Structure of the Freshened Surface Layer in the Kara Sea During Ice‐Free Periods

Abstract

The Kara Sea is a semi-enclosed sea located between the Siberian coast in the south, Novaya Zemlya in the west, and Severnaya Zemlya in the east (Figure 1). The water depth is less than 50 m at more than 40% of the Kara Sea; shallow areas are located mainly in the central and southeastern parts of the sea. This sea receives enormous freshwater discharge (∼1,500 km 3 annually) mainly from two large estuaries, namely, the Yenisei Gulf (630 km 3 from the Yenisei River) and the Gulf of Ob (530 km 3 from the Ob, Pur, and Taz rivers; Gordeev et al., 1996; Pavlov et al., 1996). Continental discharge to the Kara Sea has very large seasonal variability with a short freshet period in June-July that provides ∼50% of annual runoff and a long low discharge period in October-April caused by freezing of the inflowing rivers (Pavlov et al., 1996; Figure 1). Freshwater discharge forms the large freshened surface layer (FSL) in the Kara Sea which is among the largest freshwater reservoirs in the Arctic Ocean

Using Remotely Sensed Sea Surface Salinity and Colored Detrital Matter to Characterize Freshened Surface Layers in the Kara and Laptev Seas during the Ice-Free Season

“…Meanwhile, in September, the FSL is saltier and shallower (5-10 m) as a result of the mixing and decreasing river freshwater inputs [67,68]. The seasonal spreading patterns seen by satellite data are coherent with the seasonal variability described using in situ data [48]. An example for the years 2014 and 2017 with transects extracted data along the transect from the Ob river mouth (A: 73.5 • E, 72 • N) to the open sea (B: 73.5 • E, 77.56 • N) is shown in Figure 2.…”

“…Variations in the SSS-CDM relationships in both basins could also be attributed to geophysical characteristics (e.g., differences in vegetation, topography, and hydrology in both watersheds; differential permafrost thaw; or fires in both locations [64,65]). The delimitation of the area influenced by river plume waters-of 26 psu for the Kara Sea and 24 psu for the Laptev Sea-was chosen to illustrate the areas where the SSS and CDM co-vary as well as to be consistent with prior research characterizing these plumes [48,66].…”

“…Osadchiev et al [48] have described the seasonal variability of FSL in the Kara Sea using in situ data. They found the FSL to be shallow (5-10 m deep) and fresher in the June-July discharge peak.…”

“…The correlation was higher in July, due to being closer to the discharge peaks and the higher data availability due to lower cloud cover. The differences in the SSS-CDM relationships could be due to seasonal variability of the FSL, the different timing of discharge peaks in spring of Ob and Yenisei [48], the different CDM signatures of the rivers, or the different availability of data (September is more cloud covered). We discerned two different clouds of points, which could be due to different SSS-CDM signatures in the Ob and Yenisei rivers, or due to local sources of CDM causing a deviation above the linear fit.…”

“…These rivers alone make up about half of the freshwater flux to the Arctic Ocean [44,45]. The mixing of fresh river waters and saltier sea waters on the Siberian shelves of the Laptev and Kara Seas creates large, stable freshened surface layers (FSLs) with spatial extents of hundreds of kilometers and typical thickness of 10-20 m deep, making them among the largest freshwater reservoirs in the Arctic Ocean [4,[46][47][48]. The formation of large FSLs prevents vertical water exchange and nutrient supply from nutrient-rich waters to the euphotic zone, limiting primary production and explaining low productivity areas [49].…”