We quantify Atlantic Water heat loss north of Svalbard using year-long hydrographic and current records from three moorings deployed across the Svalbard Branch of the Atlantic Water boundary current in 2012-2013. The boundary current loses annually on average 16 W m −2 during the eastward propagation along the upper continental slope. The largest vertical fluxes of >100 W m −2 occur episodically in autumn and early winter. Episodes of sea ice imported from the north in November 2012 and February 2013 coincided with large ocean-to-ice heat fluxes, which effectively melted the ice and sustained open water conditions in the middle of the Arctic winter. Between March and early July 2013, a persistent ice cover-modulated air-sea fluxes. Melting sea ice at the start of the winter initiates a cold, up to 100-m-deep halocline separating the ice cover from the warm Atlantic Water. Semidiurnal tides dominate the energy over the upper part of the slope. The vertical tidal structure depends on stratification and varies seasonally, with the potential to contribute to vertical fluxes with shear-driven mixing. Further processes impacting the heat budget include lateral heat loss due to mesoscale eddies, and modest and negligible contributions of Ekman pumping and shelf break upwelling, respectively. The continental slope north of Svalbard is a key example regarding the role of ocean heat for the sea ice cover. Our study underlines the complexity of the ocean's heat budget that is sensitive to the balance between oceanic heat advection, vertical fluxes, air-sea interaction, and the sea ice cover. Plain Language SummaryThe Atlantic Water boundary current carries heat into the Arctic Ocean as it flows through Fram Strait and along the continental slope north of Svalbard. Using observations from bottom-mounted instruments, we investigated different processes leading to heat loss from the Atlantic Water layer in the region north of Svalbard. Most of the changes recorded over the course of 1 year from September 2012 to September 2013 at 81.5 ∘ N, 31 ∘ E are driven by changes further upstream and by air-sea heat exchange. However, significant local heat loss can be caused by mixing due to wind or tides. Seasonal differences are large and predominantly caused by absence or presence of sea ice (autumn/early winter versus spring/early summer), influence of melt water and wind on the stability of the water column, and a seasonally changing light regime.

The characteristics and seasonality of the Svalbard branch of the Atlantic Water (AW) boundary current in the Eurasian Basin are investigated using data from a six‐mooring array deployed near 30°E between September 2012 and September 2013. The instrument coverage extended to 1,200‐m depth and approximately 50 km offshore of the shelf break, which laterally bracketed the flow. Averaged over the year, the transport of the current over this depth range was 3.96 ± 0.32 Sv (1 Sv = 106 m3/s). The transport within the AW layer was 2.08 ± 0.24 Sv. The current was typically subsurface intensified, and its dominant variability was associated with pulsing rather than meandering. From late summer to early winter the AW was warmest and saltiest, and its eastward transport was strongest (2.44 ± 0.12 Sv), while from midspring to midsummer the AW was coldest and freshest and its transport was weakest (1.10 ± 0.06 Sv). Deep mixed layers developed through the winter, extending to 400‐ to 500‐m depth in early spring until the pack ice encroached the area from the north shutting off the air‐sea buoyancy forcing. This vertical mixing modified a significant portion of the AW layer, suggesting that, as the ice cover continues to decrease in the southern Eurasian Basin, the AW will be more extensively transformed via local ventilation.

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