Thermohaline patterns of intrinsic Atlantic Multidecadal Variability in MPI-ESM-LR

A vivid scientific debate exists on the nature of the Atlantic Multidecadal Variability (AMV) as an intrinsic rather than predominantly forced climatic phenomenon, and on the role of ocean circulation. Here, we use a multi-millennial unperturbed control simulation and a Holocene simulation with slow-varying greenhouse gas and orbital forcing performed with the low-resolution version of the Max Planck Institute Earth System Model to illustrate thermohaline conditions associated with twelve events of strong AMV that are comparable, in the surface anomalies, to observations in their amplitudes (similar to 0.3 degrees C) and periods (similar to 80 years). The events are associated with recurrent yet spatially diverse same-sign anomalous sea-surface temperature and salinity fields that are substantially symmetric in the warm-to-cold and following cold-to-warm transitions and only partly superpose with the long-term spatial AMV pattern. Subpolar cold-fresh anomalies develop in the deep layers during the peak cold phase of strong AMV events, often in association with subtropical warm-salty anomalies yielding a meridional dipole pattern. The Atlantic meridional overturning circulation (AMOC) robustly weakens during the warm-to-cold transition of a strong AMV event and recovers thereafter, with surface salinity anomalies being potential precursors of such overturning changes. A Holocene simulation with the same model including volcanic forcing can disrupt the intrinsic AMV-AMOC connection as post-eruption periods often feature an AMOC strengthening forced by the volcanically induced surface cooling. Overall, our results support the AMV as a potential intrinsic feature of climate, whose episodic strong anomalous events can display different shades of spatial patterns and timings for the warm-to-cold and subsequent cold-to-warm transitions. Attribution of historical AMV fluctuations thus requires full consideration of the associated surface and subsurface thermohaline conditions and assessing the AMOC-AMV relation.

Automated summary

This study investigates the nature of Atlantic Multidecadal Variability (AMV) and the role of ocean circulation using an Earth System Model. The results show that AMV events are associated with anomalous sea surface temperature and salinity, but only partly match the long-term AMV pattern. Cold-fresh anomalies develop in subpolar regions during the peak cold phase of AMV events, while warm-salty anomalies occur in subtropical regions, forming a meridional dipole pattern. The Atlantic Meridional Overturning Circulation (AMOC) significantly weakens during the warm-to-cold transition of AMV events and recovers afterward. Overall, the results support the view that AMV is a potential intrinsic feature of climate with episodic strong anomalous events displaying different spatial patterns and timings for the warm-to-cold and subsequent cold-to-warm transitions, requiring a comprehensive understanding of associated thermohaline conditions and the AMOC-AMV relationship for attributing historical AMV fluctuations.