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.
Highlights
Coincident with the retreating summer sea ice cover in the Arctic Ocean, primary production increased by 30% between 1998 and 2012, as suggested by satellite ocean color measurements (Arrigo and van Dijken, 2015)
We investigated the seasonal and interannual variability of stratification on the Laptev Sea shelf as a major Siberian ROFI (Figure 1)
Detailed hydrographic surveys were carried out in 2013 and 2014 (Figures 6, 7) under contrasting summer winds (Figure 5), which led to clear contrasts in the freshwater distribution
Summary
Coincident with the retreating summer sea ice cover in the Arctic Ocean, primary production increased by 30% between 1998 and 2012, as suggested by satellite ocean color measurements (Arrigo and van Dijken, 2015). Model projections by Slagstad et al (2015) on the future state of the Arctic Ocean ecosystem predict only regional increases in new production such as along the continental slopes. The physical processes and conditions (vertical mixing and stratification) that provide the base for these projections, are generally not well-represented in large-scale models as intense measurement efforts are needed to observe the seasonal and interannual variability of stratification. These are unknown from most Arctic regions, in particular from the Siberian shelves, which are among those regions changing fastest in recent decades
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