58
Chapter 4: Physical Control of Ecological Processes
Regional and Latitudinal Resistance
to Mixing in the Open Oceans
In this section I shall attempt to locate such discontinuities, starting with those that
may occur between the strong pycnocline of low latitudes and the weaker pycnoclines of
higher latitudes. To do this the distribution of the buoyancy or Brunt-Väisälä frequency
(N ) was examined in all oceans, with Java OceanAtlas (Osborne et al., 1992). In this
way, a maximum value for N (and hence of maximum resistance of the pycnocline
to turbulent mixing), and the depth at which it occurred in the water column, was
obtained at 1
intervals along each of 19 long sections (11 meridional and 8 zonal) from
oceanographic voyages and from 10 meridional sections synthesized from the accumulated
profile data at the U.S. National Oceanographic Data Center in Washington, DC (Levitus,
1982; Levitus et al., 1994; Levitus and Boyer, 1994a,b).
The 25
W Atlantic section that was worked by Oceanus and Melville in 1988–1989,
from Iceland to the Southern Ocean (Fig. 4.1), will serve to illustrate this approach to
the problem of locating discontinuities in the resistance of the pycnocline to mixing.
Obtained during summer in each hemisphere, it shows clearly the distinction between the
permanent tropical pycnocline, where N takes values of about 12–14 cycles hr
−1 , and the
shoaler summer thermocline (7 or 8 cycles hr
−1 ) of the subtropical gyres. The transition
occurs across the equatorward limbs of the subtropical gyres in each hemisphere; the
North Equatorial Countercurrent and the main flow of the South Equatorial Current
both lie above the tropical pycnocline, which thus lies somewhat asymmetrically to the
north in relation to the equator. The actual depth and slope of the maximum value of N
reflects (as does the depth of the pycnocline itself) the flow of the zonal currents within
the Ekman layer.
The same features are readily identifiable in climatological sections derived from
archived data along 30
W, though the seasonal summer thermocline is weaker (as it
should be in archived data that represents all seasons), taking a value of only about
5 cycles hr
−1 . The strong tropical pycnocline from 20
N to 20
S is clearer (as it should be)
in the archived data and takes the same values of N as in the Oceanus–Melville section.
Furthermore, it lies in the same relationship to the zonal tropical current systems, inferred
from the depth of the pycnocline. Because the archived data extend further poleward
they better show the effects of the low-salinity polar surface layer. At 60
N, as the section
passes over the Irminger Basin, and at 40
S across the South Subtropical Convergence
Zone, the value of N increases progressively poleward as the pycnocline approaches the
surface toward the ice edge.
This comparison gives us confidence that the Levitus sections may be used to explore
systematically the strength and depth of N at 30
latitude intervals in all oceans. Such an
investigation confirms that, by reason of its stability and depth, the tropical pycnocline
is a unique feature in all oceans and, moreover, that the transition to this feature tends
to occur at about 20
of latitude (Longhurst, 1995).
What else can be discerned about the tropical pycnocline? Certainly, the data suggest
that the relatively abrupt transition between the weak subtropical and the strong tropical
pycnocline (which is a prominent feature in most meridional sections) does not occur
always at the edge of the equatorial zonal currents, as one might expect it to do. Consider
again the Oceanus–Melville section: in the North Atlantic, the transition occurs clearly at
about 22–23
N, coincident with the northern margin of the North Equatorial Current
(NEC) flowing west around the equatorward limb of the subtropical gyre. However, in
the South Atlantic no such relationship exists, perhaps because the South Equatorial
Current (SEC) is wider and more diffuse than the NEC.
Chapter 4: Physical Control of Ecological Processes
Regional and Latitudinal Resistance
to Mixing in the Open Oceans
In this section I shall attempt to locate such discontinuities, starting with those that
may occur between the strong pycnocline of low latitudes and the weaker pycnoclines of
higher latitudes. To do this the distribution of the buoyancy or Brunt-Väisälä frequency
(N ) was examined in all oceans, with Java OceanAtlas (Osborne et al., 1992). In this
way, a maximum value for N (and hence of maximum resistance of the pycnocline
to turbulent mixing), and the depth at which it occurred in the water column, was
obtained at 1
intervals along each of 19 long sections (11 meridional and 8 zonal) from
oceanographic voyages and from 10 meridional sections synthesized from the accumulated
profile data at the U.S. National Oceanographic Data Center in Washington, DC (Levitus,
1982; Levitus et al., 1994; Levitus and Boyer, 1994a,b).
The 25
W Atlantic section that was worked by Oceanus and Melville in 1988–1989,
from Iceland to the Southern Ocean (Fig. 4.1), will serve to illustrate this approach to
the problem of locating discontinuities in the resistance of the pycnocline to mixing.
Obtained during summer in each hemisphere, it shows clearly the distinction between the
permanent tropical pycnocline, where N takes values of about 12–14 cycles hr
−1 , and the
shoaler summer thermocline (7 or 8 cycles hr
−1 ) of the subtropical gyres. The transition
occurs across the equatorward limbs of the subtropical gyres in each hemisphere; the
North Equatorial Countercurrent and the main flow of the South Equatorial Current
both lie above the tropical pycnocline, which thus lies somewhat asymmetrically to the
north in relation to the equator. The actual depth and slope of the maximum value of N
reflects (as does the depth of the pycnocline itself) the flow of the zonal currents within
the Ekman layer.
The same features are readily identifiable in climatological sections derived from
archived data along 30
W, though the seasonal summer thermocline is weaker (as it
should be in archived data that represents all seasons), taking a value of only about
5 cycles hr
−1 . The strong tropical pycnocline from 20
N to 20
S is clearer (as it should be)
in the archived data and takes the same values of N as in the Oceanus–Melville section.
Furthermore, it lies in the same relationship to the zonal tropical current systems, inferred
from the depth of the pycnocline. Because the archived data extend further poleward
they better show the effects of the low-salinity polar surface layer. At 60
N, as the section
passes over the Irminger Basin, and at 40
S across the South Subtropical Convergence
Zone, the value of N increases progressively poleward as the pycnocline approaches the
surface toward the ice edge.
This comparison gives us confidence that the Levitus sections may be used to explore
systematically the strength and depth of N at 30
latitude intervals in all oceans. Such an
investigation confirms that, by reason of its stability and depth, the tropical pycnocline
is a unique feature in all oceans and, moreover, that the transition to this feature tends
to occur at about 20
of latitude (Longhurst, 1995).
What else can be discerned about the tropical pycnocline? Certainly, the data suggest
that the relatively abrupt transition between the weak subtropical and the strong tropical
pycnocline (which is a prominent feature in most meridional sections) does not occur
always at the edge of the equatorial zonal currents, as one might expect it to do. Consider
again the Oceanus–Melville section: in the North Atlantic, the transition occurs clearly at
about 22–23
N, coincident with the northern margin of the North Equatorial Current
(NEC) flowing west around the equatorward limb of the subtropical gyre. However, in
the South Atlantic no such relationship exists, perhaps because the South Equatorial
Current (SEC) is wider and more diffuse than the NEC.