Gerard McCarthy, Guillaume Hug, David A Smeed, Ben I Moat

The Atlantic Meridional Overturning Circulation (AMOC) is key to the redistribution of heat in the climate system. It is projected to weaken due to climate change. The RAPID mooring array observes the strength of the AMOC, showing an overall weakening of 1.0 Sv/decade from 2004–2023. However, the significance of this trend is controversial. Here we consider the RAPID observations in a signal-to-noise framework to understand where low frequency, climatic signals are strongest. There is a strong signal in Lower North Atlantic Deepwater (LNADW) transports. In contrast, we find little signal and significant noise in Ekman transports and in revised Gulf Stream estimates. We remove the influence of the Ekman transport on AMOC and LNADW estimates, reducing the noise by 30% and 22% respectively. We find a simple model of LNADW, based on deep hydrography on the western boundary, has a similar signal-to-noise ratio to the full AMOC estimate, showing how climate ‘signal’ is concentrated in the deep ocean. Understanding the sources of ‘noise’ and ‘signal’ is key to timely detection of climatic change in the AMOC and in attribution of observed changes.