counter-current flow as a jet of salty water moving in the background environment.
At the same time, the mixing occurring below its core should be considered as
processes developing more or less continuously due to the turbulence and other
mechanisms.
In a flow similar to the EUC with vertical density gradients and mean vertical
shears, stability may be expressed as the ratio of the buoyancy effects of the density
change and the vertical shear arising due to the inertial effects caused by the motion
of this flow [14]. This ratio is usually expressed as the Richardson number. The
effects of the contribution of the buoyancy are given in terms of the local
Brunt-Väisälä frequency, which characterizes the static (thermohaline) stability.
The dynamic stability is defined by the mean vertical shear. This approach to
determination of stability and the mixing ability is widely used in the publications
on vertical mixing. At the same time, gravitationally stable salinity-temperature
stratification of the ocean water column may actually be unstable due to the double
diffusion convection that occurs owing to the fact that the molecular diffusivity of
heat is much greater than the diffusivity of salt [10, 11, 29, 31].
We keep the latter in mind and emphasize that the vertical thermohaline field
across the Lomonosov undercurrent is characterized by extremely high gradients of
salinity. They arise owing to the existence of the high-gradient upper halocline with
salinity in the core over 36.7 psu, while under the halocline (and above it) there is
water of relatively low salinity: 34.5–35.5 psu. The occurrence of inversion and
steps of different scales confirms the assumption of the unstable character of water
under the upper sharp halocline-thermocline. Thus, owing to continuous delivery of
high salinity water by the flow of EUC, the especially favorable conditions for
development of the convective instability and processes of convective mixing in the
form of double diffusion may exist.
As the convective mixing is caused by the local decrease in density with depth, it
propagates vertically and develops regardless the layers move or not. Therefore,
realizing inevitable influence of the turbulence and shear effects on the vertical
mixing in the case of a rapid flow, as well as the influence of internal waves, we
allocate and address a question of the convective instability leading to the vertical
convective diffusion in the form of salt fingers.
Some aspects of convective mixing in the equatorial undercurrents were
addressed in some early publications (for example, [24]). However, to the author’s
knowledge, the first comprehensive study of the equatorial convective mixing in the
Atlantic EUC started in [4]. The authors used the data from the western equatorial
Atlantic in 2010 and considered the mixing process along the countercurrent flow
axis from its sources at 43° W in the west up to 32° W in the east. They have shown
evidence that along this distance the double-diffusive mixing in the form of salt
fingers took place; and its intensity varied with longitude. They emphasize the
importance of “the combination of horizontal advection and vertical instability” in
intense “sequence of downward-convective events” probably occurring there.
In this research we follow [4] and contribute to the deep studying of fine thermohaline structures and vertical convective mixing processes in the thermocline
equatorial undercurrent area on the basis of our shipboard sADCP and CTD-cast
430
T. A. Demidova
At the same time, the mixing occurring below its core should be considered as
processes developing more or less continuously due to the turbulence and other
mechanisms.
In a flow similar to the EUC with vertical density gradients and mean vertical
shears, stability may be expressed as the ratio of the buoyancy effects of the density
change and the vertical shear arising due to the inertial effects caused by the motion
of this flow [14]. This ratio is usually expressed as the Richardson number. The
effects of the contribution of the buoyancy are given in terms of the local
Brunt-Väisälä frequency, which characterizes the static (thermohaline) stability.
The dynamic stability is defined by the mean vertical shear. This approach to
determination of stability and the mixing ability is widely used in the publications
on vertical mixing. At the same time, gravitationally stable salinity-temperature
stratification of the ocean water column may actually be unstable due to the double
diffusion convection that occurs owing to the fact that the molecular diffusivity of
heat is much greater than the diffusivity of salt [10, 11, 29, 31].
We keep the latter in mind and emphasize that the vertical thermohaline field
across the Lomonosov undercurrent is characterized by extremely high gradients of
salinity. They arise owing to the existence of the high-gradient upper halocline with
salinity in the core over 36.7 psu, while under the halocline (and above it) there is
water of relatively low salinity: 34.5–35.5 psu. The occurrence of inversion and
steps of different scales confirms the assumption of the unstable character of water
under the upper sharp halocline-thermocline. Thus, owing to continuous delivery of
high salinity water by the flow of EUC, the especially favorable conditions for
development of the convective instability and processes of convective mixing in the
form of double diffusion may exist.
As the convective mixing is caused by the local decrease in density with depth, it
propagates vertically and develops regardless the layers move or not. Therefore,
realizing inevitable influence of the turbulence and shear effects on the vertical
mixing in the case of a rapid flow, as well as the influence of internal waves, we
allocate and address a question of the convective instability leading to the vertical
convective diffusion in the form of salt fingers.
Some aspects of convective mixing in the equatorial undercurrents were
addressed in some early publications (for example, [24]). However, to the author’s
knowledge, the first comprehensive study of the equatorial convective mixing in the
Atlantic EUC started in [4]. The authors used the data from the western equatorial
Atlantic in 2010 and considered the mixing process along the countercurrent flow
axis from its sources at 43° W in the west up to 32° W in the east. They have shown
evidence that along this distance the double-diffusive mixing in the form of salt
fingers took place; and its intensity varied with longitude. They emphasize the
importance of “the combination of horizontal advection and vertical instability” in
intense “sequence of downward-convective events” probably occurring there.
In this research we follow [4] and contribute to the deep studying of fine thermohaline structures and vertical convective mixing processes in the thermocline
equatorial undercurrent area on the basis of our shipboard sADCP and CTD-cast
430
T. A. Demidova