Atlantic water transformation along its poleward pathway across the Nordic Seas

Plain Language Summary The warm and salty Atlantic Water is substantially modified along its poleward transit across the Nordic Seas, where it reaches deeper isopycnals. In particular, the Lofoten Basin, exposed to intense air-sea interactions, plays a crucial role in the transformation of Atlantic Water. Averaged over a seasonal cycle, Atlantic Water releases approximately 80W/m(2) of heat to the atmosphere over a large area, leading to winter mixed layer depths of up to 500m (locally exceeding 1,000m in the Lofoten Basin Eddy, a permanent vortex located in the basin center) and substantial water mass transformation. We investigate spiciness injection (temperature and salinity increase) by winter mixing, by performing an isopycnal analysis using a comprehensive observational data set covering the 2000-2017 period. Compared to the Atlantic Water properties at the SvinOy section, representative of the inflowing Atlantic Water, some isopycnals reveal an important warming (up to 1.5 degrees C) and salinification (up to 0.2g/kg). Key areas for spiciness injection are the western Lofoten Basin and west of Bear Island. The modified spicy Atlantic Waters coincide with low potential vorticity with strongly density-compensated layers at their base, allowing double-diffusion processes to occur farther downstream toward the Arctic. Despite its limited spatial extent, the Lofoten Basin Eddy exhibits the greatest spiciness injection, as well as the deepest mixed layer and thickest low potential vorticity layer of the Norwegian Seas. The Atlantic Water spiciness at SvinOy shows a downstream correlation in the Lofoten Basin and farther north toward the Arctic with a lag of 1 to 1.5years. The warm and salty Atlantic Water is an important component of the ocean conveyor belt transporting heat from the equator to the pole. Substantial transformations of this water mass occur along its poleward transit across the Nordic Seas. The transformed waters, in response to winter cooling from the atmosphere, become denser and can be tracked by following the properties of seawater along surfaces of same density. The Lofoten Basin is a region, where warm and salty Atlantic Water accumulates within a 500-m-thick layer. Intense atmospheric cooling in winter is able to mix Atlantic Water all the way down to 500m, thus playing a crucial role in its transformation. The area west of Bear Island also plays an important role in this process. The transformed Atlantic Water has a vertical structure allowing for slow diffusive processes to occur farther downstream in the Arctic. The Atlantic Water properties at the entrance of the Nordic Seas are correlated with the Lofoten Basin and farther north toward the Arctic with a lag of 1 to 1.5years.