Reconciling two alternative mechanisms behind bi-decadal variability in the North Atlantic

Abstract

International audienceUnderstanding the preferential timescales of variability in the North Atlantic, usually associated with the Atlantic meridional overturning circulation (AMOC), is essential for the prospects for decadal prediction. However, the wide variety of mechanisms proposed from the analysis of climate simulations, potentially dependent on the models themselves, has stimulated the debate of which processes take place in reality. One mechanism receiving increasing attention, identified both in idealized models and observations, is a westward propagation of subsurface buoyancy anomalies that impact the AMOC through a basin-scale intensification of the zonal density gradient, enhancing the northward transport via thermal wind balance. In this study, we revisit a control simulation from the Institut Pierre-Simon Laplace Coupled Model 5A (IPSL-CM5A), characterized by a strong AMOC periodicity at 20 years, previously explained by an upper ocean–atmosphere–sea ice coupled mode driving convection activity south of Iceland. Our study shows that this mechanism interacts constructively with the basin-wide propagation in the subsurface. This constructive feedback may explain why bi-decadal variability is so intense in this coupled model as compared to others

Oceanic control of multidecadal variability in an idealized coupled GCM

“…However, using observational data, Frankcombe et al (2008) observed the signature of large-scale SST and Sea Surface Height anomalies, propagating westward across the North Atlantic ocean. In addition, Sévellec and Fedorov (2013) show that, in a 2° global configuration of the OPA (Océan PArallélisé) model with realistic topography, the least damped mode of variability of the tangent adjoint linear model remains a potential candidate to explain the MOC multidecadal variability (Ortega et al 2015). This mode is characterized by large-scale temperature anomalies that propagate westward across the subpolar gyre, associated with long baroclinic Rossby waves .…”

Mechanisms Determining the Winter Atmospheric Response to the Atlantic Overturning Circulation

“…1a (colors) by its first empirical orthogonal function (EOF). The first mode of variability is an overturning cell with a large vertical extension reaching down to 4000 m. The 20-yr periodicity is primarily related to the westward propagation of subsurface density anomalies in the subpolar basin, referred to as the subsurface basin mode in Ortega et al (2015). Such variability has been identified in observations (Frankcombe et al 2008) and in idealized ocean models (Colin de Verdière and Huck 1999;Jamet et al 2016).…”

“…1b, the AMOC is followed 8 yr later by a surface warming of the subpolar basin, consistent with the increased northward heat transport from the subtropical Atlantic to the subpolar basin. Ortega et al (2015) argued that this warming is associated with negative density and anticyclonic ocean circulation anomalies in the eastern subpolar basin between 300 and 1000 m, which follow the AMOC by 5 yr. This leads to a reduction of the oceanic northward heat transport into the Nordic seas, which also occurs about 5 yr after an intensification of the AMOC (Fig.…”

Reconstructing extreme AMOC events through nudging of the ocean surface: a perfect model approach

“…This is the same model version used in previous perfect model studies focused both on surface nudging techniques for ocean initialisation (Servonnat et al 2015) and potential predictability of the AMOC (Persechino et al 2013), and has been extensively used in other studies analysing mechanisms of natural decadal variability in the North Atlantic (Escudier et al 2013;Ortega et al 2015), as well as evaluating historical decadal prediction skill in the ocean through surface nudging Mignot et al 2015). Using IPSL-CM5A-LR thus allows us to benefit from this in-depth expertise of the model, and also from its relatively cheap computational cost, ideal to run several sets of ensemble experiments.…”

“…1). This corresponds to the latitude where the full AMOC streamfunction and the northward thermal-wind transport reach their climatological maximum in the model (Ortega et al 2015). Since our interest lies in the thermohaline-driven component of the AMOC, the Ekman transport signal is removed from the MOI-48N index.…”