On the non-stationarity of Atlantic multidecadal variability and underlying mechanisms

The Atlantic Multidecadal Variability (AMV) is a climate mode of variability that affects the surface ocean state of the North Atlantic on the multidecadal time scale. It is characterized by a horse-shoe shaped pattern in the sea surface temperature (SST) anomalies, connecting the subpolar and subtropical regions. When only internal variability is considered, the Atlantic Meridional Overturning Circulation (AMOC) is recognized to be the main driver of AMV, but their connection is not stationary in time. In fact, when multicentennial pre-industrial simulations are analyzed, models robustly simulate multidecadal windows where the AMV and the AMOC are essentially uncorrelated. This thesis aims to further inspect the non-stationarity of the AMOC-AMV relationship in the idealized framework of a climate system only subject to internal variability processes. For this purpose, a multi-centennial pre-industrial climate simulation from the CESM-LME suite of experiments is used to characterize the state of the ocean under different variability regimes of the AMOC and AMV signals. Specifically, a change-point detection method is applied to identify different AMOC-AMV co-variability regimes. Next, an ocean heat budget analysis is used to identify the mechanisms underlying the different variability regimes, focusing on the Ocean Heat Content in the Subpolar Gyre region, through the evaluation of the contributions from thermodynamical and dynamical terms in the heat budget equation. The obtained results suggest that the loss of AMOC-AMV correlation can be related to changes in the ocean circulation features, with a key role being played by the meridional ocean heat transport convergence. Two differences are found between the AMOC-AMV correlated and non-correlated regimes: in the latter case, the variability of the AMOC index cannot explain the subtropical features of the horseshoe SST pattern and it appears to be less relevant to the variability of the SPG heat content.