The Atlantic Multidecadal Variability in surface and deep ocean temperature and salinity fields from unperturbed climate simulations

Large multidecadal fluctuations in basin-average sea-surface temperature (SST) are a known feature of observed, reconstructed and simulated variability in the North Atlantic Ocean. This phenomenon is often referred to as Multidecadal Atlantic Variability or AMV. Historical AMV fluctuations are associated with analog basin-scale changes in sea-surface salinity, so that warming corresponds to salinification and cooling to freshening [Polyakov et al., 2005]. The surface imprint of the AMV further corresponds to same-sign fluctuations in the shallow ocean and with opposite-sign fluctuations in the deep ocean for both temperature and salinity [Polyakov et al., 2005]. This out-of-phase behavior reflects the thermohaline overturning circulation shaping North Atlantic's low-frequency variability. Several processes contribute to the AMV, involving both ocean-atmosphere coupled processes and deep ocean circulation [e.g., Grossmann and Klotzbach, 2009]. In particular, recirculation in the North Atlantic subpolar gyre region of salinity anomalies from Arctic freshwater export may trigger multidecadal variability in the Atlantic meridional overturning circulation, and therefore may be part of the AMV [Jungclaus et al., 2005; Dima and Lohmann, 2007]. With this contribution, we aim to improve the physical interpretation of the AMV by investigating spatial and temporal patterns of temperature and salinity fields in the shallow and deep ocean. We focus on two unperturbed millennial-scale simulations performed with the Max Planck Institute Earth system model in its paleo (MPI-ESM-P) and low-resolution (MPI-ESM-LR) configurations, which provide reference control climates for assessments of pre-industrial and historical climate simulations. The two model configurations only differ for the presence, in MPI-ESM-LR, of an active module for dynamical vegetation. We use spatial-average indices and empirical orthogonal functions/principal components to track the horizontal and vertical propagation of temperature and salinity anomalies related to the AMV. In particular, we discuss the potential predictability of multidecadal fluctuations in North Atlantic SSTs based on indices derived from the sea-surface salinity field. We show how the two simulations provide AMV realizations with some distinguishable characteristics, e.g., the typical fluctuations' frequencies and the linkage with the North Atlantic meridional overturning and gyre circulations. We further show how information gained by investigating different definitions of the AMV [Zanchettin et al., 2013] helps designing numerical sensitivity studies for understanding the mechanism(s) behind this phenomenon, concerning both its origin and global impacts. References Dima, M., and G. Lohmann [2007], J. Clim., 20, 2706-2719, doi:10.1175/JCLI4174.1 Jungclaus, J.H., et al. [2005], J. Clim., 18, 4013- 4031, doi:10.1175/JCLI3462.1 Polyakov, I. V., et al. [2005], J. Clim., 18:4562-4581 Grossmann, I., and P. J. Klotzbach [2009], J. Geophys. Res., 114, D24107, doi:10.1029/2009JD012728 Zanchettin D., et al. [2013], Clim. Dyn., doi:10.1007/s00382-013-1669-0