Arctic Ocean freshwater composition, pathways and transformations from a passive tracer simulation

Abstract

Freshwater (FW) induced transformations in the upper Arctic Ocean were studied using a coupled regional sea ice-ocean model driven by winds and thermodynamic forcing from a reanalysis of data during the period 1948–2011, focusing on the mean state during 1968–2011. Using passive tracers to mark a number of FW sources and sinks, their mean composition, pathways and export were examined. The distribution of the simulated FW height reproduced the known features of the Arctic Ocean and volume-integrated FW content matched climatological estimates reasonably well. Input from Eurasian rivers and extraction by sea-ice formation dominate the composition of the Arctic FW content whilst Pacific water increases in importance in the Canadian Basin. Though pathways generally agreed with previous studies the locus of the Eurasian runoff shelf-basin transport centred at the Alpha-Mendeleyev ridge, shifting the Pacific–Atlantic front eastwards. A strong coupling between tracers representing Eurasian runoff and sea-ice formation showed how water modified on the shelf spreads across the Arctic and mainly exits through the Fram Strait. Transformation to salinity dependent coordinates showed how Atlantic water is modified by both low-salinity shelf and Pacific waters in an estuary-like overturning producing water masses of intermediate salinity that are exported to the Nordic Seas. A total halocline renewal rate of 1.0 Sv, including both shelf-basin exchange and cross-isohaline flux, was estimated from the transports: both components were of equal magnitude. The model's halocline shelf-basin exchange is dominated by runoff and sea-ice processes at the western shelves (the Barents and Kara seas) and Pacific water at the eastern shelves (the Laptev, East Siberian and Chukchi seas).

• Arctic Ocean
• freshwater
• pathways
• water mass transformation
• passive tracers

5. Acknowledgements

PP was supported by the project ‘Advanced Simulation of Arctic climate change and impact on Northern regions’ (ADSIMNOR, 214-2009-389) funded by the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS). JN acknowledges support from the Knut and Alice Wallenberg Foundation (via the SWERUS-C3 program) and the Bolin Centre for Climate Research at Stockholm University. In addition, funding from the Nordic Council of Ministers within the Top-level Research Initiative (TRI) program ‘Biogeochemistry in a changing cryosphere – depicting ecosystem-climate feedbacks as affected by changes in permafrost, snow and ice distribution’ (DEFROST) is gratefully acknowledged. The authors would like to thank Robert Newton and an anonymous reviewer for comments which significantly improved the presentation and content of the manuscript. Michael J. Pemberton is thanked for linguistic revision of the manuscript.

Notes

¹In Jahn et al. (2012) the Arctic Ocean FW balance in 10 different (four global and six regional) state-of-the-art coupled ice-ocean models was compared.

²Based on observations and an inverse model, Rabe et al. (2013) estimated the mean liquid FW export for the period 1998–2011 using S _ref=34.9.

³The area used in Yamamoto-Kawai et al. (2008) is somewhat larger (1.6×10³km²) than the BGR (1.2×10³km²) in this study.

⁴Here we used the FW storage over the total Arctic region from Fig. 3.

⁵We assume Rudels (2010) parameter values but with an area of 6.0×10⁶km² matching our region.