Chapter 5. SPATIALLY-COHERENT STRUCTURES
Munk and Armi (2001) proposed the explanation for spiral structures
illustrated in Figure 5-6. The local frontogenesis process enhances the
geostrophically balanced ambient ocean vorticity (which is of order
1
10 f
r
) and concentrates surfactants along a converging line. When the
frontal shear becomes comparable to f, instabilities lead to cross-frontal flow
accompanied by a twisting, cat’s-eye circulation pattern. The cat’s-eye
circulation twists the convergence line and neighboring linear features into a
cyclonic spiral, while stretching and further thinning the lines of surfactant
concentration.
Near the Equator, thin lines on the sea surface rather than spirals are
observed (Figure 5-16), due to diminished planetary vorticity. The next two
sections, Section 5.3 and Section 5.4, elucidate the role of nonlinear
interactions in the formation of organized structures in the near-surface
ocean for the example of equatorial warm pools.
5.3 Horizontal Mixing as a Nonlinear Diffusion Process
Atmospheric convective complexes dominate the planetary boundary
layer in the warm pool area and are responsible for the 3-5 m yr
-1 of
precipitation. Convective rains produce surface puddles of the order of 10
km diameter containing appreciable salinity, temperature, and density
anomalies. These puddles store significant potential energy, which when
released in the form of gravity currents substantially contributes to mixing.
This process is associated with vertical shearing, which involves nonlinear
dynamics. Since it is a two-dimensional system, the initial disturbances have
a tendency to self-organization.
5.3.1 Horizontal wave number statistics
We start our discussion from the analysis of some observable features of
the horizontal wavenumber statistics in the warm pool area. Figure 5-7
shows a record from a shipboard thermosalinograph (TSG) in the western
Pacific warm pool obtained during TOGA COARE from a depth of 3 m
(Delcroix et al., 1993). Rainfall was observed between 3
o N and 3.5
o N. This
rainfall produced a localized salinity anomaly. In the temperature record, the
freshwater puddle did not produce any prominent feature.
Figure 5-7 represents one of 16 sections of the R/V Le Noroit made
from 5
o S to 5
o N along 156
o E during TOGA COARE from December 1992
through March 1993. The average density spectrum calculated from all 16
sections of the TSG data is shown in Figure 5-8. In the wavenumber
range
4
4 10
k
m
-1 , the experimental spectrum is approximated by a k
-1
dependence, which is consistent with the k
-1 law that follows from the theory
of quasi-geostrophic two-dimensional turbulence for a passive tracer
295
Munk and Armi (2001) proposed the explanation for spiral structures
illustrated in Figure 5-6. The local frontogenesis process enhances the
geostrophically balanced ambient ocean vorticity (which is of order
1
10 f
r
) and concentrates surfactants along a converging line. When the
frontal shear becomes comparable to f, instabilities lead to cross-frontal flow
accompanied by a twisting, cat’s-eye circulation pattern. The cat’s-eye
circulation twists the convergence line and neighboring linear features into a
cyclonic spiral, while stretching and further thinning the lines of surfactant
concentration.
Near the Equator, thin lines on the sea surface rather than spirals are
observed (Figure 5-16), due to diminished planetary vorticity. The next two
sections, Section 5.3 and Section 5.4, elucidate the role of nonlinear
interactions in the formation of organized structures in the near-surface
ocean for the example of equatorial warm pools.
5.3 Horizontal Mixing as a Nonlinear Diffusion Process
Atmospheric convective complexes dominate the planetary boundary
layer in the warm pool area and are responsible for the 3-5 m yr
-1 of
precipitation. Convective rains produce surface puddles of the order of 10
km diameter containing appreciable salinity, temperature, and density
anomalies. These puddles store significant potential energy, which when
released in the form of gravity currents substantially contributes to mixing.
This process is associated with vertical shearing, which involves nonlinear
dynamics. Since it is a two-dimensional system, the initial disturbances have
a tendency to self-organization.
5.3.1 Horizontal wave number statistics
We start our discussion from the analysis of some observable features of
the horizontal wavenumber statistics in the warm pool area. Figure 5-7
shows a record from a shipboard thermosalinograph (TSG) in the western
Pacific warm pool obtained during TOGA COARE from a depth of 3 m
(Delcroix et al., 1993). Rainfall was observed between 3
o N and 3.5
o N. This
rainfall produced a localized salinity anomaly. In the temperature record, the
freshwater puddle did not produce any prominent feature.
Figure 5-7 represents one of 16 sections of the R/V Le Noroit made
from 5
o S to 5
o N along 156
o E during TOGA COARE from December 1992
through March 1993. The average density spectrum calculated from all 16
sections of the TSG data is shown in Figure 5-8. In the wavenumber
range
4
4 10
k
m
-1 , the experimental spectrum is approximated by a k
-1
dependence, which is consistent with the k
-1 law that follows from the theory
of quasi-geostrophic two-dimensional turbulence for a passive tracer
295