Thermohaline Structure and Salt Fingering in the Lomonosov Equatorial Undercurrent as Observed in April 2017
We analyze CTD casts accompanying ADCP velocity measurements along 33° W in the Atlantic in the region of the Lomonosov Equatorial Undercurrent (EUC) on April 14, 2017. Three CTD stations were located at the equator and 30 miles south and north of it in the core of the undercurrent. We focus on the study of the thermohaline stratification in the subsurface layer with intense development of the stepwise structures on the temperature, salinity, and density profiles at the depths below the salinity and velocity cores. The estimates of the water stratification types are based on the analysis of the profiles of temperature, salinity, and density together with the computed stability parameters such as the density ratio and vertical thermohaline stability. The high-gradient parts of steps (“sheets”) up to 15 m thick against the background of sharp negative T and S gradients were related to the small intervals of peaks of positive stable stratification E > 0 and low values of density ratio 1 < R < 2. Such intervals are statically unstable and meet the strict criteria of conditions favorable for double diffusion convection in the form of salt fingers. This provides evidence of the high probability of vertical mixing of this type. It implies that in the course of propagation of the high-salinity EUC, a convective salt fingering mechanism may provide vertical redistribution of salt and heat from the core of the EUC to the deeper layers. Variations in the intensity of the processes with latitude and indications of large horizontal scales of fingering processes are found.
The field works were supported by the Program of the Presidium of the Russian Academy of Sciences (project I3Π). Data analysis was supported by the Russian Science Foundation (project no. 14-50-00095).
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