The subpolar North Atlantic is one of the key regions in the climate system. Here, the deep water masses of the cold limb of the AMOC are formed (Labrador Sea Water) or significantly modified (overflow water masses). Paleoceanographic reconstructions and climate model studies imply a key role of Labrador Sea circulation in driving millennial-scale climate variability. On shorter timescales, comprehensive hydrographic data reveal considerable variations in convection and deep-water formation in the central Labrador Sea, which are currently only partly understood. Future changes in the strength of the AMOC will depend critically on the distribution of heat and salt in the North Atlantic. ArcTrain will establish climatologies for the subpolar North Atlantic and extended time series from paleoclimate archives for benchmarking and improving ocean-circulation models and understanding the physical processes associated with heat and salt transfer in the ocean.