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

Abstract

Atmospheric stochastic forcing associated with the North Atlantic Oscillation (NAO) and intrinsic ocean modes associated with the large-scale baroclinic instability of the North Atlantic Current (NAC) are recognized as two strong paradigms for the existence of the Atlantic multidecadal oscillation (AMO). The degree to which each of these factors contribute to the low-frequency variability of the North Atlantic is the central question in this paper. This issue is addressed here using an ocean general circulation model run under a wide range of background conditions extending from a supercritical regime where the oceanic variability spontaneously develops in the absence of any atmospheric noise forcing to a damped regime where the variability requires some noise to appear. The answer to the question is captured by a single dimensionless number Γ measuring the ratio between the oceanic and atmospheric contributions, as inferred from the buoyancy variance budget of the western subpolar region. Using this diagnostic, about two-thirds of the sea surface temperature (SST) variance in the damped regime is shown to originate from atmospheric stochastic forcing whereas heat content is dominated by internal ocean dynamics. Stochastic wind stress forcing is shown to substantially increase the role played by damped ocean modes in the variability. The thermal structure of the variability is shown to differ fundamentally between the supercritical and damped regimes, with abrupt modifications around the transition between the two regimes. Ocean circulation changes are further shown to be unimportant for setting the pattern of SST variability in the damped regime but are fundamental for a preferred time scale to emerge.

The Influence of Increased CO2 Concentrations on AMOC Interdecadal Variability Under the LGM Background

“…The intensified ocean stratification and thus the weakened AMOC interdecadal variability can also be illustrated through the propagation characteristics of temperature anomalies, as suggested by previous research (Arzel & Huck, 2020;Arzel et al, 2018;Frankcombe & Dijkstra, 2009;Sévellec & Fedorov, 2013). According to Buckley et al (2012), AMOC interdecadal variability is closely associated with buoyancy anomalies, particularly temperature anomalies, originating in the upper ocean of the subpolar gyre and propagating westward as baroclinic Rossby waves.…”

Global-scale random bottom pressure fluctuations from oceanic intrinsic variability

“…1 (e.g., homogeneous amplitudes within most basins but quite different across basins and with a clear global bipolar structure). Similar issues arise if one considers an explanation in terms of large-scale baroclinic instabilities, which also typically give rise to variability at (multi-)decadal scales ( 20 , 21 ) compared to the mostly subannual variability associated with mode 1 (fig. S1).…”

“…One process involves a spatiotemporal inverse energy cascade, in which the kinetic energy of mesoscale turbulence nonlinearly feeds larger spatial and longer time scales ( 17 – 19 ). Another process is large-scale baroclinic instability, where horizontal density gradients of the general circulation feed large-scale random intrinsic variability ( 20 , 21 ). The present study reveals a truly global-scale mode of pbi variability that involves a different mechanism to attain its coherence across several ocean basins.…”

Maximum Northern Hemisphere warming rates before and after 1880 during the Common Era

“…We believe that among the potential mechanisms that could cause persistent cycles and function as a pacemaker for cycle periods is ocean variability series. Ocean variability series show series with several cycle periods superimposed, e.g., Arzel and Huck (2020), and Arzel et al (2018) and both the Arzel papers and Seip and Grøn (2019b) offer explanations for how they are created. Thus, hypothesis H3, was only partially supported.…”