Low-frequency variability and heat transport in a low-order nonlinear coupled ocean–atmosphere model

Abstract

We formulate and study a low-order nonlinear coupled ocean-atmosphere model with an emphasis on the impact of radiative and heat fluxes and of the frictional coupling between the two components. This model version extends a previous 24-variable version by adding a dynamical equation for the passive advection of temperature in the ocean, together with an energy balance model.The bifurcation analysis and the numerical integration of the model reveal the presence of low-frequency variability (LFV) concentrated on and near a longperiodic, attracting orbit. This orbit combines atmospheric and oceanic modes, and it arises for large values of the meridional gradient of radiative input and of frictional coupling. Chaotic behavior develops around this orbit as it loses its stability; this behavior is still dominated by the LFV on decadal and multidecadal time scales that is typical of oceanic processes. Atmospheric diagnostics also reveals the presence of predominant low-and high-pressure zones, as well as of a subtropical jet; these features recall realistic climatological properties of the oceanic atmosphere.Finally, a predictability analysis is performed. Once the decadal-scale periodic orbits develop, the coupled system's short-term instabilities -as measured by its Lyapunov exponents -are drastically reduced, indicating the ocean's stabilizing role on the atmospheric dynamics. On decadal time scales, the recurrence of the solution in a certain region of the invariant subspace associated with slow modes displays some extended predictability, as reflected by the oscillatory behavior of the error for the atmospheric variables at long lead times.

The role of the ocean mixed layer on the development of the North Atlantic Oscillation: A dynamical system's perspective

“…For both values of C a clear LFV signal is visible, modulating the chaotic dynamics characterizing the “weather”‐scale variability. The LFV signal has a very long period of about 70 years, a value much larger than the LFV signal found in Vannitsem et al [].…”

“…The role of the underlying Atlantic ocean temperature dynamics has been emphasized in many investigations in the context of detailed climate models [e.g., Mosedale et al , ; Kravtsov et al , ; Gastineau et al , ; Menary et al , ], in the analysis of observations [e.g., Czaja and Frankignoul , ], and in more theoretical studies [e.g., Gallego and Cessi , ; Marshall et al , ; D'Andrea et al , ; Vannitsem et al , ]. In particular, it is well established that the upper layer of the ocean, known as the mixed layer, is the place where most of the exchanges occur between the ocean and the atmosphere, namely, heat and freshwater fluxes, radiative fluxes, and momentum fluxes through wind stresses [ Wunsch and Ferrari , ].…”

Interannual Variability in the North Atlantic Ocean’s Temperature Field and Its Association with the Wind Stress Forcing

“…The extent to which these oscillations are due to purely oceanic Berloff et al 2007, and references therein), purely atmospheric (Frankignoul and Hasselmann 1977;Frankignoul et al 1997) or coupled (Vannitsem et al 2015, and references therein) processes is still a matter of controversy. Whatever the origin of these oscillations, understanding the complex interactions between the ocean and the overlying atmosphere is crucial for the prediction of low-frequency climate variability (Kushnir et al 2002;Czaja et al 2003;Hurrell et al 2003;Visbeck et al 2003;Liu 2012;Vannitsem et al 2015;Pierini et al 2016).…”

Causal dependences between the coupled ocean-atmosphere dynamics over the Tropical Pacific, the North Pacific and the North Atlantic

“…The dynamics of the coupled ocean-atmosphere system has been recently investigated by adopting a novel approach which 10 finds its roots in the low-order modelling of such dynamical systems (Vannitsem , 2015;Vannitsem et al , 2015). It consists at projecting key fields of the large-scale dynamics of the system on a few sets of modes that are dominating its dynamics.…”

“…where Θ a,i = (ψ in a closed basin, while F i are modes used for the atmospheric fields compatible with free-slip boundaries in the meridional direction and periodic boundaries in the zonal direction, see also the paper Vannitsem et al (2015).…”