THE NEAR-SURFACE LAYER OF THE OCEAN
When the surface solar irradiance drops below a certain level that
depends on wind speed and surface heat fluxes, the diurnal mixed layer starts
its evening deepening (Phase IV). The positive buoyancy flux can no longer
stabilize the diurnal mixed layer and cannot maintain slippery conditions on
its lower boundary. The diurnal jet releases its kinetic energy, which results
in the Kelvin-Helmholtz type instability followed by overturning events
(billows). The diurnal mixed layer and diurnal thermocline rapidly deepen.
The intensive deepening of the diurnal thermocline is often associated with
jumps of temperature between the diurnal mixed layer and diurnal
thermocline. These jumps result from the Kelvin-Helmholtz instability
accompanied by overturning events.
After reaching a depth of several meters the excess kinetic energy of the
diurnal jet is pretty much spent on entrainment of colder water from below
the thermocline; the deepening of the diurnal mixed layer and thermocline
slows down. At this stage the diurnal thermocline mainly erodes from its top
due to convective cooling from the ocean surface (Phase V). Turbulent
entrainment is relatively small in this phase.
In order to illustrate the above classification, X Figure 4-17 shows a series
of temperature profiles characterizing the diurnal warming of the subtropical
ocean under conditions of low wind speed and strong insolation. This series
of measurements was made with a free-rising profiler equipped with highresolution temperature and conductivity sensors. The cold film temperature
sensor (DISA) had a response time in water of ~3 milliseconds. For the
248
Figure 4-17. Vertical profiles of temperature in the upper 10 m of the Atlantic Ocean at 28P
o
PN,
21P
o
PW.
Adapted from Soloviev and Vershinsky (1982) with permission of Elsevier. Local times
are indicated at the bottom of each profile.
When the surface solar irradiance drops below a certain level that
depends on wind speed and surface heat fluxes, the diurnal mixed layer starts
its evening deepening (Phase IV). The positive buoyancy flux can no longer
stabilize the diurnal mixed layer and cannot maintain slippery conditions on
its lower boundary. The diurnal jet releases its kinetic energy, which results
in the Kelvin-Helmholtz type instability followed by overturning events
(billows). The diurnal mixed layer and diurnal thermocline rapidly deepen.
The intensive deepening of the diurnal thermocline is often associated with
jumps of temperature between the diurnal mixed layer and diurnal
thermocline. These jumps result from the Kelvin-Helmholtz instability
accompanied by overturning events.
After reaching a depth of several meters the excess kinetic energy of the
diurnal jet is pretty much spent on entrainment of colder water from below
the thermocline; the deepening of the diurnal mixed layer and thermocline
slows down. At this stage the diurnal thermocline mainly erodes from its top
due to convective cooling from the ocean surface (Phase V). Turbulent
entrainment is relatively small in this phase.
In order to illustrate the above classification, X Figure 4-17 shows a series
of temperature profiles characterizing the diurnal warming of the subtropical
ocean under conditions of low wind speed and strong insolation. This series
of measurements was made with a free-rising profiler equipped with highresolution temperature and conductivity sensors. The cold film temperature
sensor (DISA) had a response time in water of ~3 milliseconds. For the
248
Figure 4-17. Vertical profiles of temperature in the upper 10 m of the Atlantic Ocean at 28P
o
PN,
21P
o
PW.
Adapted from Soloviev and Vershinsky (1982) with permission of Elsevier. Local times
are indicated at the bottom of each profile.