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

Per Pemberton,Johan Nilsson,H E Markus Meier

doi:10.3402/tellusa.v66.23988

Per Pemberton, Johan Nilsson + Show 1 more

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https://doi.org/10.3402/tellusa.v66.23988

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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).

Highlights

One of the key features of the Arctic Ocean is the stable stratification due to the sharp salinity gradient in the upper ocean
From the passive tracers we examine the composition of FW in the central Arctic Ocean and infer pathways of how low saline water is transported through the system
In this study we have explored how FW input and extraction, represented by a number of different passive tracers, transform the inflowing Atlantic water (AW) and Pacific water (PW) to become the upper water masses of the Arctic Ocean

Summary

Introduction

One of the key features of the Arctic Ocean is the stable stratification due to the sharp salinity gradient in the upper ocean. This effectively inhibits the heat stored in the warmer Atlantic water (AW) layer from penetrating upwards and potentially melting the sea ice The PML is strongly affected by the variations in the large-scale atmospheric forcing typically alternating between a mode of strong anti-cyclonic circulation centred over the Canada Basin and a mode with weaker anti-cyclonic circulation and stronger cyclonic circulation primarily over the shelf seas and the Eurasian side (Hunkins and Whitehead, 1992; Proshutinsky and Johnson, 1997). When atmospheric vorticity is anti-cyclonic over the shelf, runoff-diluted creativecommons.org/licenses/by/4.0/), allowing third parties to copy and redistribute the material in any medium or format and to remix, transform, and build upon the material for any purpose, even commercially, provided the original work is properly cited and states its license

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