Inertial Oscillations in the Northern Part of the Black Sea Based on the Field Observations

The characteristic properties of inertial oscillations in the active layer in the northern part of the Black Sea are discussed based on the analysis of two types of currents measurement data. The results of spectral analysis of the ~100-day series of current velocity pulsations at five horizons (35–350 m) of the autonomous buoy station showed that no less than 70 % of the kinetic energy of the internal waveband is concentrated in the frequency band 2/3f÷3/2f near the local inertial frequency f, as is in other regions of the World Ocean. A characteristic feature of the vertical distribution of the inertial oscillation energy is its fastest decrease in the vicinity of the maximum buoyancy frequency in the main pycnocline. This change in energy with depth confirms the shielding effect of the main pycnocline, which limits the penetration of inertial oscillations into the deep layers of the sea. For the first time, it was established that the ratio of inertial oscillation energy in the total kinetic energy increases with depth (from 8.5 % at the horizon of 50 m to 19 % at the horizon of 350 m). This circumstance highlights the increasing effect of inertial oscillations on the dynamics of deep sea waters. A characteristic damping time of inertial oscillations in the main pycnocline was ~170 h, above pycnocline ~55 h, lower ~70 h. This relative increase in the main pycnocline means an aggravation of the inertial peak in the spectrum, which is typical for other regions of the World Ocean and can be explained by a decrease in the group velocity of near-inertial internal waves due to their interaction with geostrophic flows. Continuous profiles obtained by Lowered Doppler Current Profiler showed a predominance of rotation of the velocity shift vector with depth in a clockwise direction, which determines the downward direction of propagation of inertial internal waves as dominant, and confirms their wind origin. The observed length of the near-inertial internal waves was 20–40 m vertically. The average duration of a wave packet is ~1.5 wavelength.

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