measurements across the EUC. We focus on the detailed discussion of vertical
thermohaline stratification of the section and on detecting favorable conditions for
the development of double diffusion and salt fingering. In order to reveal such
conditions we calculated the thermohaline stability parameters of the subsurface
water. The results provided us with a reliable pattern of spatial distribution of
convectively unstable intervals that were closely related in depth to thin
high-gradient parts of the stepwise structures of the fields. Such unstable layers that
are capable to maintain and promote vertical mixing, redistributing salt and heat
downward from the EUC core, were located within the thermocline layer, which
resists mixing.
The contribution is organized as follows. In section “Data and Methods”, we
discuss the data used for the estimates and analysis. Then, in section “Results and
Discussion”, we consider the results of sADCP describing the circulation features
of the EUC and the results of CTD casts discussing the horizontal and vertical
structure of the thermohaline field across the EUC as well as relation between
thermohaline and velocity fields. In section “Conclusions”, we consider briefly the
criteria of vertical stability stratification types and then we discuss the computed
profiles of the density ratio R ρ and vertical stability parameter E θ, S , defined from the
CTD casts. In the end, some important conclusions of the study are given.
Data and Methods
In this study, we analyze the direct high-resolution velocity current measurements
and CTD casts on April 14, 2017 in the region of the Lomonosov EUC. The CTD
stations were located in the core of the Lomonosov EUC along 33° W at latitudes
0° 30′ S (station 2698), 0° (equatorial station 2699), and 0° 30′ N (station 2700).
The data of the shipboard sADCP were processed using the software package of
the manufacturing company. The velocities that did not satisfy the reliability criteria
were discarded. The declared accuracy of velocity measurements in the
high-resolution mode was 2–3 cm/s. The measurements were carried out at a speed
of the ship of 6–10 knots. The depth range of profiling was from 22 m to 600–
800 m with a depth resolution of 10 m. We focused special attention on the
accuracy of the data and methods of data acquisition. In this study, 15 min averaged
velocities were used.
The sADCP measurements were combined with the CTD/LADCP casts to a
depth of 1500 m using the SBE 19 plus profiler and lowered Sentinel 300 kHz
LADCP. The accuracy of the temperature sensor was 0.002 °C while the salinity
was measured with an accuracy to 0.003 psu. Unfortunately, due to the conditions
of measurements and data processing, the upper boundary of the reliable LADCP
data was at a depth of ∼100 m. This restriction is unacceptable for the analysis of
the velocity measurements. Therefore, in this work we used only the ship-mounted
sADCP to plot the profiles and velocity sections up to the depths of 600–800 m.
Thermohaline Structure and Salt Fingering …
431
thermohaline stratification of the section and on detecting favorable conditions for
the development of double diffusion and salt fingering. In order to reveal such
conditions we calculated the thermohaline stability parameters of the subsurface
water. The results provided us with a reliable pattern of spatial distribution of
convectively unstable intervals that were closely related in depth to thin
high-gradient parts of the stepwise structures of the fields. Such unstable layers that
are capable to maintain and promote vertical mixing, redistributing salt and heat
downward from the EUC core, were located within the thermocline layer, which
resists mixing.
The contribution is organized as follows. In section “Data and Methods”, we
discuss the data used for the estimates and analysis. Then, in section “Results and
Discussion”, we consider the results of sADCP describing the circulation features
of the EUC and the results of CTD casts discussing the horizontal and vertical
structure of the thermohaline field across the EUC as well as relation between
thermohaline and velocity fields. In section “Conclusions”, we consider briefly the
criteria of vertical stability stratification types and then we discuss the computed
profiles of the density ratio R ρ and vertical stability parameter E θ, S , defined from the
CTD casts. In the end, some important conclusions of the study are given.
Data and Methods
In this study, we analyze the direct high-resolution velocity current measurements
and CTD casts on April 14, 2017 in the region of the Lomonosov EUC. The CTD
stations were located in the core of the Lomonosov EUC along 33° W at latitudes
0° 30′ S (station 2698), 0° (equatorial station 2699), and 0° 30′ N (station 2700).
The data of the shipboard sADCP were processed using the software package of
the manufacturing company. The velocities that did not satisfy the reliability criteria
were discarded. The declared accuracy of velocity measurements in the
high-resolution mode was 2–3 cm/s. The measurements were carried out at a speed
of the ship of 6–10 knots. The depth range of profiling was from 22 m to 600–
800 m with a depth resolution of 10 m. We focused special attention on the
accuracy of the data and methods of data acquisition. In this study, 15 min averaged
velocities were used.
The sADCP measurements were combined with the CTD/LADCP casts to a
depth of 1500 m using the SBE 19 plus profiler and lowered Sentinel 300 kHz
LADCP. The accuracy of the temperature sensor was 0.002 °C while the salinity
was measured with an accuracy to 0.003 psu. Unfortunately, due to the conditions
of measurements and data processing, the upper boundary of the reliable LADCP
data was at a depth of ∼100 m. This restriction is unacceptable for the analysis of
the velocity measurements. Therefore, in this work we used only the ship-mounted
sADCP to plot the profiles and velocity sections up to the depths of 600–800 m.
Thermohaline Structure and Salt Fingering …
431