Trends and Transport Variability of the Circulation in the Subpolar Eastern North Atlantic

The North Atlantic Current is the major pathway for warm and saline water from the subtropics into the subpolar North Atlantic. Its main branch crosses 47 degrees/48 degrees N in the western North Atlantic basin and further north the Mid-Atlantic Ridge (MAR) before entering the eastern subpolar basin where it partly feeds the subpolar gyre. To quantify the meridional exchange of water between the subtropical and subpolar regime in the interior eastern North Atlantic, long-term (1993-2017) transport time series were calculated by combining data from inverted echo sounders taken in 2016-2017 with the dynamic topography from satellite altimetry. The data reveal an additional more direct pathway from the south across 47 degrees/48 degrees N with a mean northward transport of 9.1 Sv +/- 0.8 Sv contributing about 22% to the total inflow of 41.4 Sv into the eastern subpolar basin. The transport time series of this pathway is significantly anticorrelated to the zonal transport across the MAR (R = -0.7), damping the interannual variability of the total transport into the subpolar eastern North Atlantic. Moreover, for the meridional transport in the interior eastern basin, a positive trend of 2.0 Sv +/- 1.5 Sv per decade is found, partly balancing the negative decadal trend of -6.0 Sv +/- 5.7 Sv observed in the interior western basin. Plain Language Summary Within the North Atlantic Current, warm and salty water is transported from the subtropics into the subpolar North Atlantic. The well-studied main branch of this ocean current crosses the latitude of 47 degrees/48 degrees N in the western North Atlantic, turns eastward further north and crosses the Mid-Atlantic Ridge (MAR), before entering the eastern subpolar basin. By combining data from moored instruments with satellite observations, we calculate how much water is transported north-and southward across 47 degrees/48 degrees N in the interior eastern basin. In addition to the well-known pathway into the subpolar eastern basin crossing the MAR further north, our data reveal a more direct pathway from the south crossing 47 degrees/48 degrees N in the eastern basin. This pathway contributes 22% to the total inflow into the subpolar eastern basin. The two pathways into the eastern subpolar basin are anticorrelated: the strength of the flow across the MAR decreases when the northward flow in the eastern basin across 47 degrees/48 degrees N increases and vice versa. Moreover, the northward flow across 47 degrees/48 degrees N in the interior eastern basin has increased since 1993 while the flow across 47 degrees/48 degrees N in the western basin has decreased.