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

Abstract

The overall volume of freshwater entering the Arctic Ocean has been growing as glaciers melt and river runoff increases. Since 1980, a 20% increase in river runoff has been observed in the Arctic system. As the discharges of the Ob, Yenisei, and Lena rivers are an important source of freshwater in the Kara and Laptev Seas, an increase in river discharge might have a significant impact on the upper ocean circulation. The fresh river water mixes with ocean water and forms a large freshened surface layer (FSL), which carries high loads of dissolved organic matter and suspended matter into the Arctic Ocean. Optically active material (e.g., phytoplankton and detrital matter) are spread out into plumes, which are evident in satellite data. Russian river signatures in the Kara and Laptev Seas are also evident in recent SMOS Sea Surface Salinity (SSS) Arctic products. In this study, we compare the new Arctic+ SSS products, produced at the Barcelona Expert Center, with the Ocean Color absorption coefficient of colored detrital matter (CDM) in the Kara and Laptev Seas for the period 2011–2019. The SSS and CDM are found to be strongly negatively correlated in the regions of freshwater influence, with regression coefficients between −0.72 and −0.91 in the studied period. Exploiting this linear correlation, we estimate the SSS back to 1998 using two techniques: one assuming that the relationship between the CDM and SSS varies regionally in the river-influenced areas, and another assuming that it does not. We use the 22-year time-series of reconstructed SSS to estimate the interannual variability of the extension of the FSL in the Kara and Laptev Seas as well as their freshwater content. For the Kara and Laptev Seas, we use 32 and 28 psu as reference salinities, and 26 and 24 psu isohalines as FSL boundaries, respectively. The average FSL extension in the Kara Sea is 2089–2611 km2, with a typical freshwater content of 11.84–14.02 km3. The Laptev Sea has a slightly higher mean FSL extension of 2320–2686 km2 and a freshwater content of 10.15–12.44 km3. The yearly mean freshwater content and extension of the FSL, computed from SMOS SSS and Optical data, is (as expected) found to co-vary with in situ measurements of river discharge from the Arctic Great Rivers Observatory database, demonstrating the potential of SMOS SSS to better monitor the river discharge changes in Eurasia and to understand the Arctic freshwater system during the ice-free season.

The Role of the Russian Shelf in Seasonal and Interannual Variability of Arctic Sea Surface Salinity and Freshwater Content

“…The largest SSS variance occurs in the Russian Shelf region, particularly in the Kara Sea (∼33 psu 2 ), likely forced by large interannual variations in the Ob and Yenisei rivers and at their mouths where, fresh river water meets the more saline Siberian shelf waters (Kuzin et al., 2010). Current circulation and changes in prevailing winds affect the buoyancy of river‐sourced, freshwater surface layers in this region (Umbert et al., 2021). Large SSS coastal variance in the East Siberian Sea could be attributed to the shallow Siberian Coastal Current.…”

Sea-ice-loss slowdown modulates the sea surface salinification in the Kara–Laptev Seas since the 2008 summer

“…The variables SIV and U denote the sea-ice volume and sea-ice drift velocity per unit area, respectively. According to Umbert et al (2021), the main riverinfluenced freshwater input during the ice-free season (July-September), the Ob'-Yenisey and Lena river plume, are limited to the south of 76 • N. Thus, in this study, we focus on the salinity budget in the open sea north of 76 • N of KLS, excluding the river estuary.…”

How Much Can Riverine Biogeochemical Fluxes Affect the Arctic Ocean Acidification?

“…In these regions, the anomalies in Ω and pH between the CN/CAR/NUT and CTL cases show negative or relatively small positive trends (Table 4), which is highly related to the Frfw variation in each sub‐region as discussed in Sections 4.1–4.4. Among these 3 sub‐regions, the Kara Sea had the largest significant positive trend in Ω (but insignificant positive trend in pH), especially after the 2000s, due to the enhanced ocean currents exporting riverine water toward the central Arctic basin (Polukhin, 2019; Umbert et al., 2021).…”