Séminaire
A new low-order coupled ocean–atmosphere model for mid-latitudes is derived. It is based on quasi-geostrophic equations for both the ocean and the atmosphere, coupled through momentum transfer at the interface. The systematic reduction of the number of modes describing the dynamics leads to an atmospheric low-order component of 20 ordinary differential equations, already discussed in Reinhold and Pierrehumbert (1982), and an oceanic low-order component of 4 ordinary differential equations, as proposed by Pierini (2012). The coupling terms for both components are derived and all the coefficients of the ocean model are provided. Its dynamics is explored, through the analysis of its mean field, its variability, its instability properties and its predictability. The wind-driven ocean displays a decadal variability induced by the atmospheric chaotic wind forcing. The chaotic behavior of the coupled system is highly sensitive to the ocean–atmosphere coupling, for low values of the thermal forcing affecting the atmosphere (corresponding to a weakly chaotic coupled system). But it is less sensitive for large values of the thermal forcing (corresponding to a highly chaotic coupled system).
In all the cases explored, the number of positive exponents is increasing with the coupling.Finally the dynamics of the error is explored and indicates, in particular, a rich variety of short term behaviors of the error in the atmosphere depending on the (relative) amplitude of the initial error affecting the ocean, from polynomial up to exponential-like evolutions. These features are explained and analyzed in the light of the recent findings on error growth (Nicolis, Perdigao and Vannitsem in J. Atmos. Sci. 66:766–778, 2009).