Disentangling the AMOC: influences of the gyre circulation, surface density transformations and overturning circulation on AMOC variability

A simplistic view of the Atlantic Meridional Overturning Circulation (AMOC) is that it is composed of a northward flow of warm, salty water that sinks at high latitudes as a result of wintertime surface buoyancy losses, subsequently returning southward as the cold, fresh Deep Western Boundary Current. Its strength is often expressed as the maximum of the overturning streamfunction in a depth and latitude range, the latter normally centred at around 26°N. In reality, however, the AMOC is partly “carried” by the largely wind-forced and near-barotropic horizontal gyre circulation, and the production of upper NADW has also been shown to depend on a chain of surface densification around the subpolar gyre, in addition to the deep convection localised in the Labrador and Irminger Seas. The overturning streamfunction in depth coordinates is therefore far from being a complete description of the AMOC.

We present a range of AMOC metrics using a set of centennial simulations of the HadGEM3-GC5 coupled model with a ¼° NEMO ocean component. These include a gyre index based on the barotropic streamfunction, surface-forced indices, regional mixed-layer volumes, and transport indices evaluated against a range of vertical axes. We compare these indices with the traditional overturning metric at the RAPID section at 26°N, and discuss the causal links between them. This work is carried out under the EU HORIZON25 project Explaining and Predicting the Ocean Conveyor (EPOC).