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Satellite images (Sentinel-2) of concentric rings-surface manifestations of high-frequency internal waves in river plumes-in (a) New Guinea, (b) New Zealand, (c) Peru, (d) Iceland. The red dots indicate locations of the presented river plumes. Note that the images are given in different spatial scales.
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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 p...
Citations
... Satellite observations indicate that NAEs most frequently emerge in the area between Sochi and the Iskuria Cape [22,23,29,57], and being shed after emerging, longlived NAEs propagate along the coast up to Novorossiysk with translation velocity of about 2-4 km/day [55]. The area between Novorossiysk and Iskuria Cape receives freshwater Remote Sens. 2022, 14, 4149 4 of 29 discharge from multiple rivers, which results in interaction of NAEs with buoyant river plumes in the study area [58][59][60][61][62][63][64][65][66][67]. Sometimes NAEs, after protrusion though the RC, move across the shelf towards the Black Sea interior as isolated eddies [3,23,28]. ...
... Nevertheless, numerous events associated with the RC instability creating mesoscale eddies, filaments, and frontal structures could enhance processes of the cross-shelf water and matter exchanges. These processes are very important for understanding the environmental impacts on the Caucasian coastal zone since numerous rivers flowing into the sea bring and spread suspended and dissolved constituents including pollutants over the coastal zone [58][59][60][61][62][63][64][65][66][67]. Following the RC, the matter is transferred along the Caucasian coast within a narrow near-shore zone. ...
The Northeast Caucasian Current (NCC) is the northeastern part of the cyclonic Rim Current (RC) in the Black Sea. As it sometimes approaches the narrow shelf very closely, topographically generated cyclonic eddies (TGEs) can be triggered. These eddies contribute to intense, along- and cross-shelf transport of trapped water with enhanced self-cleaning effects of the coastal zone. Despite intense studies of eddy dynamics in the Black Sea, the mechanisms of the generation of such coastal eddies, their unpredictability, and their capacity to capture and transport impurities are still poorly understood. We applied a 3-D low-dissipation model DieCAST/Die2BS coupled with a Lagrangian particle transport model supported by analysis of optical satellite images to study generation and evolution of TGEs and their effect on river plumes unevenly distributed along the northeastern Caucasian coast. Using the Furrier and wavelet analyses of kinetic energy time series, it was revealed that the occurrence of mesoscale TGEs ranges from 10 up to 50 days. We focused on one particular isolated anticyclonic TGE that emerged in late fall as a result of instability of the RC impinging on the abrupt submarine area adjoining the Pitsunda and Iskuria capes. Being shed, the eddy with a 30-km radius traveled along the coast as a coherent structure during ~1.5 months at a velocity of ~3 km/day and vertical vorticity normalized by the Coriolis parameter ~(0.1 ÷ 1.2). This eddy captured water from river plumes localized along the coast and then ejected it to the open sea, providing an intense cross-shelf transport of riverine matter.
... These gradients are especially strong at plumes formed by small rivers, which have small spatial scales and small freshwater residence time equal to hours and days [1][2][3]. In particular, horizontal (3-4 salinity units/m [2,4]) and vertical (5-6 salinity units/m [5,6]) salinity gradients observed at the outer borders of small plumes are among the largest reported in the World Ocean. ...
... Five field surveys described in this study were performed at the buoyant plumes formed by the Kodor and Bzyb rivers in the eastern part of the Black Sea ( Figure 1). General characteristics of the Kodor and Bzyb plumes and the ambient saline sea are described in detail by Osadchiev et al. [2][3][4]6]. The information about all field surveys analyzed in the study is summarized in Table 1 georeferenced by direct projection into an earth-based Cartesian coordinate system using GPS and altimetry data from the quadcopter. ...
... For this purpose, we performed five field surveys at the small Kodor and Bzyp river plumes located at the eastern part of the Black Sea in 2018-2022. This study continues our previous research of small river plumes in the Black Sea focused on the structure of small plumes [2,3,57], influence of wind forcing and river discharge variability on small plumes [4,58,59], generation of internal waves in river plumes [6,60], influence of small plumes on water quality and marine pollution [14,61]. ...
The interfaces between small river plumes and ambient seawater have extremely sharp horizontal and vertical salinity gradients, often accompanied by velocity shear. It results in formation of instabilities at the lateral borders of small plumes. In this study, we use high-resolution aerial remote sensing supported by in situ measurements to study these instabilities. We describe their spatial and temporal characteristics and then reconstruct their relation to density gradient and velocity shear. We report that Rayleigh–Taylor instabilities, with spatial scales ~5–50 m, are common features of the sharp plume-sea interfaces and their sizes are proportional to the Atwood number determined by the cross-shore density gradient. Kelvin–Helmholtz instabilities have a smaller size (~3–7 m) and are formed at the plume border in case of velocity shear >20–30 cm/s. Both instabilities induce mass transport across the plume-sea interfaces, which modifies salinity structure of the plume borders and induces lateral mixing of small river plumes. In addition, aerial observations revealed wind-driven Stokes transport across the sharp plume-sea interface, which occurs in the shallow (~2–3 cm) surface layer. This process limitedly affects salinity structure and mixing at the plume border, however, it could be an important issue for the spread of river-borne floating particles in the ocean.
... Generally, river plumes have large area but small depth. Area of a river plume exceeds its thickness by 3-5 orders of magnitude, therefore, even small rivers (with a water flow rate of several cubic meters per second) form river plumes with spatial extents of tens and hundreds of meters [1][2][3][4][5], while the spatial extents of river plumes formed by the largest rivers in the World are hundreds of kilometers [6][7][8][9]. Thus, despite the relatively small volume of global continental runoff into the World Ocean (38,000 km 3 annually) compared to the volume of shelf seawater (66,600,000 km 3 ), river plumes, depending on the season, occupy from 7% to 21% of the total shelf area of the World Ocean, i.e., several millions of square kilometers [10]. ...
Satellite altimetry is an efficient instrument for detection dynamical processes in the World Ocean, including reconstruction of geostrophic currents and tracking of mesoscale eddies. Satellite altimetry has the potential to detect large river plumes, which have reduced salinity and, therefore, elevated surface level as compared to surrounding saline sea. In this study, we analyze applicability of satellite altimetry for detection of the Ob–Yenisei plume in the Kara Sea, which is among the largest river plumes in the World Ocean. Based on the extensive in situ data collected at the study area during oceanographic surveys in 2007–2019, we analyze the accuracy and efficiency of satellite altimetry in reproducing, first, the outer boundary of the plume and, second, the internal structure of the plume. We reveal that the value of positive level anomaly within the Ob–Yenisei plume strongly depends on the vertical plume structure and is prone to significant synoptic and seasonal variability due to wind forcing and mixing of the plume with subjacent sea. As a result, despite generally high statistical correlation between the ADT and surface salinity, straightforward usage of ADT for detection of the river plume is incorrect and produces misleading results. Satellite altimetry could provide correct information about spatial extents and shape of the Ob–Yenisei plume only if it is validated by synchronous in situ measurements.
Wind forcing is the main driver of river plume dynamics. Direction and magnitude of wind determine position, shape, and size of a river plume. The response of river plumes on wind forcing was simulated in many numerical modeling studies; however, in situ measurements of this process are still very scarce. In this study, we report the first direct measurements of frontal movement of a small river plume under variable wind forcing conditions. Using quadcopters, we performed nearly continuous daytime aerial observations of the Bzyb river plume located in the non-tidal Black Sea. The aerial remote sensing was accompanied by synchronous in situ measurements of wind forcing. We assessed spreading patterns of the plume and evaluated movement velocity of its outer border with unprecedentedly high spatial (∼10 m) and temporal (∼1 min) resolution, which was not available in previous studies based on in situ measurements and satellite observations. Based on the collected data, we evaluated the time of response of plume spreading dynamics on changes in wind forcing conditions. The advection velocity of the outer plume border shows linear relation to wind speed with very small response time (10–20 min). The reversal between upstream/downstream plume spreading occurs during several hours under moderate wind forcing conditions. These reversals involve only near-field part of the plume, which cause detachment of the far-field part of the plume. The obtained results are crucial for understanding and simulating spreading dynamics of small river plumes worldwide.
