Long-term variability of NAC and DWBC volume transports and their relation to the AMOC at 47°N

The Atlantic Meridional Overturning Circulation (AMOC) is important for the climate in Western Europe and models predict a decrease in the overturning circulation due to climate change. It is the fundamental mechanism for the transports of heat, freshwater, and dissolved gases. In the region east of the Grand Banks of Newfoundland and Flemish Cap, the Deep Western Boundary Current (DWBC) is part of the lower limb of the AMOC and brings cold and fresh water to the south, which is balanced by the North Atlantic Current (NAC), which brings warm and salty water to the north. In a long-term study we investigated the transport variability along a transect at 47°N that includes DWBC and NAC and lies in the transition zone between the subpolar and subtropical gyres, where decadal changes in the AMOC are transferred southward. The interactions between the DWBC and the NAC and its influence to the AMOC variability are subject to current research. We use 6 years of moored current meter observations between 2014 and 2020 within the DWBC as well as shipboard hydrographic and current meter measurements from 15 cruises between 2003 and 2020. The shipboard and mooring data were used to calculate volume transports. The shipboard data show that the DWBC consists of two cores, one in close proximity to the continental slope with maximum velocities at mid-depth and a bottom-intensified core further offshore. The correlation of both the hydrographic properties and absolute current measurements with sea surface height was used to reconstruct time series of geostrophic transport variability from satellite altimetry alone. The sea surface height and moored current meter volume transport time series are compared to estimate the reliability of the sea surface height time series. For the offshore core a higher correlation between the time series is found than for the slope core. To estimate the relation between the DWBC cores and the NAC, the correlation between their volume transport time series is examined. The two DWBC cores are not correlated, while a comparison with a NAC time series shows that the offshore core is significantly correlated with the NAC. A combination of these two DWBC time series was constructed to cover the entire DWBC. The entire DWBC is significantly anti-correlated with the NAC, which leads to larger volume transport of the NAC when the transport of the DWBC is smaller and vice versa. Overall, the sea surface height time series show no long-term trend in DWBC volume transport. When comparing the reconstructed monthly mean DWBC transports with a time series of AMOC variability at 47°N, a significant anti-correlation is found. This indicates that AMOC variability could be characterized to a large extent by the variability of the DWBC-NAC system.