Pathways and variability of the circulation in the subpolar eastern North Atlantic studied with inverted echo sounders and model data

Abstract

The North Atlantic Current (NAC) as part of the Atlantic Meridional Overturning Circulation (AMOC) is the major pathway for warm and saline water from the subtropics into the subpolar North Atlantic. Due to buoyancy loss along its flow path and subsequent deep water formation, it connects the upper warm limb of the AMOC with the deeper cold limb. Associated volume fluxes and their variability are thus of great interest, especially in the context of climate change. The main branch of the NAC and related transports are widely studied. The NAC crosses 47°/48°N in the western North Atlantic and further north the Mid-Atlantic Ridge (MAR) before entering the eastern subpolar basin where it partly feeds the Subpolar Gyre or flows into the Nordic Seas. To quantify the meridional exchange of water between the subtropical and subpolar regime in the interior eastern North Atlantic where studies are scarce, in this work, long-term (1993 to 2017) transport time series were calculated by combining data from inverted echo sounders taken in 2016 and 2017 with satellite altimetry. The results obtained from observational data are complemented with transport time series calculated from high resolution model output of the ANHA12 configuration of the NEMO model and with the analysis of particle trajectories calculated from the Lagrangian model ARIANE. The observational data reveal an additional more direct pathway from the south across 47°/48°N into the subpolar eastern North Atlantic 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 basin. The meridional transport of this pathway is significantly anticorrelated to the transport across the MAR (R = −0.7), damping the interannual variability of the total inflow 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 for the interior western basin. The mean transport imbalance at the 47°/48°N transect between Newfoundland and 15°W was found to be −2.2 Sv which is likely to be compensated by the flow east of 15°W. In the model, the overall circulation pattern in the subpolar North Atlantic as well as the main regions for water mass transformation are very similar to what is found from observations. However, also substantial differences between the model and observations were found such as a surplus northward flow across 47°/48°N in the western basin, a weaker coupling between the western and eastern basin, and a smaller total inflow into the eastern subpolar North Atlantic of +24.2 Sv. Moreover, the analysis of particle trajectories reveals that about 60% of the water at 47°/48°N and the MAR originates in the subtropics and about 11% flows into the Nordic Seas.