56
Chapter 4: Physical Control of Ecological Processes
high latitudes (Hastenrath, 1985) and (ii) the irradiance field is dominated by a simple
and predictable meridional gradient, which migrates with the seasons and is modified by regionally variable cloudiness and atmospheric clarity that respond to processes
above adjacent continents. At this point in the discussion, that is all that needs to
be said.
Prominent in any textbook on biological oceanography are the ecological consequences
of the balance between turbulence and mixing caused by wind stress at the surface and
stratification induced by solar heating. That this balance is relevant only at higher latitudes
is rarely mentioned, and the student may be left to discover that the physics required
by the Sverdrup model is quite different at low latitudes. There, seasonal changes in
depth of the surface mixed layer are due not to seasonal changes in turbulence induced
by varying local wind stress but rather, through geostrophic adjustment, to changes in
circulation that may occur in distant regions (Philander, 1985; Katz, 1987; Hastenrath
and Merle, 1987). To understand this it is necessary to start with the essential, but often
overlooked, difference between the effects of wind stress at the sea surface at high and low
latitudes.
Simply stated, wind stress is translated mostly into potential energy at high latitudes,
but into kinetic energy at low latitudes. This is not the place for a formal discussion of
this effect, for which a good modern source is Tomczak and Godfrey (1994), but, simply
stated, flow is induced by the pressure-gradient force at the sea surface, and because
geostrophic balance is maintained between this gradient and the Coriolis parameter
induced by Earth’s rotation (f, with dimensions of 1/sec), the slope of the sea surface
required to produce motion diminishes equatorward as a direct function of latitude, and
without discontinuity (Lighthill, 1969; Philander, 1985). It is perhaps characteristic of the
evolution of oceanographic theory that this was not understood until the 1960s.
Consequently, seasonal moderation of wind stress over the North Atlantic in summer
has a negligible effect on the flow of the Gulf Stream, whereas seasonal reversal of
monsoon winds in the Arabian Sea reverses the Somali Current to almost 1000 m depth in
just a few weeks. Only after reversal of the North Atlantic westerly winds for about 10 years
would the same effect be produced on the Gulf Stream. At the extreme, geostrophic
balance ceases to function at less than about 2
from the equator, where motion is
determined by nonrotational fluid dynamics; the most striking manifestation of this effect
is the undercurrent flowing eastward in the pycnocline below the equator.
That seasonal changes in wind stress should quickly modify the flow in major
barotropic currents at low latitudes, such as that along the coast of Somalia and Arabia,
is a fundamental characteristic of tropical oceans; moreover, since the spatial scale of the
adjustment of barotropic currents is similar at all latitudes, seasonal geostrophic adjustments in the mixed-layer depths can be identified across whole ocean basins and have
important consequences when considering the Sverdrup model. This, of course, is usually
invoked where it is changes in local wind mixing and irradiance that force changes in
mixed-layer depth, although it has also been used to interpret phytoplankton seasonality
in lower latitudes (Obata et al., 1996). This investigation of the interaction between relative changes in mixed-layer depth and Z cr and the concomitant increase or decrease of
surface chlorophyll used Levitus mixed-layer depth (Z m ), CZCS surface chlorophyll, and
the surface light field. The results suggested that whenever and wherever Z m shoals up
through Z cr , whether by changes in wind stress or by geostrophic adjustment, a bloom
follows. The reverse effect was also simulated: whenever and wherever Z m became deeper
than Z cr , a decrease in surface chlorophyll was produced in the following month.
Also consequent upon the equatorward vanishing of the Coriolis parameter are changes
in the scale taken by internal wave trains, implicated in wind mixing of the surface layers
of the ocean. The period of these waves changes from only 12 hours at the poles to much
longer at the equator (Garrett, 2003). Their propagation into the interior of the ocean
Chapter 4: Physical Control of Ecological Processes
high latitudes (Hastenrath, 1985) and (ii) the irradiance field is dominated by a simple
and predictable meridional gradient, which migrates with the seasons and is modified by regionally variable cloudiness and atmospheric clarity that respond to processes
above adjacent continents. At this point in the discussion, that is all that needs to
be said.
Prominent in any textbook on biological oceanography are the ecological consequences
of the balance between turbulence and mixing caused by wind stress at the surface and
stratification induced by solar heating. That this balance is relevant only at higher latitudes
is rarely mentioned, and the student may be left to discover that the physics required
by the Sverdrup model is quite different at low latitudes. There, seasonal changes in
depth of the surface mixed layer are due not to seasonal changes in turbulence induced
by varying local wind stress but rather, through geostrophic adjustment, to changes in
circulation that may occur in distant regions (Philander, 1985; Katz, 1987; Hastenrath
and Merle, 1987). To understand this it is necessary to start with the essential, but often
overlooked, difference between the effects of wind stress at the sea surface at high and low
latitudes.
Simply stated, wind stress is translated mostly into potential energy at high latitudes,
but into kinetic energy at low latitudes. This is not the place for a formal discussion of
this effect, for which a good modern source is Tomczak and Godfrey (1994), but, simply
stated, flow is induced by the pressure-gradient force at the sea surface, and because
geostrophic balance is maintained between this gradient and the Coriolis parameter
induced by Earth’s rotation (f, with dimensions of 1/sec), the slope of the sea surface
required to produce motion diminishes equatorward as a direct function of latitude, and
without discontinuity (Lighthill, 1969; Philander, 1985). It is perhaps characteristic of the
evolution of oceanographic theory that this was not understood until the 1960s.
Consequently, seasonal moderation of wind stress over the North Atlantic in summer
has a negligible effect on the flow of the Gulf Stream, whereas seasonal reversal of
monsoon winds in the Arabian Sea reverses the Somali Current to almost 1000 m depth in
just a few weeks. Only after reversal of the North Atlantic westerly winds for about 10 years
would the same effect be produced on the Gulf Stream. At the extreme, geostrophic
balance ceases to function at less than about 2
from the equator, where motion is
determined by nonrotational fluid dynamics; the most striking manifestation of this effect
is the undercurrent flowing eastward in the pycnocline below the equator.
That seasonal changes in wind stress should quickly modify the flow in major
barotropic currents at low latitudes, such as that along the coast of Somalia and Arabia,
is a fundamental characteristic of tropical oceans; moreover, since the spatial scale of the
adjustment of barotropic currents is similar at all latitudes, seasonal geostrophic adjustments in the mixed-layer depths can be identified across whole ocean basins and have
important consequences when considering the Sverdrup model. This, of course, is usually
invoked where it is changes in local wind mixing and irradiance that force changes in
mixed-layer depth, although it has also been used to interpret phytoplankton seasonality
in lower latitudes (Obata et al., 1996). This investigation of the interaction between relative changes in mixed-layer depth and Z cr and the concomitant increase or decrease of
surface chlorophyll used Levitus mixed-layer depth (Z m ), CZCS surface chlorophyll, and
the surface light field. The results suggested that whenever and wherever Z m shoals up
through Z cr , whether by changes in wind stress or by geostrophic adjustment, a bloom
follows. The reverse effect was also simulated: whenever and wherever Z m became deeper
than Z cr , a decrease in surface chlorophyll was produced in the following month.
Also consequent upon the equatorward vanishing of the Coriolis parameter are changes
in the scale taken by internal wave trains, implicated in wind mixing of the surface layers
of the ocean. The period of these waves changes from only 12 hours at the poles to much
longer at the equator (Garrett, 2003). Their propagation into the interior of the ocean