On the Variability of Stratification in the Freshwater-Influenced Laptev Sea Region

Abstract

In this paper, we investigate the seasonal and spatial variability of stratification on the Siberian shelves with a case study from the Laptev Sea based on shipboard hydrographic measurements, year-round oceanographic mooring records from 2013 to 2014 and chemical tracer-based water mass analyses. In summer 2013, weak onshore-directed winds caused spreading of riverine waters throughout much of the eastern and central shelf. In contrast, strong southerly winds in summer 2014 diverted much of the freshwater to the northeast, which resulted in 50% less river water and significantly weaker stratification on the central shelf compared with the previous year. Our year-long records additionally emphasize the regional differences in water column structure and stratification, where the northwest location was well-mixed for 6 months and the central and northeast locations remained stratified into spring due to the lower initial surface salinities of the river-influenced water. A 26 year record of ocean reanalysis highlights the region’s interannual variability of stratification and its dependence on winds and sea ice. Prior the mid-2000s, river runoff to the perennially ice-covered central Laptev Sea shelf experienced little surface forcing and river water was maintained on the shelf. The transition toward less summer sea ice after the mid-2000s increased the ROFI’s (region of freshwater influence) exposure to summer winds. This greatly enhanced the variability in mixed layer depth, resulting in several years with well-mixed water columns as opposed to the often year-round shallow mixed layers before. The extent of the Lena River plume is critical for the region since it modulates nutrient fluxes and primary production, and further controls intermediate heat storage induced by lateral density gradients, which has implications for autumnal freeze-up and the eastern Arctic sea ice volume.MAIN POINTS1.CTD surveys and moorings highlight the regional and temporal variations in water column stratification on the Laptev Sea shelf.2.Summer winds increasingly control the extent of the region of freshwater influence under decreasing sea ice.3.Further reductions in sea ice increases surface warming, heat storage, and the interannual variability in mixed layer depth.

Comment on bg-2020-462

“…Nevertheless, the area of freshwater influence (ROFI) extends far north to the outer LS shelf under predominantly easterly winds, similar to the conditions observed in 2008. This was also observed in 2011, when strong southeasterly summer winds (Janout et al, 2020), coincided with mixed layer salinities below 16 ~ 300 km north of the Lena delta. The northern part of the ROFI should thus contain a significant amount of tDOM-rich freshwater from the spring freshet.…”

“…In 2011, large areas of the southeastern LS were covered by melting land-fast ice until the end of July. Exceptionally strong southeasterly winds in August 2011 (Janout et al, 2020) directed the meltwater plume to the northwest resulting in an additional admixture of CDOM-poor meltwater to the shelf waters north of the Lena Delta, and a further dilution of shelf waters with low-tDOM meltwater late in August and September. Although a decrease of the tDOM concentration due to photochemical or biogeochemical processes cannot be excluded, our study indicates that mixing of plural water masses rather than removal controls the tDOM distribution on this shelf.…”

“…During our sampling in the ESS in 2019, winds from the east (ERA5; Copernicus Climate Change Service, 2017) pushed the inflow from the Pacific Ocean westward far onto the ESS shelf. At the same time, southeasterly winds over the LS might have blocked the Lena ROFI from extending into the western ESS (Anderson et al, 2011;Janout et al, 2020). In addition, summer 2019 was characterized by anomalously low river discharge into the LS and ESS (ArcticGRO;Shiklomanov et al, 2020).…”

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

“…where S 0 is the reference salinity. We used S 0 = 32 as a reference in the Kara Sea and S 0 = 28 in the Laptev Sea, based on previous studies [58][59][60]. The total liquid FWC (km 3 ) was then calculated by integrating f over the horizontal area and the FSL depth.…”

“…We considered the FSL depth to be constant, based on previous references: 12 m for the Kara Sea [61] and 10 m for the Laptev Sea [59,60].…”