The Atlantic meridional overturning circulation in multi-decadal end of century climate predictions

For climate predictions on decadal to multi-decadal time scales, the ocean circulation has been found to carry a substantial portion of the memory from initialisations. In this study, we analyse the global ocean overturning circulation, in particular the Atlantic meridional overturning circulation (AMOC), in climate simulations with the global coupled model MPI-ESM for the time period 1960-2100. We compare an ensemble of multi-decadal predictions, initialised from a coupled assimilation simulation, and an ensemble of uninitialised simulations, both with CMIP6 historical and SSP2-45 external forcing. We find three distinct time scales for the evolution of the AMOC strength at 26N after the initialisation time. On a time scale up to 5 years after initialisation, the AMOC reacts to the initialised state with a rapid under- or overshooting when compared to uninitialised simulations, depending on the initialisation time. On a time scale of 30 to 140 years after initialisation, the AMOC by and large maintains this bias between initialised predictions and uninitialised simulations. We also find these distinct time scales in the characteristics of the AMOC cells, in both the overturning and re-circulation cells. In addition, we show that the AMOC evolution is related to the global ocean circulation. Specifically, we find a strong connection of the AMOC cell with the global Southern Ocean circulation, and we also find that multi-decadal AMOC trends are being partly compensated by changes in the strength of the Indo-Pacific meridional overturning. Our results show that the ocean circulation, in particular the AMOC, may carry the information about initialisation over multi-decadal time scales, up to 140 years. While this does not necessarily imply good prediction skill on the multi-decadal time scale, it adds another dimension on how we asses the uncertainty of climate projections until 2100.