Extreme Hydrometeorological Conditions of Sediment Waves' Formation and Migration in Peter the Great Bay (The Sea of Japan)

New data were obtained on the formation mechanisms of relief sediment waves (SWs) in Peter the Great Bay of the Sea of Japan. From the studies, we can assume that the sediment waves in the bay originate from two types of turbidity (TB) currents: (1) fine-dispersed TB currents caused by disintegrating internal waves and entrained by the Primorsky current along the shelf through SWs troughs; (2) fine- and coarse-grained TB currents, formed and entrained by the Primorsky current, intensified by the typhoons. This work presents the geological structure of the shelf area containing SWs. We established that the area of SWs location is on the border of change in facies conditions of sediment bedding in the bay. The transition from the inner shelf facies to the outer shelf facies occurs at a depth of 50-60 m. We analyzed the space variability of temperature and sound speed fields peculiar to the region on the results of hydrological CTD measurements and long-term measurements with moored vertical thermostrings. In the course of measurements, seven typhoons during their passage off the Primorye coast of the Russian Federation were registered. We found a significant change in the hydrological situation in the bay under the influence of extreme external factors. Under calm meteorological conditions, up to three mild thermoclines were observed in the bay, located at depths of 8-15 m, 30-35 m, and 45-60 m. During the passage of typhoons, they combined into one powerful thermocline at a depth of approximately 50-60 m, and in some cases, a high-gradient thermocline formed at the depths of 30-40 m (about 12-15 degrees C per 10 m of depth). The high-gradient layer significantly increases the amplitude and strength of internal gravity waves, thereby increasing the speed of currents in the water media. We should note that this period of transformation of the hydrological profile on the shelf under the influence of typhoons covers the period from August to October-November. We established that in the SWs location area, firstly, the process of internal waves disintegration intensifies in the autumn season, resulting in the formation of the waves with lengths close to the SWs horizontal scales (SWs wavelengths); secondly, under the influence of the typhoons entering the Sea of Japan, the kinetic energy of the shelf branch of the Primorsky current increases. In this case, it acts as a source of TB currents, including medium- and coarse-grained sedimentary material that forms SWs.