The inflow of warm and saline Atlantic water from the North Atlantic to the Western Arctic is provided by two branches, namely, the Fram Strait branch water and the Barents Sea branch water. The pathways of these branches merge at the St. Anna Trough, and then both branches propagate eastward along the continental slope, albeit at different depths. As a result, the local interaction between these branches in the trough affects the properties of the large-scale Atlantic water flow to the Eastern Arctic and the deep Arctic basins. In this study, we report extensively in situ measurements with high spatial coverage (56 hydrological stations organized into 7 transects) in the St. Anna Trough, obtained in August and October 2021. Based on these data, we reconstructed the thermohaline structure and circulation in this area and obtained new insights, which are crucial for the assessment of the interaction and heat balance of water masses in the trough. First, we state that the majority of the Fram Strait branch water is recirculated in the trough within the stable cyclonic gyre, while a smaller fraction returns to the continental slope. The formation of this gyre increases the residence time of the Fram Strait branch water in the trough and decreases the intensity of water and heat exchange between the trough and the continental slope. Second, we describe the dynamic interaction between the northward flow of the Barents Sea branch water and the surface layer. It causes intense transport of warm surface water from the Kara and Barents seas adjacent to the Novaya Zemlya toward the continental slope and its mixing with the Barents Sea branch water along the eastern part of the trough. These processes result in increased surface temperature at the eastern part of the trough, which enhances ice melting at the study area and increases the duration of the ice-free period.

The Gulf of Ob is among the largest estuaries in the World Ocean in terms of area, watershed basin, and freshwater discharge. In this work, we describe the roles of river discharge and wind forcing on the water exchange between the Gulf of Ob and the Kara Sea during ice-free seasons. This work is based on the extensive in situ measurements performed during 10 oceanographic surveys in 2007–2019. Due to large river runoff (∼530 km3 annually) and low tidal forcing (<0.5 m/s), the estuarine processes in the Gulf of Ob during the ice-free season are generally governed by gravitational circulation. Local wind forcing significantly affects general estuarine circulation and mixing only in rare cases of strong winds (∼10 m/s). On the other hand, remote wind forcing over the central part of the Kara Sea regularly intensifies estuarine—sea water exchange. Eastern winds in the central part of the Kara Sea induce upwelling in the area adjacent to the Gulf of Ob, which increases the barotropic pressure gradient between the gulf and the open sea. As a result, intense and distant (120–170 km) inflows of saline water to the gulf occur as compared to the average conditions (50–70 km). Remote wind forcing has a far stronger impact on saltwater intrusion into the Gulf of Ob than the highly variable river discharge rate. In particular, saltwater reaches the shallow central part of the gulf only during upwelling-induced intense inflows. In the other periods (even under low discharge conditions), fresh river water occupies this area from surface to bottom. The upwelling-induced intense inflows occur on average during a quarter of days (July to October) when the gulf is free of ice. They substantially increase the productivity of phytoplankton communities in the gulf and modify the taxa ratio toward the increase of brackish water species and the decrease of freshwater species.

The major Siberian rivers form large river plumes in the Arctic Ocean, which govern structure of the sea surface layer at the Arctic shelf. These river plumes were explicitly studied during the warm period in summer and early autumn characterized by high river runoff and ice-free conditions. However, little is known about processes, which occur within these river plumes at the beginning of the cold season, i.e., during late autumn shortly before sea ice formation. In this study, we report in situ measurements performed in the Kara Sea in late October in 2020, 2021, and 2022. We reveal that intense convection occurs in the Ob-Yenisei plume due to heat loss from the surface layer, which is caused by transport of cold air from land to the central part of the Kara Sea. This process induces homogenization of the Ob-Yenisei plume and results in extremely sharp salinity jump (up to 10-12 at vertical distance of 1-2 m) between the plume and the subjacent seawater. This sharp gradient is not formed at the whole area of the plume except, first, at the Ob and Yenisei gulfs due to low surface salinities and the related high temperatures of maximal density and, second, at the lateral boundary of the plume due to intense horizontal mixing across the plume-sea border. As a result, autumn convection significantly modifies vertical structure of the Ob-Yenisei plume that could affect its further spreading below sea ice during winter season.

This study is focused on concentric rings, which are regularly observed by remote sensing of small river plumes located in different regions worldwide. We report new aerial observations of these features obtained by quadcopters and supported by synchronous in situ measurements, which were collected during the recent field survey at the Bzyb river plume in the eastern part of the Black Sea. Joint analysis of remote sensing imagery and in situ data suggest that the observed concentric rings are surface manifestations of high-frequency internal waves generated in the vicinity of the river mouth. The obtained results demonstrate that the propagation of these waves does not induce offshore material transport within the plume induced by shear instability, which was hypothesized in a recent numerical modeling study of this process. We provide an explanation for the appearance of misleading material features in the numerical simulations discussed above. Finally, we discuss directions for future research of high-frequency internal waves generated in small river plumes.