Density and sea level changes weaken the Atlantic Overturning: mechanistic insights from ocean bottom pressure

Abstract

Observations from the RAPID array near 26.5$^\circ$N indicate a linear decline in the AMOC over the past two decades, linked to contrasting boundary changes: a weakening western boundary contribution partly compensated by strengthening at the eastern boundary. The dynamical processes responsible for this partial compensation, however, remain unclear. Because cross-slope gradients in ocean bottom pressure (OBP) provide a dynamical measure of overturning transport, we use a high-resolution ocean model to diagnose OBP gradients across the mid-latitude North Atlantic and investigate the processes governing these boundary-specific changes. The model reproduces a meridionally consistent decline in western boundary overturning transport and a partially compensating strengthening at the eastern boundary, consistent with observations at 26.5$^\circ$N. These opposing trends arise from a vertically coherent structure in the OBP trend, shaped by two competing drivers: rising coastal sea level and decreasing density in the ocean interior. Through geostrophic balance, this dual-driver mechanism produces partial compensation between western and eastern boundary trends while also extending across latitudes, leading to a basin-wide decline of the AMOC throughout the mid-latitude North Atlantic.