On the physics of the Atlantic Multidecadal Oscillation

Abstract

The Atlantic Multidecadal Oscillation (AMO) is a pronounced signal of climate variability in the North Atlantic sea-surface temperature field. In this paper, we propose an explanation of the physical processes responsible for the timescale and the spatial pattern of the AMO. Our approach involves the analysis of solutions of a hierarchy of models. In the lowest member of the model hierarchy, which is an ocean-only model for flow in an idealized basin, the variability shows up as a multidecadal oscillatory mode which is able to destabilize the mean thermohaline circulation. In the highest member of the model hierarchy, which is the Geophysical Fluid Dynamics Laboratory R30 climate model, multidecadal variability is found as a dominant statistical mode of variability. The connection between both results is established by tracing the spatial and temporal expression of the multidecadal mode through the model hierarchy while monitoring changes in specific quantities (mechanistic indicators) associated with its physics. The proposed explanation of the properties of the AMO is eventually based on the changes in the spatial patterns of variability through the model hierarchy.

Insights into Atlantic multidecadal variability using the Last Millennium Reanalysis framework

“…Several dynamical studies using both oceanonly and fully coupled models have suggested that zonal and meridional oscillations in the AMOC on multidecadal timescales may drive changes in North Atlantic SSTs (for a description of the dynamical mechanism, see Raa and Dijkstra, 2002;Dijkstra et al, 2006Dijkstra et al, , 2008. While long-term observational records of the AMOC state are unavailable, observational evidence of sea surface height appears to support the idea that North Atlantic SSTs covary with changes in sea surface height along the eastern seaboard of the United States, which is consistent with changes in the AMOC (McCarthy et al, 2015).…”

Frequency- or amplitude-dependent effects of the Atlantic meridional overturning on the tropical Pacific Ocean

“…van Oldenborgh (oldenborgh@knmi.nl) variations of the AMOC (Delworth and Mann, 2000;Knight et al, 2005;Dijkstra et al, 2006). In particular, a positive AMO (relatively high North Atlantic SSTs) has been found to be associated with a strong AMOC (Knight et al, 2005), possibly with a phase lag between the maximum AMO and the maximum AMOC (Dijkstra et al, 2006). Using a coupled ocean-atmosphere GCM, Dong et al (2006) suggest that latent heat anomalies associated with a positive phase of the AMO lead to a deeper equatorial thermocline in the Pacific through anomalous easterly winds.…”

An experimental study of the Atlantic variability on interdecadal timescales

“…However, the divergence of the oscillation period for even smaller values of T could not be resolved in this experiment, as under such conditions the period would already have been larger than the upper limit of the selected spectral band of our interest, which was determined by the maximum measurement length. As a "mechanistic indicator" associated with the spatial pattern of the multidecadal variability, Dijkstra et al (2006) proposed to measure the phase lag between east-west and north-south temperature differences. Inspired by this idea, we processed the "meridional" and "zonal" temperature difference anomaly signals (δT M and δT Z ) accordingly.…”

“…Inspired by this idea, we processed the "meridional" and "zonal" temperature difference anomaly signals (δT M and δT Z ) accordingly. It is worth mentioning that Dijkstra et al (2006) obtained these differences using zonally and meridionally averaged temperature signals in their study, instead of pure differences of a pair of pointwise temperature records. This definitely yielded a more robust indicator of the dynamics; however, in the case of our setup such averaging would have introduced a significant bias.…”