The Kara Sea is a semi-enclosed sea located between the Siberian coast in the south, Novaya Zemlya in the west, and Severnaya Zemlya in the east (Figure 1). The water depth is less than 50 m at more than 40% of the Kara Sea; shallow areas are located mainly in the central and southeastern parts of the sea. This sea receives enormous freshwater discharge (∼1,500 km 3 annually) mainly from two large estuaries, namely, the Yenisei Gulf (630 km 3 from the Yenisei River) and the Gulf of Ob (530 km 3 from the Ob, Pur, and Taz rivers; Gordeev et al., 1996; Pavlov et al., 1996). Continental discharge to the Kara Sea has very large seasonal variability with a short freshet period in June-July that provides ∼50% of annual runoff and a long low discharge period in October-April caused by freezing of the inflowing rivers (Pavlov et al., 1996; Figure 1). Freshwater discharge forms the large freshened surface layer (FSL) in the Kara Sea which is among the largest freshwater reservoirs in the Arctic Ocean

The Kara and Laptev seas receive about one half of total freshwater runoff to the Arctic Ocean from the ob, Yenisei, and Lena rivers. Discharges of these large rivers form freshened surface water masses over wide areas in these seas. these water masses, i.e., the ob-Yenisei and Lena river plumes, generate an eastward buoyancy boundary current that accounts for the large-scale zonal freshwater transport along the Siberian part in the Arctic ocean. in this study we investigate spreading of the ob-Yenisei plume from the Kara Sea to the Laptev Sea through the Vilkitsky Strait and of the Lena plume from the Laptev Sea to the east-Siberian Sea through the Laptev and Sannikov straits during ice-free season. Large horizontal density gradient between freshened plume water and salty ambient sea water is the main driver of these processes, however, their intensity strongly depends on local wind forcing. the ob-Yenisei plume is spreading to the Laptev Sea in a narrow alongshore current which is induced by strong and long-term southwesterly winds. Under other wind forcing the plume does not reach the Vilkitsky Strait. the Lena plume is almost constantly spreading to the east-Siberian Sea as a large-scale surface water mass which intensity is governed by eastward ekman transport and is prone to large synoptic variability. The Ob, Yenisei and Lena rivers contribute large volumes of freshwater discharge into the Kara (~ 1,500 km 3 annually from the Ob and Yenisei rivers) and Laptev (~ 800 km 3 annually from the Lena River) seas that account for approximately one half of the total river runoff into the Arctic Ocean 1-3. Most of the continental runoff to the Kara and Laptev seas is discharged during ice-free period in June-September and forms sea-wide Ob-Yenisei and Lena river plumes which are among the largest freshwater reservoirs in the Arctic Ocean 4-6. River plumes are freshened surface-advected water masses, which seasonally form a relatively thin surface layer, compared to ambient saline sea. As a result, dynamics of river plumes is buoyancy-driven and wind-driven 7-12 , which is also true for large Ob-Yenisei and Lena plumes 3,13-18. It was previously investigated, that in the absence of strong wind forcing the Coriolis force and large salinity gradient between river plumes and ambient shelf water induce a baroclinic flow along the coast, which was previously addressed in many studies 7,19-21. This flow can be favored or impeded by local wind forcing. The associated eastward freshwater transport along large segments of northern shores of Eurasia and North America is an important part of large-scale freshwater transport pathways in the Arctic Ocean 22-27. Eastward alongshore spreading of the Ob-Yenisei and Lena plumes as a buoyancy-driven boundary current was described as a part of the Siberian Coastal Current (SCC) 22 which is the Eurasian branch of the Riverine Coastal Domain (RCD) 26. However, only few previous studies specifically addressed spreading of these river plumes from their source sea to an eastward sea, i.e., ...

Abstract. The Yenisei and Khatanga rivers are among the largest estuarine rivers that inflow to the Arctic Ocean. Discharge of the Yenisei River is 1 order of magnitude larger than that of the Khatanga River. However, spatial scales of buoyant plumes formed by freshwater runoff from the Yenisei and Khatanga gulfs are similar. This feature is caused by different tidal forcing in these estuaries, which have similar sizes, climate conditions, and geomorphology. The Khatanga discharge experiences strong tidal forcing that causes formation of a diluted bottom-advected plume in the Khatanga Gulf. This deep and weakly stratified plume has a small freshwater fraction and therefore occupies a large area on the shelf. The Yenisei Gulf, on the other hand, is a salt-wedge estuary that receives a large freshwater discharge and is less affected by tidal mixing due to low tidal velocities. As a result, the low-salinity and strongly stratified Yenisei plume has a large freshwater fraction and its horizontal size is relatively small. The results show that estuarine tidal mixing determines freshwater fraction in these river plumes, which governs their depth and area after they spread from estuaries to coastal sea. Therefore, the influence of estuarine mixing on spatial scales of a large river plume can be of the same importance as the roles of river discharge rate and wind forcing. In particular, plumes with similar areas can be formed by rivers with significantly different discharge rates, as illustrated by the Yenisei and Khatanga plumes.