Active and passive ocean regimes in a low-order climate model

A low-order climate model is studied which combines the Lorenz-84 model for the atmosphere on a fast time scale and a box model for the ocean on a slo w time scale. In this climate model, the ocean is forced strongly by the at mosphere, The feedback to the atmosphere is weak. The behaviour of the mode l is studied as a function of the feedback parameters. We find regions in p arameter space with dominant atmospheric dynamics, i.e.. a passive ocean, a s well as regions with an active ocean. where the oceanic feedback is essen tial for the qualitative dynamics. The ocean is passive if the coupled syst em is fully chaotic. This is illustrated by comparing the Kaplan-Yorke dime nsion and the correlation dimension of the chaotic attractor to the values found in the uncoupled Lorenz-84 model. The active ocean behaviour occurs a t parameter values between fully chaotic and stable periodic motion. Here. intermittency is observed. By means of bifurcation analysis of periodic orb its, the intermittent behaviour. and the role played by the ocean model, is clarified. A comparison of power spectra in the active ocean regime and th e passive ocean regime clearly shows an increase of energy in the low frequ ency modes of the atmospheric variables. The results are discussed in terms of itinerancy and quasi-stationary states observed in realistic atmosphere and climate models.