Under Pressure: Telltale Signs of the Weakening of the AMOC

Abstract

The Atlantic Meridional Overturning Circulation (AMOC) is widely expected to weaken under greenhouse warming, and climate models and proxy reconstructions suggest that this decline may already be underway. Yet direct observational evidence has remained elusive: the monitoring arrays are few, relatively short-lived, and each relies on its own methods, making it hard to tell a genuine trend from decadal variability. In this talk, I will show that the ocean may have been keeping a record of its own, written in pressure along the seafloor of the western boundary. Applying a common method based on cross-slope gradients of ocean bottom pressure to four mooring arrays spanning 16.5°N to 42.5°N, we find a meridionally consistent decline in the deep western overturning transport over the past two decades. At 26.5°N, this decline is stronger than the AMOC trend measured by the RAPID array, because a strengthening contribution at the eastern boundary partly offsets it. Is this compensation a regional quirk or a basin-wide feature, and what drives it? Using a high-resolution ocean model that reproduces the observed changes, I will show that the opposing boundary trends share a single origin: a vertically coherent trend in bottom pressure shaped by two competing processes, rising coastal sea level and decreasing ocean interior density. Through geostrophic balance, these changes produce partial compensation between the boundaries at all latitudes and a basin-wide AMOC decline across the mid-latitude North Atlantic. Together, the results point to the western boundary as a sensitive indicator of AMOC change, and to bottom pressure as a promising way to watch it.