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

The Meridional Overturning Circulation (MOC) is key to the redistribution of heat and is projected to weaken due to climate change. The RAPID mooring array observes the strength of the MOC, showing an overall weakening of 1.4 Sv/decade from 2004–2022. 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 tranpsorts. We remove the influence of the Ekman transport on MOC and LNADW estimates, reducing the noise by 30% and 22% respectively. We find a simple model of LNADW has a comparable signal-to-noise ratio as the full MOC estimate. Understanding the sources of ‘noise’ and ‘signal’ is key to timely detection of climatic change in the MOC.