Chapter 4: FINE STRUCTURE AND MICROSTRUCTURE
also referred to as the seasonal mixed layer wherever the climate has
seasons.
During daytime, a warmer diurnal mixed layer and a diurnal thermocline
form on the background of the mixed layer (X Figure 4-1X b), due to the
absorption of solar radiation in the upper ocean. From Chapter 1 the reader
knows that solar forcing
R
I z is a volume source of heat and strictly
speaking should not be considered as a surface flux. The surface flux
consists of latent ( E
Q ) and sensible ( T
Q ) heat fluxes and the net longwave
radiation ( L
I ). Wind-wave mixing and convection produce a diurnal mixed
layer. The diurnal mixed layer is clearly pronounced (as in the sketch shown
in X Figure 4-1X b) when QB 0 B , the sum of E
Q , T
Q , and L
I , is positive (i.e., net
surface cooling takes place) and its magnitude is not too small compared to
the solar heating.
For 0 0
Q (for example, in cases when air is warmer than water and
relative humidity is close to 100%), the diurnal mixed layer can hardly be
seen in temperature profiles (X Figure 4-1X c) but may still be resolved from
turbulence or gradient Richardson number profiles. Similar temperature
profiles can be observed under calm weather conditions during peak
insolation. In this case, the sea surface can lose heat to the atmosphere
( 0 0
Q ! ) but the volume source of heat (absorption of solar radiation) is so
strong that convective mixing ceases, and there is no surface mixed layer
(Soloviev and Lukas, 1997a).
Freshwater cycling may affect the ocean diurnal cycle in two ways: 1)
Evaporation from the sea surface or ice formation results in increased
surface salinity that adds to the convection due to surface cooling; 2)
Precipitation, river discharge, or sea ice melting inhibits convection. These
effects are considered elsewhere in this chapter (as well as in Chapter 2 in
relation to the sea surface microlayer).
The structure of the diurnal mixed layer and diurnal thermocline depends
on atmospheric forcing (wind speed, solar radiation, rain, heat fluxes), which
is generally spatially inhomogeneous. The internal processes associated with
pressure gradient equilibration and mass redistribution in the upper ocean are
often quasi-two-dimensional and may exhibit features of organization. As a
result, the diurnal thermocline can reveal spatial patterns in the form of
nonlinear internal waves, billows, fronts, etc. Examples of the vertical and
horizontal structure of the diurnal thermocline are given in the next section.
Spatially coherent organized structures in the near-surface layer of the ocean
are considered in Chapter 5.
221
also referred to as the seasonal mixed layer wherever the climate has
seasons.
During daytime, a warmer diurnal mixed layer and a diurnal thermocline
form on the background of the mixed layer (X Figure 4-1X b), due to the
absorption of solar radiation in the upper ocean. From Chapter 1 the reader
knows that solar forcing
R
I z is a volume source of heat and strictly
speaking should not be considered as a surface flux. The surface flux
consists of latent ( E
Q ) and sensible ( T
Q ) heat fluxes and the net longwave
radiation ( L
I ). Wind-wave mixing and convection produce a diurnal mixed
layer. The diurnal mixed layer is clearly pronounced (as in the sketch shown
in X Figure 4-1X b) when QB 0 B , the sum of E
Q , T
Q , and L
I , is positive (i.e., net
surface cooling takes place) and its magnitude is not too small compared to
the solar heating.
For 0 0
Q (for example, in cases when air is warmer than water and
relative humidity is close to 100%), the diurnal mixed layer can hardly be
seen in temperature profiles (X Figure 4-1X c) but may still be resolved from
turbulence or gradient Richardson number profiles. Similar temperature
profiles can be observed under calm weather conditions during peak
insolation. In this case, the sea surface can lose heat to the atmosphere
( 0 0
Q ! ) but the volume source of heat (absorption of solar radiation) is so
strong that convective mixing ceases, and there is no surface mixed layer
(Soloviev and Lukas, 1997a).
Freshwater cycling may affect the ocean diurnal cycle in two ways: 1)
Evaporation from the sea surface or ice formation results in increased
surface salinity that adds to the convection due to surface cooling; 2)
Precipitation, river discharge, or sea ice melting inhibits convection. These
effects are considered elsewhere in this chapter (as well as in Chapter 2 in
relation to the sea surface microlayer).
The structure of the diurnal mixed layer and diurnal thermocline depends
on atmospheric forcing (wind speed, solar radiation, rain, heat fluxes), which
is generally spatially inhomogeneous. The internal processes associated with
pressure gradient equilibration and mass redistribution in the upper ocean are
often quasi-two-dimensional and may exhibit features of organization. As a
result, the diurnal thermocline can reveal spatial patterns in the form of
nonlinear internal waves, billows, fronts, etc. Examples of the vertical and
horizontal structure of the diurnal thermocline are given in the next section.
Spatially coherent organized structures in the near-surface layer of the ocean
are considered in Chapter 5.
221