Oceanic control of multidecadal variability in an idealized coupled GCM

Abstract

International audienceIdealized ocean models are known to develop intrinsic multidecadal oscillations of the meridional overturning circulation (MOC). Here we explore the role of ocean–atmosphere interactions on this low-frequency variability. We use a coupled ocean–atmosphere model set up in a flat-bottom aquaplanet geometry with two meridional boundaries. The model is run at three different horizontal resolutions (4°, 2° and 1°) in both the ocean and atmosphere. At all resolutions, the MOC exhibits spontaneous variability on multidecadal timescales in the range 30–40 years, associated with the propagation of large-scale baroclinic Rossby waves across the Atlantic-like basin. The unstable region of growth of these waves through the long wave limit of baroclinic instability shifts from the eastern boundary at coarse resolution to the western boundary at higher resolution. Increasing the horizontal resolution enhances both intrinsic atmospheric variability and ocean–atmosphere interactions. In particular, the simulated atmospheric annular mode becomes significantly correlated to the MOC variability at 1° resolution. An ocean-only simulation conducted for this specific case underscores the disruptive but not essential influence of air–sea interactions on the low-frequency variability. This study demonstrates that an atmospheric annular mode leading MOC changes by about 2 years (as found at 1° resolution) does not imply that the low-frequency variability originates from air–sea interactions

Mechanisms Determining the Winter Atmospheric Response to the Atlantic Overturning Circulation

“…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).…”

Contributions of Atmospheric Stochastic Forcing and Intrinsic Ocean Modes to North Atlantic Ocean Interdecadal Variability

“…To provide a quantitative estimate of the contribution of each of these two processes (i.e., atmospheric versus oceanic energy source) in the variability we refer to the buoyancy variance budget, which has proven to be a powerful tool to infer the origins of the variability. Such an approach has been previously and successfully applied to the interdecadal climate variability problem in either oceanic (Colin de Verdière and Huck 1999;Arzel et al 2006Arzel et al , 2018 or coupled models (Arzel et al 2007(Arzel et al , 2012Buckley et al 2012;Jamet et al 2016;Gastineau et al 2018) with complexities ranging from idealized to fully coupled and realistic. We consider the linearized buoyancy variance equation 1 2…”

The Internal Generation of the Atlantic Ocean Interdecadal Variability

“…As a whole, these studies are inconclusive about the respective role of the ocean and the atmosphere for the AMO because of the large diversity of the proposed mechanisms and the comparisons of the patterns and time scales of the variability with observations. Two strong paradigms emerged: 1) the first one is related to the integration of the atmospheric white noise by the ocean along with its large heat capacity giving rise to a reddened spectrum (Hasselmann 1976); and 2) the second one has dynamical origins and is related to intrinsic unstable interdecadal ocean modes that spontaneously develop under steady surface buoyancy fluxes, a hypothesis that has only been tested in models ranging in complexity from idealized to intermediate (Greatbatch and Zhang 1995;Colin de Verdière and Huck 1999;te Raa and Dijkstra 2002;Arzel et al 2007;Jamet et al 2016).…”