Ocean Sci., 16, 781–798, 2020 https://doi.org/10.5194/os-16-781-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Influence of estuarine tidal mixing on structure and spatial scales of large river plumes Alexander Osadchiev1,2,3, Igor Medvedev1,4, Sergey Shchuka1,3, Mikhail Kulikov1, Eduard Spivak5, Maria Pisareva1, and Igor Semiletov5,6 1Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia 2Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences, Moscow, Russia 3Moscow Institute of Physics and Technology, Dolgoprudny, Russia 4Fedorov Institute of Applied Geophysics, Roshydromet, Moscow, Russia 5Ilyichov Pacific Oceanological Institute, Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia 6National Research Tomsk Polytechnic University, Tomsk, Russia Correspondence: Alexander Osadchiev ([email protected]) Received: 29 October 2019 – Discussion started: 20 December 2019 Revised: 13 May 2020 – Accepted: 20 May 2020 – Published: 3 July 2020 Abstract. The Yenisei and Khatanga rivers are among the 1 Introduction largest estuarine rivers that inflow to the Arctic Ocean. Dis- charge of the Yenisei River is 1 order of magnitude larger than that of the Khatanga River. However, spatial scales of River plumes play an important role in land-ocean interac- buoyant plumes formed by freshwater runoff from the Yeni- tions. Despite their relatively small volume as compared to sei and Khatanga gulfs are similar. This feature is caused by adjacent coastal seas, they significantly affect global fluxes different tidal forcing in these estuaries, which have similar of buoyancy, heat, terrigenous sediments, nutrients, and an- sizes, climate conditions, and geomorphology. The Khatanga thropogenic pollutants, which are discharged to the coastal discharge experiences strong tidal forcing that causes for- ocean with continental runoff (Dagg et al., 2004; Milliman mation of a diluted bottom-advected plume in the Khatanga and Farnsworth, 2011; Lebreton et al., 2017; Schmidt et al., Gulf. This deep and weakly stratified plume has a small 2017). As a result, dynamics and variability of river plumes freshwater fraction and therefore occupies a large area on the are key factors for understanding mechanisms of spreading, shelf. The Yenisei Gulf, on the other hand, is a salt-wedge transformation, and redistribution of continental discharge estuary that receives a large freshwater discharge and is less and river-borne constituents in coastal seas and their influ- affected by tidal mixing due to low tidal velocities. As a re- ence on adjacent continental shelves (Geyer et al., 2004; sult, the low-salinity and strongly stratified Yenisei plume Hickey et al., 2010; Hetland and Hsu, 2013). World river has a large freshwater fraction and its horizontal size is rel- plumes are characterized by wide variety of structure, mor- atively small. The results show that estuarine tidal mixing phology, and dynamical characteristics caused by large dif- determines freshwater fraction in these river plumes, which ferences in regional features (Chant, 2011; Horner-Devine governs their depth and area after they spread from estuaries et al., 2015; Osadchiev and Korshenko, 2017; Osadchiev and to coastal sea. Therefore, the influence of estuarine mixing Sedakov, 2019; Zavialov et al., 2020), in particular, estuarine on spatial scales of a large river plume can be of the same conditions (Guo and Valle-Levison, 2007; Nash et al., 2009; importance as the roles of river discharge rate and wind forc- Lai et al., 2016; Osadchiev, 2017). ing. In particular, plumes with similar areas can be formed River estuaries are areas where freshwater discharge ini- by rivers with significantly different discharge rates, as illus- tially interacts with saline seawater. The related processes of trated by the Yenisei and Khatanga plumes. mixing of river runoff with seawater and formation of river plumes in estuaries determine their structure and govern their Published by Copernicus Publications on behalf of the European Geosciences Union. 782 A. Osadchiev et al.: Influence of estuarine tidal mixing on large river plumes subsequent spreading and mixing in open sea. Intensity of es- tween Sibiryakov Island and the Taymyr Peninsula steadily tuarine mixing varies from negligible, when mostly undiluted increases towards the open sea to 25–30 m and connects the freshwater discharge inflows directly to coastal sea, to domi- Yenisei Gulf with the central part of the Kara Sea. The Yeni- nant, which results in significant dilution of river discharge in sei Gulf is covered by ice in October–July. well-mixed enclosed basins before being released to the open Freshwater discharge from the Khatanga River (105 km3 sea (Schettini et al., 1998; Halverson and Palowicz, 2008; annually or 3300 m3 s−1 on average) is much smaller than MacCready and Geyer, 2010; Geyer and MacCready, 2014). that from the Yenisei River. Approximately one-half of this The Yenisei and Khatanga rivers are among the largest es- volume is discharged to the Laptev Sea during a freshet pe- tuarine rivers that flow into the Arctic Ocean (Fig. 1). These riod in June, and then the river discharge steadily decreases Yenisei and Khatanga gulfs are closely located and have until September (Pavlov et al., 1996). The lower part of similar sizes, geomorphology, and climatic conditions, albeit the Khatanga River is completely frozen in October–May significantly different tidal forcing. In this study, we focus and river discharge is negligible during this period (Pavlov on transformation of discharge of the Yenisei and Khatanga et al., 1996). The Khatanga River inflows into the Khatanga rivers in their estuaries and spreading of their buoyant plumes Gulf, which is located at the southwestern part of the Laptev that occupy wide areas in the Kara and Laptev seas. Dis- Sea at the eastern side of the Taymyr Peninsula (Fig. 1a). charge of the Yenisei River is 1 order of magnitude larger Shape, bathymetry, spatial scales, and climatic conditions of than that of the Khatanga River. However, spatial scales of the Khatanga Gulf are similar to those of the Yenisei Gulf, buoyant plumes formed by freshwater runoff from the Yeni- which is located approximately 800 km to the west from the sei and Khatanga gulfs are similar. Using in situ hydro- Khatanga Gulf. The Khatanga Gulf is 250 km long; its width graphic data, we reveal that this feature is caused by differ- is 25–50 km. The shallow (5–20 m deep) inner and deep (20– ence in intensity of estuarine tidal mixing that greatly affects 30 m deep) outer parts of the gulf are connected by a nar- spatial scales of these river plumes. row strait (15–20 km wide) between the Khara-Tumus and The paper is organized as follows. Section 2 provides the Taymyr peninsulas (Fig. 1c). Bolshoy Begichev Island is lo- detailed information about the study area, the in situ, satel- cated in the outer part of the Khatanga Gulf and divides it lite, and wind reanalysis data used in this study. Section 3 de- into two straits. The southern strait is narrow (8 km wide) scribes the vertical structures and spatial extent of the Yeni- and shallow (10 m deep), while depth of the northern strait sei and Khatanga plumes, as well as tidal and wind forcing (15–20 km wide) between Bolshoy Begichev Island and the conditions in the study area. The influence of estuarine tidal Taymyr Peninsula steadily increases towards the open sea to forcing on spreading and mixing of the Yenisei and Khatanga 25–30 m and connects the Khatanga Gulf with the western plumes is analyzed in Sect. 4, followed by the conclusions in part of the Laptev Sea. The Khatanga Gulf is covered by ice Sect. 5. from October to July–August. Tides in the Khatanga Gulf are among the largest in the Eurasian part of the Arctic Ocean (Pavlov et al., 1996; Kulikov et al., 2018). 2 Study area and data 2.2 Data 2.1 Study area Freshwater discharge from the Yenisei River (630 km3 an- Hydrographic in situ data used in this study were collected nually or 20 000 m3 s−1 on average) is the largest among during three oceanographic field surveys in the Kara and the Arctic rivers and accounts for 20 % of total freshwater Laptev seas including the fourth cruise of the R/V Niko- runoff to the Arctic Ocean (Gordeev et al., 1996; Carmack, lay Kolomeytsev on 27–29 August 2000 in the southwestern 2000). The hydrological regime of the Yenisei River is gov- part of the Laptev Sea, the 66th cruise of the R/V Akademik erned by a distinct freshet peak in June–July (half of the total Mstislav Keldysh on 24–26 July 2016 in the Yenisei Gulf and annual discharge), moderate discharge in May and August– the central part of the Kara Sea, the 69th cruise of the R/V September, and a drought in October–April (Pavlov et al., Akademik Mstislav Keldysh on 17–18 September 2017 in the 1996; Guay et al., 2001). The Yenisei River inflows into the Khatanga Gulf and the southwestern part of the Laptev Sea Yenisei Gulf located at the southeastern part of the Kara Sea (Fig. 1). Field surveys included continuous measurements at the western side of the Taymyr Peninsula (Fig. 1a). The of salinity in the surface sea layer (2–3 m depth) performed Yenisei Gulf is 250 km long; its width is 35–50 km. Average along the ship track using a shipboard pump-through sys- depth of the southern (inner) part of the gulf increases off the tem equipped by a thermosalinograph (Sea-Bird Electron- river mouth from 5 to 15 m. The large Sibiryakov Island is ics SBE 21 SeaCAT). Vertical profiles of salinity were per- located in northern (outer) part of the gulf and divides it into formed using a conductivity–temperature–depth (CTD) in- two 40–50 km long and 30–35 km wide straits (Fig.