The Eastern Atlantic Basin Pathway for the Export of the North Atlantic Deep waters

The North Atlantic deep water (NADW), according to the classic ocean circulation theory, moves southward as a deep western boundary current (DWBC) even though it may veer into interior and then rejoin DWBC when encountering regional circulation features, such as eddy-driven recirculation. In potential vorticity dynamics, the eastern side of the Mid-Atlantic Ridge (MAR) may provide a similar topographic support as the continental slope off the western boundary for a southward transport of NADW. In this article, we quantify the mean meridional NADW transports on both sides of the MAR using a data-assimilated product and find that the flow in the eastern basin contributes about 38 +/- 14% of the net southward transport of NADW from 50 degrees to 35 degrees N. Our study points to the importance of observing NADW transport variations on the eastern side of the MAR in order to monitor the transport strength of Atlantic Meridional Overturning Circulation. Plain Language Summary Classic theory suggested that the Deep Western Boundary Current (DWBC) carries the cold and fresh North Atlantic deep water (NADW) from the high latitude to the south. However, float and tracer observations, albeit sparse, have revealed a possible southward pathway in the eastern North Atlantic, that is, east of the Mid-Atlantic Ridge (MAR). Using an ocean reanalysis product, the Estimating the Circulation and Climate of the Ocean (ECCOV4r4), we show that the steady-state NADW meridional transport in the basin east of the MAR accounts for about 38 +/- 14% of the meridional transport in the whole basin between 35 degrees and 50 degrees N. It is found that the NADW circulation pathways are strongly influenced by ocean bottom topography. For example, the eastern flank of the MAR acts as a "western boundary" in the eastern basin against which NADW flows southward. The topographic constraint from the MAR weakens south of 35 degrees N due to changes in the MAR height and smaller planetary vorticity f . The current study has improved our understanding of NADW transport structure at mid-latitude by quantifying the equatorward transport east of the MAR.