redistribution of salt and heat down from the EUC core within the thermo-halocline,
which, on the contrary, resists any mixing.
The fact of the reality of salt-fingering convection as a consequence of the
propagation of the high salinity flow of EUC, which occurred in the thermocline
below its core along the section at 33
o W on April, 2017 agrees with the results in
[4] along the EUC axis based on the data in 2010.
Conclusions
Our study showed that in the region of the Lomonosov EUC that transports water of
very high salinity, the vertical structure of the thermohaline field below the
boundaries of the current core (approximately between the depths of 150 and
500 m) is characterized by vertical inhomogeneities of the stepwise type. The
estimates of the vertical water stratification types there showed “oscillating”
numbers of thermohaline stability E θ, s with alternating thick intervals of the minimal positive to negative values (neutral to unstable stratification) and thin “peak”
intervals of positive stability. The high-gradient portions (“sheets”) of the steps with
negative T and S gradients were related to these peaks of higher stable stratification
and low values of density ratio 1 < R ≤ 2. Such intervals 3–4 to 10–15 m thick meet
the strict criteria of the development of double diffusion convection of the salt finger
type and provide evidence of the high probability of the development of vertical
convective mixing. It implies that in the course of the propagation of the
high-salinity current the fingering mechanism is responsible for the redistribution of
salt and heat from the core of the EUC to the deeper layers of the water column. The
manifestation of salt fingering effects is strongest at the southern station and
decreases from south to north of our section. Some indications of large horizontal
scale of the fingering processes are found.
Thus, as a result, we obtained clear evidence of numerous thin, convectively
unstable layers able to induce and maintain vertical mixing and redistribution of salt
and heat downward from the EUC core, within the main thermo-halocline that
resists any mixing. Our assessment of the spatial distribution of such unstable
intervals relative to depth and latitude with some evidence of their large horizontal
scale is also very important.
Here, we need to emphasize that convective mixing of the salt fingering type is a
part of the overall vertical mixing in the area, which also includes other convective
forms as well as the processes of the dynamic mechanisms of mixing, related to
turbulence, internal waves, and intense vertical shear. The importance of such types
of mixing at the depth intervals above and below the boundaries of the EUC cores
in the Atlantic and Pacific oceans is shown in [12–14, 20, 22–24, 27], etc. At the
same time, it was also found that the existence of a shear current not only does not
impede the existence of salt fingers, but it is even an additional mechanism of
facilitating conditions for their formation [27]. Thus, the convective instability in
the form of salt-fingering double diffusion in the region of the Lomonosov EUC
444
T. A. Demidova
which, on the contrary, resists any mixing.
The fact of the reality of salt-fingering convection as a consequence of the
propagation of the high salinity flow of EUC, which occurred in the thermocline
below its core along the section at 33
o W on April, 2017 agrees with the results in
[4] along the EUC axis based on the data in 2010.
Conclusions
Our study showed that in the region of the Lomonosov EUC that transports water of
very high salinity, the vertical structure of the thermohaline field below the
boundaries of the current core (approximately between the depths of 150 and
500 m) is characterized by vertical inhomogeneities of the stepwise type. The
estimates of the vertical water stratification types there showed “oscillating”
numbers of thermohaline stability E θ, s with alternating thick intervals of the minimal positive to negative values (neutral to unstable stratification) and thin “peak”
intervals of positive stability. The high-gradient portions (“sheets”) of the steps with
negative T and S gradients were related to these peaks of higher stable stratification
and low values of density ratio 1 < R ≤ 2. Such intervals 3–4 to 10–15 m thick meet
the strict criteria of the development of double diffusion convection of the salt finger
type and provide evidence of the high probability of the development of vertical
convective mixing. It implies that in the course of the propagation of the
high-salinity current the fingering mechanism is responsible for the redistribution of
salt and heat from the core of the EUC to the deeper layers of the water column. The
manifestation of salt fingering effects is strongest at the southern station and
decreases from south to north of our section. Some indications of large horizontal
scale of the fingering processes are found.
Thus, as a result, we obtained clear evidence of numerous thin, convectively
unstable layers able to induce and maintain vertical mixing and redistribution of salt
and heat downward from the EUC core, within the main thermo-halocline that
resists any mixing. Our assessment of the spatial distribution of such unstable
intervals relative to depth and latitude with some evidence of their large horizontal
scale is also very important.
Here, we need to emphasize that convective mixing of the salt fingering type is a
part of the overall vertical mixing in the area, which also includes other convective
forms as well as the processes of the dynamic mechanisms of mixing, related to
turbulence, internal waves, and intense vertical shear. The importance of such types
of mixing at the depth intervals above and below the boundaries of the EUC cores
in the Atlantic and Pacific oceans is shown in [12–14, 20, 22–24, 27], etc. At the
same time, it was also found that the existence of a shear current not only does not
impede the existence of salt fingers, but it is even an additional mechanism of
facilitating conditions for their formation [27]. Thus, the convective instability in
the form of salt-fingering double diffusion in the region of the Lomonosov EUC
444
T. A. Demidova