River discharge is the main source of terrigenous sediments in many coastal areas adjacent to estuaries and deltas of large rivers. Spreading and mixing dynamics of river plumes governs transport of suspended sediments and their deposition at sea bottom at these areas. Generally river plumes have very large synoptic and seasonal variability, which cannot be reconstructed from structure of bottom sediments due to their small accumulation velocity. However, bottom sediments can be indicative of variability of river plumes on inter-annual and decadal time scales. In this study we focus on the large Ob and Yenisei buoyant plumes formed in the central part of the Kara Sea. These plumes interact and mix in the area adjacent to the closely located Ob and Yenisei gulfs. Suspended sediments carried by these river plumes have significantly different geochemical characteristics that can be used to detect Ob or Yenisei origin of bottom sediments. Using new geochemical methods we revealed dependence between spreading patterns of these plumes and spatial distribution and vertical structure of bottom sediments in the study area. This relation is confirmed by a good agreement between local wind and discharge conditions reconstructed for 1948–2001 and vertical structure of bottom sediments.

The Sea of Azov is a small, shallow, and freshened sea that receives a large freshwater discharge. Under certain external forcing conditions low-salinity waters from the Sea of Azov flow into the north-eastern part of the Black Sea through the narrow Kerch Strait and form a surface-advected buoyant plume. Water flow in the Kerch Strait also regularly occurs in the opposite direction, which results in the spreading of a bottom-advected plume of saline and dense waters from the Black Sea into the Sea of Azov. In this study we focus on the physical mechanisms that govern water exchange through the Kerch Strait and analyse the dependence of its direction and intensity on external forcing conditions. Analysis of satellite imagery, wind data, and numerical modelling shows that water exchange in the Kerch Strait is governed by a wind-induced barotropic pressure gradient. Water flow through the shallow and narrow Kerch Strait is a one-way process for the majority of the time. Outflow from the Sea of Azov to the Black Sea is induced by moderate and strong north-easterly winds, while flow into the Sea of Azov from the Black Sea occurs during wind relaxation periods. The direction and intensity of water exchange have wind-governed synoptic and seasonal variability, and they do not depend on the rate of river discharge to the Sea of Azov on an intraannual timescale. The analysed data reveal dependencies between wind forcing conditions and spatial characteristics of the buoyant plume formed by the outflow from the Sea of Azov.Published by Copernicus Publications on behalf of the European Geosciences Union.

Abstract. The Sea of Azov is a small, shallow and freshened sea that receives a large freshwater discharge and, therefore, can be regarded as a large river estuary. Under certain external forcing conditions low-saline waters from the Sea of Azov inflow to the northeastern part of the Black Sea through the narrow Kerch Strait and form a surface-advected buoyant plume. Water flow in the Kerch Strait also regularly occurs in the opposite direction, which results in spreading of a bottom-advected plume of saline and dense waters from the Black Sea in the Sea of Azov. In this study we focus on physical mechanisms that govern water exchange through the Kerch Strait, analyse dependence of its direction and intensity on external forcing conditions. Based on ocean color satellite imagery and wind reanalysis data, we show that water transport from the Sea of Azov to the Black Sea is induced by moderate and strong northeastern winds, while water transport in the opposite direction occurs during wind relaxation periods. Thus, direction and intensity of water exchange through the Kerch Strait has wind-govern synoptic and seasonal variability, and do not show dependence on river discharge rate to the Sea of Azov on intra-annual time scale. Finally, we determined numerical dependencies between discharge rate from the Sea of Azov to the Black Sea and spatial characteristics of the related surface-advected plume in the Black Sea, on the one hand, and wind forcing conditions, on the other hand.

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We strongly appreciate the reviewers' suggestions and comments that served to improve the article. In response to them, first, we added results of numerical modelling that supported the results obtained from analysis of satellite imagery. Second, we clarified methods of detection of surface-advected outflows from the Sea of Azov to the Black Sea and bottom-advected inflows in the opposite direction based on agreement between Chl-a and TSM satellite imagery. Third, we modified the interpretation of influence of wing forcing on inflows from the Black Sea to the Sea of Azov and provided a more thorough physical background based on both satellite data and numerical mod- C1 OSD Interactive commentPrinter-friendly version Discussion paper