Influence of Estuarine Tidal Mixing on Structure and Spatial Scales of Large River Plumes

Abstract. The Yenisei and Khatanga rivers are among the largest estuarine rivers that inflow to the Arctic Ocean. Discharge of the Yenisei River is one order of magnitude larger than that of the Khatanga River. However, spatial scales of buoyant plumes formed by freshwater runoffs 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 exhibits strong tidal forcing that causes formation of a diluted bottom-advected plume in the Khatanga Gulf. This anomalously 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 obtained 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, 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, rivers with similar discharge rates can form plumes with significantly different areas, while plumes with similar areas can be formed by rivers with significantly different discharge rates.