Wind-Driven Coastal Upwelling near Large River Deltas in the Laptev and East-Siberian Seas

Abstract

The Lena, Kolyma, and Indigirka rivers are among the largest rivers that inflow to the Arctic Ocean. Their discharges form a freshened surface water mass over a wide area in the Laptev and East-Siberian seas and govern many local physical, geochemical, and biological processes. In this study we report coastal upwelling events that are regularly manifested on satellite imagery by increased sea surface turbidity and decreased sea surface temperature at certain areas adjacent to the Lena Delta in the Laptev Sea and the Kolyma and Indigirka deltas in the East-Siberian Sea. These events are formed under strong easterly and southeasterly wind forcing and are estimated to occur during up to 10%-30% of ice-free periods at the study region. Coastal upwelling events induce intense mixing of the Lena, Kolyma, and Indigirka plumes with subjacent saline sea. These plumes are significantly transformed and diluted while spreading over the upwelling areas; therefore, their salinity and depths abruptly increase, while stratification abruptly decreases in the vicinity of their sources. This feature strongly affects the structure of the freshened surface layer during ice-free periods and, therefore, influences circulation, ice formation, and many other processes at the Laptev and East-Siberian seas.Remote Sens. 2020, 12, 844 2 of 25 seas and are among the largest freshwater reservoirs in the Arctic Ocean [17][18][19][20]. Spreading and transformation of these river plumes determine vertical stratification and, therefore, strongly affect circulation and ice formation in the Laptev and East-Siberian seas, as well as many other physical, geochemical, and biological processes [21][22][23][24][25][26][27][28][29][30][31][32].In this study we focus on upwelling events which regularly occur at coastal areas adjacent to the deltas of the Lena, Kolyma, and Indigirka rivers. Surface manifestations of these upwelling events are visible on ocean color satellite imagery due to elevated turbidity and on sea surface temperature (SST) satellite imagery due to reduced temperature. However, correct identification of the origin of SST and turbidity features observed on satellite imagery is not a straightforward task. SST features in the study region are formed as a result of interaction between water masses with different temperature, namely, warm river plumes and cold saline sea water, and are associated with spreading of river plumes, mixing of surface layer with subjacent sea, and ice melting. Areas of elevated sea surface turbidity in coastal and shelf regions are commonly associated with four different processes: spreading of turbid river plumes, coastal erosion, resuspension of bottom sediments penetrated to sea surface, and algal blooms [33]. The first three processes are common features of the Laptev and East-Siberian seas [25,27,34,35], while algal blooms do not occur in these seas [32,[36][37][38]. Turbid regions associated with river plumes are adjacent to river estuaries and deltas. Spatial and temporal variability of these regions is def...

Freshwater transport between the Kara, Laptev, and East-Siberian seas

“…In order to study spreading of the Ob-Yenisei plume to the Laptev Sea we analyzed sea surface temperature (SST) products acquired from Terra/Aqua MODIS satellite data. Large river plumes formed in the Kara and Laptev seas during summer and early autumn are significantly warmer than surrounding sea due to the large temperature difference between river and sea water 33 – 35 . Due to very low solar radiation in the study areas, surface temperature contrast formed between shallow and deep water is much lower, than the contrast between the plumes and ambient sea 33 .…”

“…Note that elevated surface turbidity in many coastal areas in the Laptev and East-Siberian seas (including the Laptev Strait) is formed as a result of extremely intense coastal erosion due to active thermal abrasion 43,44 . Turbid regions associated with coastal erosion are adjacent to long segments of sea coast and do not correspond to spreading areas of river plumes 35 . In particular, narrow alongshore stripe of turbid water in the Laptev Strait formed by coastal erosion does not correspond to low-saline waters of the Lena plume, which occupies the whole strait from the southern to the northern coast that is visible at SST images.…”

Water Exchange Between the Gulf of Ob and the Kara Sea During Ice-Free Seasons: The Roles of River Discharge and Wind Forcing

“…Coastal upwelling events are often manifested at optical and thermal satellite imagery by areas of cold (and sometimes turbid) water along the shore once the bottom layer penetrates up to the sea surface near the coast (Osadchiev et al, 2020c). However, during the upwelling events in the Kara Sea, the cold and saline bottom layer does not reach the surface layer near the Gulf of Ob because it is blocked by the Ob plume in this area.…”

North to South Variations in the Suspended Sediment Transport Budget within Large Siberian River Deltas Revealed by Remote Sensing Data

“…Sudden changes in the base flow observed in the estuary section of Kolyma may intensify towards the sea. Additional impacts on the sediment budget in the Arctic deltas are related to coastal upwelling events, which are regularly increased sea surface turbidity at certain areas adjacent to the Lena delta in the Laptev Sea and the Kolyma deltas in the East-Siberian Sea [78]. These events are formed under strong easterly and southeasterly wind forcing and are estimated to occur during up to 10%-30% of ice-free periods.…”