Spreading dynamics of small river plumes off the northeastern coast of the Black Sea observed by Landsat 8 and Sentinel-2

Abstract

We use near simultaneous ocean color satellite imagery from NASA's Landsat 8 and ESA's Sentinel-2 missions to reconstruct surface currents along the northeastern shore of the Black Sea and study the spread of a small river plume formed in this area. Several times a year Landsat 8 and Sentinel-2 satellites both pass over the study area within a time interval of <10 min. Analysis of near simultaneous ocean color composites obtained during these periods provides an opportunity for accurate reconstruction of surface currents. It is especially efficient for detecting motion of frontal zones associated with river plumes which are visible in optical satellite imagery. Using an optical flow algorithm we reconstructed surface currents associated with the motion of a small plume formed by the Mzymta River in response to different wind forcing conditions. We show that the dynamics of the Mzymta plume are significantly different to those of large plumes. First, a near-field jet of the small plume is abruptly decelerated by friction with the subjacent ocean and does not form a recirculating bulge even under low wind forcing conditions. As a result, freshwater discharge does not accumulate near the river mouth and is transported to the far-field part of the plume. Second, under certain external forcing conditions the angle between wind direction and Ekman transport within the plume takes anomalously large values of 60–80°. As a result, onshore winds cause upstream accumulation of the river plume, while offshore/downwelling and upwelling winds result in downstream and offshore transport of freshwater discharge, respectively.