184
Chapter 9: The Atlantic Ocean
the Pacific. The equilibrium time of tropical oceans to low-frequency wind forcing is
a function of how long the baroclinic waves require to pass along the equatorial wave
guide. The Pacific basin (15,000 km) is too wide for this to occur seasonally as it does
in the Atlantic (5000 km) although the seasonal changes in wind forcing are similar. In
the Pacific, therefore, westward tilting of the pycnocline occurs—and can only occur—
at the scale of ENSO events, with their long-sustained modification of regional wind
stress over the ocean (Longhurst, 1993). These facts have fundamental consequences for
the seasonality of phytoplankton growth—and all that stems therefrom—in the tropical
oceanic biome.
North Atlantic Tropical Gyral Province (NATR)
Extent of the Province
The North Atlantic Tropical Gyral Province (NATR) comprises that part of the North
Atlantic gyre that lies south of the subtropical convergence that runs zonally across the
ocean at about 30
N (see North Atlantic Subtropical Gyral Province). The southern
boundary of NATR is the limit of westerly flow along the thermocline ridge associated
with the North Equatorial Current (NEC) at about 12–14
N. The western boundary is
taken to be the edge of the coastal boundary biome seaward of the Antilles and the
Bahamas. NATR thus includes the offshore segment of the Canary Current south of the
Canaries, the flow of this into the NEC, and then the continuation of this flow into
the western limb of the gyre. The only shoal water in this province is the platform of
the Cape Verde Islands.
Defining Characteristics of Regional Oceanography
Here, we lack the wealth of information that is available for the provinces to the north
and for the equatorial ocean to the south. Perhaps because NATR consistently has the
lowest surface chlorophyll of the North Atlantic—well seen as a big blue hole in seasonal
chlorophyll images—it has attracted little attention from biological oceanographers. It
was, for instance, ignored by McClain et al. (1990) in their CZCS-based models of
phytoplankton growth in 11 boxes intended to characterize the regional ecology of the
North Atlantic. Like many other provinces that follow in this and later chapters, we shall
just have to do the best we can with what we have.
It is in this province that we first encounter the open ocean regions where phytoplankton biomass and productivity are minimal at the global scale. Although the usage is
incorrect oceanographic terminology, these regions are often known as the “subtropical
gyres,” as by McClain et al. (2004), who specify the seasonally changing size of each
“gyre” by reference to chlorophyll concentrations of >007 mg m
−3 . This isoline defines
a region that is very much smaller than the subtropical gyres with which we are familiar
in descriptive physical oceanography; this, of course, in the Atlantic includes flow around
both boundary currents together with flow in the Azores and the North Equatorial Current (NEC). I believe that this has led to some confusion in the past in how biological
oceanographers think about subtropical gyres. Although NATR is, ideally, one of the
regions studied so helpfully by MacClain et al. (see later discussion), it will not be useful
to restrict our attention to the much smaller box used by those authors.
The NATR lies below the trade-wind belt of the North Atlantic, separated from the
westerlies by the Azores High, which is often expressed as a series of high-pressure
cells lying southwest-northeast across the ocean at about 30
N and is best developed
in boreal summer. Thus, NATR comprises the ageostrophic, 200-m-deep, wind-driven
flow (Fiekas et al., 1992) around the southern half of the anticyclonic gyre of the North
Chapter 9: The Atlantic Ocean
the Pacific. The equilibrium time of tropical oceans to low-frequency wind forcing is
a function of how long the baroclinic waves require to pass along the equatorial wave
guide. The Pacific basin (15,000 km) is too wide for this to occur seasonally as it does
in the Atlantic (5000 km) although the seasonal changes in wind forcing are similar. In
the Pacific, therefore, westward tilting of the pycnocline occurs—and can only occur—
at the scale of ENSO events, with their long-sustained modification of regional wind
stress over the ocean (Longhurst, 1993). These facts have fundamental consequences for
the seasonality of phytoplankton growth—and all that stems therefrom—in the tropical
oceanic biome.
North Atlantic Tropical Gyral Province (NATR)
Extent of the Province
The North Atlantic Tropical Gyral Province (NATR) comprises that part of the North
Atlantic gyre that lies south of the subtropical convergence that runs zonally across the
ocean at about 30
N (see North Atlantic Subtropical Gyral Province). The southern
boundary of NATR is the limit of westerly flow along the thermocline ridge associated
with the North Equatorial Current (NEC) at about 12–14
N. The western boundary is
taken to be the edge of the coastal boundary biome seaward of the Antilles and the
Bahamas. NATR thus includes the offshore segment of the Canary Current south of the
Canaries, the flow of this into the NEC, and then the continuation of this flow into
the western limb of the gyre. The only shoal water in this province is the platform of
the Cape Verde Islands.
Defining Characteristics of Regional Oceanography
Here, we lack the wealth of information that is available for the provinces to the north
and for the equatorial ocean to the south. Perhaps because NATR consistently has the
lowest surface chlorophyll of the North Atlantic—well seen as a big blue hole in seasonal
chlorophyll images—it has attracted little attention from biological oceanographers. It
was, for instance, ignored by McClain et al. (1990) in their CZCS-based models of
phytoplankton growth in 11 boxes intended to characterize the regional ecology of the
North Atlantic. Like many other provinces that follow in this and later chapters, we shall
just have to do the best we can with what we have.
It is in this province that we first encounter the open ocean regions where phytoplankton biomass and productivity are minimal at the global scale. Although the usage is
incorrect oceanographic terminology, these regions are often known as the “subtropical
gyres,” as by McClain et al. (2004), who specify the seasonally changing size of each
“gyre” by reference to chlorophyll concentrations of >007 mg m
−3 . This isoline defines
a region that is very much smaller than the subtropical gyres with which we are familiar
in descriptive physical oceanography; this, of course, in the Atlantic includes flow around
both boundary currents together with flow in the Azores and the North Equatorial Current (NEC). I believe that this has led to some confusion in the past in how biological
oceanographers think about subtropical gyres. Although NATR is, ideally, one of the
regions studied so helpfully by MacClain et al. (see later discussion), it will not be useful
to restrict our attention to the much smaller box used by those authors.
The NATR lies below the trade-wind belt of the North Atlantic, separated from the
westerlies by the Azores High, which is often expressed as a series of high-pressure
cells lying southwest-northeast across the ocean at about 30
N and is best developed
in boreal summer. Thus, NATR comprises the ageostrophic, 200-m-deep, wind-driven
flow (Fiekas et al., 1992) around the southern half of the anticyclonic gyre of the North