Chapter 4: FINE STRUCTURE AND MICROSTRUCTURE
The subsequent temperature profiles demonstrate development (1452
LST) and evening deepening (1622 and 2121 LST) of the diurnal mixed
layer and diurnal thermocline. These are Phase IV (16 22 LST) and Phase V
(2121 LST). At 0418 LST next morning, the diurnal thermocline could not
be seen within the upper 10 m. A slight temperature inversion near the
surface was related to the convective cooling of the ocean surface. This is
Phase I. The temperature profile taken after sunrise (0838 LST) indicated
that a new warm layer had begun to form (Phase II).
The section of temperature profile marked by rectangle a in X Figure 4-17 is
shown in X Figure 4-18 in more detail (including conductivity, salinity, and
density). There are numerous small-scale temperature inhomogeneities
within the diurnal thermocline, associated with intermittent turbulent mixing,
which can be classified as microstructure. Local vertical gradients of
temperature reach 6P
o
PC
mP
-1
P.
Above and below the diurnal thermocline, the
temperature gradients are considerably smaller.
The conductivity profile in X Figure 4-18 almost repeats the corresponding
temperature profile. The conductivity profile reveals finer structure than the
temperature profile, mainly because of a better spatial resolution of the
conductivity sensor. These small-scale features partially disappear in the
salinity profiles because, in order to avoid spikes, salinity was calculated
from smoothed (over 1 cm) temperature and conductivity signals.
The salinity profile in X Figure 4-18 shows a small (~0.02-psu) increase of
salinity toward the surface, associated with evaporation from the ocean
surface. The salinity stratification is unstable and contributes to convective
mixing within the diurnal mixed layer. Within the diurnal thermocline, the
unstable salinity stratification is compensated by the stable stratification of
temperature; the density profile is dominated by the temperature contribution
and is therefore stable. Strong stratification in the diurnal thermocline makes
it more difficult for the excess near-surface salinity to penetrate into deeper
layers. This results in a slightly elevated level of salinity within the diurnal
mixed layer and, in part, within the diurnal thermocline. In this situation
(stable temperature and unstable salinity stratification), double diffusion
convection may develop in the form of salt fingers.
The section of the temperature profile marked by rectangle b (Figure
4-17X ) is shown in X Figure 4-19 in more detail. In this example, the diurnal
thermocline reveals a remarkable step-like structure. This profile was taken
at the beginning of the evening deepening of the diurnal thermocline (Phase
IV). Step-like structures often appear in the diurnal thermocline in this phase
of the diurnal cycle. The next temperature profile taken at 16:22 also reveals
step-like structures but with larger vertical scale (X Figure 4-17). In Section
5.5, we consider two possible mechanisms leading to overturning and steplike structure in the diurnal thermocline, which are the Kelvin-Helmholtz (K251
The subsequent temperature profiles demonstrate development (1452
LST) and evening deepening (1622 and 2121 LST) of the diurnal mixed
layer and diurnal thermocline. These are Phase IV (16 22 LST) and Phase V
(2121 LST). At 0418 LST next morning, the diurnal thermocline could not
be seen within the upper 10 m. A slight temperature inversion near the
surface was related to the convective cooling of the ocean surface. This is
Phase I. The temperature profile taken after sunrise (0838 LST) indicated
that a new warm layer had begun to form (Phase II).
The section of temperature profile marked by rectangle a in X Figure 4-17 is
shown in X Figure 4-18 in more detail (including conductivity, salinity, and
density). There are numerous small-scale temperature inhomogeneities
within the diurnal thermocline, associated with intermittent turbulent mixing,
which can be classified as microstructure. Local vertical gradients of
temperature reach 6P
o
PC
mP
-1
P.
Above and below the diurnal thermocline, the
temperature gradients are considerably smaller.
The conductivity profile in X Figure 4-18 almost repeats the corresponding
temperature profile. The conductivity profile reveals finer structure than the
temperature profile, mainly because of a better spatial resolution of the
conductivity sensor. These small-scale features partially disappear in the
salinity profiles because, in order to avoid spikes, salinity was calculated
from smoothed (over 1 cm) temperature and conductivity signals.
The salinity profile in X Figure 4-18 shows a small (~0.02-psu) increase of
salinity toward the surface, associated with evaporation from the ocean
surface. The salinity stratification is unstable and contributes to convective
mixing within the diurnal mixed layer. Within the diurnal thermocline, the
unstable salinity stratification is compensated by the stable stratification of
temperature; the density profile is dominated by the temperature contribution
and is therefore stable. Strong stratification in the diurnal thermocline makes
it more difficult for the excess near-surface salinity to penetrate into deeper
layers. This results in a slightly elevated level of salinity within the diurnal
mixed layer and, in part, within the diurnal thermocline. In this situation
(stable temperature and unstable salinity stratification), double diffusion
convection may develop in the form of salt fingers.
The section of the temperature profile marked by rectangle b (Figure
4-17X ) is shown in X Figure 4-19 in more detail. In this example, the diurnal
thermocline reveals a remarkable step-like structure. This profile was taken
at the beginning of the evening deepening of the diurnal thermocline (Phase
IV). Step-like structures often appear in the diurnal thermocline in this phase
of the diurnal cycle. The next temperature profile taken at 16:22 also reveals
step-like structures but with larger vertical scale (X Figure 4-17). In Section
5.5, we consider two possible mechanisms leading to overturning and steplike structure in the diurnal thermocline, which are the Kelvin-Helmholtz (K251