66
Chapter 4: Physical Control of Ecological Processes
The photic depth lies deeper, and in some cases much deeper, than the mixed layer.
This situation obtains essentially year-round, and the pattern breaks down with deepening
of the mixed layer only briefly and only in some regions. Nowhere is there any evidence
of accumulation of chlorophyll unaccompanied by a simultaneous increase in the rate of
primary production: growth and loss terms are closely coupled, with the biomass values
varying over a relatively small dynamic range.
It is not simple to devise a poleward boundary for the region where Case 3 is appropriate
that excludes the effects of some increased wind stress at the surface from winter westerlies.
For this reason, in some zones that we might want to think of otherwise as Case 3 models,
winter winds may force the mixed layer deeper than the photic zone for several months
seasonally. The equatorial regions of the Pacific Ocean have less seasonality in mixed-layer
depth, rate of primary production, and chlorophyll accumulation than comparable Atlantic
regions because of the differences in east-west dimension of the two oceans.
Case 5—Large-Amplitude Response to Monsoon-like
Reversal of Trade Winds
This model describes those parts of the trade-wind zone of the oceans where a smallamplitude Case 4 response is modified by strong seasonality in local trade-wind forcing.
These are the monsoon regions where, in the extreme case of the northwest Indian
Ocean, seasonal reversal of the trade winds results in seasonal reversal of the monsoon
currents and seasonal alternation between eutrophic and oligotrophic biological systems.
The small zonal dimension of the tropical Atlantic permits a strong seasonal response to
the surge of southerly trades across the equator that is analogous to the effect of monsoon
reversal in the Arabian Sea.
Though the seasonal variance in frictional wind stress at the sea surface in these regions
is equivalent to that in the latitudes of winter westerlies, the consequence is quite different.
In the northwest Indian Ocean the onset of both monsoons is accompanied by increases
in mixed-layer depth only about one-tenth of that achieved in the North Atlantic by the
onset of similar seasonal winds. But the mixed layer does deepen sufficiently with the
onset of each monsoon that the pycnocline is alternately within, or below, the photic
zone. Four seasons may therefore be distinguished.
As in Case 3 waters, productivity is not light limited and responds rapidly to nutrient
entrainment into the photic zone by coastal upwelling, by offshore Ekman suction, and
by geostrophic adjustment of the pycnocline associated with reversal of the monsoon
winds. The pycnocline is illuminated during the oligotrophic season, but the mixed
layer shoals above the photic zone during upwelling episodes. Changes in phytoplankton
biomass match the seasonal changes in production rate, consistent with consumption
and production rates being closely matched.
Case 6—Intermittent Production at Coastal
Divergences
This model is appropriate to the four canonical eastern boundary current upwelling areas,
and a few other places where seasonal coastal upwelling occurs in low to moderate latitudes.
The mixed layer shoals, and primary production rate takes high values when coastal winds
are appropriate for upwelling, usually in summer, so that deep nitrate-rich water is entrained
into the photic zone. Chlorophyll accumulation coincides with duration of upwelling and
accumulation of chlorophyll is balanced by advection and consumption loss terms. Herbivores, as in high latitudes, enter a resting phase rather deep off upwelling regions in periods
Chapter 4: Physical Control of Ecological Processes
The photic depth lies deeper, and in some cases much deeper, than the mixed layer.
This situation obtains essentially year-round, and the pattern breaks down with deepening
of the mixed layer only briefly and only in some regions. Nowhere is there any evidence
of accumulation of chlorophyll unaccompanied by a simultaneous increase in the rate of
primary production: growth and loss terms are closely coupled, with the biomass values
varying over a relatively small dynamic range.
It is not simple to devise a poleward boundary for the region where Case 3 is appropriate
that excludes the effects of some increased wind stress at the surface from winter westerlies.
For this reason, in some zones that we might want to think of otherwise as Case 3 models,
winter winds may force the mixed layer deeper than the photic zone for several months
seasonally. The equatorial regions of the Pacific Ocean have less seasonality in mixed-layer
depth, rate of primary production, and chlorophyll accumulation than comparable Atlantic
regions because of the differences in east-west dimension of the two oceans.
Case 5—Large-Amplitude Response to Monsoon-like
Reversal of Trade Winds
This model describes those parts of the trade-wind zone of the oceans where a smallamplitude Case 4 response is modified by strong seasonality in local trade-wind forcing.
These are the monsoon regions where, in the extreme case of the northwest Indian
Ocean, seasonal reversal of the trade winds results in seasonal reversal of the monsoon
currents and seasonal alternation between eutrophic and oligotrophic biological systems.
The small zonal dimension of the tropical Atlantic permits a strong seasonal response to
the surge of southerly trades across the equator that is analogous to the effect of monsoon
reversal in the Arabian Sea.
Though the seasonal variance in frictional wind stress at the sea surface in these regions
is equivalent to that in the latitudes of winter westerlies, the consequence is quite different.
In the northwest Indian Ocean the onset of both monsoons is accompanied by increases
in mixed-layer depth only about one-tenth of that achieved in the North Atlantic by the
onset of similar seasonal winds. But the mixed layer does deepen sufficiently with the
onset of each monsoon that the pycnocline is alternately within, or below, the photic
zone. Four seasons may therefore be distinguished.
As in Case 3 waters, productivity is not light limited and responds rapidly to nutrient
entrainment into the photic zone by coastal upwelling, by offshore Ekman suction, and
by geostrophic adjustment of the pycnocline associated with reversal of the monsoon
winds. The pycnocline is illuminated during the oligotrophic season, but the mixed
layer shoals above the photic zone during upwelling episodes. Changes in phytoplankton
biomass match the seasonal changes in production rate, consistent with consumption
and production rates being closely matched.
Case 6—Intermittent Production at Coastal
Divergences
This model is appropriate to the four canonical eastern boundary current upwelling areas,
and a few other places where seasonal coastal upwelling occurs in low to moderate latitudes.
The mixed layer shoals, and primary production rate takes high values when coastal winds
are appropriate for upwelling, usually in summer, so that deep nitrate-rich water is entrained
into the photic zone. Chlorophyll accumulation coincides with duration of upwelling and
accumulation of chlorophyll is balanced by advection and consumption loss terms. Herbivores, as in high latitudes, enter a resting phase rather deep off upwelling regions in periods