THE NEAR-SURFACE LAYER OF THE OCEAN
Brief conclusions for Section 5.3 are as follows:
1) The TSG data from the western Pacific warm pool reveal the k x
-3
spectral subrange.
2) The hypothesis of an equilibrium spectrum leads to wavenumber
dependence E(k x ) ~ k x
-3 , which is consistent with the field data.
3) Numerical experiments illustrate how nonlinear diffusion transforms
initial horizontal density anomalies into conic structures with spike
type buoyancy curvature at their corners, which leads to a k x
-3
wavenumber spectrum.
4) For the equilibrium state, the nonlinear diffusion problem then
reduces to a linear problem with the constant horizontal diffusion
coefficient.
5) The increased horizontal curvature of buoyancy drives
frontogenesis.
The last conclusion thus links the process of nonlinear diffusion to the
problem of oceanic fronts. As we have seen from previous sections of this
chapter, the process of self-organization involves a cascade of energy from
smaller to larger scales. At the same time, horizontal gradients must
simultaneously increase to satisfy the conservation law (5.3). One
interpretation is that in the process of self-organization, boundaries of the
spatially coherent organized structures become sharper. The data presented
in the next section indeed show that sharp frontal interfaces are an
observable feature of the near-surface layer of the ocean. The FY97 model
is, however, hydrostatic and is not capable of simulating fronts.
5.4 Sharp Frontal Interfaces
Recent global surveys using the Pathfinder SST dataset reveal persistent
fronts in many parts of the World Ocean (Belkin et al., 2001). Oceanic fronts
have been linked to the process of subduction, where one water-mass sinks
below another without substantial mixing (see for instance Rudnick and
Luyten, 1996). Subduction appears to be an important process in maintaining
the salt-stratified barrier layer often found below the warm, fresh mixed
layer of the western Pacific warm pool (Lukas and Lindstrom, 1991;
Shinoda and Lukas, 1995; Tomczak, 1995; You, 1995; Ando and McPhaden,
1997; Vialard and Delecluse, 1998).
Woods (1980) and Fedorov (1986) linked ocean fronts to dissipation of
large-scale and mesoscale horizontal inhomogeneities of the physical fields
in the upper ocean. Fronts sometimes are so narrow that in images from
space they may look like “cracks” on the sea surface (Figure 5-16).
312
Brief conclusions for Section 5.3 are as follows:
1) The TSG data from the western Pacific warm pool reveal the k x
-3
spectral subrange.
2) The hypothesis of an equilibrium spectrum leads to wavenumber
dependence E(k x ) ~ k x
-3 , which is consistent with the field data.
3) Numerical experiments illustrate how nonlinear diffusion transforms
initial horizontal density anomalies into conic structures with spike
type buoyancy curvature at their corners, which leads to a k x
-3
wavenumber spectrum.
4) For the equilibrium state, the nonlinear diffusion problem then
reduces to a linear problem with the constant horizontal diffusion
coefficient.
5) The increased horizontal curvature of buoyancy drives
frontogenesis.
The last conclusion thus links the process of nonlinear diffusion to the
problem of oceanic fronts. As we have seen from previous sections of this
chapter, the process of self-organization involves a cascade of energy from
smaller to larger scales. At the same time, horizontal gradients must
simultaneously increase to satisfy the conservation law (5.3). One
interpretation is that in the process of self-organization, boundaries of the
spatially coherent organized structures become sharper. The data presented
in the next section indeed show that sharp frontal interfaces are an
observable feature of the near-surface layer of the ocean. The FY97 model
is, however, hydrostatic and is not capable of simulating fronts.
5.4 Sharp Frontal Interfaces
Recent global surveys using the Pathfinder SST dataset reveal persistent
fronts in many parts of the World Ocean (Belkin et al., 2001). Oceanic fronts
have been linked to the process of subduction, where one water-mass sinks
below another without substantial mixing (see for instance Rudnick and
Luyten, 1996). Subduction appears to be an important process in maintaining
the salt-stratified barrier layer often found below the warm, fresh mixed
layer of the western Pacific warm pool (Lukas and Lindstrom, 1991;
Shinoda and Lukas, 1995; Tomczak, 1995; You, 1995; Ando and McPhaden,
1997; Vialard and Delecluse, 1998).
Woods (1980) and Fedorov (1986) linked ocean fronts to dissipation of
large-scale and mesoscale horizontal inhomogeneities of the physical fields
in the upper ocean. Fronts sometimes are so narrow that in images from
space they may look like “cracks” on the sea surface (Figure 5-16).
312