Freshwater variability in the Arctic Eurasian Shelf seas: satellite sea surface salinity and inter-annual variability to predict Arctic system change

Abstract

Eurasian Rivers provide a quarter of total fresh water to the Arctic, maintaining a persistent fresh layer that covers the surface Arctic Ocean. This freshwater export controls Arctic Ocean stratification, circulation, and basin-wide sea ice concentration. The Russian Arctic receives around 2/3 of the river runoff to the Arctic, primarily from the Ob, Yenisei and Lena Rivers which outflow into the Kara and Laptev Sea as a particularly shallow plume.

Previous in-situ and modelling studies suggest that local wind forcing is a driver of variability in Eurasian Arctic sea surface salinity (SSS) but there is no consensus on the roles river runoff and sea ice cover have in contributing to this variability or on the dominant driver of variability. The dominant controls on SSS variability have also been suggested to vary regionally, with suggested differences in the Kara and Laptev Sea.

Until recently, satellite SSS retrievals were insufficiently accurate for use in the Arctic. However, retreating sea ice cover and continuous progress in satellite product development have significantly improved SSS retrievals. This thesis first shows the value and potential of satellite SSS as a useful tool to strengthen our understanding of Arctic SSS dynamics. Satellite SSS is found to agree well with in situ data (r ≥ 0.81) with notably better agreement than reanalysis products and in situ data (r ≤ 0.76).

Satellite SSS is then used in combination with reanalysis and in-situ products to first compare and contrast the processes controlling the interannual variability of summer SSS, sea surface temperature (SST) and sea ice concentration (SIC) variability and their interactions in the Laptev and Kara Sea and in the Vilkitsky Strait.

In the Laptev Sea, zonal wind is the dominant driver of offshore/alongshore Lena River plume transport, with eastward wind driving alongshore transport (and westward wind driving offshore transport). This drives differences in both SSS and SST and spatial variability in SIC across the Laptev and East Siberian Sea. Conversely, Lena runoff does not appear to play a role in controlling interannual variability in SSS, SST, or SIC in the Laptev and East Siberian Sea. In the Kara Sea, zonal (and meridional) wind and Ob and Yenisei runoff all appear to be key drivers of whether the fresh plume is transported offshore or alongshore. Whilst eastward wind forcing is the dominant driver of alongshore transport, as is true in the Laptev Sea, a high ratio of summer Yenisei runoff/ spring Ob runoff can accentuate the low SSS anomalies offshore driven by westward wind forcing. Zonal wind forcing also has an influence on SST but is not the dominant driver of variability in SST, as it is in the Laptev Sea. Therefore, SSS and SST are notably less closely coupled in the Kara Sea and the zonal wind does not drive differences in Kara SIC.

In the Vilkitsky Strait, strong eastward wind drives buoyancy driven transport of the Ob-Yenisei plume through Vilkitsky Strait and into the western Laptev Sea and can occur over one summer season. Plume transport has a consistent SST signature, suggesting co-variability between SSS and SST but is not drive a notable difference in SIC. After a summer of westward wind forcing, plume transport through the strait appears to occur over winter. However, differences in timing drive very different SSS/SST patterns and in turn stratification dynamics.

The dominant controls on Eurasian-wide interannual variability are then identified and the implications of these on sea ice persistence and Arctic-wide freshwater storage are assessed. The Arctic Oscillation Index (AOI) is found to be a dominant control on local wind forcing in all three regions, and drives a consistent pattern of freshwater transport across all Eurasian shelf seas, which appears to be accelerating in recent decades. This pattern of freshwater transport persists until at least the following year and appears to have implications on autumn and spring sea ice persistence.

Finally, the implications of these findings are inferred in the context of climate change. The dominance of zonal wind and the AOI as a key driver of SSS (and SST) interannual variability suggests that understanding variability in wind stress and its changes is key to predicting future freshwater transport from the Eurasian shelf seas and its impacts on Arctic circulation.