Chapter 5. SPATIALLY-COHERENT STRUCTURES
At 16:37L (Figure 5-39), the beginning of the evening deepening of the
diurnal thermocline, vertical profiles reveal increased scatter. This time
interval is analyzed further in the contour plot shown in Figure 5-40. The
corresponding section of the records is denoted in Figure 5-38 by a
rectangle. There are high-frequency oscillations of T, S and V t in this record
(Figure 5-40, b-d) because of the depth variation of the probes (Figure
5-40a). There are also variations in temperature of about 2
o C in b, which are
visible over several kilometers. The t p
V contour plot (Figure 5-40d)
reveals wavelike disturbances of the diurnal thermocline with a typical scale
beginning from ~200 m.
The
t
T V
contour plot in Figure 5-40e reveals an asymmetric anomaly
between 5300 m and 6000 m associated with a salinity feature (Figure
5-40d). It is not clear, however, whether it has any connection with the largeamplitude shortwave train observed in the t p
V contour plot (Figure
5-40d).
The t p
V contour plot in Figure 5-40d shows an intermittent wavelike
pattern. The intermittence in space and time, with energy predominantly in
the first mode, is a typical feature of short-period internal waves trapped in
the thermocline that is close to the surface (Brekhovskikh et al., 1975).
There is also asymmetry in the perturbations in Figure 5-40d. When a
thermocline is close to the surface, it is typical for the wave crest to flatten
and the troughs to become sharper (Thorpe, 1968).
These observations revealed a near-surface spatial structure that looks
like a packet of large amplitude internal waves. For the data shown in Figure
5-40d the amplitude of the wavelike perturbations is much larger than the
uncertainty of the pressure-to-depth conversion (~0.1 m). The 30-day data
set analyzed by Soloviev and Lukas (1996) contained several other cases of
large amplitude internal waves within the diurnal thermocline to the
amplitude comparable with the depth of the thermocline.
For comparison, Voropaev et al. (1981) observed a wave-like
perturbation of the diurnal thermocline of ~200 m length and ~1 m height
under 5 m s
-1 wind speed. Imberger (1985) presented detailed near-surface
measurements from a freshwater lake during 21.5 hours, including
observation of internal waves of ~1 m amplitude and billows associated with
the diurnal thermocline.
Wijesekera and Dillon (1991) and Lien et al. (1996) observed packets of
internal waves with approximately 200 m wavelength associated with
diurnal cycling. Interestingly, Wijesekera and Dillon (1991) found for a
large-amplitude wave packet that the wave-induced Reynolds stress below
the mixed layer was of the same order of magnitude as the surface wind
stress.
349
At 16:37L (Figure 5-39), the beginning of the evening deepening of the
diurnal thermocline, vertical profiles reveal increased scatter. This time
interval is analyzed further in the contour plot shown in Figure 5-40. The
corresponding section of the records is denoted in Figure 5-38 by a
rectangle. There are high-frequency oscillations of T, S and V t in this record
(Figure 5-40, b-d) because of the depth variation of the probes (Figure
5-40a). There are also variations in temperature of about 2
o C in b, which are
visible over several kilometers. The t p
V contour plot (Figure 5-40d)
reveals wavelike disturbances of the diurnal thermocline with a typical scale
beginning from ~200 m.
The
t
T V
contour plot in Figure 5-40e reveals an asymmetric anomaly
between 5300 m and 6000 m associated with a salinity feature (Figure
5-40d). It is not clear, however, whether it has any connection with the largeamplitude shortwave train observed in the t p
V contour plot (Figure
5-40d).
The t p
V contour plot in Figure 5-40d shows an intermittent wavelike
pattern. The intermittence in space and time, with energy predominantly in
the first mode, is a typical feature of short-period internal waves trapped in
the thermocline that is close to the surface (Brekhovskikh et al., 1975).
There is also asymmetry in the perturbations in Figure 5-40d. When a
thermocline is close to the surface, it is typical for the wave crest to flatten
and the troughs to become sharper (Thorpe, 1968).
These observations revealed a near-surface spatial structure that looks
like a packet of large amplitude internal waves. For the data shown in Figure
5-40d the amplitude of the wavelike perturbations is much larger than the
uncertainty of the pressure-to-depth conversion (~0.1 m). The 30-day data
set analyzed by Soloviev and Lukas (1996) contained several other cases of
large amplitude internal waves within the diurnal thermocline to the
amplitude comparable with the depth of the thermocline.
For comparison, Voropaev et al. (1981) observed a wave-like
perturbation of the diurnal thermocline of ~200 m length and ~1 m height
under 5 m s
-1 wind speed. Imberger (1985) presented detailed near-surface
measurements from a freshwater lake during 21.5 hours, including
observation of internal waves of ~1 m amplitude and billows associated with
the diurnal thermocline.
Wijesekera and Dillon (1991) and Lien et al. (1996) observed packets of
internal waves with approximately 200 m wavelength associated with
diurnal cycling. Interestingly, Wijesekera and Dillon (1991) found for a
large-amplitude wave packet that the wave-induced Reynolds stress below
the mixed layer was of the same order of magnitude as the surface wind
stress.
349