the annual cycle exceeds the diel at