The Xiamen Symposium on Marine Environmental Sciences (XMAS 2025)

This study examines some topographic effects on seasonal variations of the southward transport of the North Atlantic Deep Water (NADW). On the seasonal and longer time scales, the ocean circulation responds to an atmospheric forcing through the propagation of Rossby and boundary waves. Such waves carry the information of pressure anomalies and influence geostrophic velocity along their pathways. Rossby waves propagate along geostrophic contours, i.e., isolines of the background potential vorticity (PV) whose distribution is shaped directly by the bathymetry. Our hypothesis is that the seasonal variability of velocity in the NADW layer between the equatorial waveguide and the Greenland-Iceland-Scotland Ridge is primarily driven by wind stress and propagated by topographic Rossby waves. To test this hypothesis, we examine seasonal changes of the ocean bottom pressure (OBP) and velocity in the NADW layer by using satellite gravimetric observations, an ocean state estimate and a wind-driven process model. The spatiotemporal evolutions of both OBP and velocity in the NADW layer are strongly influenced by the bathymetry. The magnitudes of seasonal variations increase significantly when a realistic model bathymetry is replaced by a constant water depth. The changes can be explained in terms of the topographic Sverdrup balance. Large changes in velocity and OBP are typically found along major topographic features such as continental slopes, mid-ocean ridges and trenches, in contrast with along the western boundary in a constant depth model. The results suggest that observations of the deep western boundary current (DWBC) alone may not be sufficient in accounting for the net AMOC variability.

topographic Rossby waves, topographic effect, seasonality, NADW